Q. J. R. Meteorol. Soc. (2006), 132, pp. 3035–3058 doi: 10.1256/qj.06.36

Fohn/cold-pool interactions in the Rhine valley during MAP IOP 15

By C. FLAMANT1∗, P. DROBINSKI1, M. FURGER2, B. CHIMANI3, S. TSCHANNETT3,R. STEINACKER3, A. PROTAT4, H. RICHNER5, S. GUBSER5 and C. HABERLI3,6

1Institut Pierre-Simon Laplace, SA, Paris, France2Paul Scherrer Institute, Villigen, Switzerland

3Department of Meteorology and Geophysics, University of Vienna, Austria4Institut Pierre-Simon Laplace, CETP, Velizy, France

5Institute for Atmospheric and Climate Science, ETH, Zurich, Switzerland6MeteoSwiss, Zurich, Switzerland

(Received 20 March 2006; revised 10 July 2006)

SUMMARY

The fohn/cold-pool interactions in the lower Alpine Rhine valley documented in the framework of theIntensive Observing Period (IOP) 15 of the Mesoscale Alpine Programme (MAP) on 5 November 1999 areanalysed. The present study focuses on the water vapour mixing ratio measurements acquired with the airbornedifferential absorption lidar LEANDRE 2 which enabled detailed documentation of the along-valley structureof the cold pool. LEANDRE 2 and microbarograph measurements revealed the presence of Kelvin–Helmholtzwaves (KHW) at the top of the cold pool. The characteristics of the waves were different in the region of the cold-pool leading edge (the southernmost part of the cold pool) and in the vicinity of the Bodensee (Lake Constance),further to the north. Gravity waves were also observed above the cold pool in the in situ aircraft data acquired inthe vicinity of the Bodensee. The gravity waves are suspected to be triggered by the KHW at the top of the coldpool. We also investigate the respective role of the three known processes likely to control the structure of the coldpool and its erosion along the Rhine valley, namely (i) convection within the cold pool, (ii) turbulent erosion atthe top of the cold pool due to the presence of KHW, and (iii) dynamic displacement of the cold pool by fohn air.The former two processes are likely not to play a role in the erosion of the cold pool observed in the course of thisIOP. Finally, the temporal evolution of the heat budget advection term in the lower Rhine valley was investigatedusing temperature profiles derived from balloon soundings acquired at two sites which were overpassed by thecold-pool edge in the course of its displacement northwards during the early afternoon as the result of the actionof the fohn, and then southwards in the late afternoon as the fohn weakened and cold air from the Bodensee areawas filling the lower Rhine Valley.

KEYWORDS: Airborne water vapour lidar Cold-pool erosion Heat budget Kelvin–Helmholtz wavesVERA analyses

1. INTRODUCTION

The data collected recently in the framework of sub-project P5 (FORM, Fohn in theRhine valley during MAP) of the Mesoscale Alpine programme (MAP, Bougeault et al.2001) Special Observing Period (SOP) has led to improved understanding of numerousaspects of fohn-related phenomena, such as the unsteadiness and inhomogeneousaspects of the fohn in the area (Jaubert and Stein 2003; Drobinski et al. 2003a), the fohnsplitting between the Rhine and Seez valleys and related mass flux budget (Drobin-ski et al. 2001; Beffrey et al. 2004a; Drobinski et al. 2006), the turbulence duringfohn events (Lothon et al. 2003) and the pollution mechanisms associated with fohn(Baumann et al. 2001; Frioud et al. 2003, 2004). Nevertheless, to this day, there remaina number of open questions related to the interaction of the upper-level fohn flow andthe so-called cold pool covering the floor of most of the Alpine valleys, as a resultof radiative cooling during the nighttime and cold-air advection. Cold-air pools areusually defined as a surface-based layer of high static stability that is not dissolvedduring the daytime heating period. Cold-air pools tend to be particularly long lived invalleys and basins, where the surrounding topography reduces the advective air-mass

∗ Corresponding author: Service d’Aeronomie du CNRS, Institut Pierre-Simon Laplace, Tour 45, Boıte 102,Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France.e-mail: cyrille.flamant@aero.jussieu.frc© Royal Meteorological Society, 2006.

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exchange with the environment. The cold pools often filling the floor of Alpine valleysprevent the upper-level fohn flow from reaching the ground during most of the durationof fohn episodes. Only when the fohn is sufficiently intense does it touch the floor ofAlpine valleys. However, the mechanisms by which the cold pool is removed when fohndescends into the Rhine valley are still not known exactly. Furthermore, the locationin the valley as well as the time at which fohn touchdown occurs is highly variable, asshown by Drobinski et al. (2003a), among others, in the framework of MAP.

Some aspects of cold-pool removal in basins and valleys have been addressed inthe literature by numerical simulations (Lee et al. 1989; Petkovsek 1992; Vrhovec andHrabar 1996; Zhong et al. 2001) as well as by observational studies (Wolyn and McKee1989; Savoie and McKee 1995; Mayr and McKee 1995; Whiteman et al. 1999).The mechanisms affecting the evolution of wintertime cold-air pools in these studiesinclude surface radiative cooling and heating, large-scale subsidence, temperatureadvection, downslope warming in the lee of a major mountain barrier, and low-levelcloudiness. However, the observational studies mentioned above mostly concern thebuild-up and destruction of cold pools related to the passage of weather systems andtheir associated warm- and cold-air advection above the pools (conditions leading tothe cessation of fohn events in the Rhine valley). Some numerical studies providedevidence that dissipation of a cold pool from above can be initiated provided that thewind speed shear at the cold-pool top was large and/or increased with time (Petkovsek1992; Vrhovec and Hrabar 1996; Rakovec et al. 2002), while others found this mech-anism to be insignificant (Lee et al. 1989; Zhong et al. 2001). In these studies, surfacethermal forcing was also often insufficient to lead to cold-pool erosion, especially whenthe ground is covered by snow (Lee et al. 1989; Vrhovec and Hrabar 1996; Zhong et al.2001). Finally, the impact of low-level cloudiness was found to negligible (e.g. Zhonget al. 2001).

Three mechanisms are likely to govern the removal of the cold pool in the Rhinevalley during a fohn event (Gubser and Richner 2001):

(i) Convection within the cold pool. In the absence of clouds, solar radiation willheat the valley floor, which will in turn warm the atmosphere close to the surface. As aresult, convective plumes may be generated. Buoyancy-induced entrainment at the topof the cold pool and warming at the surface will lead to the gradual erosion of the stablystratified cold pool.

(ii) Turbulent erosion at the top of the cold pool. The strong wind shear between thefohn air and the cold pool may trigger Kelvin–Helmholtz instability (KHI) at the top ofthe cold pool (Nater et al. 1979), which, in turn, may produce the mixing necessary todeplete the cold pool.

(iii) Dynamic displacement of the cold pool by fohn air. In addition to the changingmesoscale pressure gradient caused by very short waves aloft, which lead to a dynamicalreaction within the cold-air pool, the occasional intensification of the mountain wave(for instance in the case of a breaking wave aloft) at the upper level may force the fohnflow down to the ground level, and flush the cold pool downstream.

Despite the unprecedented dataset acquired on cold pools from instruments andplatforms gathered in the framework of FORM (see Richner et al. 2006 for an overviewof the instrumentation), studies have only been conducted so far on fohn/cold-poolinteractions using data collected in the Rhine valley during intensive observing periods(IOPs) 8 and 9 (21–22 October 1999, Gubser and Richner 2001) and IOP 15 (Jaubertet al. 2005).

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The warming rate due to heat flux in the cold pool under fohn conditions(mechanism (i)) was estimated by Gubser and Richner (2001) from airborne in situmeasurements obtained with the Metair light research aircraft Dimona close to theinterface between cold pool and fohn flow. They found that the heat flux at the top of thecold pool (determined using a covariance technique) was of the same order of magnitudeas the heat flux from the surface during daytime. Using a number of assumptions, theyestimated a warming rate of about 25 K day−1, which appears to be considerably toohigh and not compatible with cold-pool persistence. As part of the Vienna EnhancedResolution Analysis (VERA, Steinacker et al. 2000; Chimani et al. 2006), surface mea-surements have been extremely useful to monitor the displacement of the cold-poolleading edge (Drobinski et al. 2003a; Zangl et al. 2004) in the Rhine valley, with ahigh temporal resolution, and to better understand the processes involved in mecha-nism (iii). However, neither the balloon soundings nor the ground-based remote-sensinginstruments could provide high spatio-temporal resolution information on the verticalstructure of the cold pool at the scale of the Rhine valley, needed to tackle mechanisms(i) and (ii).

Numerical simulations conducted in the framework of the FORM project providedevidence that the large- and meso-scale aspects of the fohn events observed duringseveral of the MAP IOPs could be satisfactorily reproduced using non-hydrostaticmesoscale models forced by operational analyses (Jaubert and Stein 2003; Zangl et al.2004; Drobinski et al. 2003a, 2006), whereas the interaction of the cold pool with thefohn flow could not (Beffrey et al. 2004b), due to the lack of a cold pool in the initialanalyses as the result of smoothed topography. Jaubert et al. (2005) have overcome thisdeficiency by replacing operational analyses with higher-resolution analyses (producedas described in Calas et al. 2000), thereby taking explicitly into account the initial coldpool at the valley scale. They were able to detail the heat budget of the cold pool atthe scale of the Rhine valley on 5 November 1999 (IOP 15 of the MAP SOP), basedon results obtained on a 2.5 km horizontal resolution nested domain. They showedthat the main mechanisms leading to the removal of the cold pool appeared to be theadvection by the mean flow and turbulence, whereas radiative effects could be neglected.Nevertheless, a number of open questions remain to be answered, concerning the roleof numerical diffusion in such simulations, as well as the origin of the turbulenceresponsible for the removal of the cold pool. Concerning the latter, Jaubert et al. (2005)assumed, based on the large values of wind shear simulated near the top of the cold pool,that these conditions lead to KHI even though there was no further evidence for this inthe simulation. Given the resolution of the simulation, it is not clear how the impact ofthe Kelvin–Helmholtz waves (KHW) is accounted for, provided that the wavelength ofsuch waves may be less that 2.5 km in the early stage of their development.

This paper also focuses on the 5 November 1999 fohn case of the MAP SOP.The objective of the study is two-fold:

(i) Analyse the structure of the cold pool at the scale of the Rhine valley, as wellas fohn/cold-pool interactions using high spatio-temporal resolution measurements, andassess the existence of KHW at the top of the cold pool and gravity waves above, whichcould not be confirmed in the simulations of Jaubert et al. (2005), and

(ii) Determine which of the above-cited mechanisms are responsible for cold-poolremoval in the vicinity of the cold-pool edge to the south. In particular, we show thatadvection was the main mechanism leading to cold-pool removal in the southernmostpart of the Lower Rhine valley, and that KHW in this case did not induce mixing at thetop of the cold pool, unlike that proposed in Jaubert et al. (2005).

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TABLE 1. GROUND-BASED SITES AND DATA USED IN THIS STUDY OF THE 5 NOVEMBER 1999 FOHNEVENT

Height Types of measurementName Longitude Latitude (m amsl) (times of operation, frequency)

Altenrhein 9.56◦E 47.48◦N 398 Microbarograph (1400–1500 UTC, 1 sec)Buchs-Grabs 9.47◦E 47.18◦N 445 Sounding (1100, 1400, 1700, 2000 and

2315 UTC), surface (hourly)Diepoldsau 9.66◦E 47.37◦N 411 Sounding (1100 and 1500 UTC), surface (hourly)Feldkirch 9.64◦E 47.26◦N 438 Sounding (1112, 1428, 1826, 1934 and 2315 UTC)Flaescherberg 9.49◦E 47.04◦N 918 Surface (hourly)Heiligkreuz 9.41◦E 47.06◦N 475 Sounding (1100 and 1400 UTC), surface (hourly)Lustenau 9.68◦E 47.46◦N 417 Anemometer (0000–2400 UTC, hourly)Maienfeld 9.52◦E 47.00◦N 502 Surface (hourly)Malans 9.58◦E 46.98◦N 533 Sounding (0200, 0500, 0800, 1100, 1416, 1700,

2000 and 2300 UTC), surface (hourly)Sevelen 9.49◦E 47.12◦N 465 Scintillometer (0000–2400 UTC, 10 min)Vaduz 9.53◦E 47.13◦N 460 Microbarograph (1400–1500 UTC, 1 sec)

The deployment of numerous ground-based remote-sensing instruments in theRhine valley during MAP has enabled detailed studies of the cold pool. Nevertheless,because of the great variability observed during MAP in terms of location and timingof fohn touchdown, improved knowledge of processes leading to the formation anddestruction of cold pools can only be obtained using a combination of ground-basedand airborne remote-sensing instruments.

We take advantage of the unique opportunity given to the multi-agency Avionde Recherche Atmospherique et Teledetection (ARAT, equipped with the downward-looking differential absorption lidar (DIAL) LEANDRE 2) to fly over the Rhine valley,to document the structure of the cold pool at the scale of the Rhine valley, usinghigh-resolution DIAL-derived two-dimensional (2D) water vapour fields in the lowertroposphere. A second aircraft (the Merlin IV) also flew in the Rhine valley on this dayand documented the thermodynamics of the fohn flow above the cold pool. The otherin situ and remote-sensing instruments/platforms used in this study are summarized inTable 1.

2. THE MAP IOP 15 FOHN EPISODE

(a) Synoptic environmentOn 5 November 1999, an intense North Atlantic short-wave trough deepened and

the associated cold front propagated south-eastwards (Fig. 1) over central Europe witha pronounced southerly flow on the eastern flank. The mean sea level pressure gradientprogressively increased between 0000 and 1800 UTC, as the North Atlantic troughmoved east. Associated with this pressure field, a south-westerly synoptic flow gavebirth to a fohn episode in the Rhine valley. The ‘fohn nose’ (Brinkmann 1971) isapparent in the mean sea level field over Italy at 1200 and 1800 UTC. The fohn episodeappeared to weaken on 6 November at 0000 UTC when the ‘fohn nose’ disappeared(Fig. 1(c)). However, there remained a significant pressure gradient across the Alps atthis time. Finally, the fohn episode terminated in the early hours of 6 November as thecold front ahead of the trough reached the FORM target area (also see Jaubert et al.2005; Richner et al. 2006).

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Figure 1. Synoptic situation at 12-hourly intervals from ECMWF analyses at (a) 0600 and (b) 1800 UTC on5 November and (c) 0600 UTC on 6 November 1999, showing mean sea level pressure (solid lines at 2 hPa

intervals) and geopotential height at 500 hPa (bold dashed lines at 50 m intervals).

(b) Operations in the FORM target areaThe instruments and platforms operating in the Rhine valley during MAP IOP 15

consisted of a dense network of up to eight radiosonde stations, several remote-sensinginstruments (sodars, wind profilers, lidars, crosswind scintillometers), three microbaro-graph stations, and numerous surface stations. The observations used in the paper aredetailed in Table 1. In addition, two aircraft (based in Milan) operated in the Rhine valleyduring IOP 15. The ARAT took off from Milan at 1253 UTC on 5 November. It operatedfrom an altitude of 4.8 km above mean sea level (amsl) over the target area between 1412and 1518 UTC, before returning to Milan at 1550 UTC. The Merlin mission consisted oftwo flights, the first of which was dedicated to the in situ documentation of the fohnflow in the target area, whereas the second was merely a ferry flight back to Milan.In the target area, the flight patterns of the aircraft consisted of two types of leg: (i) longlegs running south-south-west/north-north-east, roughly parallel to the axis of the lowerAlpine Rhine valley and (ii) shorter legs running roughly perpendicular to the first type.In this study, we shall focus on the former type (see Fig. 2 where this leg is referred toas leg AB). The ARAT overflew the Rhine valley once, between 1445 and 1500 UTC,whereas the Merlin made three passes along leg AB at three levels: 880 m amsl(1533–1553 UTC), 1310 m amsl (1555–1612 UTC), and 1815 m amsl (1317–1337 UTC)(i.e. at 450, 880 and 1385 m above ground level (agl), respectively).

(c) Fohn characteristics in the Rhine valley on 5 November 1999The onset and end times of the IOP 15 fohn episode have been determined by

Richner et al. (2006) to be 0610 UTC on 5 November 1999 and 0940 UTC on 6 November

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Figure 2. Topography of the Rhine valley target area with the main landmarks mentioned in the text(e.g. the Bodensee and the Rhine river) and the locations of the balloon launching sites (diamonds),the surface measurements sites (asterisks), the sodar and anemometer (Lustenau, triangle) and the wind pro-filer/RASS (Rankweil, cross). The shading indicates topography above 500 m amsl. The ‘v-shaped’ dotted linenear Sevelen indicates the geometry of the scintillometer light beams. The square indicates the location of thesite where three cameras were installed, with the arrows indicating the directions (NE, SE, S) in which the threecameras pointed. The straight bold solid and dashed lines AB indicate the legs flown by the ARAT and Merlinaircraft along the lower Alpine Rhine valley on 5 November 1999. Political boundaries are indicated by the bolddashed lines. Finally, ‘LI’ near Vaduz denotes Liechtenstein and ‘Seez’ and ‘Walgau’ refer to the Seez and Walgau

valleys.

1999, respectively. As discussed in Richner et al. (2006), the method used was basedon the fohn detection algorithm developed by Gutermann (1970), modified to takeadvantage of the high temporal resolution of the SOP data.

The focus of this paper being the analysis of the detailed vertical structure ofthe fohn/cold-pool interactions as documented by two aircraft in the afternoon of5 November, in the following we shall discuss of the evolution of fohn conditions onthis day only.

Unlike what was observed by Drobinski et al. (2003a) during the IOP 12 of MAP,this fohn onset does not coincide with a transition from shallow fohn to deep fohn, asshown by the wind measurements obtained from balloon soundings at Malans (Fig. 3)

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Figure 3. Time–altitude presentation of the diurnal evolution of the horizontal wind (arrows) at Malans.Isentropes (solid lines) are shown at 2 K intervals.

between 0200 and 2300 UTC. The flow in the lower 3 km amsl appeared to be decoupledfrom the south-westerly synoptic flow above 3 km amsl which, in the Alps, is generallyindicative of shallow fohn conditions (Seibert 1990). Similar behaviour was observed inHeiligkreuz, Buchs-Grabs, and Feldkirch as well (not shown). The shallow fohn wasobserved below 2.4 km amsl at Heiligkreuz and below 1.8 km amsl at Buchs-Grabsand Feldkirch. Furthermore, the upper-level measurements in the vicinity of Malansshow that the wind speed above 3 km amsl was significant after 0800 UTC which is alsoindicative of fohn conditions.

Scintillometer measurements of the wind speed and direction across the lowerAlpine Rhine valley entrance region at an altitude of 500 m agl (1000 m amsl) (Furgeret al. 2001, see Fig. 2 for location) support the fact that the fohn was present inthe Rhine valley as early as 0700 UTC on 5 November (Fig. 4). The three-mirrorconfigurations allowed for the simultaneous measurement of the horizontal and verticalcrosswind components. The light paths of the two scintillometers were arranged in theshape of a horizontal ‘V’ with the transmitters at the intersection of the two legs ofthe V. The centres of the two light paths were approximately 2 km apart. From thehorizontal crosswind components of the two scintillometers, a ‘true’ horizontal windcomponent can be calculated. For this approximation, the horizontal wind field isassumed to be homogeneous in the area. This was approximately the case for the well-developed fohn flow after 0700 UTC (Fig. 4), when both scintillometers show a similarbehaviour in horizontal wind speed development. Before 0700 UTC the measurementswere unreliable (low signal-to-noise ratio), and the fluctuations in wind direction werepurely random. Between 0700 and 1300 UTC, the wind direction stabilized aroundan average value of 150◦, indicative of channelled south-easterly flow in the entranceregion of the Lower Rhine valley. After 1300 UTC, the wind direction slowly decreasedfrom 140 to 120◦, also suggesting the existence of channelled south-easterly flow in theentrance region of the lower Alpine Rhine valley. (The lower Alpine Rhine valley in thevicinity of Sevelen is oriented roughly 335◦.)

Surface relative humidity measurements along the Rhine valley at Maienfield,Flaescherberg, Balzers, Vaduz, Buchs-Grabs and Diepoldsau are shown in Fig. 5.

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Figure 4. Scintillometer measurements of wind speed (solid) and direction (dashed) near Sevelen at 500 m aglon 5 November 1999.

They illustrate that the fohn touched the ground much later at Balzers than at Flaescher-berg, even though the two stations are a few kilometers apart. The reason for that isbelieved to be the valley orientation, which changes significantly between Maien-field and Flaescherberg on the one hand, and Flaescherberg and Balzers on the otherhand (Fig. 2). When the fohn blows from the south-east in the upper Rhine valley, itblows nearly perpendicular to the Flaescherberg–Balzers valley section, which hinderspenetration. On the contrary, this direction is parallel to the valley orientation betweenMaienfield and Flaescherberg, such that the fohn can penetrate down to the valley floormore easily. Furthermore, it looks as if the Flaescherberg is an obstacle that stronglyseparates the Rhine valley between Sargans and the Bodensee from the upstream part(i.e. Malans).

At Maienfield, Flaescherberg and Balzers, fohn air mass characteristics wereobserved at the surface from the time of touchdown onwards (Fig. 5(a)). Interestingly,just a few kilometers to the north, at Vaduz, the fohn was observed to touch the groundmuch later in the day (Fig. 5(b)), and only briefly. At Buchs-Grabs, the fohn touched theground about an hour later than in Vaduz, while it appears that at Diepoldsau the fohnnever touched the ground.

This may be explained by the fact that of all the fohn events analysed in theframework of MAP (Jaubert and Stein 2003 (IOP 2); Drobinski et al. 2001, 2003a(IOPs 5 and 12); Zangl et al. (2004) (IOP 10); Lothon et al. 2003 (IOP 8); Beffreyet al. 2004b (IOP 8)), the IOP 15 fohn episode is the least penetrative.

Finally, the three digital cameras located in Hoherkasten overlooking the lowerAlpine Rhine valley in different directions confirmed the presence of a thick stratocu-mulus deck until 0700 UTC. This cleared after 0830 UTC and clear-air conditions werethen observed in the lower Alpine Rhine valley until sunset.

3. THE COLD POOL AT THE SCALE OF THE VALLEY

(a) Surface analysesAt 1300 UTC, the VERA analysis shows a sharp surface front in potential temper-

ature (Fig. 6(a)), oriented approximately north-east/south-west, in the triangle formed

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(a)

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Figure 5. Diurnal evolution of the surface relative humidity at (a) Maienfeld (solid line), Flaescherberg (dashed)and Balzers (dash-dotted) and (b) Vaduz (solid), Buchs-Grabs (dashed) and Diepoldsau (dash-dotted). The shaded

areas represent the range of relative humidity (�45%) indicative of fohn occurrence in the Rhine valley.

by the stations Bad Ragaz, Sargans and Balzers. This surface front represented thesouthernmost extent of the cold pool near the surface. Fohn air was observed to touch theRhine valley floor to the south of this front. Associated with the cold pool, a persistingup-valley northerly cold flow was observed in the lower Alpine Rhine valley (notshown). Conversely, a warmer southerly down-valley flow was observed in the upperAlpine Rhine valley. Finally a westerly up-valley flow was observed in the Seez valley.The temperature field was associated with a pressure field that exhibited a minimum inpressure roughly centred on Bad Ragaz. The patterns in the pressure field (not shown)appeared to be extremely favourable to flow convergence for the three above-mentionedvalleys in the region of Bad Ragaz.

At 1500 UTC, the surface potential temperature front deformation was furtheraccentuated, with a bulge in the front progressing north in the upper Alpine Rhinevalley (Fig. 6(b)). In the lower Alpine Rhine valley, cold air was still advected up-valleyfrom the Bodensee region (not shown). Persisting up-valley and down-valley flows wereobserved in the Seez and upper Alpine Rhine valleys, respectively. At 1700 UTC, thesurface potential temperature front has moved north slightly (Fig. 6(c)). In the Bodensee

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(a) (b) (c)

(d) (e) (f)

Figure 6. VERA analyses of potential temperature (contours at 1 K intervals) on 5 November 1999 at (a) 1300,(b) 1500, (c) 1700, (d) 2000, (e) 2200 and (f) 2300 UTC, superimposed on the topography of the Rhine valleytarget area with the main landmarks mentioned in the text: Bad Ragaz (BR), Sargans (S), Balzers (B), Buchs-Grabs (BG), Ruggel (R) and Feldkirch (F). The domain is the same as in Fig. 2. For clarity, the names of mostlandmarks do not appear, but can be seen in Fig. 2. Light (dark) grey shading denotes topography above 500 m

(1500 m) amsl.

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Figure 7. Water vapour mixing ratio field (see key shading) obtained from LEANDRE 2 along leg AB between1445 and 1500 UTC, with the 5.5 g kg−1 contour (centre white solid line) outlining the cold-pool structure. Thetop curve is the vertical velocity as measured by the Merlin IV along the leg AB between 1533 and 1553 UTC at880 m amsl (480 m agl). Also shown are the locations along the Rhine valley of some of the measurement sites:Maienfeld (Ma), Flaescherberg (Fl), Vaduz (V), Buchs-Grabs (B), Feldkirch (F), Rankweil (R), Diepoldsau (D),

Lustenau (L), Altenrhein (A), Bodensee (BS) and Friedrichshafen (Fr).

region, the flow was not now directed up-valley, but exhibited a marked easterly com-ponent (not shown). At 2000 UTC, the surface potential temperature front was locatedbetween Buchs-Grabs and Ruggell in the Rhine valley (Fig. 6(d)), and southerly flowwas observed as far north as Feldkirch (not shown). The displacement northwards wasconnected to the strong fohn blowing in the Rhine valley, as seen in the scintillometerdata (Fig. 4). At 2200 UTC, the surface potential temperature front exhibited its north-ernmost location on that day, between Ruggell and Feldkirch (Fig. 6(e)), as the result ofthe strong fohn (over 20 m s−1 as measured by the scintillometer, Fig. 4). At 2300 UTC,a strong southerly flow was observed south of the front, and strong northerly flow wasobserved in the cold pool just north of the front (not shown). At the same time, the fohnabove weakened (Fig. 4). This strong flow, combined with the weaker fohn eventuallyled to displacement of the front towards the south, where it was located south of Buchs-Grabs, as shown in Fig. 6(f).

(b) Vertical structure of the cold poolThe water vapour mixing ratio field, monitored at 732 nm by the downward-

pointing DIAL LEANDRE 2 on the ARAT, was used to document the structure of thecold pool. The lidar-derived water vapour mixing ratio field along the Rhine valley isshown in Fig. 7. Values obtained were between 6 and 8 g kg−1 in the cold pool andapproximately equal to 4 g kg−1 above. Vertical and horizontal resolutions are 30 and150 m, respectively.

3046 C. FLAMANT et al.

Figure 7 shows that the leading edge of the cold pool was located to the southof Vaduz, as also seen in the 1500 UTC VERA analysis (Fig. 6(b)). This is confirmedby the surface temperature and relative humidity measurements made on both sides ofthe front seen in the LEANDRE 2 data. At Maienfeld and Flaescherberg (which are tothe south of the front), relative humidities measured at 1500 UTC indicate that the fohnindeed touched the ground, and hence that these stations were not in the cold pool at thattime (Fig. 5(a)). On the other hand, relative humidities measured at 1500 UTC at Vaduzand Buchs-Grabs (Fig. 5(b)) are not typical of fohn conditions, which is an indicationthat these stations were in the cold pool at the time of the ARAT overpass. Importantwater vapour mixing ratio modulations within and at the top of the cold pool wereobserved in Fig. 7 between 47.13◦N and 47.26◦N. As discussed later, these fluctuationsare a manifestation of the presence of KHW at the cold-pool top. KHW are knownto occur near the leading edge of laboratory and atmospheric density currents (Britterand Simpson 1978). Furthermore, LEANDRE water vapour mixing ratio measurementssuggest that the leading edge of the cold pool (47.13◦N) is deeper than the cold poolfurther north (i.e. 47.22◦N) which is also expected in the case of a gravity current(e.g. Simpson 1987).

Between 47.22◦N and 47.33◦N, the depth of the cold pool increased from 150 to250 m. Between 47.33◦N and 47.43◦N, the depth of the cold pool was approximatelyconstant and equal to 250 m. The high-resolution lidar measurements also revealed watervapour mixing ratio modulations within and at the top of the cold pool in this region.This type of fluctuation is reminiscent of lidar-observed thermals in the convectiveatmospheric boundary layer. However, given the strong stable conditions associatedwith the presence of the cold pool, thermals are not expected to be observed here.Furthermore, surface sensible heat flux measured in Lustenau (covered by the cold pool)decreased from 14 to 0 W m−2 between 1400 and 1500 UTC. The maximum sensibleheat flux observed on this day was 80 W m−2 at 1200 UTC. Hence, the water vapourmixing ratio fluctuations could be related to wind-shear-induced instability at the top ofthe cold pool.

Between 47.45◦N and 47.57◦N, in the Bodensee basin region, the structure ofthe cold pool appeared to be dramatically affected by the presence of waves; the topof the cold pool underwent important wave-type fluctuations. North of 47.57◦N, thedepth of the cold pool was observed to diminish over the sloping terrain. Here also,the modulations of the water vapour mixing ratio field are suspected to be caused by thepresence of KHW.

Surface pressure measurements (1 s temporal resolution) made by two microbaro-graphs (Fig. 8) revealed the presence of waves characterized by a period, T , of 300 s atVaduz (near the cold-pool leading edge) and 650 s at Altenrhein (in the Bodensee basin)between 1400 and 1500 UTC on 5 November 1999, as determined from Fast FourierTransform (FFT) analysis. The wave-like pattern in the surface pressure measurementswas more pronounced at Vaduz.

In the following, using lidar and balloon sounding measurements, we assesswhether KHI is triggered in the present case. We also compare the frequency of wavesobserved by lidar at the top of the cold pool with that obtained from the microbarographmeasurements.

(c) KHW at the top of the cold poolKHI is produced by shear at the interface between two fluids with different physical

properties, i.e. different densities and velocities. Provided that the angle α characterizingthe slope of the interface associated with the waves at the interface is much smaller

FOHN/COLD-POOL INTERACTIONS 3047

(a)

(b)

Figure 8. Surface pressure measurements (1 sec temporal resolution) from microbarographs at (a) Vaduz and(b) Altenrhein between 1400 and 1500 UTC on 5 November 1999.

than unity (i.e. α � 1), the instability condition leading to the presence of KHW ina linearized framework can be expressed as (Drazin and Reid 1981)

V �√

(ρ2 + ρ1)(ρ2 − ρ1)

ρ1ρ2

g

k, (1)

where V = U1 − U2 is the difference of velocity between the upper and the lower fluids,ρ1 and ρ2 are the densities of the upper and lower fluids, k is the wave number, and g isthe acceleration due to gravity. In Region III (where waves have the greatest amplitude),the condition on α is verified with tan α ≈ α ≈ A/(λ/2) ≈ 0.05, where A is the waveamplitude (50 m in this case) and λ is the wavelength (also see Table 2).

3048 C. FLAMANT et al.

Furthermore, the dispersion equation for KHW in the absence of background windis given by (Drazin and Reid 1981)

ω = kV (ρ2 − ρ1)/2 ± ik√

V 2ρ1ρ2 − (g/k)(ρ2 − ρ1)/(ρ2 + ρ1)

(ρ2 + ρ1), (2)

where ω is the wave frequency. The rate of development of the perturbation is givenby the imaginary part of ω, and the real part of the wave frequency also verifiesωreal = 2π/T , where T is the period of the KHW.

As the dominant wavelength characterizing the water vapour mixing ratio fluctua-tions at the top of the cold pool was not the same throughout the length of the Rhinevalley, three regions have been defined according to the dominant wavelength identifiedusing FFT on the time/distance series of the height of the cold-pool top derived by lidar.Furthermore, each region includes at least one upper-level sounding station (see Fig. 7).These regions are:

Region I. Cold-pool leading edge: 47.10◦–47.24◦N which includes the soundingstation at Buchs-Grabs, and the surface station at Vaduz, as well as the microbarographstation at Vaduz.

Region II. Cold pool confined in the Rhine valley: 47.25◦–47.44◦N which includesthe sounding stations at Feldkirch and Diepoldsau.

Region III. Cold pool in the Bodensee area: 47.45◦–47.65◦N which does not includeany sounding station but was probed by the Merlin IV during operations (refuelling inFriedrichshafen), as well as the microbarograph station at Altenrhein.

In Eqs. (1) and (2), V , ρ1 and ρ2 are obtained from balloon sounding measurementsmade at the closest available upper-air station, while k is derived from the lidar-measured water vapour mixing ratio fluctuations at the top of the cold pool. Finally,T is derived from the microbarograph measurements (Regions I and III only). Table 2summarizes the values of the variables in the three regions. On each sounding, V and(ρ2 − ρ1) are determined around the very sharp fohn/cold-pool interface defined bythe change in wind direction with height. Because the above-mentioned variables variedfrom one region to the other, the relationship given by Eq. (1) also varied, as illustrated inFig. 9. Nevertheless, it appears that conditions were favourable for KHI to be producedin all three regions (each pair (λ, V ) lying in the unstable part of the stability diagram,Fig. 9, with λ = 2π/k the wavelength of the perturbation), even though conditions arebarely met in Region II.

However, as shown in Table 2, values of ωreal derived from Eq. (2) (computed usingthe values of λ and V determined from lidar and sounding measurements in Regions Iand III) did not match the values of 2π/T derived from wave period measurementsobtained from the microbarograph stations at Vaduz and Altenrhein. Values of ωrealindicate that KHW were quasi-stationary. Instead, it is believed that the advection of theKHW by the mean wind is responsible for periodicity observed in the microbarographdata. From a fixed point at the surface, the period of a wave of wavelength λ advectedat the mean wind speed of v at the interface level (i.e. cold-pool top) is equal to λ/v.This ratio yields 290 s and 667 s in Regions I and III, respectively—in fair agreementwith the values inferred from microbarographs (see Table 2).

In conclusion, we believe that KHW triggered by wind shear at the top of the coldpool and advected with the mean wind had a pronounced influence on the structure of thecold pool. For example, water vapour mixing ratio fluctuations near the top of the coldpool were of the order of 3.4 g kg−1 in Region III (Table 2) as a result of the presenceof waves. Also, as discussed below, these waves may also have affected the fohn flowabove the cold pool.

FOHN/COLD-POOL INTERACTIONS 3049

TABLE 2. VARIABLES RELEVANT TO THE ASSESSMENT OF THE EXISTENCEOF KELVIN–HELMHOLTZ WAVES IN THE RHINE VALLEY, AS COMPUTED

AROUND 1500 UTC ON 5 NOVEMBER 1999

Variable Units Region I Region II Region III

λ km 1.6 1.0 2.0V m s−1 11 5 6v m s−1 5.5 2.5 3ρ1 kg m−3 1.14 1.15 1.17ρ2 kg m−3 1.125 1.16 1.165T s 300 – 650

2π/T s−1 2.1 ×10−2 – 0.96 ×10−2

ωreal s−1 1.4 ×10−4 6.1 ×10−5 2.1 ×10−5

q g kg−1 6.1 6.5 7.1σq g kg−1 0.55 0.60 0.65�q g kg−1 2.7 2.8 3.4Rit 0.7 1.3 0.6

q, σq and �q are the mean value, standard deviation and maximum peak-to-peakvalue of the water vapour mixing ratio derived from lidar near the top of the coldpool (600 m amsl). See text for definitions of the other variables and the threeregions.

Figure 9. Stability diagram (velocity difference, V , between the two fluids as a function of wavelength, λ): thethree curves (I, II and III) are obtained using Eq. (1) for which parameters ρ1 and ρ2 are determined from balloonsounding measurements (see Table 2). The symbols indicate the position of the (λ, V ) pair determined from lidarand sounding measurements, respectively, in Region I (triangle), Region II (asterisk), and Region III (diamond).

(d) Gravity waves above the cold poolAs shown in Fig. 7, a very distinct wave-like feature was observed in the Bodensee

area (Region III) on the vertical velocity measurements made by the Merlin on thelowest overpass (880 m amsl). In Regions I and II, such a signature was not as obviousin the vertical velocity data. Note that the average flight level for the Merlin IV on thelower overpass was 450 m agl, hence the Merlin never flew into the cold pool. A FFTanalysis of vertical velocity data in Region III (Region II) revealed that the dominantwavelength was ≈2 km (≈1 km), the same wavelength as was obtained from the lidar-derived cold-pool top heights (see Table 2).

3050 C. FLAMANT et al.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 10. Measurements of vertical velocity, w, made by the Merlin aircraft along the leg AB (a) between1555 and 1607 UTC (880 m agl) and (b) between 1533 and 1553 UTC (450 m agl). The lower panels showmeasurements made by the Merlin along AB between 1533 and 1553 UTC (450 m agl, solid line) and between1555 and 1607 UTC (880 m agl, dashed line): (c) water vapour mixing ratio R, (d) wind direction WD, (e) windspeed WS, (f) temperature T , and (g) relative humidity RH . Vertical dotted lines indicate the positions of the

vertical velocity maxima at 450 m agl in Region III.

Figure 10 shows the vertical velocity, horizontal wind speed and direction, temper-ature and relative humidity measurements made by the Merlin along the leg AB between1533 and 1553 UTC at 450 m agl and between 1555 and 1607 UTC at 850 m agl. At thelower level, in Region III, wind direction, temperature and relative humidity exhibitedwell-marked wave-like fluctuations. The wavelength of these fluctuations was also of theorder of 2 km, as derived from FFT analysis. At the higher level, wave-like fluctuationsof smaller amplitude were also observed. The vertical velocity fluctuations at 850 and

FOHN/COLD-POOL INTERACTIONS 3051

Figure 11. Scorer parameter profile derived from the Merlin IV soundings at Friedrichshafen. The horizontaldotted, dash-dotted and dash-triple dotted lines represent the altitude of the Merlin during the three Rhine valleyoverpasses, i.e. at 880, 1310 and 1815 m amsl, respectively. The vertical dashed line denotes the wave numberof the gravity waves observed in situ by the Merlin IV above the cold pool and the wave number of the KHW

observed by lidar at the top of the cold pool.

450 m agl appear to be in phase in some instances, which could be an indication of thepresence of trapped lee waves. However, given the time lag between the two Merlinlegs (20 min on average) and the length of the legs (15 min on average), fluctuationsat both levels cannot be expected to be observed in phase over the entire Region III.Nevertheless, in order to verify whether the atmospheric stratification in this regioncould support trapped lee waves, we computed the Scorer parameter profile in Region IIIusing the Merlin IV sounding at Friedrichshafen (Fig. 11).

The horizontal, k, and vertical, m, wave numbers of the wave over the cold pool arecoupled through the Scorer parameter, l, given by:

l2 = N2

U2− 1

U

∂2U

∂z2, (3)

where l2 = k2 + m2, N is the Brunt–Vaisala frequency and U the wind speed. If k < l,the wave is vertically propagating through a depth of the atmosphere where l remainsgreater than k. If k > l, the wave is damped with height, as m is imaginary. If the verticalvariations of N and U are such that l decreases significantly with height, then the flowis prone to favour wave trapping.

In Region III, the wave number of the waves observed in situ with the Merlin IVabove the cold pool and observed by lidar at the top of the cold pool (corresponding to awavelength of 2 km) is smaller than the Scorer parameter below 1.6 km amsl (Fig. 11).Furthermore, l decreases significantly with height between the top of the cold pooland 880 m agl (altitude of the lower Merlin overpass). The 1385 m agl Merlin leg waslocated in an atmospheric layer also characterized by a significant decrease of the Scorerparameter. Interestingly, there exists a layer of constant Scorer parameter (of depth200 m) between the lower two levels flown by the Merlin, which could explain the waveamplitude damped with height seen on temperature and relative humidity. Above 1.6 kmamsl, the Scorer parameter is nearly constant and its value is close to the value of the

3052 C. FLAMANT et al.

Figure 12. Horizontal wind field measured by the Merlin IV along leg AB at 450 m agl superimposed on thetopography of the Rhine valley target area.

wave number of the waves observed below. In this layer, the waves are damping withheight, consistent with the fact that no gravity waves were observed at 1835 m amsl.

In Regions I and II, there was some (no) evidence of wave activity above the coldpool at 450 m (880 m) agl, as shown in Fig. 10. The Scorer parameter was observed todecrease significantly with height between the top of the cold pool and 880 m agl, butwas smaller than the wave number corresponding to the wavelength of 1 km observedby lidar at the top of the cold pool above 0.7 km agl (not shown), suggesting that thewaves did not propagate above this altitude and hence could not be observed in situ.In Regions I and II, the flow was reasonably well channelled at 450 and 880 m agl(i.e. varying between 200 and 240◦). The fluctuations observed in the wind direction at450 m agl are believed to be connected to the presence of very weak flow splitting andwake effects (Fig. 12) and possibly hydraulic jumps in the vicinity of the Walgau valley(intersecting the Rhine valley in the vicinity of Feldkirch, i.e. around 47.3◦N). Further tothe north (Region III), the flow at 450 m agl is no longer influenced by the topography.

FOHN/COLD-POOL INTERACTIONS 3053

(e) On the relationship between KHW and gravity waves aboveLidar measurements provide evidence of the existence of waves at the top of

the cold pool. The wave characteristics verify the instability condition leading to thepresence of KHW given by Eq. (1). It can then be argued that the deformation of thecold-pool top induced by the KHW is responsible for generation of gravity waves inthe atmosphere above the cold pool. This argument can made by analogy with whatis referred to as ‘convection waves in the atmosphere’ (e.g. Hauf 1993), i.e. that insome conditions (most often daytime in the summer), strong thermals in the planetaryboundary layer act as obstacles for the flow in the free troposphere above, just likea mountain. It follows that in this situation large fluctuations of the cold-pool depthinduced by KHW would produce obstacle-like forcing likely to generate gravity wavesin the fohn flow above. Based on a Scorer profile determined from a sounding in thevicinity of the Bodensee, we show that conditions are favourable for waves generatedin the vicinity of the cold-pool top to actually propagate upwards and be trapped in aportion of the fohn layer.

4. COLD-POOL EROSION

(a) Radiative and turbulent processesAt the time of the lidar measurements of the cold-pool structure (1500 UTC), the

surface sensible heat flux measured at Lustenau was negative. Such conditions arefavourable to the reformation or maintenance of the cold pool. High-resolution VERAanalyses in the Rhine valley show that Lustenau was covered by the cold pool as farsouth at Vaduz, and the potential temperature field in the vicinity of the valley floorlooked relatively homogeneous. Surface measurements along the Rhine valley alsoimply that, north of Balzers, the fohn did not touch the ground until late in the day(Fig. 5(b)). Finally, the northerly surface wind measured up to 1900 UTC at Vaduz(not shown) indicates that the cold-air pool was continuously replenished with air fromthe north (Bodensee area). Based on this, we may argue that the single-point fluxmeasurement at Lustenau is representative of a larger area, i.e. the whole cold poolsouth of Vaduz. Hence, we may conclude that mechanism (i) (i.e. convection in the coldpool) did not play a role at the time of LEANDRE 2 measurements.

To assess whether the KHW at the top of the cold pool could have contributed tothe turbulent erosion of the cold pool, we have computed the profiles of the Richardsonnumber in Regions I, II and III. Richardson number profiles have been derived fromsounding data, even though it is widely recognized that radiosounding data collectedalong a skewed path within a few minutes provide a ‘snapshot-like’ profile that can be alimiting factor in estimating planetary boundary layer characteristics (e.g. Parlange andBrutsaert 1989).

We looked for the smallest Richardson number value, Rit, in the wind speed shearregion above the cold pool, turbulence generally being assumed to be produced below acritical value. This value is usually taken as 0.25, although suggestions in the literaturerange from 0.2 to 1.0 (see discussion by Jericevic and Grisogono 2006), and even largerwhen vigorous, intermittent turbulence occurs in stable conditions (e.g. Poulos andBurns 2003). There is also some suggestion of hysteresis, where laminar airflow mustdrop below Ri = 0.25 to become turbulent, but turbulent flow can exist up to Ri = 1.0before becoming laminar.

Hence, a single value of Ri at a given time cannot be used to assess whether mixingcan occur. Rather, one has to monitor the temporal evolution of Ri associated with the

3054 C. FLAMANT et al.

air mass of interest in order to reach a conclusion on the likeliness of mixing associatedwith KHW. Therefore, we have computed the temporal evolution of Rit between 0500and 2300 UTC (whenever there were adequate data) in Regions I and II. This was notpossible in Region III as the calculation of Ri relies on a single aircraft sounding.Maximum mixing between the fohn layer and the cold pool is expected where and whenthe fohn is strongest, i.e. the wind shear at the top of the cold pool is largest. Basedon scintillometer and surface measurements (Figs. 4 and 5, respectively), it appears thatthe fohn is strongest around 2000 UTC in Regions I and II. In Region I, Rit valueswere found to decrease with time from a value of 3 to a value of 0.3 between 0500 and1700 UTC, and increase afterwards. (Note that the 2000 UTC profile did not enable thecomputation of Ri in the region of interest.) In Region II, Rit values decreased from 8to 1.3 between 0500 and 1500 UTC, and increased afterwards reaching a value of 3.9at 2315 UTC. Hence it appears that the flow was laminar early in the day and since Rit

values did not drop below 0.25, it is likely that mixing between the fohn layer and thecold pool did not occur.

(b) Dynamic displacement of the cold pool by fohn airIn the light of the above finding, it appears that mechanism (iii) should mainly be

responsible for the cold-pool removal observed in the VERA analyses between 1500and 2200 UTC (Fig. 6). In the following, we investigate the evolution of the heat budgettendency terms in the lower Rhine valley from about 1100 to 2300 UTC. The evolutionof the potential temperature results from an imbalance between the contribution ofseveral processes, mainly advection, turbulence and radiation. The major sources ofturbulence are wind shear and buoyancy. In the light of the above discussion, it appearsthat wind shear at the top of the cold pool and buoyancy may be neglected in the budget.Furthermore, the cold pool was characterized by low wind speeds, and friction also isnot expected to be a factor in this case. As the fog had dissipated early in the day, andclear-air conditions were experienced throughout the afternoon, the impact of radiationcan be neglected as shown by Jaubert et al. (2005) using high-resolution numericalsimulations. Finally, the contribution of the phase changes is also negligible in this case.

As a result, the advection tendency term in the heat budget can be written as

−(u · ∇) θ = ∂

∂tθ, (4)

where θ is potential temperature and t is time.The temporal evolution of this heat storage term at Buchs-Grabs was determined

using temperature profiles derived from five balloon soundings performed at 1100, 1400,1700, 2000 and 2315 UTC. Buchs-Grabs was chosen because is was closest to the cold-pool nose in the early afternoon of 5 November 1999, and because the cold-pool noseadvected over the station at Buchs-Grabs in the course of its displacement northwardsin the early afternoon (as the result of the action of the fohn) and then southwards inthe late afternoon (as the fohn weakened and cold air from the Bodensee area filled thelower Rhine valley).

The evolution of the storage term is shown in Fig. 13. Between 1100 and 1400 UTC,cooling was observed below 1.5 km amsl (1 km agl), which is consistent with the VERAanalyses showing that the air behind the cold-pool edge (located south of Balzers) wasincreasingly cold with time until 1500 UTC. Between 1400 and 1700 UTC, Fig. 13shows cooling at the surface (≈ −1 K h−1) and pronounced warming at the top ofthe cold pool (≈ 2 K h−1) due to the action of the fohn. This is consistent with thesurface measurements (Fig. 5(b)) which show that at Buchs-Grabs, at this time, the

FOHN/COLD-POOL INTERACTIONS 3055

Figure 13. Heat storage term (K h−1) computed using soundings at Buchs-Grabs between 1100 and 1400 UTC(solid line), 1400 and 1700 UTC (dashed line), 1700 and 2000 UTC (dash-dotted line) and 2315 and 2000 UTC

(dash-triple dotted line).

fohn had not yet touched down. Between 1700 and 2000 UTC, Fig. 13 shows thatpronounced warming was observed at the surface (≈ 3 K h−1), consistent with thesurface measurements which showed that the fohn had touched down shortly before2000 UTC, and with the VERA analysis at 2000 UTC (Fig. 6(d)) showing that the cold-pool edge was indeed to the north of Buchs-Grabs. Finally, between 2000 and 2300 UTC,Fig. 13 shows pronounced cooling at the surface (≈ −2.5 K h−1) as the result of theweakening of the fohn and the cool air from the Bodensee filling the lower Rhine valley,as seen in the VERA analysis at 2200 and 2300 UTC (Figs. 6(e), (f)), Buchs-Grabs beingagain covered by the cold pool.

The temporal evolution of the heat storage term at Feldkirch (another locationoverpassed by the cold-pool nose during its displacement northwards, and roughly11 km north of Buchs-Grabs) was also computed using soundings performed at 1112,1428, 1826, 1934 and 2315 UTC (not shown). The evolution of the heat storage term wasobserved to be very similar to the one at Buchs below 1.2 km amsl. Above that altitude,the down-valley flow from the Walgau tributary led to significant warming between 1.3and 2 km amsl.

Finally, it was not possible to carry out a comparable analysis for moisture, as not asingle water vapour mixing ratio profile could be retrieved from the balloon soundings.

5. SUMMARY AND CONCLUSIONS

The fohn/cold-pool interactions in the lower Alpine Rhine valley documented inthe framework of IOP 15 of MAP on 5 November 1999 have been analysed by meansof airborne differential absorption lidar, microbarograph and in situ (balloon, surfaceand aircraft) measurements. The presence of KHW at the top of the cold pool has beenassessed. The characteristics of the waves were different in the region of the cold-poolleading edge (the southernmost part of the cold pool) and in the vicinity of the Bodensee,further to the north. The wavelength associated with these waves varied between 1

3056 C. FLAMANT et al.

and 2 km. Gravity waves were also observed above the cold pool in the vicinity of theBodensee. Scorer parameter analysis shows that wave trapping was likely in this regionabove the cold pool. Gravity waves are suspected to be triggered by the KHW at the topof the cold pool. In their simulation, Jaubert et al. (2005) showed that the turbulent termin the heat budget was non-negligible near the cold-pool top in the vicinity of the cold-pool leading edge and north of Rankweil. They assumed, based on the large values ofwind shear simulated near the top of the cold pool, that these conditions were conduciveto KHI even though there was no further evidence for this in the simulation. In thepresent study, the presence of KHW at the top of the cold pool is clearly establishedfrom observations.

We also investigated the respective role of the three known processes likely tocontrol the structure of the cold pool and its erosion along the Rhine valley, namely(i) convection within the cold pool, (ii) turbulent erosion at the top of the cold pooldue to the presence of KHW, and (iii) dynamic displacement of the cold pool by fohnair. As discussed above, the former two processes are likely not to play a role in theerosion of the cold pool observed between 1500 and 2000 UTC. The minor role of (i)and (ii) should not be generalized but seem valid in the case of IOP 15. One argumentthat the radiative heating and convective removal of the cold-air pool in the Rhinevalley is not very important in general can be seen in the fact that there is no diurnalamplitude for fohn breakthroughs (in contrast to the Wipptal and Reusstal, where thereis a maximum probability of fohn penetration during noon hours) (e.g. Waibel 1984).Despite the presence of wind shear near the cold-pool top, an analysis of the temporalevolution of Richardson number near the cold-pool top in Regions I and II showed thatthe flow was laminar early in the day and, since Rit values did not drop below 0.25,it is likely that mixing between the fohn layer and the cold pool did not occur. This isin agreement with previous findings by Lee et al. (1989), Petkovsek (1992) and Zhonget al. (2001), considering that the wind shear above the cold pool did not exceed 9 m s−1

over a depth of 150 m, except in the region of the cold-pool leading edge.Finally, we investigated the temporal evolution of the heat budget advection term at

two locations in the lower Rhine valley (Buchs-Grabs and Feldkirch). Both these siteswere overpassed by the cold-pool edge in the course of its displacement northwardsduring the early afternoon as the result of the action of the fohn, and then southwardsin the late afternoon as the fohn weakened and cold air from the Bodensee area wasfilling the lower Rhine valley. The different phases of the fohn breakthrough in this partof the Rhine valley determined from the temporal evolution of the advective term of theheat budget were consistent with those derived from surface measurements. Maximumheating and cooling rates were observed to be of the order of 3 K h−1 and –2.5 K h−1,and to be associated with fohn touchdown and cold-pool reformation, respectively.

ACKNOWLEDGEMENTS

This paper is dedicated to the memory of Tomaz Vrhovec who died in an avalanchein December 2004. Not only was Tomaz a much-appreciated colleague and collabo-rator in the framework of MAP, he also conducted pioneer work in the area of cold-pool destruction in Alpine valleys in the early 1990s. This research has been fundedby the Centre National de Recherche Scientifique (CNRS) through the ProgrammeATmosphere Ocean Meso-echelle (PATOM), the Institut National des Sciences del’Univers (INSU), and by the Centre National d’Etudes Spatiales (CNES). GenevieveJaubert (Meteo-France) is acknowledged for helpful discussions.

FOHN/COLD-POOL INTERACTIONS 3057

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