A 10-year climatology of warm-season cloud patterns over Europe and the Mediterranean from Meteosat...

Atmospheric Research 97 (2010) 555–576

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Atmospheric Research

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A 10-year climatology of warm-season cloud patterns over Europe and theMediterranean from Meteosat IR observations

Vincenzo Levizzani a,⁎, Francesca Pinelli a,b, Massimiliano Pasqui c, Samantha Melani c,Arlene G. Laing d, Richard E. Carbone d

a ISAC-CNR, Bologna, Italyb University of Bologna, Italyc Ibimet-CNR, Sesto Fiorentino, Italyd NCAR, Boulder, CO, USA

a r t i c l e i n f o

⁎ Corresponding author. ISAC-CNR, via Gobetti 101,Tel.: +39 051 6398015; fax: +39 051 6399749.

E-mail address: [email protected] (V. Levizzan

0169-8095/$ – see front matter © 2010 Elsevier B.V.doi:10.1016/j.atmosres.2010.05.014

a b s t r a c t

Article history:Received 5 December 2009Received in revised form 16 April 2010Accepted 28 May 2010

Thermal infrared (IR, 10.5–12.5 μm) images from the Meteosat Visible and InfraRed Imager(MVIRI) of cold cloud episodes (cloud top brightness temperatureb241 K) are used as a proxy ofprecipitating clouds to derive a warm-season (May–August) climatology of their coherency,duration, span, and propagation speed over Europe and the Mediterranean. The analysis focusesover the 30°–54° N, 15°W–40°E domain in May–August 1996–2005. Harmonic analysis usingdiscrete Fourier transforms is applied together with a statistical analysis and an investigation ofthe diurnal cycle. The objective of the study is tomake available a set of results on the propagationdynamics of the cloud systems with the aim of assisting numerical modellers in improvingsummer convection parameterization.The zonal propagation of cold cloud systems is accompanied by a weak meridional componentconfined to narrow latitude belts. The persistence of cold clouds over the area evidences therole of orography, the Pyrenees, the Alps, the Balkans and Anatolia. A diurnal oscillation isfound with a maximum marking the initiation of convection in the lee of the mountains andshifting from about 1400 UTC at 40°E to 1800 UTC at 0°. A moderate eastward propagation ofthe frequency maximum from all mountain chains across the domain exists and the diurnalmaxima are completely suppressed west of 5°W. The mean power spectrum of the cold cloudfrequency distribution evidences a period of one day all over Europe disappearing over theocean (west of 10°W). Other maxima are found in correspondence of 3 to 7 day synopticactivity. A median zonal phase speed of 16.1 m s−1 is found for all events≥1000 km and≥20 hand a full set of results divided by year and recurrence categories is also presented. Themaximaof the diurnal signal are in phase with the presence of elevated terrain and with land masses.

© 2010 Elsevier B.V. All rights reserved.

Keywords:CloudsWarm seasonMediterranean SeaRemote sensingSatellite meteorologyRegional climate

1. Introduction

With the increasing spatial resolution of numerical weatherprediction (NWP) models and the advent of non-hydrostaticand explicit models, the low skills of quantitative precipitationforecasts (QPF) in the warm season (e.g., Olson et al., 1995)

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in absolute terms or with respect to those in other seasonsbecome a more stringent problem (see Section 1.1). Researchneeds for improving convective precipitation predictability inthewarm seasonwere recognized (Fritsch and Carbone, 2004),stressing the necessity of improving the process representationthrough adequate observing strategies. A better understandingof organized convection includes factors associated withpropagation, dissipation and regeneration.

Carbone et al. (2002) have first addressed the mecha-nisms governing the sustained motion of organized convective

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556 V. Levizzani et al. / Atmospheric Research 97 (2010) 555–576

systems that may be external or internal to a moist mesoscalecirculation. Several mechanisms were proposed offering ex-planations for convective system propagation that may exceedambient winds. However, a systematic global examination ofthe life cycle of coherent convective rainfall episodes is notavailable, including the aspects dealingwith their regeneration.Regional and global observing strategies must be adopted thatlead to an improved description of the genesis and propagationcharacteristics of organized long-lived convective systems at alllatitudes. Systematic observations from remote sensing sys-tems are needed to ensure the adequate space/time resolutionat all scales. Fundamental studies and observations in variousparts of the world (Section 1.2) were conducted. However, aEuropean climatology is still lacking.

The paper wants to contribute filling this gap by presentingthe dynamic characteristics of warm-season (May–August)cloudiness over Europe and the Mediterranean using 10 year(1996–2005) of Meteosat thermal infrared (IR) imagery at halfhourly interval. Basic dynamicparameters arequantified for thefirst time such as span, duration, zonal phase speed, and thediurnal cycle of cloud occurrence. For August 2002 a sampleanalysis of storm track, soil moisture and precipitable wateris also included. These rather fundamental quantities are atpresent not adequately measured over the region and they areneeded when conceiving model parameterizations. Meteosatdata make it possible to gather a complete dataset over thecontinent and the Mediterranean, which cannot be ensured byradar or rain gauge coverage.

1.1. Representation of summer convection in NWP models

The occurrence, characteristics, propagation and diurnalcycle of mesoscale convective systems (MCS) are generallypoorly represented in NWP models (e.g., Davis et al., 2003),which perform reasonably well only when rainfall maximizesin the late afternoon and remains a local phenomenon. Modelsimulations of summer precipitation with different cumulusparameterization schemes (Dai et al., 1999) pointed to defi-ciencies in capturing the diurnal cycle of precipitation and toan overestimation of precipitation frequency and underesti-mation of its intensity. The problem of the too early triggeringof convective precipitation over land after sunrise is sharedby many general circulation models (GCM) (Bechtold et al.,2004) depending on the parameterizations. Simulations usingcloud resolvingmodels (CRM) and single columnmodels (SCM)(Guichard et al., 2004) indicated that the representation of thediurnal cycle of deep convection still remains a challenge.

A comparison of satellite-derived rainfall rates and modelforecasts during the North America Monsoon Experiment(NAME) (Janowiak et al., 2007) showed that the models over-forecast the frequency of rainfall events in the 1–75 mm day−1

range and underforecast heavy events (N 85mm day−1); theirdiurnal cycle peaks 3–6 h earlier than the observed one.

The need for a better description of summer organizedconvection into the last NWP model generation is exemplifiedby the performance of daily convection forecasts of theWeather Research and Forecast (WRF) model (Done et al.,2004). Although forecasts using a 10-km grid spacing andparameterized convection do not lack in prediction of convec-tive rainfall, fully explicit forecasts with a 4-km grid spacingmore often predict identifiable MCSs that correspond to

observed systems in time and space. Furthermore, the explicitforecasts more accurately predict the number of MCSs dailyand type of organization. The explicit treatment of convectionin NWP does not necessarily provide a better point specific-forecast, but rather a more accurate depiction of the physics ofconvective systems.

Problems arise also when investigating the predictabilityof an extremewarm-season precipitation event, which leads toflooding conditions (Zhang et al., 2006). Substantial improve-ments of the forecasting accuracy could be gained by refiningthe initial analysis and adopting better assimilation techniquesor enhanced observations, together with the introductionof better resolved or parameterized physical processes.

1.2. Observations

Studies of MCSs were conducted by several authors (e.g.,Maddox, 1980; Fritsch et al., 1986; Augustine and Caracena,1994; Anderson and Arritt, 1998; Trier et al., 2000) whoinvestigated their life cycle, the initiation in the lee of theRocky Mountains, and the eastward propagation with anovernightmaximum of precipitation across the central plains.Other studies have focused in time on the internal structureand evolution of storms in various parts of the world bymeans of field studies, satellite observations and modellingefforts (e.g., Maddox, 1983; Kane et al., 1987; Leary andRappaport, 1987; Velasco and Fritsch, 1987; Houze et al.,1990; Keenan and Rutledge, 1993; Laing and Fritsch, 1993;Laing et al., 1999).

The data of the US WSR-88D Next Generation WeatherRadar (NEXRAD) Doppler radar network were used (Carboneet al., 2002) to derive a climatology of warm-season precipi-tation episodes over the continental US in the 1998–2000 timeperiod. Key findings are: 1) episodes exhibit coherent rainfallpatterns, characteristic of propagating events, under a broadrange of atmospheric conditions, 2) the patterns are mostfrequent under “weakly-forced”midsummer conditions, 3) thelongevity of the episodes (up to 60 h) suggests an intrinsicpredictability significantly exceeding the lifetime of a singleconvective system, 4) the episodes are initiated mostly due todiurnal and semidiurnal forcings, 5) the propagation speed ofmajor episodes is often higher than that attributable to large-scale forcing or to the advection from low- tomidlevel steeringwinds. A further studyon the rainfall diurnal cycle overwesternand central US (Ahijevych et al., 2004) found a coherent phasetransition of the precipitation maximum between the RockyMountains and the Great Plains from afternoon to night time,thus evidencing the long-range influence of orography onprecipitation frequency. Two further studies over NorthAmerica concentrated on the temporal structure of precipita-tion (Hsu et al., 2006) and on the diurnal occurrence of rainfall(Carbone and Tuttle, 2008). Other studies over North Americawere conducted by means of: 0.1°×0.1° geostationary (GEO)satellite infrared (IR) radiances at 11 μm in combination with2.5°×2.0° hourly surface precipitation observations (Tian et al.,2005); cloud to ground lightning, kinematics and precipitation(Tadesse and Anagnostou, 2009); rain gauges, GEO IR imageryand the Precipitation Radar (PR) aboard the Tropical RainfallMeasuring Mission (TRMM) (Gebremichael et al., 2007).

A study of the diurnal variation of cloudiness over EastAsia using 3-hourly thermal IR data from the Geostationary

557V. Levizzani et al. / Atmospheric Research 97 (2010) 555–576

Meteorological Satellite (GMS-3) for the summer 1987 (Asaiet al., 1998) concluded that the diurnal cycle results from thesuperposition of a large diurnal cycle with a small semidiur-nal cycle and that there is a systematic phase delay of thediurnal cycle variation east of the Tibetan Plateau. The GMS IRimagery was used to derive a climatology of warm-seasoncloud patterns over East Asia for 1998–2001 (Wang et al.,2004) showing coherent patterns of summer convection andcharacteristics of propagating events in the longitude–timespace. The same dataset was used to investigate the regionaland intraseasonal variability of East Asian warm-seasoncloud episodes due to land–sea contrast and latitudinal effects(Wang et al., 2005). Other studies over Asia used a varietyof precipitation products from satellites and the ground: hourlyor 3-hourly data from the Precipitation Estimation fromRemotely Sensed Information using Artificial Neural Networks(PERSIANN; Hsu et al., 1997) algorithm, TRMM3B42 (Huffmanet al., 2007) and rain gauges (Zhou et al., 2008) over East Asia;hourly, 0.05°×0.05° TRMM PR (Bhatt and Nakamura, 2005)data around the Himalayas, and successively with emphasis onits southern slopes (Bhatt and Nakamura, 2006); TRMM Real-Time Multi-Satellite Precipitation Analysis (TMPA-RT) dataover the Bay of Bengal and the adjacent coastal region (Liu et al.,2008).

Harmonic analysis of 3 h means from the TRMM PR 2A25rain rates product (DeAngelis et al., 2004) over Tropical andsub-Tropical South America revealed the extent and propaga-tion of rain bands from the western and eastern Amazontowards the interior, and from western subtropical SouthAmerica towards the east. TRMMPRdata and shipboardC-bandradar data were applied to explore the diurnal cycle of pre-cipitation over Tropical East Pacific (Cifelli et al., 2008) findingdistinct diurnal features of MCSs with increasing intensity inthe afternoon and a phase lag response in the oceanic regions toafternoon–evening convection over the Central Americanlandmass. Warm-season cloudiness/precipitation using GMSIR data for the 1996–2001 time period was used to studyconvective events over Australia and the maritime continent(Keenan and Carbone, 2008).

Five years of Meteosat IR data May–August (1999–2003)were recently analyzed (Laing et al., 2008) to study the pro-pagation and diurnal cycle of organized convection in northerntropical Africa. A satellite-based climatology of European andMediterranean cloud systems is available from TIROS-NOperational Vertical Sounder (TOVS) data (Chaboureau andClaud, 2006). A limited study over 5 years of Meteosat IR datawas also conducted (Levizzani et al., 2006) and subsequentlyexpanded to 10 years (Pinelli, 2010) as a baseline for the presentstudy.

1.3. The present study structure

In the following section a brief description is given ofthe analysis methodology. Section 3 presents the character-istics of the zonal and meridional propagation and dynamics/thermodynamics of coherent cloud patterns for the year 2002as an indication of summer propagation of cloud systemsin the area. Section 4 reports the results on frequency andperiodicity of the cloud systems, and Section 5 the completestatistics of cloud streaks for the whole period. Conclusionsand summary are detailed in Section 6.

2. Satellite data and methods of analysis

2.1. Satellite data and model analyses

Given that a suitable dataset of rainfall rates from groundradarnetworks covering the areaof the studywasnot available,cold cloud tops were identified through low Meteosat IRbrightness temperatures (Tb) as the closest proxy to precipi-tating clouds. Note that the IR Tb is usually less correlated withrainfall amount with respect to radar reflectivity and does notalways indicate deep convection. The correlation betweenradar-estimated rainfall and themean fractional cloudcoveragefor a certain area improves as the spatial and temporal scalesare increased (Richards and Arkin, 1981); for averaging scalesof 1.5° or larger correlations exceeding 0.8 were found for abroad range of threshold temperatures. Nevertheless, IRthresholding techniques have long been used to identify cloudsand to estimate precipitation (e.g., Arkin, 1979; Arkin andMeisner, 1987) and therefore can serve as a proxy for rainfallalthough their precise relationship is not well determined.Cirrus contamination is certainly present in this kind ofdatasets, but Wang et al. (2004) have demonstrated that cirrusclouds do not significantly affect the convective cloud toptemperature determination by comparing the structure of datain the thermal IR and the water vapour channels. Surfacetemperature variations in cloud free areas are another issue tobe considered and the threshold cloud top temperature takescare of this aspect. However, note that in the present paper“cold cloud episodes” refer to cloud clusters with cold cloudtops in the distance-time space and are to be considered only asproxies to rainfall episodes.

Radiances from the Meteosat Visible and InfraRed Imager(MVIRI) IR spectral band (10.5–12.5 μm) were extracted fromthe archive of the European Organization for the Exploitationof Meteorological Satellites (EUMETSAT) for the 1996–2005warm-season period, May to August, and Tbs were computedusing the instrument's calibration. The IR images have a spatialresolution of 5 km×5 km at the sub-satellite point (0°, 0°) andare available at 30 min intervals; the resolution at SouthernEuropean latitudes degrades down to about 7 km×8 km.These resolutions are both worse than those of the radar datautilized byCarboneet al. (2002) (2 km×2 km,15 min), butwereconsidered sufficient for the identification of cold cloud systems.Moreover, they are similar to the GMS-5 IR data used by Wanget al. (2004) (5 km×5 km, 1 h).

The domain of analysis was selected taking into consid-eration the prevailing low-level flow, the continental bound-aries, and tracks of precipitating systems. The latitude andlongitude ranges of the domain are 30–54°N and 15°W–40°E,respectively (Fig. 1). The northernmost latitude limit is deter-mined by the increasing deformation of the Meteosat imagepixels, which gradually degrades the spatial resolution whenmoving away from the equator.

A limited analysis using the reanalyses of the NationalCenters for Environmental Prediction/National Center forAtmospheric Research (NCEP/NCAR; Kalnay et al., 1996) wascarried out for August 2002 in order to gain a first insighton storm tracks, precipitable water and dynamic character-istics underlying the convective activity. The exercise is byno means exhaustive and is part of the ongoing researchactivity.

Fig. 1. Computational domain for satellite cold cloud top Hovmöller diagrams. The area was divided into 1100 vertical strips of 0.05° width (∼4 km).


2.2. Analysis method

The analysis methodology is based on longitude–timeHovmöller diagrams as already applied over other geographicareas (North America— Carbone et al., 2002; East Asia—Wanget al., 2004; northern tropical Africa — Laing et al., 2008;Australia— Keenan and Carbone, 2008; the Bay of Bengal— Liuet al., 2008) so that results can be intercompared. The methodaims at reducing the dimensions of the dataset from three totwo in order to examine the zonal component of the cloudsystem propagation at the expense of the meridional compo-nent. The zonal component is in fact the dominant propaga-tion direction of the cloud systems over Europe. The domainis divided into 1100 vertical strips of 0.05° width (∼ 4 km)spanning from 30 to 54°N in the N–S direction.

Carbone et al. (2002) averaged the radar-derived precip-itation data along each longitudinal strip (for more detailssee also Ahijevych et al., 2001). Wang et al. (2004) replicatedthe analysis for satellite-derived Tbs below 273.15 K over EastAsia and reported the resulting longitude–time pairs onto aHovmöller diagram applying a threshold value of 241 K toidentify likely precipitating systems.

For the present study the results of a preliminary sensitivityanalysis (not shown) indicated that a cloud top Tb threshold of241 K effectively identifies the convective systems that spanover Europe and the Mediterranean region against the warmerbackground. The data points in the Hovmöller diagrams don'trepresent the mean of all Tb values falling within each strip ofthe latitude/longitude grid since the average refers only toTb≤241 K. Convective systems are thus better identified thanusing the 265 K threshold applied by Levizzani et al. (2006)over the same domain. On the other hand, the 241 K thresholdvalue revealed much more effective for the European andMediterranean area than the 230 K chosen by other authors(e.g., Tian et al., 2005) and deriving from theGOES Precipitation

Index (Arkin and Meisner, 1987). The reason is that it capturesmany more convective systems, which would otherwise goundetected by using the colder threshold.

The quantitative analysis is based on the application ofthe two-dimensional autocorrelation function introduced byCarbone et al. (2002) to the Tb data in the Hovmöller lon–time space. The function is rectangular in one dimension andweighted by a negative cosine in the other (Fig. 2) since lowercloud top temperatures correspond to higher rainrates or,moreproperly in this case, to stronger convection. The convectiveevents are quantified as to their coherency, longevity, zonaldistance (hereinafter referred to as “span”), and propagationspeed. The 2D function is stepped through all grid points in theHovmöller space and rotated until the correlation coefficient ismaximized. Sequences of contiguous “fits” define the coherentspan, duration and propagation characteristics for each cloudpattern (cloud streak, see Fig. 2). The fit relies upon the cloudtop temperature threshold of 241 K and a correlation coefficientof 0.35 is required for the fit to become part of the statistics. Thestart and endpoints of each streak define its span, duration, andaveraged propagation speed. The rectangular pulse of thefunction ∼4° in longitude and the time dimension 3 h,comparable to the size of individual MCSs.

Note that the span and duration statistics derived fromsatellite IR Tbswith thismethod are highly dependent upon theTb threshold used, but the appropriate choice of the thresholdcan be brought into good agreement with those based uponradar data (Tuttle et al., 2008). Upon comparing radar- andsatellite-derived speed statistics over the United States theseauthors found that the propagation-speed statistics of satelliteevents are on average ∼4 m s−1 faster than radar events andare relatively insensitive to the Tb threshold.

To gain a better graphical representations of cloud streaksthe Hovmöller diagrams are plotted using a threshold Tb=251 K, a value higher than the threshold applied to the 2D

Fig. 2. Example of brightness temperature Hovmöller diagram (June 2002)with autocorrelation fits superimposed using a dimension of autocorrelation function of4° and 3 h. The negative cosine-rectangular weighting function is shown that is rotated until the autocorrelation coefficient is maximized.


autocorrelation function to identify the cloud streaks. A set ofaveraged Hovmöller diagrams is created to examine the phase-locked behaviour of cold cloud signals at diurnal and higherfrequencies. Every cold cloud top with Tbb241 K constitutes an“event” at a given longitude–time coordinate pair. The cumula-tive event frequency is then averaged for monthly and seasonalperiods for the investigation of regional, intraseasonal, and in-terseasonal variations.

The second step of the analysis concerns the identificationof the spatial distribution of cold clouds for the whole 10-yearperiod during the four summer months as a frequency analysisof cold cloud episodes. This contributes to identify the areaswhere cold clouds are preferentially found especially withrespect to the orography. The average diurnal cycle is then

used to identify phase-locked behaviours and the initiation ofconvection with respect to the diurnal heating both for theoverall period and for each month.

Discrete Fourier transforms (DFT) are successively appliedto various longitudinal bands of the domain obtaining thepower spectrum of time series of averaged cloud top Tbsin the lon–time Hovmöller space. The longitude bands areconceived to identify the contribution to the spectra of themajor mountain chains. The harmonic decomposition of thepower spectrum is also conducted to quantify the amplitudeof the diurnal and semidiurnal component of the cycle.

Finally, a statistical analysis of the zonal characteristics ofthe cold cloud propagation is done as regards to span, durationand phase speed. A complete set of results is reported on the

image of Fig.�2

Fig. 3. Mean cross section of the zonal wind component over the area ofinterest from NCEP/NCAR reanalyses for the May–August period 1996–2005.a) Latitude belt 32.5–37.5 N; b) latitude belt 37.5–42.5 N. See map on top forthe two latitudinal belts amplitude.


whole sample of cloud systems over the 10 summer seasonsand on the various categories of cloud systems subdividedwithrespect to their recurrence frequency (1 per day, 1 per 2 days, 2per week, 1 per week, 2 per month, and 1 per month) and totheir duration (N3 h and N1000 km and 20 h).

2.3. Zonal wind

The zonal wind component at 700 hPa is extracted fromthe NCEP/NCAR reanalyses (Kalnay et al., 1996). The crosssections of the zonal wind over the area of the study arederived at several latitude belts for the period May–Augustof the 1996–2005 time frame. The objective of such aninspection of the zonal wind at this height was to verify theorder of magnitude of the westerlies over the area in order tocompare with the speed of motion of the cloud systems asidentified using the methodology described in Section 2.2and reported later in the present paper. Fig. 3 shows the crosssections over the entire area and for the whole period at thelatitude belts 32.5–37.5 N and 37.5–42.5 N. The cross sectionsat different latitude belts computed on amonthly basis showedvery similar results and are thus not shown here. Note that theorder of magnitude of the zonal wind at 700 hPa is betweenabout 2 and 7 m s−1, well below the zonal tropospheric speedspresented in Section 5.2 thus confirming that the analysisdetects strong relative propagation asnoted alreadybyCarboneet al. (2002) over north America.

3. Examples of coherent cloud patterns

The summer of 2002 is examined in detail to gain an idea ofthe cloud systems propagating over the area. The correspon-dence between the cloud streaks in the Hovmöller diagrams ofthe various months with the cloud systems in the IR imagery isfirst. The zonal and meridional propagations are analyzed so asto gain an idea of the correspondence of the various types ofcloud streaks and the meteorological systems they syntheti-cally describe. Moreover a dynamic and thermodynamic studyof the same period is presented to describe the structure ofthe atmosphere through the NCEP/NCAR reanalysis data.

3.1. Cloud streaks and intraseasonal variations: the year 2002

Coherent cloudpatterns during thewarm season are shownin Fig. 4–7 where the longitude–time Hovmöller diagrams forthe May–August period of 2002 are presented together withexamples of typical cloud systems depicting the meteorologyin the area in the various months.

Fig. 4a shows the monthly Hovmöller diagram for the period1–31 May 2002. First, note that almost all cloud patterns aresloped from the upper left to the lower right of the diagramthus indicating a clear coherent eastward propagation as is to beexpected over Europe where the circulation is dominated bywesterly wind systems. Four snapshots of typical IR imagesduring themonth are illustrated in Fig. 4b–e. Thefirst fewdays ofthe month are still characterized by a spring circulation drivenby Atlantic fronts that bring large convective systems embeddedin the frontal stratiform clouds (Fig. 4b, 2 May 1330 UTC). Thesituation on 8May 0400UTC (Fig. 4c) depicts an evolution in thecirculation with large frontal systems travelling from the Africancontinent towards Europe; the convective cloud systems are

continuously fed bywarmmoist airwith a large cyclone standingin the middle of the Mediterranean. When the cyclonic activityfades away the area starts to be dominated by local summerconvective phenomena driven by solar heating all over thecontinent, which propagate eastward downwind of the mainmountain chains (Fig. 4d, 12 May 1830 UTC); at the same timefronts continue to break in over Western Europe from theAtlantic. The summer anticyclone then establishes itself overCentral Europe and the Mediterranean with fronts travellinghigher up over northern Europe (not shown). The last part of themonth is dominatedbya temporarydisruptionof the anticyclonewith frontal activity and widespread convection over thecontinent as seen in Fig. 4e for 21 May1200 UTC.

Fig. 5 describes the propagation of cloud systems for theperiod 1–30 June, 2002. The first three days of the month arecharacterized by a weak activity in the western sector with

image of Fig.�3

Fig. 4. May 2002. a) Longitude–time Hovmöller diagram of cloud top Tb (in K). Note that all data pairs characterized by a Tb N 241 K fall in the top category in white asthey are not retained for the quantitative analysis. b) 2 May 1330 UTC, IR Tb of cloud tops N 241 K. c) 8 May 0400 UTC. d) 12 May 1830 UTC. e) 21 May 1200 UTC.


cloud systems travelling north of the Alps and convectiondeveloping over Turkey and Eastern Europe, but scarcelypropagating. On June 4 (see Fig. 5b, 1830 UTC) a large lowpressure system develops over Western Europe and extendsall over the continent and the Mediterranean till June 6. Thisopens up the area to the influence of a successive systemfrom North Africa (not shown). The successive strong cycloneoriginates from an Atlantic disturbance on June 8 andproduces large cloud systems, which propagate eastwardover the Mediterranean (Fig. 5c, 9 June 0100 UTC), whileother systems continue to propagate over Eastern Europe. Aperiod of relative scarce activity then follows with more localcloud systems or confined at high latitudes, as is typical ofsummer conditions in the area. Around the middle of themonth several convective cloud systems develop over Centraland Eastern Europe influenced by the Carpathians (Fig.5d, 16June 1530 UTC). Towards the last part of the month (Fig. 5e,24 June 1500 UTC) other large convective systems developover the Alps, the Apennines and the Balkans and shifteastward.

The successive 1–31 July 2002 period is shown in Fig. 6. Julystarts with a frontal system that travels high in the area and amesoscale convective system that occupies the Central Medi-terraneanmoving eastward on July 2 (Fig. 6b, 2 July 0500 UTC).The high latitude cloud systems continue tomove eastward till

a frontal system breaks in on 8 July determining a widespreadinstability over Western Europe with storms distributed fromNorth Africa to France and Germany; at the same time severalconvective systems develop over Eastern Europe and Turkey(Fig. 6c, 9 July 1430 UTC). The central part of the month ischaracterized by an intense convective activity over EasternEurope from the Balkans to the Ukrainian plains with a dailymodulation and at various intensity degrees. A low pressuresystem develops over Central Mediterranean on 14 July, whichdrives the circulation with intense convection in the lees of theAlps and the Balkans (Fig. 6d, 15 July 2100 UTC). The EasternEuropean convection continues for several days afterwardswith heavy storms over Romania, Bulgaria, the Black Sea andUkraine; at the same time fast moving systems travel overnorthern Europe. A depression over the Balkans and the Greekpeninsula characterizes the lastportionof themonthwithseveralconvective systems developing in the area for several days(Fig. 6e, 27 July 1400 UTC).

Fig. 7 shows the Hovmöller diagram and four snapshots ofthe 1–31 August 2002 period. The first decade of the month ismarked by fast moving fronts over France and Central Europefollowed by a depression centred over France and a series ofconvective systems moving from North Africa towards theCentral Mediterranean (Fig. 7b, 3 August 0200 UTC). In thefollowing days several convective systems pass over Central

image of Fig.�4

Fig. 5. Same as in Fig. 4 but for June 2002. a) Longitude–time Hovmöller diagram. b) 4 June 1830 UTC. c) 9 June 0100 UTC. d) 16 June 1530 UTC. e) 24 June 1500 UTC


Europe traveling fast towards the east, some of them very large(Fig. 7c, 6 August 0300UTC). The situation then continues to bedominated by awesterlyflowwith successive fronts and a largefrontal system develops moving across the continent andthe Mediterranean (Fig. 7d, 10 August 0500 UTC), eventuallytransforming into a large cyclonic system in transit to EasternEurope. Short-living convective systems characterize themeteo-rology of the next days and a system of African origin takesover the scene in the last part of the month (Fig. 7e, 26 August0330 UTC).

3.2. Meridional propagation

The latitude–time Hovmöller diagrams for May, June, Julyand August of selected years in Fig. 8 show examples of themeridional component of the cloud system motion. Thepropagation is much less evident and spans shorter distancesover the analysis domain than that in the zonal direction asalready noted by other authors (Ahijevych et al., 2001;Carbone et al., 2002). Convection is generally delimited bylatitude belts as is the case of July 2002, which showsconvection above 35 N at all latitudes slightly propagatingtowards the north. Note that these examples are among thosedisplaying the maximum of meridional propagation; theother months of the ten-year period show even less or nopropagation at all thus indicating the zonal character of themotion as it is to be expected in the area. The inspection of

.

movie loops for the entire period under scrutiny reveals thatthe propagation is in fact zonal with the westerlies as theclear driving factor. This in principle explains the motion ofthe system along latitudinal bandswith little or nomeridionalcomponents.

3.3. Dynamics and thermodynamics of August 2002

As an example of the dynamic structure of the atmosphereover the area of interest, August 2002 is examined in somedetail using the NCEP/NCAR reanalysis data (Kalnay et al.,1996). During August 2002, a large negative storm trackanomaly occurs over Central Europe (Fig. 9; in m2 s−2 andindicating the sum over the month of the square difference ofthe meridional velocity at 300 hPa between one day and theprevious one; e.g., Chang et al., 2002). It is associated with atropospheric jet stream split into two branches: the Scandi-navian, located approximately at 70°N, and the Subtropicallocated approximately at 38°N (Fig. 10a). Only two majorAtlantic depressions are able to cross Central Europe: thefirst one lasts from 1 to 6 August, and a second one, morepersistent and intense, from 8 to 17 spanning from the BritishIsles to Romania. From 10 August a southern warm low-level airflux coming from the Mediterranean injects new moisture intothis system. Furthermore, due to apositive soilmoisture anomalyat the end of July, evaporation contributes to the perturbedsystem regeneration along its path. The latitudinal bandbetween

image of Fig.�5

Fig. 6. Same as in Fig. 4 but for July 2002. a) Longitude-time Hovmöller diagram. b) 2 July 0500 UTC. c) 9 July 1430 UTC. d) 15 July 2100 UTC. e) 27 July 1400 UTC.


42°N and 55°N is thus characterized by a persistent updraftmotion highlighted by a clear negative pressure vertical velocity(Fig. 10b) in the monthly mean latitudinal cross section be-tween 10°E and 20°E. On the northern flank of this updraft cell aweak easterly low-level flow between 54°N and 57°N is present.

It is of some use to consider the cloud work function(Arakawa and Schubert, 1974), which is an integratedmeasure of the difference between the moist static energyin the cloud and that in the environment; in general, it can beviewed as an indicator of whether or not the cloud is likely togrow and its dimensions are J kg−1. A time–lon diagram ofthe function averaged over the 46°N–53°N belt (Fig. 11a)reveals an intense convection activity all over the period withan eastward propagation from 8 to 17 August associated witha strong daily latent heat release over the area between 5°Eand 40°E. During the subsequent 5 days the less intenseconvective footprint moves back westward. In Fig. 11b theprecipitablewater time–lon diagram, as in Fig. 11a, highlights,along with the negative pressure vertical velocity contours,the extension and some propagation characteristics of deepconvective systems during August 2002. Four long westwardmoving episodes are present, two of them associated withmajor cyclonic systems over Central Europe (1 to 6 and 8 to17) plus two minor episodes in the western part (18 to 21,10°W–10°E; 26 to 28, 5°E–10°E). The most intense (August8 to 17) is also stationary for more than 5 days over a wideregion. As revealed by the cloudwork function, such large and

persistent precipitable water footprint could be associatedwith a daily convective regenerationmechanismable to sustainconvective cells through local evaporation.

4. Frequency and periodicity of cloudiness

The spatial distribution of cold clouds over the domain isfirst examined. Subsequently, two different methods are usedto analyze the periodicity and phase of cloudiness over theselected domain (Ahijevych et al., 2001; Carbone et al., 2002):1) frequency of cloud Tbsb241 K is averaged over months,seasons and thewhole period 1996–2005 to examine coherentstructures within the diurnal cycle, and 2) discrete Fouriertransforms (DFTs) are applied along the time dimension ofthe frequency diagrams to identify those signals in the powerspectra that point to diurnal and shorter-period oscillations.

4.1. Spatial distribution of cold clouds

The average spatial distribution of cold cloud frequencyin Fig. 12 shows distinct variations from May to August.Maximum percentage values of cold clouds are found in May(Fig. 12a), especially in correspondence of and in the lees ofthe Pyrenees, the Alps and Anatolia, indicating a clear roleof these mountain chains in the initiation of convection; highvalues are also found over thewesternMediterranean pointing

image of Fig.�6

Fig. 7. Same as in Fig. 4 but for August 2002. a) Longitude–time Hovmöller diagram. b) 3 August 0200 UTC. c) 6 August 0300 UTC. d) 10 August 0500 UTC. e) 26August 0330 UTC.


to an influence of the Atlas mountains of North Africa, and inthe wake of the Balkans and the Carpathians.

The percentage frequency values decrease in magnitude inJune (Fig. 12b) with a general northward shift of the maximaabove 45°N while still maintaining a maximum in the lee ofthe Pyrenees and some activity over North Africa; as to the restof the domain, the cloudiness is mainly located over centraland eastern Europe.

The June trend intensifies in July (Fig. 12c)with themaximumof cold clouds all located in central and eastern Europe. Asubstantial decrease of the percent cold cloud coverage overEurope is then found in August (Fig. 12d), which also sees areturning maximum over the Pyrenees and the Alps.

The cold cloud distribution seems thus connected with thefrontal passage over the area from the Atlantic, which is morefrequent during the last part of spring and almost totally ceasesin summer over the Mediterranean due to the anticycloniccirculation. A certain amount of cold cloud developmentremains at higher latitudes where the fronts still break in andinduce local cloud development.

4.2. Frequency diagrams

The fraction of time that Tbb241 K at each longitude–timecoordinate pair over the analysis domain at the resolution of0.05° and 1/2 h is computed. As noted by Carbone et al. (2002),coherent patterns of average frequency in this coordinate

system can represent “phase-locked” occurrence of cold cloudtops and thus rainfall. Local phase-locked events would beassociated with convection over diurnally heated fixed sourcessuch as mountain chains, which would eventually transforminto more global events when propagating along a certaindirection and at a preferred speed.

In Fig. 13 the averagediurnal cycle for the 1996–2005periodfor the domain shown in Fig. 1 is shown by repeating the cycletwice (one on top of the other) to identify those features thatspan over more than one day; the scale is in percent and 45%means that a Tbb241k is detected at that specific longitude–time coordinate∼45% of the whole time period. Note thefollowing features:

• a diurnal oscillation across the analysis domain with amaximum marking the initiation of convection in the lee ofthe mountains and shifting from about 1400 UTC at 40°E to1800 UTC at 0°;

• an amplitude of the diurnal cyclemore or less homogeneousacross the 5°W–40°E longitude domain with larger differ-ences between day and night towards the eastern sector,east of 30°E;

• no clear semidiurnal maximum with only a slight presenceof it around 0000 UTC between 0° and 20°E;

• a moderate eastward propagation of the frequency maxi-mum from all mountain chains across the domain;

• diurnal maxima completely suppressed west of 5°W.

image of Fig.�7

Fig. 8. Latitude–time Hovmöller diagram showing the meridional propagation of cloud systems in a) May 2000, b) June 1999, c) July 2002, and d) August 1999.

Fig. 9. Storm track anomaly from NCEP/NCAR reanalysis data on August 2002 with respect to the 1971–2000 reanalysis II dataset. The dimensions are m2 s−2 andthe anomaly indicates the sum over the month of the square difference of the meridional velocity at 300 hPa between one day and the previous one. The twovertical dashed lines at 10E and 20E delimit the latitude portion where the average cross sections of Fig. 10 were computed.


image of Fig.�8

image of Fig.�9

Fig. 10. Mean latitudinal cross sections between 10°E and 20°E from thereanalysis II on August 2002 of a) the zonal windmonthlymean (m s−1), andb) pressure vertical velocity (Pa s−1).


Fig. 14 shows the average diurnal cycle for 1996–2005 on amonthly basis; for example thefirst graph shows the average ofall contributions from the month of May of each of the tenyears. Note that the largest contribution to the maximum ofcloudiness comes from themonth ofMay,which showsalso thestrongest diurnal signal. A semidiurnal signal is also detected,though notably weaker, centred between 0000 and 0600 UTCbetween 0° and 20°E, i.e. in the wake of the Pyrenees, the Alpsand the Balkans.

The two central month, June and July, exhibit a verysimilar average diurnal cycle with comparable amplitude andlocation of maxima in correspondence of the mountain chains.The evidence hints to the existence of a phase-locked day-2propagation.

The eastward propagation of cloud patterns (includingboth frequency maxima and minima) is maximum in Mayand is still evident in June and July. The propagation remainsconfined to the western part of the continental domain, the

Pyrenees and Western Alps, in August while it completelydisappears togetherwith the diurnal cycle in the eastern sector.

This frequency analysis is completed by the computation ofthe power spectrum of the cloud percentage with Tbb241 K.The spectra are computed using a Fourier power spectrumanalysis applied to the frequency distribution of Fig. 13 at fivelongitudinal bands: 2°W–5°E (Pyrenees), 7°–10°E (westernAlps), 10°–18°E (eastern Alps), 22°–28°E (Balkans), and 33°–40°E (Anatolia). The idea is to examine the amplitude ofthe frequencies with respect to themonth and to the longitude(see the fourmeanmonthlyplots in Fig. 15).Note theamplitudeof the diurnal cycle (wave number 30), which is maximum inMay and June at all longitudes and gradually declines betweenJune and August when it reaches its minimum. A weaker andmore or less constant semidiurnal peak (wave number 60) isalso found and will be discussed later in Section 4.3. It isinteresting tonote also that thediurnal cycle is alwaysmaximumin the 22–28°E longitudinal bands where the Balkans andCarpathians are located. It remains to be demonstrated that thisis due to the north–south direction of suchmountains. A series oflower peaks are found at longer frequencies (more than 1 day)over the western part of the domain, to be associated withincomingAtlantic frontal activity; suchpeaksaremoreevident inMay and June when the frontal activity is higher before thesummer minimum.

4.3. Harmonic analysis

One-dimensional DFTs are applied to the average diurnalfrequencies of Fig. 13. The scope is to evidence the diurnalfrequency signal and exploit the sensitivity of such analysisto spectral maxima higher than diurnal. The power spectracomputed using the DFTs and plotted as a function of longitudeand logarithm of period (days) in Fig. 16 for the four monthaverages over the ten years of the study show activities atwavelengths up to 30 days. A period of one day is evidentall over Europe while it disappears over the ocean (west of10°W) in agreementwith the findings ofWang et al. (2004) forAsia. Local maxima peaking between 3 and 7 days indicatethe activity of the westerlies with frontal passage over thecontinent as is also evident from the movie loops of the 1/2hourly Meteosat IR imagery and as already documentedfrom the analysis of Fig. 15. Longer period maxima appearstronger in May and June between 15 and 30 days over thecentral part of the continent presumably connected withthe large-scale activity. These latter weaken during July andAugust, consistently with the anticyclone establishing itselfduring summer.

Muchweaker semidiurnal maxima are to be found over thesame longitude range over the continent. This semidiurnalsignal is most probably associated with the superposition of“delayed-phase diurnal” convection with local diurnal convec-tion (Carbone et al., 2003) and seems not to be involvingsemidiurnal forcing such as atmospheric tidal lifting. On theother hand, Carbone et al. (2003) have proved that the signal isneither tidal nor continental so that the interpretation of thesemidiurnal signal as a forcingmechanism is very questionablefor North America in subtropical conditions. There is no ap-parent physical reason to believe that the conditions are dif-ferent in the present case.

image of Fig.�10

Fig. 11. Time–lon (46°N–53°N) diagram for August 2002 of a) cloud work function (shaded) and positive latent heat flux in Wm−2 (contour), and b) precipitablewater content (shaded) and negative pressure vertical velocity in m s−1 (contour).


Fig. 17a shows the diurnal and semidiurnal maxima com-puted from the average of the power spectrum over the wholeperiod and located with respect to the height of mountainsalong the longitude strips of the domain. While the maximaof the diurnal signal are well in phase with the presence ofelevated terrain and generally with land masses, the interpre-tation of the result is not quite as direct as is in the NorthAmerican case (Carbone et al., 2002). In other words, the verycomplex European orography does not lend itself to straight-forward identification of maxima and minima associated withnorth-to-south mountain chains and plains, respectively, as isthe case of North America.

Fig. 17b–f shows phase and amplitude of the diurnal andsemidiurnal cycle by harmonic decomposition in the same fivelongitude bands of Fig. 15. The harmonic decomposition isperformed for the 0th (mean), 1st (diurnal) and 2nd (semidi-urnal) harmonics. The combined variance ofwaves 1–2 accountsfor N90% of the total variance along the considered longitudebands. The original data are also plotted as crosses. There is ageneral tendency to increase the diurnal amplitude from westto east while this is not true for the semidiurnal signal. At thesame time the diurnal signal peak shifts earlier in the day fromwest to east (from 2000 UTC to 1600 UTC) as is to be expected;the semidiurnal peaks do the same, though a bit less evidently.

image of Fig.�11

Fig. 12. Percentage of cloud coverage with Tbb241 K averaged over the 1996–2005 period: a) May, b) June, c) July, and d) August.

Fig. 13. Average diurnal cold cloud (Tbb241 K) frequency Hovmöllerdiagram for the entire period of record (1996–2005). The diurnal cycle isrepeated twice for clarity across the UTC day boundary as in Carbone et al.(2002). The scale corresponds to the percentage of days during which cloudsare present at the given longitude–UTC hour coordinate. The local noon isindicated by the dashed lines.

1 An event corresponds to a cold cloud streak as identified by the boxautocorrelation function applied to the Hovmöller diagram (see Section 2.2for details).


5. Zonal cold cloud streaks statistics

As alreadydone forNorthAmerica (Carbone et al., 2002), Asia(Wang et al., 2004, 2005), Africa (Laing et al., 2008), Australia(Keenan and Carbone, 2008), the Bay of Bengal (Liu et al., 2008),and to a limited extent for Europe (Levizzani et al., 2006),hereafter the coherence and phase speed characteristics of thecold cloud episodes over Europe and the Mediterranean arequantified using the two-dimensional autocorrelation functiondescribed in Section 2.2. Zonal span and duration of a totalnumber of 20152 “events”1 over 1230 warm-season days areexamined, approximately 16 per day; the zonal phase speed isthen calculated. Note that only the zonal component of motionis considered and thus comparison with other data should belimited to it.

As it appears from the analysis of all Hovmöller diagrams forthe entire period the cold cloud episodes have a continuouscharacter while some of them show an intermittent behaviour,which is most likely attributable to regeneration of convectiondownstream of the main cloud system.

5.1. Span-duration statistics

The span and duration statistics for the overall 10-yearperiod are summarized in Table 1 where categories from 1event per day to 1 event per month are adopted. The conceptof “recurrence frequency” (Carbone et al., 2002) refers to the

image of Fig.�12

image of Fig.�13

Fig. 14. Same as in Fig. 13 but for a) all May, b) all June, c) all July, and d) all August of the 1996–2005 period. The local noon is indicated by the dashed lines.

Fig. 15. Mean power spectrum of the cold cloud diurnal frequency distribution of Fig. 13 averaged over five longitudinal bands for a) May, b) June, c) July, andd) August. The longitudinal bands are. Pyrenees (2°W–5°E), western Alps (7°–10°E), eastern Alps (10°–18°E), Balkans (22°–28°E), and Anatolia (33°–40°E).


image of Fig.�14

image of Fig.�15

Fig. 16. Distribution of mean power spectrum of time series of fraction of IR Tbb241 K in the Hovmöller space over the area of the study (at 0.05° interval) plottedas a function of logarithm of period (day) and longitude (degree) for the whole period 1996–2005: a) May, b) June, c) July, and d) August.


average time interval for recurrence of an event that equalsor exceeds a specified span or duration. This means, forexample, that at the recurrence frequency of 1 per day thereare events of span≥646 km and duration≥12 h, which clearlyexceed the span and duration of an individual local convectivesystem. Note that long events have necessarily to occur onmost days of the examined period in order to achieve this dailyfrequency.

The span and duration within each category of the 10-yearstatistical sample are quite consistent with each other alsoconsidering that a substantial variability in the meteorologyof the various summers was registered with summerscharacterized by strong temperature and humidity anomaliesin one direction or the other. The mean value of span andduration varies between 646 km/12 h of the 1 per day

category to 2204 km/34 h of the 1 per month systems, withthe 1 per week category spanning 1576 km and lasting 25 h. Itis thus conceivable that the cloud systems are subject to asubstantial degree of regeneration over long distances andperiods reaching as long as almost 1 1/2 days.

Table 2 reports a comparison between the mean values ofspan and duration from Table 1 with the correspondingfindings of Carbone et al. (2002) for the continental US, Wanget al. (2004) for East Asia and Laing et al. (2008) for northerntropical Africa. The present figures are very similar to those ofEast Asia, while a bit shorter in span and duration than theones over US and considerably shorter than the African ones.

From the climatological point of view the distribution ofexceedance frequencies (see Fig. 18) is also very useful tofind out what is the probability of occurrence of convective

image of Fig.�16

Fig. 17. Power spectrum (a) and harmonic decomposition (b), (c), (d), (e) and (f) of the cold cloud diurnal frequency distribution of Fig. 13. The power spectrumdensity vs longitude exhibits a strong diurnal signal in correspondence of all mountain chains and a weaker semidiurnal signal more or less superimposed. Theharmonic decompositions quantify the amplitude of diurnal and semidiurnal oscillations in the same five longitude bands of Fig. 15. Dotted, thin solid, dashed, andthick solid curves refer to wave numbers 0, 1, 2 and their summation, respectively, and crosses depict the original data before decomposition.


episodes of given span and duration. Note that, as pointedout by Laing et al. (2008) for northern tropical Africa, in thepresent case the exceedance frequency distributions of spanand duration are approximately exponential, which is to beexpected in the case that the locations (in time and space) ofthe convection are reasonably random. Local conditions are

Table 1Recurrence frequencies of rain streak zonal span (km) and duration (h).

Recurrence frequencyZonal span (km)Duration (h)

1996 1997 1998 1999 2000

1 per day 586.3 647.8 602.7 664.2 60611.5 13.0 11.0 12.0 11

1 per 2 days 815.9 975.8 938.9 1004.5 86116.0 17.5 14.5 16.5 15

2 per week 1152.1 1316.1 1168.5 1283.3 122520.0 20.5 17.0 19.5 18

1 per week 1373.5 1541.6 1586.7 1615.4 157424.5 26.0 26.0 23.5 22

2 per month 1660.5 1799.9 2062.3 2050.0 192728.5 32.5 32.0 29.0 30

1 per month 1877.8 2390.3 2308.3 2173.0 216432.0 41.0 37.0 30.5 33

surely favourable for the initiation of convection, which theneventually ceases due to changing conditionswith time. Suchlocal conditions are certainly not entirely random as they areconnected with the general circulation patterns, but theireffect is not distinguishable from that of a random model(Fig. 18) (Laing et al., 2008).

2001 2002 2003 2004 2005 Mean

.8 680.6 676.5 602.7 664.2 729.8 646.2

.5 12.0 13.0 11.5 13.0 12.5 12.1

.0 1000.4 959.4 873.3 922.5 1029.1 938.1

.5 16.5 16.5 15.0 16.0 18.0 16.2

.9 1295.6 1197.2 1123.4 1234.1 1320.2 1231.6

.5 19.5 20.5 18.0 20.0 22.5 19.6

.4 1578.5 1553.9 1373.5 1836.8 1730.2 1576.5

.0 25.0 24.0 23.5 28.0 28.0 25.1

.0 1832.7 1795.8 1693.3 2107.4 1968.0 1889.7

.0 28.0 30.0 28.0 34.5 33.0 30.6

.8 2173.0 2328.0 1836.8 2337.0 2455.9 2204.5

.5 32.0 32.0 32.0 37.0 36.5 34.4

image of Fig.�17

Table 2Mean values of cloud streak zonal span (km) and duration (h) from Table 1compared with the corresponding values of Carbone et al. (2002) for thecontinental United States, Wang et al. (2004) for East Asia, and Laing et al.(2008) for northern tropical Africa.

Recurrence frequencyZonal span (km)Duration (h)

Mean Carbone et al.(2002)

Wang et al.(2004)

Laing et al.(2008)

1 per day 646.2 838.0 620.0 147212.1 18.5 11.6 36.4

1 per 2 days 938.1 1250.0 1008.8 227216.2 24.5 17.5 52.6

2 per week 1231.6 1588.0 1327.7 279619.6 30.5 22.0 62.5

1 per week 1576.5 2000.0 1706.8 355225.1 40.3 26.6 79.3

2 per month 1889.7 2325.0 2178.8 410430.6 49.0 34.3 90.0

1 per month 2204.6 2500.0 2664.034.4 55.5 39.1


5.2. Zonal phase speed

The phase speed characteristics of the cold cloud streaks inthe area are explored for the opportunity they offer to betterunderstand the dynamic organization of the summer seasoncloud systems. The phase speed statistics of the events as

Fig. 18. Exceedance frequency (vertical bars, number per day), best fi

exponential (bold black line; equation reported in the graph) and recurrenceinterval (black dots), for a) duration and b) span of cold cloud episodesduring May–August, 1996–2005.

t

described above is summarized in Table 3 for all events and twocategories,whichserve thepurposeof separating thebehaviourof the longest duration events, i.e. those that start exceedingthe duration of a single local storm cell (durationN3 h) andthose that belong to the category of likely regeneratingevents (spanN1000 km and durationN20 h). It is noteworthythat thefigures are verymuch consistent fromyear to year thussuggesting inherent dynamicmechanisms linked to the variousforms of propagation of convection over the area. These sta-tistical properties are instrumental for the improvement ofprediction of convective precipitation in a probabilistic sense(Laing et al., 2008) as they can serve as benchmarks for QPF.

Some degree of interannual variation is to be found, butthis is more present for the longer events while almost non-detectable for all events and for the N3 h ones. This suggeststhat the largest variations are to be found among the mostrelevant regenerating storms while in the majority of cloudsystems the varying synoptic conditions exert smaller influ-ences. From the climatological point of view this would some-how also indicate that the peak events are those that seem tobe most influenced by changes in the circulation and/or bychanges in the dry/moist or warm/cold character of the indi-vidual summer season. However, it must be considered thatthe present population of events and duration of the statisticalanalysis are by no means enough to draw any final conclusionon this important subject.

The bivariate distribution (correlation coefficient of 0.91) andthe median zonal phase speed of 16.1 m s−1 for cold cloudstreaks spanning≥1000 km and≥20 h duration are reported inFig. 19. This median zonal phase speed is higher than that foundover North America (14 m s−1, Carbone et al., 2002), northerntropical Africa (13.6 m s−1, Laing et al., 2008), Australia for theeastward moving streaks (13.4 m s−1, Keenan and Carbone,2008),while it is in linewith thatof thewestwardmoving streaksover Australia (16 m s−1) and comparable with the one of thenorthern sector over East Asia (17.1 m s−1, Wang et al., 2004).However in this latter case the two categories of events do notentirely match as in the Asian case the category refers to theevents≥500 km/10 h. Note that the 25th and the 75th percen-tiles (dash-dotted lines in Fig. 19) of the distribution indicate thatthe zonal phase speeds have values within a relatively narrowrange, i.e. between ∼13.6 and 18.9 m s−1. The large majority ofthe events are enclosed between the 7 and 30 m s−1 zonal phasespeed lines as also found by other authors (Carbone et al., 2002;Laing et al., 2008; Wang et al., 2004).

It must also be considered that a comparison betweensatellite-derived median zonal phase speed with the radar-derived one cannot be done without considering the differ-ences between the two instruments, the satellite passive IRsensor and the active radar, i.e. based on different observingprinciples. Most important of all, the two instruments sensedifferent parts of the clouds, the Tb at cloud top from satelliteand the precipitation from radar. Tuttle et al. (2008) havefound that the mean satellite-radar difference in the zonalpropagation speed at the 241 K threshold chosen for this studyranges between 3.7 and 4 m s−1. If this difference is applied totheabove reported16.1 m s−1medianphase speed, the resultingspeed attains a value comparable with the radar-derived oneover North America.

Table 4 shows three different sets of zonal phase speedsdivided according to the recurrence frequencies already

image of Fig.�18

Table 3Relevant statistics for all events (top rows). Number of events andmedianphase speed for eventswith durationN3 h (middle rows) and for eventswith span N1000 kmand duration N20 h.

1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Mean

Number of events 1910 2225 1940 1972 1962 1957 2016 2047 2173 1950 2015Mean span (km) 164.6 165.2 163.9 179.5 167.2 172.8 170.6 163.7 169.3 182.9 170.0Median span (km) 53.3 49.2 45.1 53.3 53.3 53.3 53.3 53.3 49.2 53.3 46.3Stand. deviation (km) 265.2 286.8 297.5 304.6 291.8 300.5 288.9 259.4 296.4 319.4 291.1Mean duration (h) 3.6 3.6 3.3 3.6 3.5 3.6 3.7 3.6 3.7 3.8 3.6Median duration (h) 1.5 1.5 1.5 1.5 1.5 1.5 2.0 2.0 1.5 1.5 1.6Stand. deviation (h) 4.8 5.0 4.7 4.7 4.7 4.8 4.8 4.5 4.9 5.3 4.8Mean speed (m s−1) 10.3 10.0 10.5 10.8 10.1 10.2 10.2 10.0 9.7 9.9 10.2Median speed (m s−1) 9.1 9.1 9.1 9.6 9.1 9.1 9.1 9.1 9.1 9.1 9.2Stand. deviation (m s−1) 6.8 6.5 7.3 7.2 6.6 6.6 6.6 6.3 6.1 6.0 6.6Number of eventsN3 h 713 825.0 669.0 743.0 722.0 721.0 764.0 792.0 805.0 744.0 750Median speedN3 h 12.5 11.8 12.9 13.3 12.6 12.7 12.0 12.4 12.5 12.6 12.5Number of eventsN1000 kmN20 h 24 33.0 22.0 31.0 22.0 31.0 32.0 20.0 27.0 40.0 28Median speedN1000 kmN20 h 14.3 15.9 17.6 16.2 16.7 16.7 16.6 14.3 17.1 15.4 16.1


adopted for Table 1 and following the categorization ofCarbone et al. (2002). The top row in each recurrencefrequency is the speed obtained by dividing up span andduration couplets of Table 1. The bottom row within eachrecurrence frequency is the median span and duration ratiocalculated from the data plotted in Fig. 19, i.e. the median ofthe span and duration ratio for each recurrence category. Thecentral row is an “exceedance speed”, defined as the averagezonal phase speed equalled or exceeded for each recurrencefrequency. For example, approximately 1230 cold cloudstreaks equalled or exceeded the phase speed of 19.1 m s−1

(i.e., an average of one per day). The span/duration andmedian phase speed categories show very comparable valuesthus confirming a general homogeneity of the sample as alsofound by Wang et al. (2004) over East Asia. The values of the

Fig. 19. Scatter plot of zonal span (km) vs duration (h) of cold cloud streaksfor May–August, 1996–2005 over the analysis domain. The thick solid lineindicates the median phase speed (16.1 m s−1) and the dashed-dotted linesthe 25th (13.6 m s−1) and the 75th (18.9 m s-1) percentile of the distributionof the streaks longer than 1000 km and lasting more than 20 h. The dashedlines at 7 and 30 m s−1 constant phase speed encompass themajority of “long”events during the warm season over Europe and the Mediterranean.

exceedance speeds (middle row) are considerably larger asthey represent the threshold speeds exceeded only by thefastest moving events within each recurrence category. Notethat the exceedance speed of the categories recurring morethan 1 per week are close or even exceed the 30 m s−1 valuethat delimits most of the events as noted from Fig. 19.

A comparison of span/duration and exceedance speeds of thepresent analysis and the results over continental US (Carboneet al., 2002), East Asia (Wang et al., 2004) and northern tropicalAfrica (Laing et al., 2008, only span/duration are compared) isdetailed in Table 5. The present values and those over East Asiaare considerably larger than those over the US and Africa. Thiscould be due to the shorter duration of the events over Europeand Easy Asia compared with the other two cases (see Table 2).However, as to the comparison with the US case, the alreadydiscusseddifferencesbetween the satellite andradarobservationmethods could account for part of the difference.

Finally, the longitudinal dependence of the number ofcold cloud systems during the observation period is shownin Fig. 20, divided into three space–time categories, i.e.≥500 km/10 h, ≥1000 km/17 h, and ≥1500 km/25 h. Thefigures refer to the number of streaks in each category withina 2.5° longitude strip. The largest number of streaks is foundover the Atlantic Ocean where it is connected with the frontalactivity, especially in the first part of the season. As is to beexpected, themajority of the streaks, including the long lastingones, is concentrated in the central part of the domain wherethemountain chains are located thuspointing to an influence ofthe orography on generation and evolution mechanisms. Acomplete set of graphs for the number of streaks within eachlongitude banddivided on thebasis of span, duration and speedis provided by Pinelli (2010).

6. Summary and conclusions

6.1. Summary

Meteosat MVIRI IR Tbs in the 10.5–12.5 μm spectral bandin May–August 1996–2005 are used over Europe and theMediterranean to investigate the regional and intraseasonalvariability of cold cloud episodes during late spring and summermonths. Note that cold clouds (cloud top Tbb241 K) are hereconsidered correlated to precipitation even though limitations

image of Fig.�19

Table 4Cold cloud streak zonal phase speeds (m s−1). For each category the top row refers to span/duration ratio of the values from Table 1; the central row is the“exceedance” speed, i.e. the average zonal phase speed equalled or exceeded for each recurrence frequency; the bottom row is the median span/duration ratiocomputed from the data in Fig. 19.

Recurrence speeds (number of samples)Span/durationExceedanceMedian

1996(1910)

1997(2225)

1998(1940)

1999(1972)

2000(1962)

2001(1957)

2002(2016)

2003(2047)

2004(2173)

2005(1950)

Mean(2015)

1 per day 14.2 13.8 15.2 15.4 14.7 15.8 14.5 14.6 14.2 16.2 14.818.8 18.5 19.7 20.0 18.9 19.6 19.2 19.0 18.9 18.8 19.114.4 15.0 17.8 16.7 15.6 16.7 15.8 15.8 15.8 16.1 14.4

1 per 2 days 14.29 15.5 18.0 16.9 15.4 16.8 16.2 16.2 16.0 15.9 16.122.0 21.80 23.8 23.9 23.0 23.0 22.8 22.4 22.8 21.9 22.715.3 16.4 18.1 17.8 18.3 17.5 16.1 16.7 16.9 16.4 17.0

2 per week 16.0 17.8 19.1 18.3 18.4 18.4 16.2 17.3 17.1 16.3 17.524.9 23.6 26.0 27.3 26.1 25.4 25.7 24.5 24.9 24.8 25.315.8 16.0 17.8 18.6 18.3 17.5 16.9 16.7 17.2 16.4 17.2

1 per week 15.6 16.5 17.1 19.1 19.9 17.5 18.0 16.2 18.2 17.2 17.528.3 27.7 30.4 31.0 28.9 28.8 29.1 28.3 27.3 27.2 28.716.1 15.3 17.2 19.4 17.5 17.5 17.8 16.7 17.2 16.7 17.1

2 per month 16.2 15.4 17.9 19.6 17.8 18.2 16.6 16.8 17.0 16.6 17.232.4 29.4 34.5 37.5 30.4 31.9 32.9 31.7 28.9 29.4 31.916.1 16.1 17.6 19.6 17.5 18.8 18.5 15.8 17.1 17.9 17.5

1 per month 16.3 16.2 17.3 19.8 17.9 18.9 20.2 15.9 17.5 18.7 17.934.5 30.6 36.4 40.1 34.0 32.8 34.8 36.0 31.7 32.3 34.316.4 16.8 18.5 19.6 17.4 18.8 19.2 16.6 17.4 18.9 18.0


exist related to cirrus contamination and to the indirect linkbetween the cloud top structure and the precipitation at theground.

The latitude–longitude domain under scrutiny is enclosedbetween 30–54°N and 15°W–40°E. The zonal component ofmotion of the cold cloud systems is examined by dividing thearea into 1100 vertical strips of 0.05° width (∼4 km) spanningfrom 30 to 54°N in the N–S direction. The quantitative analysisis based on the application of a two-dimensional autocorrela-tion function to the Tb data in the Hovmöller lon–time space.The convective events are quantified as to their coherency,duration, span, and propagation speed. Harmonic analysisusing DFTs is used as an analysis tool together with a statistical

Table 5Meanvalues of cloud streak zonal phase speeds (m s−1) at the same recurrencefrequencies of Table 1 comparedwith the corresponding valuesof Carbone et al(2002) for the continental United States, Wang et al. (2004) for East Asia, andLaing et al. (2008) for northern tropical Africa.. The values in the top rows oeach category are computed as the ratio of span and duration (see Table 1), andthose in bottom rows are “exceedance phase speed”, defined as the thresholdexceeded only by the fastest streaks at the specified recurrence frequency.

Recurrence frequencySpan/duration (m s−1)Exceedance speed(m s−1)

Mean Carbone et al.(2002)

Wang et al.(2004)

Laing et al.(2008)

1 per day 14.8 12.6 14.9 11.219.1 23.9 17.3

1 per 2 days 16.1 14.3 16.0 12.022.7 26.6 21.0

2 per week 17.5 14.5 16.8 12.425.3 28.7 24.4

1 per week 17.5 13.8 17.8 12.428.7 30.8 28.2

2 per month 17.2 13.3 17.7 12.731.9 33.8 33.3

1 per month 17.9 13.3 19.034.3 37.7 37.9

Fig. 20. Number of streaks of the period May–August, 1996–2005, belongingto three classes: ≥500 km/10 h, ≥1000 km/17 h, and ≥1500 km/25 h. Thedomain is divided into 2.5° longitude strips.

.

f

analysis and diurnal cycle investigation. The major findings aresummarized hereafter:

1. The propagation of cold cloud systems is zonal as is to beexpected for the European region dominated by the wester-lies. The meridional component is very weak and confined torelatively narrow latitude belts as found by other authors.

2. The analysis of the persistence of cold clouds over the areaevidences the influence on the generation and propagationof cloud systems of the Pyrenees and the Alps in May andpart of June. In May the Balkans and Anatolia play also arole, though of lower magnitude. In July cold clouds areconfined more over Eastern Europe with a moderateclustering in the eastern wake of the Alps. In August theactivity is at a minimum with the Pyrenees and the Alpsstarting again to show their influence.

image of Fig.�20


3. A diurnal oscillation is found across the analysis domainwith a maximum marking the initiation of convection inthe lee of the mountains and shifting from about 1400 UTCat 40°E to 1800 UTC at 0°. The amplitude of the diurnal cycleismore or less homogeneous across the 5°W–40°E longitudedomain with larger differences between day and night eastof 30°E. There is a moderate eastward propagation of thefrequency maximum from all mountain chains across thedomain and the diurnal maxima are completely suppressedwest of 5°W. The eastward propagation of cloud patterns(including both frequency maxima and minima) is maxi-mum in May and is still evident in June and July.

4. One-dimensional DFT analysis shows a period of one dayall over Europe while it disappears over the ocean (west of10°W). Local maxima peaked between 3 and 7 days indicatethe activity of the westerlies with frontal passage over thecontinent.

5. The maxima of the diurnal signal are well in phase withthe presence of elevated terrain and generally with landmasses, but the result is difficult to interpret because of thevery complex European orography.

6. A much weaker semidiurnal signal is found over the conti-nent, but the interpretation is somewhat not straightforward.

7. The harmonic decomposition of the signal shows that thereis a general tendency to increase the diurnal amplitude fromwest to east while the diurnal signal peak shifts earlier inthe day from west to east (from 2000 UTC to 1600 UTC).

8. A median zonal phase speed of 16.1 m s−1 is found for allevents ≥1000 km and ≥20 h in agreement with the EastAsia and Australia (westwardmotion) results obtainedwiththe same satellite-basedmethod. A full set of results dividedby year and recurrence category is also presented.

6.2. Orographic effects

The results show that the Atlantic westerlies drive thecirculation over thewhole period as is tobe expected.However,the Atlantic influence is more relevant in the first part of thewarm season (May and part of June) when the fronts stillbreak in over the continental areas and the Mediterranean. Inmidsummer central Europe (north of the Alps) continues to beinterested by the frontal activity, although less intense. Thefrontal circulation reappears at lower latitudes at the end of theperiod (August) when the Azores high starts weakening.

As a consequence of this circulation patterns, the Pyreneesand the Alps seem to exert theirmaximum impact on the cloudsystems in May. The Alps alone continue to affect the cloudformation during the summer while the Pyrenees lower theirinfluence in June and July and start over again in August inconnection with the lowering of the frontal activity.

A noteworthy finding concerns the cloud system distri-bution over eastern Europe, which registers a maximum inJuly when the cloud systems are concentrated especially inthe eastern plains. It appears that the Carpathians play a rolein this direction by contributing to the genesis and evolutionof regenerating systems in their wake. The Balkans play amore local role as is the case of the Apennine chain in Italy.However, the effects exerted by the Balkans are still unclearand may well be masked by superimposed signals from theAlps and the Apennines.

The Atlas mountains in north Africa necessarily contributeto the cloud systemmodification inMay and part of Junewhenthe fronts travel towards the Mediterranean at these latitudes.A much weaker signal is detected in August. Note that theAnatolia orography shows some signal in May while this latterdisappears in the other months. This fact may be due to thecirculation of the westerlies, which travels at higher latitudesthus enhancing the cloud formation over eastern Europe.

The present results are relevant for the detailed statisticson span, duration and phase speed that can be used as bench-marks for the model development on one side, and also as astarting point for regional climatology of precipitating events onthe other.

6.3. Research perspectives

The results of the present paper are to be regarded as afurther step towards the understanding of summer seasonclouds and precipitation mechanisms over Europe and theMediterranean. Studies are to be carried out to explore in detailthe effects of the mountain chains and of the Europeancontinent itself. In particular, the geographic domain analyzedin the present study needs to be divided in appropriate sub-domains where a more thorough analysis of the effects of thevarious mountain chains be conducted. The areas in the wakeof the Pyrenees, the Alps, the Balkans and the Carpathians arecandidates for such analyses.

A study using satellite-derived precipitation data shouldalso tell more on the propagation of precipitating systems.Global precipitation datasets of controlled quality are avail-able and can be very helpful in this respect. However, theirspatial resolution is mostly conceived for climate applicationsand thus high-resolution retrievals by means of state-of-the-art retrieval methods are necessary over the above men-tioned areas.

Finally, long term runs of mesoscale numerical models arerequired for a high-resolution investigation of the physicalprocesses behind the propagating phenomena and the evolutionof precipitation. A non-hydrostatic model such as the WeatherResearch and Forecasting model (http://wrf-model.org/) ispreferable so as to be able to simulate convection and itsevolution in presence of complex orography.

Acknowledgments

The senior author dedicates the paper to honour the 80thbirthday of Prof. Dr. Hans R. Pruppacherwho taught him a greatdeal on how to do research in atmospheric sciences. Supportis gratefully acknowledged from the projects AEROCLOUDSof the Italian Ministry of Education, University and ScientificResearch (MIUR) and Progetto Strategico “Nowcasting avanzatocon l'uso di tecnologie GRID e GIS” of Regione Puglia. NCAR issponsored by the National Science Foundation. Meteosatimagery was made available and is copyrighted by EUMETSAT.NCEP Reanalysis data were provided by the NOAA/OAR/ESRLPSD, Boulder, Colorado, USA, from their web site at http://www.esrl.noaa.gov/psd/. R. Ginnetti is acknowledged for hissupport in Meteosat data analysis. Two anonymous reviewerssignificantly contributed to improve the manuscript with theirsuggestions and criticism.

http://wrf-model.org/

http://www.esrl.noaa.gov/psd/

http://www.esrl.noaa.gov/psd/


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