Categorization of cold period weather types in Greece on the basis of the photointerpretation of...

Categorization of cold period weather types in Greece on the basis ofthe photointerpretation of NOAA/AVHRR imagery

C. CARTALIS

University of Athens, Department of Applied Physics, Building PHYS-V,Athens, GR-15784 Greece

N. CHRYSOULAKIS*

Foundation for Research and Technology—Hellas, Institute of Applied andComputational Mathematics, Regional Analysis Division, P.O. Box 1527,Heraklion, Crete, GR-71110 Greece; e-mail: [email protected]

H. FEIDAS

University of the Aegean, Department of Geography, University Hill,Building of Geography, GR-81100 Mytilene, Greece

and N. PITSITAKIS

National Meteorological Service, Hellinikon, Athens, Greece

(Received 9 July 2002; in final form 11 July 2003 )

Abstract. The Advanced Very High Resolution Radiometer (AVHRR) onboard the National Oceanic and Atmospheric Administration (NOAA) satellitesmay be used to detect and monitor prevailing synoptic weather systems and todescribe the state of the atmosphere with very good spatial and temporalresolution. In this study, the synoptic weather types which are responsible forsevere cold and rainy weather conditions in Greece during the cold period of theyear were examined with the use of NOAA/AVHRR satellite images in con-junction with weather charts. An attempt was made to develop a classificationscheme for these synoptic weather types on the basis of their tracks and cloudpatterns inferred by satellite images. Forty-two visible and thermal infraredAVHRR images were interpreted with emphasis on the analysis of cloud patternsformed in synoptic weather systems in order to relate cloud features in everyimage to physical processes. Synoptic maps (12:00 UTC) from the EuropeanCentre for Medium-Range Weather Forecasts (ECMWF) were used as auxiliarydata. The final product was a categorization of the cloud features formed by eachof the four prevailing weather types in the area of Greece for the winter rainyperiod (three Depressional Weather Types and one Mixed Weather Type).

1. Introduction

The Mediterranean basin is well known as a region of frequent cyclone

formation and is affected by moving depressions generated either in the Atlantic

International Journal of Remote SensingISSN 0143-1161 print/ISSN 1366-5901 online # 2004 Taylor & Francis Ltd

http://www.tandf.co.uk/journalsDOI: 10.1080/01431160310001632684

*Corresponding author.

INT. J. REMOTE SENSING, 10 AUGUST, 2004,VOL. 25, NO. 15, 2951–2977

Ocean or in north-western Europe. Preferred regions for cyclogenesis in the

Mediterranean region were identified by Radinovic (1987). The depressions

occurring in specific areas of the Mediterranean region and the cyclonic tracks

have been the subject of extensive climatological research (e.g. Maheras 1979, 1983,

1988a, Katsoulis 1980, Prezerakos 1985, Flocas 1988, Kassomenos et al. 1998). In

these studies, the depressions were identified and classified manually on the basis of

synoptic charts. An automatic classification of circulation types in Greece has been

recently developed using spatial methods of topology and geometry by Maheras

et al. (2000, 2001).The central Mediterranean is affected by depressions in the westerly circulation

and at the same time is largely influenced by meridional circulations and depres-

sions formed over the western and central Mediterranean or over the Sahara

Desert. The meridional circulation is the main factor governing most of the pre-

cipitation over the whole of the Mediterranean basin (Maheras 1988a, b, Maheras

et al. 1992).

The cold period in Greece occurs between late October and early April. The

duration of the cold period is of significant importance to the country, because

during this period the largest amount of rain is accumulated. The main synoptic

weather types which affect the broader area of Greece in winter are classified by

Maheras (1979, 1983, 1988a) as follows:

. Anticyclonic Weather Type, in which an anticyclone is positioned peri-

pherally to or over Greece. This weather type favours fair and dry weather;

. Depressional Weather Type, in which a depression follows a zonal track from

west to east or a semi-meridional track from north-west to south-east. This

weather type favours cold and rainy weather with strong winds; and

. Mixed Weather Type, in which an anticyclone is combined with a depression

causing a tied pressure gradient over Greece. Mixed Weather Types cause

severe cold weather, with rainfall and snowfall mainly over central and

northern Greece, with very low temperatures and very strong north-easterly

winds.

Despite the considerable research on the climatology of Mediterranean cyclones,

there is still a need to correlate depressional weather types with cloud types and

patterns developed over an area with various terrain features like Greece. In this

study, weather types which are responsible for severe cold and rainy weather

conditions in Greece during the cold period are identified on the basis of their

tracks and cloud patterns inferred by satellite images in conjunction with weather

charts. Afterwards, the interpretation of a large number of related satellite images

was carried out, in order to categorize the cloud features in the area of Greece

formed by each prevailing weather type.

2. Data and methodology

For the purposes of this study, high spatial resolution Advanced Very High

Resolution Radiometer (AVHRR) images (Local Area Coverage) were used as

acquired from the National Oceanic and Atmospheric Administration (NOAA)

ground receiving station of the National Meteorological Service. AVHRR has a

spatial resolution of 1.1 km at the nadir and a swath coverage of 2700 km. AVHRR

records incoming radiation using five spectral channels: channel 1, 0.58–0.68 mm

(visible); channel 2, 0.72–1.10 mm (near-infrared); channel 3, 3.55–3.93 mm (mid-

infrared); channel 4, 10.5–11.3 mm (thermal infrared) and channel 5, 11.5–12.5 mm

2952 C. Cartalis et al.

https://www.researchgate.net/publication/227820466_The_Northwest_African_depressions_affecting_the_South_Balkans?el=1_x_8&enrichId=rgreq-015e7a8c0b29ea80fa46ede68821d65c-XXX&enrichSource=Y292ZXJQYWdlOzIzMzIxODIzMTtBUzo5OTAxMTE1NTQ2NDIwMkAxNDAwNjE3NTA3ODg5

https://www.researchgate.net/publication/260892074_The_Synoptic_Weather_Types_and_Objective_Delimitation_of_the_Winter_Period_in_Greece?el=1_x_8&enrichId=rgreq-015e7a8c0b29ea80fa46ede68821d65c-XXX&enrichSource=Y292ZXJQYWdlOzIzMzIxODIzMTtBUzo5OTAxMTE1NTQ2NDIwMkAxNDAwNjE3NTA3ODg5



(thermal infrared). In this study, only channels 1 and 4 were used for the photo-

interpretation. The spatial resolution of AVHRR data (1.1 km) enables the

identification of cloud patterns linked with small-scale physical processes and

permits a more detailed cloud analysis in comparison to the coarse spatial

resolution of the geostationary data (6.5–7.5 in the area of Greece).

Forty-two cases were selected for days when well-developed low-pressure

systems were located over the broader area of Greece (table 1). Fifty-nine AVHRR

images (one to two images per day) were used, depending on the passage of NOAA-

14, NOAA-12 and NOAA-9 over the area of east Mediterranean, during the

periods January to April and October to December 1995. At least one image per day

was daytime (reception times close to 12:00 UTC), therefore the best distinctness

was achieved for the visible channel.

Synoptic analysis maps (12:00 UTC) from the European Centre for Medium-

Range Weather Forecasts (ECMWF) have also been used. These maps presented

graphically the horizontal distribution (over Europe and the Mediterranean Sea) of

the following parameters:

. 500 hPa contour lines;

. 500 hPa isotherms;

. 500 hPa isotachs of vertical velocity;

. surface isobars; and

. 850 hPa isotherms.

Synoptic analysis maps were used as auxiliary data for the photointerpretation.

The analysis of cloud patterns was used to estimate the synoptic system types as

well as their location. The type and the location of the synoptic systems were

verified using the respective synoptic analysis maps. The analysis of cloud patterns

on satellite images was also related to physical processes. This relation was also

verified using the synoptic analysis maps, e.g. the detection of cumulonimbus clouds

on the satellite image was verified using the 500 hPa isotachs map of vertical motion

in which convective motion areas can be located.

The combination of visible with thermal infrared imagery photointerpretation

was necessary to define the atmospheric mechanisms. Clouds and cloud system

identification on satellite images were based on shape, texture and pattern recogni-

tion relating to the spatial arrangement of cloud elements, on indications of height

revealed directly in the infrared imagery and deducible in visible imagery, and the

relation to local topography. Clouds viewed in satellite imagery were classified into

three general categories based on appearance: cumuliform, stratiform and cirriform.

The characteristics of the individual cloud types which form the cumuliform,

stratiform and cirriform categories are summarized in tables 2, 3 and 4 (Rao et al.

1990). Detection of fronts has been accomplished using the basic principles of

identification of weather systems on satellite imagery (Rao et al. 1990, Bader et al.

1995). Briefly, a classical cold frontal cloud band is defined as continuous, relatively

broad cloud formation with mostly cold tops, bright in the visible images, char-

acterized by a distinct long axis with or without curvature. It has a sharp rear edge

and sometimes a distinct forward edge where the surface front is located. The wide

zone of overcast cloudiness is on the cold side of the surface frontal position.

Concerning warm frontal bands, if there is little or no high cloud in the warm

sector, the warm frontal position may be well defined by a band of cold clouds in

the infrared image. However, high cloud in the warm sector may merge with the

Categorization of weather types using satellite imagery 2953

frontal band, making the warm front difficult to locate. In this case, the distance

between the cold clouds’ edge and surface front for an active system is typically

about 500 km but this will depend, for instance, on the slope of the front.

Table 1. Date and time of AVHRR images used for the categorization of the cloud featuresin the area of Greece for the 42 cases examined in this study. The correspondingsynoptic weather type for each case is also included.

Day Time (UTC) Weather type

1 4 January 1995 8:20, 19:45 WDWT2 9 January 1995 8:57 WDWT3 12 January 1995 6:31, 19:52 WDWT4 14 January 1995 7:53 SWDWT5 16 January 1995 9:06 MWT6 27 January 1995 11:27 WDWT7 31 January 1995 12:23 WDWT8 4 February 1995 11:40 MWT9 5 February 1995 11:29 NWDWT

10 15 February 1995 11:23 SWDWT11 19 February 1995 12:19 NWDWT12 2 March 1995 and 12:02 SWDWT

3 March 1995 11:42 SWDWT13 5 March 1995 and 11:29 WDWT

6 March 1995 11:19, 12:59 WDWT14 9 March 1995 12:27 NWDWT15 14 March 1995 11:39 SWDWT16 23 March 1995 and 11:37 MWT

24 March 1995 11:27 MWT17 28 March 1995 and 12:23, 12:12 NWDWT

29 March 1995 12:12 NWDWT18 30 March 1995 12:02 NWDWT19 1 April 1995 11:40 MWT20 7 April 1995 12:16 SWDWT21 10 April 1995 11:44 MWT22 11 April 1995 and 11:34 NWDWT

12 April 1995 11:22 NWDWT23 16 April 1995 12:19 WDWT24 25 April 1995 12:23 WDWT25 26 April 1995 12:12 WDWT26 27 October 1995 12:36 WDWT27 3 November 1995 13:01, 11:21 NWDWT28 7 November 1995 12:18 MWT29 9 November 1995 11:56 SWDWT30 14 November 1995 01:07, 12:43 WDWT31 15 November 1995 12:32 WDWT32 18 November 1995 00:23, 12:00 NWDWT33 21 November 1995 and 01:32, 11:27 NWDWT

22 November 1995 01:21, 12:57 NWDWT34 25 November 1995 00:48, 12:24 WDWT35 1 December 1995 01:24, 13:00 WDWT36 4 December 1995 00:51, 12:27 WDWT37 6 December 1995 and 12:06 WDWT

7 December 1995 11:55 WDWT38 13 December 1995 00:54, 12:30 SWDWT39 18 December 1995 01:41, 11:36 SWDWT40 19 December 1995 01:31, 11:26 SWDWT41 28 December 1995 11:29, 13:09 WDWT42 29 December 1995 01:23, 12:58 WDWT


3. Results

Depressional weather types are mainly responsible for cold and rainy weather

with strong winds in Greece during the cold period of the year. These weather types

are characterized by warm and cold air sequences as well as by potential and kinetic

energy exchanges. A typical example is a well-organized depression passing over

north Greece. In this case, the warm air initially moves cyclonically towards the

Balkan Peninsula and the Black Sea and then, after the intrusion of a polar or

Table 2. Cumuliform cloud characteristics in satellite imagery according to Rao et al. (1990).

Cloud type Typical shape or pattern

Typical grey tone

Visible Infrared

Cumulus (Cu) Small elements orgroups of elements

Medium brightness. Dark tone,usually difficultto detect.

Cumulonimbus(Cb)

Globular or carrotshaped with upwindedge sharp andopposite downwindanvil edge indistinct.

Bright. Relatively bright.

Stratocumulus(Sc)

Closed cellular patternin visible imagery oversea in areas of coldadvection. Cell sizevaries depending oninversion height.

Centre of cells inpattern bright,become grey towardedges where cloudthins. If cells arebellow sensor resolutionthey appear smoothas if stratus.

Surface uniform darkgrey with cellularstructure not present.Can be difficult todetect when cloudtops are low.

Table 3. Stratiform cloud characteristics in satellite imagery according to Rao et al. (1990).


Typical grey tone

Visible Infrared

Fog-stratus(St)

Smooth tops, boundariesoften sharp definedby topography.

Uniform mediumgrey tone ifthick and mottletone if thin.

Uniform darkgrey tone,usually difficultto detect.

Altostratusand altocumulus(As and Ac)

Smooth tops, boundariescan be ragged or smoothed,often in layers. When incombination with deepconvection, tops appearbumpy, if Sun anglelow. Cellular structureof altocumulus too smallfor sensors to resolve socannot be distinguishedfrom altostratus.

Light grey, appearsmottled or striateddepending onthickness orlayered structure.

Uniform mediumgrey tonedependingon height.


arctic air mass, it comes back as a north-west wind, resulting in a surface pressure

increase and in a decrease in surface temperature. Weather conditions depend on

evolving air masses, on wind direction at 500 hPa, on the trajectory of the low-

pressure system and of the upper trough, on the geographical characteristics of each

specific area of interest and on surface roughness.

Thirty-six cases for the cold period with a depression passing over the broader

Greek area have been examined in this study for the periods January to April and

October to December 1995. The selection was accomplished by means of cloud

system identification in satellite images in combination with the analysis of mean-

sea-level pressure synoptic maps. For each case, the mean depression trajectory was

determined by tracking the associated cloud system in successive satellite images.

The temporal resolution of the AVHRR images used for the determination of the

systems’ trajectory depended on the number of available images covering the areas

close to the position of the systems. The outcome of the above-mentioned analysis

was the classification of the depressional weather types in three main categories

according to the depression’s mean track: West Depressional Weather Type

(WDWT), North-west Depressional Weather Type (NWDWT) and South-west

Depressional Weather Type (SWDWT). The above-mentioned classification scheme

is in accordance with the categorization of depressions made by Maheras (1979,

1983, 1988a). The main trajectories of depression movement over the Mediterranean

and southern Europe and the related weather types as they are derived by the

analysis of the NOAA/AVHRR images, are illustrated in figure 1.

Six cold period cases, which could not be classified as depressional weather

types, were also examined. These cases were classified according to Maheras (1979,

1983, 1988a), in a separate category: the Mixed Weather Type (MWT). This weather

coincides with severe cold weather, with rainfall and snowfall mainly over central

and northern Greece, with very low temperatures and very strong north-easterly

winds.

In the following sections, each winter weather type is analysed and the most

representative case studies of each type are demonstrated. A summary of the cloud

characteristics of each weather type in Greece, based on the photointerpretation of

NOAA/AVHRR visible and infrared imagery, in combination with the character-

istics of the systems in synoptic analysis maps, is presented in tables 5–8. Taken that

Table 4. Cirriform cloud characteristics in satellite imagery according to Rao et al. (1990).


Typical grey tone

Visible Infrared

Cirrus (Ci) Banded fibrous structure.Features of underlyingterrain and cumulus cloudsare sometimes detectablethrough clouds.

Dark grey to greytone depending onunderlying surface.

Light grey in tone.Fibrous structurenot as evident asin visible.

Cirrostratus(Cs)

Generally smooth anduniform tops butsometimes fibrous inappearance. May bein long bands oran extensive sheet.

Appears light greywhen thin and whiteras thickness increases.

Varies from whiteto grey. Difficultto distinguishfrom middleclouds.




no single representative case includes all the observations presented in tables 5–8,

some of these observations are not referred to in the text. The frequency of

occurrence (in percentage) of each observation associated with each weather type in

satellite imagery for the 42 cases is also included in these tables.

3.1. West Depressional Weather Type (WDWT)

This category includes all the synoptic situations characterized by the presence

of a depression over Greece moving from west to east (the contour lines at the

isobaric surface of 500 hPa are parallel to west–east direction). Surface temperature

variations in these frontal depressions are substantially lower compared to the

respective differences in the depressions coming from the north-west because both

sectors of the corresponding atmospheric disturbance contain maritime air masses

coming from mid-latitudes. In addition, the zonal circulation in the entire

troposphere blocks the meridional air mass exchanges, which are responsible for

abrupt weather variations. Thus, the zonal momentum and energy advection

dominates not only in the surface, but also in the entire troposphere.

WDWT may be divided in two sub-categories related to the trajectory of the

depression systems: WDWT with north track and WDWT with south track.

(a) WDWT with north track: The depression follows a zonal track from west to

east at latitude greater than 45‡ (Maheras 1979, 1983, 1988a). Only the southern

Figure 1. Main trajectories of depression movement over the Mediterranean and southernEurope and the related weather types as they are derived by analysis of the NOAA/AVHRR images.



edge of such depression systems affects Greece. The weather is clement, with south

winds, variable clouds and very little rainfall.

(b) WDWT with south track: This weather type is generated by depressions with

Atlantic or Mediterranean origin moving on a zonal track but at a latitude lower

than 45‡. This weather type brings cold weather with rain all over Greece; it is a

weather type typical of the winter period (Maheras 1979, 1983, 1988a). In the first

case, the depressions with Atlantic origin are attenuated when reaching Greece due

to their long path over the Atlantic, west and central Mediterranean; therefore they

are associated with occluded fronts. In the second case, the depressions are formed

over west or central Mediterranean in association with a south-west extension of

the Siberian cold anticyclone, which interrupts the zonal circulation (blocking

situation).A summary of the cloud characteristics of WDWT in Greece, based on

photointerpretation of NOAA/AVHRR visible and infrared imagery, in combina-

tion with the characteristics of the systems in synoptic analysis maps, is presented in

table 5. In the following sections, two representative case studies of WDWT are

demonstrated.

Table 5. Summary of the characteristics of West Depressional Weather Types over Greece,based on photointerpretation of NOAA/AVHRR visible and infrared imagery, incombination with synoptic analysis maps. Percentages in satellite observationsindicate frequency of occurrence.

Synoptic situation500 hPa analysis (contour lines,

temperature, vertical velocity)SFC analysis (MSL), 850 hPa

analysis (temperature). A trough, frequently located

over Italy, with sequentialdisturbances moving towardsNE and E

. Depressions associated with cold and warmfronts (or occluded front). The direction of frontsare usually N–S

. Cold air masses (withtemperatures around230‡C) over Greece

. Weak pressure gradients

. W–SW wind currentsover Greece

. High levels of relative humidity in specific areas

. Intensive convectivemotions over mountainousregions

. Weak temperature gradients at 850 hPa

. Convective motions overthe warm sea in cold sector

AVHRR Imagery photointerpretation. Cb embedded in thick Cs and As over the convergence zones of cold and warm air

masses (frontal zones of the depressions) 100%. Massive Cs ahead of the cold front of the depressions (75%). Low clouds (St, Sc) over the west side of Pindos mountains due to south-east winds

usually ahead of the cold front (100%). Sc orographic clouds over the west side of mountainous areas of Greece due to

north-west winds prevailing usually after the cold front of the depression has crossedcontinental Greece (88%)

. Few Cs above Ac over the warm sector of the depressions (72%)

. Low clouds (St, Sc) over the warm sector of the depressions (27%)

. Streets of Cu and Cb over marine regions when there is cold air advection in the troughof the upper troposphere (50%)

. Sc or St over sea in front of the warm or occluded front due to the low-level warmadvection caused by the south winds (70%)



3.1.1. Case study 1: 5 March 1995

The synoptic situation is presented in figure 2. The 500 hPa height analysis

shows that there is a cut-off-low associated with a trough over Italy containing a

very cold air mass (230‡C). The trough is affecting west Greece while moving

towards the east-north-east. The combination of the surface analysis and 850 hPa

temperature analysis shows that there is a depression (not very well developed) over

the same area associated with a cold front along the western coastline of Greece.

Figures 3 and 4 present the AVHRR visible and thermal infrared images,

respectively. Both images were recorded on 5 March 1995 at 11:29 UTC. The

analysis of consecutive images showed a depression which has just started to

develop along a frontal cloud zone north of Greece, moving in a zonal track from

west to east. A part of the frontal cloud band associated with the cold front of the

depression is evident in the images. It is a classical cold frontal cloud band in the

infrared images determined as continuous, relatively broad cloud formation with

mostly cold tops, bright in the visible image, and characterized by a distinct long

axis with a slight curvature. The position of the cold front has been superimposed

in both images. The frontal cloud layer consisting of thick Cs with the overshooting

tops of Cb in area A, is clearly depicted in both figures 3 and 4. Cbs are triggered by

low-level forcing for ascent along the cold front in combination with considerable

forcing associated with an upper level trough. They can be easily recognized from

Table 6. Summary of the characteristics of North-west Depressional Weather Types overGreece, based on photointerpretation of NOAA/AVHRR visible and infraredimagery, in combination with synoptic analysis maps. Percentages in satelliteobservations indicate frequency of occurrence.



analysis (temperature). Sequential disturbances moving

from NW to SE. Well-developed depressions associated with

cold and warm fronts. Cold air masses (with

temperatures around 225‡C). Low pressure gradients

. Convective motions ahead ofthe disturbances, especiallyover the marine regions

. Weak temperature advection at 850 hPa

AVHRR imagery photointerpretation. Cu or Cb over the marine regions behind the cold front as a result of a cold advection

and sometimes MCSs and Cb associated with a disturbance in the 500 hPa air flow (80%). Clouds of cold front are composed of either(a) mesoscale convective systems (MCSs)

and several Cb (isolated or embedded in thick Cs) (88%) or (b) constitute a typical coldfrontal band defined as continuous, relatively broad cloud formation with mostly coldtops (thick Cs and Ci with embedded Cb lowered to Sc at the tail of the front), bright inthe visible images, characterized by a distinct long axis with or without curvature (12%)

. Warm cloud band comprises cloud layers of Ci and Cs and Ac, As underneath andfrequently Sc (100%)

. The direction of high clouds (Ci and Cs) indicates the location of a jet stream whichdrives the circulation when there is a breakdown of clouds on the right (38%)

. Sc orographic clouds over the west side of the main mountain range of Greece(Pindos Mountains) and breakdown of clouds over the east side due to north-westwinds prevailing, usually after the passage of a cold front (75%)

. Sc lee waves on the west side of the mountainous Greece and islands in the case ofsouth-west winds over the warm sector of the depression (50%)


the shadow of overshooting tops on the underlying Cs layer in the visible image, as

well as from the high brightness values (very low cloud top temperatures) of their

tops in the thermal infrared image. There are also Ac and As in the area A, as well

as lower clouds as Sc or St. The formation of low clouds is favoured along the west

coastline of Greece because the south-easterly winds, which are blown in front of

the cold front, transfer warm and wet air masses over this area. The adiabatic

cooling of these air masses, as they rise over the Pindos mountains (a mountain

chain along the west coastline of Greece), is the main reason for the low clouds’

formation in the area A. Cs are located in the area B, ahead of the cold front. These

clouds are transparent in the visible (figure 3). The areas C and D correspond to

thin Ci and Cs, which are characteristic cloud types of the warm section of the

depression (Bader et al. 1995). The fibrous nature of Ci can be seen more clearly in

figure 3. The very low top temperatures of Ci are obvious in figure 4 (very bright

tones).

3.1.2. Case study 2: 25 November 1995

The synoptic situation is presented in figure 5. The 500 hPa analysis shows that

there is a deep (5580 m) low near Sicily associated with a trough over the Ionian

Table 7. Summary of the characteristics of South-west Depressional Weather Types overGreece, based on photointerpretation of NOAA/AVHRR visible and infraredimagery, in combination with synoptic analysis maps. Percentages in satelliteobservations indicate frequency of occurrence.



analysis (temperature). A trough is located over

central or eastern Mediterranean,frequently with sequentialdisturbances moving from SWto E–NE

. Well-developed depressions locatedSW of Greece moving towards E–NE

. Intensive upward motionsextending from SW and SGreece to west coastlineof Turkey

. The depressions are associated withcold and warm fronts or, frequently,with occluded fronts

. SW winds, frequently verystrong

. High temperature gradients at 850 hPa

. High pressure gradients, resulting in verystrong winds (frequently stronger than8 Beaufort over the Aegean)

AVHRR imagery photointerpretation. The appearance of Ac at the downwind sides of the mountains (perpendicular to the

south winds), indicates the development of lee waves (85%). Massive cloud layers of Sc, As, Ac and Cs are usually observed over the Aegean Sea

along a warm front moving to the north (100%). Cb embedded in a thick Cs layer over the cold front, especially when the depressions

pass over marine regions (60%). Cu or Cb behind the cold front, especially when the depressions pass over marine

regions (100%). Sc formation over the east side of the main mountain range of Greece (Pindos

Mountains), in cases in which the depressions are located at the southern Aegean Sea(75%)

. The appearance of Cs indicates the presence of strong winds in the higher levels oftroposphere (location of the jet stream) (60%)


Sea. The presence of a depression at the surface in the same area generates strong

south winds over the west coastline of Greece.


respectively. Both images were recorded on 25 November 1995 at 12:14 UTC. The

exact locations of possible cold and warm fronts are very difficult to determine in

this case. In area A, there is a Cs cloud layer above Ac, As and, probably, Sc layers.

The stratiform nature of the above-mentioned cloud layers implies that there is

synoptic scale air mass convergence without any convective activity in the area A.

An inspection of the isotherms (dashed lines at 850 hPa temperature analysis) shows

a cold intrusion in the Aegean Sea and a warm advection in the Ionian Sea,

consistent with the warm air rising over the cold air in the area A. There is no

evidence, however, to indicate that this convergence is associated with an occlusion,

warm front or stationary front. The 500 hPa analysis shows that there are

descending air mass movements associated with a ridge at mid-tropospheric heights.

In area B, there is an extensive opaque cloud layer with very low top tem-

peratures as can be seen in figure 7 (very bright tones). Rough cloud tops in the

visible image (figure 6) indicate also convective activity. These vertical motions can

be easily explained by the existence of the trough at 500 hPa located over Italy

(figure 5(a)) in combination with the low-layer advection of warm air masses

(850 hPa temperature analysis, figure 5(c)). This combination is responsible for the

high atmospheric instability in area B.

Table 8. Summary of the characteristics of Mixed Weather Types over Greece, based onphotointerpretation of NOAA/AVHRR visible and infrared imagery, in combinationwith synoptic analysis maps. Percentages in satellite observations indicate frequencyof occurrence.


temperature, vertical velocity)SFC analysis (MSL), 850 hPa analysis temperature)

. Low geopotential heights basinover Greece, with perturbationsmoving from N–NW to S–SE

. Extended anticyclone covers the BalkanPeninsula, whilst low-pressure systemsare located at its south-east edge

. Cold air temperatures overGreece (air masses withtemperatures around 235‡C)

. The combination of the high-pressureanticyclone with the low-pressure systems,which are located S–SE, generates stormN–NE winds over the Aegean

. Upward motions over marineregions, especially over thelocation of the depressions

. Low temperatures at 850 hPa due to theprevailing N winds over the Aegean

. A blocking ridge is situatedover west Europe

. The perturbations at 500 hPa enforce thesurface winds

. The depressions are associated withcold and warm fronts which are movingover the Southern part of Greece

AVHRR imagery photointerpretation. Streets of Cu, Cb embedded in As layers and Sc, windward over the Aegean islands,

as well as along the east coastline of continental Greece (75%). The appearance of orographic Ac and Sc clouds at the downwind sides of the

mountains (perpendicular to the north winds) (50%). Isolated Cu or Cb behind the cold fronts, especially over marine regions (100%). Cu or Cb development over marine regions indicating the locations of 500 hPa

perturbations over these areas (100%)


(a)

(b)

(c)

Figure 2. Synoptic analysis for 5 March 1995 (12:00 UTC). (a) 500 hPa height analysis,contour lines with 60 m interval (solid lines) and temperature analysis (dashed lines with5‡C intervals). Disturbances in the trough described in the text are indicated with thickdashed lines. (b) 500 hPa vertical velocity analysis with 0.2 Pa s21 intervals; downwardisotachs with solid lines and upward isotachs with dashed lines. (c) Surface meso-scalemap with 4 hPa intervals (solid lines) and 850 hPa temperature analysis (dashed lineswith 2‡C intervals). Systems described in the text are indicated with thick solid lines.


In the area C, there is a compact low-level St and Sc layer, probably fog,

associated with the cold air intrusion over the Aegean Sea and Greece (figure 5(c)).

These low-level clouds or fog are the result of the mixing of this cold air with the

warm surface layer over the Mediterranean.

3.2. North-west Depressional Weather Type (NWDWT)

This category comprises all the synoptic situations characterized by the presence

of a depression over Greece moving on a semi-meridional track from north-west to

south-east. This weather type is generated by depressions with Atlantic or

Mediterranean origin.

Depressions originating from the Atlantic Ocean may reach Greece when the

polar front obtains meridional (or semi-meridional) direction over central Europe.

Actually, this refers to a secondary branch of the polar front, which is developed

when the track of the depressions coming from the Atlantic is oriented in a south-

east direction due to the presence of the Atlantic anticyclone. This type of depres-

sion is infrequent in Greece, affecting mainly the west Mediterranean or west

Europe.

Depressions originating from the Mediterranean travel over the central

Mediterranean and Italy, moving from north-west to south-east before reaching

Figure 3. NOAA/AVHRR channel 1 image (visible) for 5 March 1995 (11:29 UTC). In thiscase, Greece is affected by a West Depressional Weather Type. The location of thecold front is superimposed in the images. The prevailing cloud systems in areas A, B,C and D are described in the text.


Greece. In this case, upper level airflow obtains a meridional direction from north

or north-west. Mediterranean origin depressions play the most important role for

weather and climate in Greece, because they occur frequently and they generate

high levels of accumulated precipitation for the whole country.

NWDWTs in winter move over Greece along a track situated to the west of the

country. If the depressions have Atlantic origin, their frontal systems will reach

Greece as occlusions driven by the upper airflow (500 hPa). If the depressions have

Mediterranean origin, they will be well developed when they reach Greece. Their

impact on weather conditions in Greece is related to the intensity of the cold air

intrusion as well as to depth of the associated low-pressure system. This weather

type causes cold weather with strong winds and it is also typical of the winter

period (Maheras 1979, 1983, 1988a). In the northern part of Greece, this type of

depression rarely causes floods, whereas in central and southern Greece, where

orographic effects are significant, floods are not unusual.

A summary of the cloud characteristics of NWDWTs in Greece, based on the


tion with the characteristics of the systems in synoptic analysis maps, is presented in

table 6. The case study, which is presented in the following section, has been

selected as a representative one.

Figure 4. NOAA/AVHRR channel 4 image (infrared) for 5 March 1995 (11:29 UTC). Inthis case, Greece is affected by a West Depressional Weather Type. The location ofthe cold front is superimposed in the images. The prevailing cloud systems in areas A,B, C and D are described in the text.



(a)

(b)

(c)

Figure 5. Synoptic analysis for 25 November 1995 (12:00 UTC). (a) 500 hPa height analysis,contour lines with 60 m interval (solid lines) and temperature analysis (dashed lines with5‡C intervals). Disturbances in the trough described in the text are indicated with thickdashed lines. (b) 500 hPa vertical velocity analysis with 0.2 Pa s21 intervals; downwardisotachs with solid lines and upward isotachs with dashed lines. (c) Surface meso-scalemap with 4 hPa intervals (solid lines) and 850 hPa temperature analysis (dashed lineswith 2‡C intervals). Systems described in the text are indicated with thick solid lines.


Figure 6. NOAA/AVHRR channel 1 image (visible) for 25 November 1995 (12:24 UTC). Inthis case, Greece is affected by a West Depressional Weather Type. The location ofthe occluded front is superimposed in the images. The prevailing cloud systems inareas A, B and C are described in the text.

Figure 7. NOAA/AVHRR channel 4 image (infrared) for 25 November 1995 (12:24 UTC).In this case, Greece is affected by a West Depressional Weather Type. The location ofthe occluded front is superimposed in the images. The prevailing cloud systems inareas A, B and C are described in the text.


3.2.1. Case study 3: 12 April 1995

The synoptic situation is presented in figure 8. The synoptic analysis shows a

depression system moving from north-west to east. The associated cold front is

located over north-west Greece, whereas the warm front is located over Thrace

(north-eastern Greece). The locations of the above-mentioned fronts have been

superimposed to the AVHRR visible and thermal infrared images, which are

presented in figures 9 and 10, respectively. Both images were recorded on 12 April

1995 at 11:12 UTC.The stratiform cloud layers in area A (figures 9 and 10) is probably associated

with the 500 hPa trough which is oriented in a north-east to south-west direction

over Greece (figure 8(a)). There are Cs on top, which obtain very bright tones in

visible (figure 9), whereas Ac, As and Sc are located underneath. Ac and As appear

as massive opaque clouds in figure 9, whereas Sc obtain grey tones in infrared

(figure 10) due to their higher top temperatures. Area B is dominated by a wedge-

shaped mesoscale convective system. A narrow cloud band associated with the

warm front lies to the north of area B consisting of Cs on top, whereas Ac, As and

Sc are located underneath.

Frontal clouds in area C are composed of mesoscale convective systems (MCSs)

and several Cb, either isolated or embedded in thick Cs, and do not constitute a

typical cold frontal band. In satellite images MCSs appear as large circular shields

having typical diameters of 100–500 km, or as elliptical shields with their long axis

being up to 1000 km in length (Bader et al. 1995). Few Sc are detected underneath

the high cloud tops. Cloud tops in area C of the visible image appear bright with a

rugged texture (figure 9) and present relatively low temperatures in the infrared

image (figure 10). Altogether are indications of atmospheric instability in this area

and therefore of convective activity induced by the upper level forcing in

combination with the adiabatic cooling of the corresponding air masses, as they lift

over the mountainous west Greece. However, these clouds appear broken in the

leeward side of the mountains, due to the downward air motion. Cb and small

MCSs behind the cold front indicate convective activity related to the disturbance

in the trough at 500 hPa (solid contour lines in figure 8(a)) in combination with the

cold advection over the warm sea.

3.3. South-west Depressional Weather Type (SWDWT)

This category includes all the synoptic situations characterized by the presence of a

depression over Greece following a semi-meriditional track from south-west to

north-east. SWDWT may be divided into two sub-categories related to the trajectory

of the depression systems: WDWT with West Track and WDWT with East Track.

(a) SWDWT with west track: The depression follows a semi-meridional track

from south-west to north-east, situated to the west of the Aegean Sea. The synoptic

situations related to this category vary according to the location of the low-pressure

system. If the warm section of a depression moving slowly towards the north-east

affects Greece, the weather will be unstable with moderate cloudiness. If the centre

of the depression passes over Greece, a discontinuous middle cloud layer (As) will

be developed underneath the high cloud bases.

Surface winds are generally southerly, light or moderate. Winds are also

southerly at 500 hPa level but much stronger. The descending winds along the west

Aegean coastline, which are developed in the leeward main mountain range of


(a)

(b)

(c)

Figure 8. Synoptic analysis for 12 April 1995 (12:00 UTC). (a) 500 hPa height analysis,contour lines with 60 m interval (solid lines) and temperature analysis (dashed lines with5‡C intervals). Disturbances in the trough described in the text are indicated with thickdashed lines. (b) 500 hPa vertical velocity analysis with 0.2 Pa s21 intervals; downwardisotachs with solid lines and upward isotachs with dashed lines. (c) Surface meso-scalemap with 3 hPa intervals (solid lines) and 850 hPa temperature analysis (dashed lineswith 3‡C intervals). Systems described in the text are indicated with thick solid lines.


Greece, contribute to the reduction of relative humidity and of atmospheric instabi-

lity into the lower atmospheric levels.

It has to be mentioned that the cold front associated with the depression reaches

Greece from the south. The intrusion of the cold air masses behind the cold front

favours the atmospheric instability and the formation of convective clouds over the

Aegean Sea.

(b) SWDWT with east track: The depression follows a semi-meridional track

from south-west to north-east, situated to the east of mainland Greece across the

Aegean Sea. This weather type is associated with low temperatures and widespread

rainfall with snow in winter (Maheras 1988a).

A summary of the cloud characteristics of SWDWT in Greece, is presented

in table 7. In the following section a representative case study of SWDWT is

demonstrated.

3.3.1. Case study 4: 18 December 1995

The synoptic situation is presented in figure 11. The surface analysis shows that

two low-pressure systems are located north-west and west of Greece, respectively

(figure 11(c)). The two surface lows are combined with two troughs at 500 hPa,

located in different latitudes (figure 11(a)).


respectively. Both images were recorded on 18 December 1995 at 11:26 UTC. Five

specific areas (A, B, C, D and E) are marked in both images.

Figure 9. NOAA/AVHRR channel 1 image (visible) for 12 April 1995 (11:22 UTC). In thiscase, Greece is affected by a North-west Depressional Weather Type. The locationsof the cold and warm fronts of the depressions are superimposed in the images. Theprevailing cloud systems in areas A, B and C are described in the text.



The cloud zones in areas A and B are parts of the depression which is crossing

southern Greece in a south-west to north-east direction. The areas A and C may be

characterized as intense convective zones with Cb clouds embedded into a Cs layer.

In the area A, there is a cold front at the surface in conjunction with a disturbance

at the 500 hPa level. In the area C, only a disturbance at 500 hPa can be observed.

The veering of the geostrophic wind from south-east at surface to south-west at

500-hPa between area A and C indicates warm advection. This appears to reinforce

the upper-level forcing associated with the trough. An inspection in both images

shows that a Cs layer with stratiform clouds As underneath are located ahead of

the areas A and C.In front of the area D, there are isolated Cb that delimit the cold air mass at

500 hPa. These clouds are the results of the thermodynamic instability in the

boundary layer caused by the cold air advection over the warmer sea.

In the area E, there are mainly St. These clouds present a smooth texture in

visible and obtain grey tones in infrared as can be seen in figures 12 and 13,

respectively. In the west part of the area E there are also Ac over St.

In the area B, there are higher clouds, mainly Ci, related to the warm front,

which has been outlined in both images.

3.4. Mixed Weather Type (MWT)

The classification of mixed types is made by considering the direction of isobars

between an anticyclone and a depression and the wind flow at 500 hPa (Maheras

Figure 10. NOAA/AVHRR channel 4 image (infrared) for 12 April 1995 (11:22 UTC). Inthis case, Greece is affected by a North-west Depressional Weather Type. Thelocations of the cold and warm fronts of the depressions are superimposed. Theprevailing cloud systems in areas A, B and C are described in the text.


(a)

(b)

(c)

Figure 11. Synoptic analysis for 18 December 1995 (12:00 UTC). (a) 500 hPa height analysis,contour lines with 60 m interval (solid lines) and temperature analysis (dashed lines with5‡C intervals). Disturbances in the trough described in the text are indicated with thickdashed lines. (b) 500 hPa vertical velocity analysis with 0.2 Pa s21 intervals; downwardisotachs with solid lines and upward isotachs with dashed lines. (c) Surface meso-scalemap with 4 hPa intervals (solid lines) and 850 hPa temperature analysis (dashed lineswith 2‡C intervals). Systems described in the text are indicated with thick solid lines.


1979, 1983, 1988a). The high-pressure system is located over the broader Greek

area, whilst the low is situated at the edge of the anticyclone. The synoptic situation

is inverted at 500 hPa level: an extended trough, or a cut-off-low covers the Greek

area, whilst a ridge is located at the west endings of the anticyclone. MWT is

frequently the result of a jet stream blocking situation in west Europe. The weather

conditions frequently depend on the season, similarly to the conditions produced by

the depressional weather type.

This weather type may be classified in two main sub-categories. The synoptic

situation in each sub-category is described as follows (Maheras 1979, 1983, 1988a).(a) In the first case, the anticyclone is located to the north-west or west of

Greece and the depression to the north-east or east. Isobars obtain meridional

direction at the surface, defining a very strong current of north winds over Greece.

(b) In the second case, the anticyclone is positioned to the north-west, north or

north-east of Greece, the depression to the south-west, south or south-east. Isobars

obtain zonal direction at the surface.

A summary of the cloud characteristics of MWTs in Greece, based on


tion with the characteristics of the systems in synoptic analysis maps, is presented

in table 8. In the following section a representative case study of MWT is

demonstrated.

Figure 12. NOAA/AVHRR channel 1 image (visible) for 18 December 1995 (11:36 UTC).In this case, Greece is affected by a South-west Depressional Weather Type. Thelocations of the cold and warm fronts of the depression are superimposed in theimages. The prevailing cloud systems in areas A, B, C, D and E are described inthe text.



3.4.1. Case study 5: 23 March 1995

The synoptic situation is presented in figure 14. The surface analysis shows that

a low-pressure system associated with a warm and a cold front is located south-east

of Greece, whilst an anticyclone is covering the countries of the Balkan Peninsula

and the continental part of Greece. The 500 hPa analysis shows that a trough with

cold air masses (235‡C) is covering the broader Greek area whereas a blocking

ridge is situated over west Europe. The curvature of its contour lines indicates high

levels of vorticity, which implies high atmospheric instability, especially over the

Aegean Sea.


respectively. Both images were recorded on 23 March 1995 at 11:26 UTC. Three

specific areas (A, B and C) are marked in both images.

In the area A, there is a low-pressure system moving north-eastward. The

stratiform cloud layers are obvious in both figures 15 and 16 (Cs on top and

underneath Ac, As and probably Sc). Especially in figure 15, the higher clouds are

quite massive. This cloud pattern is associated with the warm advection ahead of

the warm front. The white arrow in figure 16 shows Cu zones, which determine the

location of the cold front.North of area B the north-east winds, which prevail over the Aegean, favour the

formation of streets of Cu and Sc as the cold air is warmed flowing over the warm

Aegean Sea and the windward side of the islands. The white arrows in both

Figure 13. NOAA/AVHRR channel 4 image (infrared) for 18 December 1995 (11:36 UTC).In this case, Greece is affected by a South-west Depressional Weather Type. Thelocations of the cold and warm fronts of the depression are superimposed in theimages. The prevailing cloud systems in areas A, B, C, D and E are described inthe text.


(a)

(b)

(c)

Figure 14. Synoptic analysis for 23 March 1995 (12:00 UTC). (a) 500 hPa height analysis,contour lines with 60 m interval (solid lines) and temperature analysis (dashed lines with5‡C intervals). (b) 500 hPa vertical velocity analysis with 0.2 Pa s21 intervals; downwardisotachs with solid lines and upward isotachs with dashed lines. (c) Surface meso-scalemap with 4 hPa intervals (solid lines) and 850 hPa temperature analysis (dashed lineswith 2‡C intervals). Systems described in the text are indicated with thick solid lines.


Figure 15. NOAA/AVHRR channel 1 image (visible) for 23 March 1995 (11:37 UTC). Inthis case, Greece is affected by Mixed Weather Type. The locations of the cold andwarm fronts of the depression are superimposed in the images. The prevailing cloudsystems in areas A, B and C are described in the text.

Figure 16. NOAA/AVHRR channel 4 image (infrared) for 23 March 1995 (11:37 UTC). Inthis case, Greece is affected by Mixed Weather Type. The locations of the cold andwarm fronts of the depression are superimposed in the images. The prevailing cloudsystems in areas A, B and C are described in the text.


figures 15 and 16 show well developed Sc and Cb over Crete. The formation of Cb,

embedded in a Cs layer, is favoured by the high mountains of Crete, which are

perpendicular to the north-easterly current.

In the area C, according to figure 14, there is a cold air mass at 500 hPa, with

low temperature (230‡C), over the warm Ionian Sea. The combination of the cold

air with the warm air produces high atmospheric instability in this area, and

therefore isolated thunderstorms, which are shown by the arrows in both figures 15

and 16.

4. Conclusions

The geographical location, as well as the surface topography, of Greece plays

the most important role with regards to the weather types affecting the country.

Likewise, the marine region of the central Mediterranean, which is a huge reservoir

of heat and moisture, plays the most important role concerning cloud formation

over Greece. During the winter, cold continental air masses encounter warmer and

moister Mediterranean air masses over Greece. As a result, the air mass

convergence generates atmospheric instability and therefore cloud formation. The

atmospheric instability is enforced by the evaporation, which takes place over the

relative warm Greek maritime areas.Greece is affected by the depressions of the westerly circulation and at the same

time is influenced by meridional circulations and depressions formed over the

Mediterranean. The main synoptic weather types, which are responsible for the

severe cold and rainy weather conditions in Greece during the cold period of

the year, are the Depressional Weather Types and the Mixed Weather Types.

For a more detailed categorization of the cold period weather types in Greece,

NOAA/AVHRR satellite images (visible and thermal infrared) as well as synoptic

analysis data (surface, 850 hPa and 500 hPa analysis) were used. After the examina-

tion of 42 case studies, the outcome of the AVHRR imagery photointerpretation

was the classification of the cold period weather types in Greece in four categories

on the basis of their tracks and cloud patterns inferred by satellite images in

conjunction with weather maps:

(a) WDWT: West Depressional Weather Type;

(b) NWDWT: North-west Depressional Weather Type;

(c) SWDWT: South-west Depressional Weather Type; and

(d) MWT: Mixed Weather Type.

The main scope of this photointerpretation on satellite images was to categorize

the cloud features in the area of Greece formed by each prevailing weather type. The

main characteristics of all categories are summarized in tables 5–8. Each table presents

the synoptic situation for the respective weather type, together with the results of the

AVHRR imagery photointerpretation for the corresponding case studies.

This categorization aims to be a first link between the weather types affecting

Greece and the related cloud features as being extracted by satellite images. It could

be a useful guide for operational forecasters for interpreting satellite images and

weather systems in Greece and it should be considered as a basis for future

climatological study.

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