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

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1 Q. J. R. Meteorol. Soc. (2006), 132, pp doi: /qj Föhn/cold-pool interactions in the Rhine valley during MAP IOP 15 By C. FLAMANT 1, P. DROBINSKI 1,M.FURGER 2, B. CHIMANI 3, S. TSCHANNETT 3, R. STEINACKER 3,A.PROTAT 4,H.RICHNER 5, S. GUBSER 5 and C. HÄBERLI 3,6 1 Institut Pierre-Simon Laplace, SA, Paris, France 2 Paul Scherrer Institute, Villigen, Switzerland 3 Department of Meteorology and Geophysics, University of Vienna, Austria 4 Institut Pierre-Simon Laplace, CETP, Vélizy, France 5 Institute for Atmospheric and Climate Science, ETH, Zurich, Switzerland 6 MeteoSwiss, Zurich, Switzerland (Received 20 March 2006; revised 10 July 2006) SUMMARY The föhn/cold-pool interactions in the lower Alpine Rhine valley documented in the framework of the Intensive Observing Period (IOP) 15 of the Mesoscale Alpine Programme (MAP) on 5 November 1999 are analysed. The present study focuses on the water vapour mixing ratio measurements acquired with the airborne differential absorption lidar LEANDRE 2 which enabled detailed documentation of the along-valley structure of the cold pool. LEANDRE 2 and microbarograph measurements revealed the presence of Kelvin Helmholtz waves (KHW) at the top of the cold pool. The characteristics of the waves were different in the region of the coldpool 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 in the vicinity of the Bodensee. The gravity waves are suspected to be triggered by the KHW at the top of the cold pool. We also investigate the respective role of the three known processes likely to control 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 pool due to the presence of KHW, and (iii) dynamic displacement of the cold pool by föhn air. The former two processes are likely not to play a role in the erosion of the cold pool observed in the course of this IOP. Finally, the temporal evolution of the heat budget advection term in the lower Rhine valley was investigated using temperature profiles derived from balloon soundings acquired at two sites which were overpassed by the cold-pool edge in the course of its displacement northwards during the early afternoon as the result of the action of the föhn, and then southwards in the late afternoon as the föhn weakened and cold air from the Bodensee area was filling the lower Rhine Valley. KEYWORDS: Airborne water vapour lidar Cold-pool erosion Heat budget Kelvin Helmholtz waves VERA analyses 1. INTRODUCTION The data collected recently in the framework of sub-project P5 (FORM, Föhn in the Rhine valley during MAP) of the Mesoscale Alpine programme (MAP, Bougeault et al. 2001) Special Observing Period (SOP) has led to improved understanding of numerous aspects of föhn-related phenomena, such as the unsteadiness and inhomogeneous aspects of the föhn in the area (Jaubert and Stein 2003; Drobinski et al. 2003a), the föhn splitting between the Rhine and Seez valleys and related mass flux budget (Drobinski et al. 2001; Beffrey et al. 2004a; Drobinski et al. 2006), the turbulence during föhn events (Lothon et al. 2003) and the pollution mechanisms associated with föhn (Baumann et al. 2001; Frioud et al. 2003, 2004). Nevertheless, to this day, there remain a number of open questions related to the interaction of the upper-level föhn flow and the so-called cold pool covering the floor of most of the Alpine valleys, as a result of radiative cooling during the nighttime and cold-air advection. Cold-air pools are usually defined as a surface-based layer of high static stability that is not dissolved during the daytime heating period. Cold-air pools tend to be particularly long lived in valleys and basins, where the surrounding topography reduces the advective air-mass Corresponding author: Service d Aéronomie du CNRS, Institut Pierre-Simon Laplace, Tour 45, Boîte 102, Université Pierre et Marie Curie, 4 Place Jussieu, Paris Cedex 05, France. cyrille.flamant@aero.jussieu.fr c Royal Meteorological Society,

2 3036 C. FLAMANT et al. exchange with the environment. The cold pools often filling the floor of Alpine valleys prevent the upper-level föhn flow from reaching the ground during most of the duration of föhn episodes. Only when the föhn is sufficiently intense does it touch the floor of Alpine valleys. However, the mechanisms by which the cold pool is removed when föhn descends into the Rhine valley are still not known exactly. Furthermore, the location in the valley as well as the time at which föhn touchdown occurs is highly variable, as shown 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 in the literature by numerical simulations (Lee et al. 1989; Petkovsek 1992; Vrhovec and Hrabar 1996; Zhong et al. 2001) as well as by observational studies (Wolyn and McKee 1989; Savoie and McKee 1995; Mayr and McKee 1995; Whiteman et al. 1999). The mechanisms affecting the evolution of wintertime cold-air pools in these studies include surface radiative cooling and heating, large-scale subsidence, temperature advection, downslope warming in the lee of a major mountain barrier, and low-level cloudiness. However, the observational studies mentioned above mostly concern the build-up and destruction of cold pools related to the passage of weather systems and their associated warm- and cold-air advection above the pools (conditions leading to the cessation of föhn events in the Rhine valley). Some numerical studies provided evidence that dissipation of a cold pool from above can be initiated provided that the wind speed shear at the cold-pool top was large and/or increased with time (Petkovsek 1992; Vrhovec and Hrabar 1996; Rakovec et al. 2002), while others found this mechanism to be insignificant (Lee et al. 1989; Zhong et al. 2001). In these studies, surface thermal forcing was also often insufficient to lead to cold-pool erosion, especially when the 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. Zhong et al. 2001). Three mechanisms are likely to govern the removal of the cold pool in the Rhine valley during a föhn event (Gubser and Richner 2001): (i) Convection within the cold pool. In the absence of clouds, solar radiation will heat the valley floor, which will in turn warm the atmosphere close to the surface. As a result, convective plumes may be generated. Buoyancy-induced entrainment at the top of the cold pool and warming at the surface will lead to the gradual erosion of the stably stratified cold pool. (ii) Turbulent erosion at the top of the cold pool. The strong wind shear between the föhn air and the cold pool may trigger Kelvin Helmholtz instability (KHI) at the top of the cold pool (Nater et al. 1979), which, in turn, may produce the mixing necessary to deplete the cold pool. (iii) Dynamic displacement of the cold pool by föhn air. In addition to the changing mesoscale pressure gradient caused by very short waves aloft, which lead to a dynamical reaction 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 föhn flow down to the ground level, and flush the cold pool downstream. Despite the unprecedented dataset acquired on cold pools from instruments and platforms gathered in the framework of FORM (see Richner et al for an overview of the instrumentation), studies have only been conducted so far on föhn/cold-pool interactions 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 (Jaubert et al. 2005).

3 FÖHN/COLD-POOL INTERACTIONS 3037 The warming rate due to heat flux in the cold pool under föhn conditions (mechanism (i)) was estimated by Gubser and Richner (2001) from airborne in situ measurements obtained with the Metair light research aircraft Dimona close to the interface between cold pool and föhn flow. They found that the heat flux at the top of the cold pool (determined using a covariance technique) was of the same order of magnitude as the heat flux from the surface during daytime. Using a number of assumptions, they estimated a warming rate of about 25 K day 1, which appears to be considerably too high and not compatible with cold-pool persistence. As part of the Vienna Enhanced Resolution Analysis (VERA, Steinacker et al. 2000;Chimani et al. 2006),surface measurements have been extremely useful to monitor the displacement of the cold-pool leading edge (Drobinski et al. 2003a; Zängl et al. 2004) in the Rhine valley, with a high temporal resolution, and to better understand the processes involved in mechanism (iii). However, neither the balloon soundings nor the ground-based remote-sensing instruments could provide high spatio-temporal resolution information on the vertical structure 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 provided evidence that the large- and meso-scale aspects of the föhn events observed during several of the MAP IOPs could be satisfactorily reproduced using non-hydrostatic mesoscale models forced by operational analyses (Jaubert and Stein 2003; Zängl et al. 2004; Drobinski et al. 2003a, 2006), whereas the interaction of the cold pool with the föhn flow could not (Beffrey et al. 2004b), due to the lack of a cold pool in the initial analyses as the result of smoothed topography. Jaubert et al. (2005) have overcome this deficiency by replacing operational analyses with higher-resolution analyses (produced as described in Calas et al. 2000), thereby taking explicitly into account the initial cold pool at the valley scale. They were able to detail the heat budget of the cold pool at the scale of the Rhine valley on 5 November 1999 (IOP 15 of the MAP SOP), based on results obtained on a 2.5 km horizontal resolution nested domain. They showed that the main mechanisms leading to the removal of the cold pool appeared to be the advection by the mean flow and turbulence, whereas radiative effects could be neglected. Nevertheless, a number of open questions remain to be answered, concerning the role of numerical diffusion in such simulations, as well as the origin of the turbulence responsible 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 in the simulation. Given the resolution of the simulation, it is not clear how the impact of the Kelvin Helmholtz waves (KHW) is accounted for, provided that the wavelength of such waves may be less that 2.5 km in the early stage of their development. This paper also focuses on the 5 November 1999 föhn 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 well as föhn/cold-pool interactions using high spatio-temporal resolution measurements, and assess the existence of KHW at the top of the cold pool and gravity waves above, which could not be confirmed in the simulations of Jaubert et al. (2005), and (ii) Determine which of the above-cited mechanisms are responsible for cold-pool removal in the vicinity of the cold-pool edge to the south. In particular, we show that advection was the main mechanism leading to cold-pool removal in the southernmost part of the Lower Rhine valley, and that KHW in this case did not induce mixing at the top of the cold pool, unlike that proposed in Jaubert et al. (2005).

4 3038 C. FLAMANT et al. TABLE 1. GROUND-BASED SITES AND DATA USED IN THIS STUDY OF THE 5NOVEMBER 1999 FÖHN EVENT Height Types of measurement Name Longitude Latitude (m amsl) (times of operation, frequency) Altenrhein 9.56 E N 398 Microbarograph ( UTC, 1 sec) Buchs-Grabs 9.47 E N 445 Sounding (1100, 1400, 1700, 2000 and 2315 UTC), surface (hourly) Diepoldsau 9.66 E N 411 Sounding (1100 and 1500 UTC), surface (hourly) Feldkirch 9.64 E N 438 Sounding (1112, 1428, 1826, 1934 and 2315 UTC) Flaescherberg 9.49 E N 918 Surface (hourly) Heiligkreuz 9.41 E N 475 Sounding (1100 and 1400 UTC), surface (hourly) Lustenau 9.68 E N 417 Anemometer ( UTC, hourly) Maienfeld 9.52 E N 502 Surface (hourly) Malans 9.58 E N 533 Sounding (0200, 0500, 0800, 1100, 1416, 1700, 2000 and 2300 UTC), surface (hourly) Sevelen 9.49 E N 465 Scintillometer ( UTC,10min) Vaduz 9.53 E N 460 Microbarograph ( UTC, 1 sec) The deployment of numerous ground-based remote-sensing instruments in the Rhine 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 timing of föhn touchdown, improved knowledge of processes leading to the formation and destruction of cold pools can only be obtained using a combination of ground-based and airborne remote-sensing instruments. We take advantage of the unique opportunity given to the multi-agency Avion de Recherche Atmosphérique et Télédétection (ARAT, equipped with the downwardlooking 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, using high-resolution DIAL-derived two-dimensional (2D) water vapour fields in the lower troposphere. A second aircraft (the Merlin IV) also flew in the Rhine valley on this day and documented the thermodynamics of the föhn flow above the cold pool. The other in situ and remote-sensing instruments/platforms used in this study are summarized in Table THE MAP IOP 15 FÖHN EPISODE (a) Synoptic environment On 5 November 1999, an intense North Atlantic short-wave trough deepened and the associated cold front propagated south-eastwards (Fig. 1) over central Europe with a pronounced southerly flow on the eastern flank. The mean sea level pressure gradient progressively increased between 0000 and 1800 UTC, as the North Atlantic trough moved east. Associated with this pressure field, a south-westerly synoptic flow gave birthtoaföhn episode in the Rhine valley. The föhn nose (Brinkmann 1971) is apparent in the mean sea level field over Italy at 1200 and 1800 UTC. Theföhn episode appeared to weaken on 6 November at 0000 UTC when the föhn nose disappeared (Fig. 1(c)). However, there remained a significant pressure gradient across the Alps at this time. Finally, the föhn episode terminated in the early hours of 6 November as the cold front ahead of the trough reached the FORM target area (also see Jaubert et al. 2005; Richner et al. 2006).

5 1014 Latitude ( N) (a) Longitude ( E) FÖHN/COLD-POOL INTERACTIONS Latitude ( N) Latitude ( N) (b) Longitude ( E) (c) Longitude ( E) 1016 Figure 1. Synoptic situation at 12-hourly intervals from ECMWF analyses at (a) 0600 and (b) 1800 UTC on 5 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 area The 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-sensing instruments (sodars, wind profilers, lidars, crosswind scintillometers), three microbarograph stations, and numerous surface stations. The observations used in the paper are detailed in Table 1. In addition, two aircraft (based in Milan) operated in the Rhine valley during IOP 15. The ARAT took off from Milan at 1253 UTC on 5 November. It operated from an altitude of 4.8 km above mean sea level (amsl) over the target area between 1412 and 1518 UTC, before returning to Milan at 1550 UTC. The Merlin mission consisted of two flights, the first of which was dedicated to the in situ documentation of the föhn flow 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) long legs running south-south-west/north-north-east, roughly parallel to the axis of the lower Alpine 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 to as 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 ( UTC), 1310 m amsl ( UTC), and 1815 m amsl ( UTC) (i.e. at 450, 880 and 1385 m above ground level (agl), respectively). (c) Föhn characteristics in the Rhine valley on 5 November 1999 The onset and end times of the IOP 15 föhn episode have been determined by Richner et al. (2006) to be 0610 UTC on 5 November 1999 and 0940 UTC on6november

6 3040 C. FLAMANT et al. 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 profiler/rass (Rankweil, cross). The shading indicates topography above 500 m amsl. The v-shaped dotted line near Sevelen indicates the geometry of the scintillometer light beams. The square indicates the location of the site where three cameras were installed, with the arrows indicating the directions (NE, SE, S) in which the three cameras pointed. The straight bold solid and dashed lines AB indicate the legs flown by the ARAT and Merlin aircraft along the lower Alpine Rhine valley on 5 November Political boundaries are indicated by the bold dashed 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 based on the föhn detection algorithm developed by Gutermann (1970), modified to take advantage of the high temporal resolution of the SOP data. The focus of this paper being the analysis of the detailed vertical structure of the föhn/cold-pool interactions as documented by two aircraft in the afternoon of 5 November, in the following we shall discuss of the evolution of föhn conditions on this day only. Unlike what was observed by Drobinski et al. (2003a) during the IOP 12 of MAP, this föhn onset does not coincide with a transition from shallow föhn to deep föhn, as shown by the wind measurements obtained from balloon soundings at Malans (Fig. 3)

7 FÖHN/COLD-POOL INTERACTIONS 3041 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 decoupled from the south-westerly synoptic flow above 3 km amsl which, in the Alps, is generally indicative of shallow föhn conditions (Seibert 1990). Similar behaviour was observed in Heiligkreuz, Buchs-Grabs, and Feldkirch as well (not shown). The shallow föhn was observed below 2.4 km amsl at Heiligkreuz and below 1.8 km amsl at Buchs-Grabs and Feldkirch. Furthermore, the upper-level measurements in the vicinity of Malans show that the wind speed above 3 km amsl was significant after 0800 UTC which is also indicative of föhn conditions. Scintillometer measurements of the wind speed and direction across the lower Alpine Rhine valley entrance region at an altitude of 500 m agl (1000 m amsl) (Furger et al. 2001, see Fig. 2 for location) support the fact that the föhn was present in the Rhine valley as early as 0700 UTC on 5 November (Fig. 4). The three-mirror configurations allowed for the simultaneous measurement of the horizontal and vertical crosswind components. The light paths of the two scintillometers were arranged in the shape of a horizontal V with the transmitters at the intersection of the two legs of the V. The centres of the two light paths were approximately 2 km apart. From the horizontal crosswind components of the two scintillometers, a true horizontal wind component can be calculated. For this approximation, the horizontal wind field is assumed to be homogeneous in the area. This was approximately the case for the welldeveloped föhn flow after 0700 UTC (Fig. 4), when both scintillometers show a similar behaviour in horizontal wind speed development. Before 0700 UTC the measurements were unreliable (low signal-to-noise ratio), and the fluctuations in wind direction were purely random. Between 0700 and 1300 UTC, the wind direction stabilized around an average value of 150, indicative of channelled south-easterly flow in the entrance region of the Lower Rhine valley. After 1300 UTC, the wind direction slowly decreased from 140 to 120, also suggesting the existence of channelled south-easterly flow in the entrance region of the lower Alpine Rhine valley. (The lower Alpine Rhine valley in the vicinity 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.

8 3042 C. FLAMANT et al. Figure 4. Scintillometer measurements of wind speed (solid) and direction (dashed) near Sevelen at 500 m agl on 5 November They illustrate that the föhn touched the ground much later at Balzers than at Flaescherberg, even though the two stations are a few kilometers apart. The reason for that is believed to be the valley orientation, which changes significantly between Maienfield and Flaescherberg on the one hand, and Flaescherberg and Balzers on the other hand (Fig. 2). When the föhn blows from the south-east in the upper Rhine valley, it blows nearly perpendicular to the Flaescherberg Balzers valley section, which hinders penetration. On the contrary, this direction is parallel to the valley orientation between Maienfield and Flaescherberg, such that the föhn can penetrate down to the valley floor more easily. Furthermore, it looks as if the Flaescherberg is an obstacle that strongly separates the Rhine valley between Sargans and the Bodensee from the upstream part (i.e. Malans). At Maienfield, Flaescherberg and Balzers, föhn air mass characteristics were observed at the surface from the time of touchdown onwards (Fig. 5(a)). Interestingly, just a few kilometers to the north, at Vaduz, the föhn was observed to touch the ground much later in the day (Fig. 5(b)), and only briefly. At Buchs-Grabs, the föhn touched the ground about an hour later than in Vaduz, while it appears that at Diepoldsau the föhn never touched the ground. This may be explained by the fact that of all the föhn events analysed in the framework of MAP (Jaubert and Stein 2003 (IOP 2); Drobinski et al. 2001, 2003a (IOPs 5 and 12); Zängl et al. (2004) (IOP 10); Lothon et al (IOP 8); Beffrey et al. 2004b (IOP 8)), the IOP 15 föhn episode is the least penetrative. Finally, the three digital cameras located in Hoherkasten overlooking the lower Alpine Rhine valley in different directions confirmed the presence of a thick stratocumulus deck until 0700 UTC. This cleared after 0830 UTC and clear-air conditions were then observed in the lower Alpine Rhine valley until sunset. 3. THE COLD POOL AT THE SCALE OF THE VALLEY (a) Surface analyses At 1300 UTC, the VERA analysis shows a sharp surface front in potential temperature (Fig. 6(a)), oriented approximately north-east/south-west, in the triangle formed

9 FÖHN/COLD-POOL INTERACTIONS 3043 (a) (b) 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 föhn occurrence in the Rhine valley. by the stations Bad Ragaz, Sargans and Balzers. This surface front represented the southernmost extent of the cold pool near the surface. Föhn air was observed to touch the Rhine valley floor to the south of this front. Associated with the cold pool, a persisting up-valley northerly cold flow was observed in the lower Alpine Rhine valley (not shown). Conversely, a warmer southerly down-valley flow was observed in the upper Alpine 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 in pressure 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-mentioned valleys in the region of Bad Ragaz. At 1500 UTC, the surface potential temperature front deformation was further accentuated, with a bulge in the front progressing north in the upper Alpine Rhine valley (Fig. 6(b)). In the lower Alpine Rhine valley, cold air was still advected up-valley from the Bodensee region (not shown). Persisting up-valley and down-valley flows were observed in the Seez and upper Alpine Rhine valleys, respectively. At 1700 UTC, the surface potential temperature front has moved north slightly (Fig. 6(c)). In the Bodensee

10 3044 C. FLAMANT et al. (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 valley target 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 most landmarks do not appear, but can be seen in Fig. 2. Light (dark) grey shading denotes topography above 500 m (1500 m) amsl.

11 FÖHN/COLD-POOL INTERACTIONS 3045 Figure 7. Water vapour mixing ratio field (see key shading) obtained from LEANDRE 2 along leg AB between 1445 and 1500 UTC, with the 5.5 g kg 1 contour (centre white solid line) outlining the cold-pool structure. The top curve is the vertical velocity as measured by the Merlin IV along the leg AB between 1533 and 1553 UTC at 880 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 component (not shown). At 2000 UTC, the surface potential temperature front was located between Buchs-Grabs and Ruggell in the Rhine valley (Fig. 6(d)), and southerly flow was observed as far north as Feldkirch (not shown). The displacement northwards was connected to the strong föhn blowing in the Rhine valley, as seen in the scintillometer data (Fig. 4). At 2200 UTC, the surface potential temperature front exhibited its northernmost location on that day, between Ruggell and Feldkirch (Fig. 6(e)), as the result of the strong föhn (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 was observed in the cold pool just north of the front (not shown). At the same time, the föhn above weakened (Fig. 4). This strong flow, combined with the weaker föhn eventually led 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 pool The water vapour mixing ratio field, monitored at 732 nm by the downwardpointing DIAL LEANDRE 2 on the ARAT, was used to document the structure of the cold pool. The lidar-derived water vapour mixing ratio field along the Rhine valley is shown in Fig. 7. Values obtained were between 6 and 8 g kg 1 in the cold pool and approximately equal to 4 g kg 1 above. Vertical and horizontal resolutions are 30 and 150 m, respectively.

12 3046 C. FLAMANT et al. Figure 7 shows that the leading edge of the cold pool was located to the south of Vaduz, as also seen in the 1500 UTC VERA analysis (Fig. 6(b)). This is confirmed by the surface temperature and relative humidity measurements made on both sides of the front seen in the LEANDRE 2 data. At Maienfeld and Flaescherberg (which are to the south of the front), relative humidities measured at 1500 UTC indicate that the föhn indeed touched the ground, and hence that these stations were not in the cold pool at that time (Fig. 5(a)). On the other hand, relative humidities measured at 1500 UTC at Vaduz and Buchs-Grabs (Fig. 5(b)) are not typical of föhn conditions, which is an indication that these stations were in the cold pool at the time of the ARAT overpass. Important water vapour mixing ratio modulations within and at the top of the cold pool were observed in Fig. 7 between N and N. As discussed later, these fluctuations are a manifestation of the presence of KHW at the cold-pool top. KHW are known to occur near the leading edge of laboratory and atmospheric density currents (Britter and Simpson 1978). Furthermore, LEANDRE water vapour mixing ratio measurements suggest that the leading edge of the cold pool (47.13 N) is deeper than the cold pool further north (i.e N) which is also expected in the case of a gravity current (e.g. Simpson 1987). Between N and N, the depth of the cold pool increased from 150 to 250 m. Between N and N, the depth of the cold pool was approximately constant and equal to 250 m. The high-resolution lidar measurements also revealed water vapour 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 convective atmospheric boundary layer. However, given the strong stable conditions associated with 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 sensible heat flux observed on this day was 80 W m 2 at 1200 UTC. Hence, the water vapour mixing ratio fluctuations could be related to wind-shear-induced instability at the top of the cold pool. Between N and N, in the Bodensee basin region, the structure of the cold pool appeared to be dramatically affected by the presence of waves; the top of the cold pool underwent important wave-type fluctuations. North of N, the depth 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 the presence of KHW. Surface pressure measurements (1 s temporal resolution) made by two microbarographs (Fig. 8) revealed the presence of waves characterized by a period, T, of 300 s at Vaduz (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 Fourier Transform (FFT) analysis. The wave-like pattern in the surface pressure measurements was more pronounced at Vaduz. In the following, using lidar and balloon sounding measurements, we assess whether KHI is triggered in the present case. We also compare the frequency of waves observed by lidar at the top of the cold pool with that obtained from the microbarograph measurements. (c) KHW at the top of the cold pool KHI 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 characterizing the slope of the interface associated with the waves at the interface is much smaller

13 FÖHN/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 than unity (i.e. α 1), the instability condition leading to the presence of KHW in a linearized framework can be expressed as (Drazin and Reid 1981) (ρ 2 + ρ 1 )(ρ 2 ρ 1 ) g V ρ 1 ρ 2 k, (1) where V = U 1 U 2 is the difference of velocity between the upper and the lower fluids, ρ 1 and ρ 2 are the densities of the upper and lower fluids, k is the wave number, and g is the 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 wave amplitude (50 m in this case) and λ is the wavelength (also see Table 2).

14 3048 C. FLAMANT et al. Furthermore, the dispersion equation for KHW in the absence of background wind is given by (Drazin and Reid 1981) ω = kv (ρ 2 ρ 1 )/2 ± ik V 2 ρ 1 ρ 2 (g/k)(ρ 2 ρ 1 )/(ρ 2 + ρ 1 ) (ρ 2 + ρ 1 ), (2) where ω is the wave frequency. The rate of development of the perturbation is given by the imaginary part of ω, and the real part of the wave frequency also verifies ω real = 2π/T,whereT is the period of the KHW. As the dominant wavelength characterizing the water vapour mixing ratio fluctuations at the top of the cold pool was not the same throughout the length of the Rhine valley, three regions have been defined according to the dominant wavelength identified using 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: N which includes the sounding station at Buchs-Grabs, and the surface station at Vaduz, as well as the microbarograph station at Vaduz. Region II. Cold pool confined in the Rhine valley: N which includes the sounding stations at Feldkirch and Diepoldsau. Region III. Cold pool in the Bodensee area: N which does not include any sounding station but was probed by the Merlin IV during operations (refuelling in Friedrichshafen), as well as the microbarograph station at Altenrhein. In Eqs. (1) and (2), V, ρ 1 and ρ 2 are obtained from balloon sounding measurements made at the closest available upper-air station, while k is derived from the lidarmeasured 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 2 summarizes the values of the variables in the three regions. On each sounding, V and (ρ 2 ρ 1 ) are determined around the very sharp föhn/cold-pool interface defined by the change in wind direction with height. Because the above-mentioned variables varied from one region to the other, the relationship given by Eq. (1) also varied, as illustrated in Fig. 9. Nevertheless, it appears that conditions were favourable for KHI to be produced in 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 are barely met in Region II. However, as shown in Table 2, values of ω real derived from Eq. (2) (computed using the values of λ and V determined from lidar and sounding measurements in Regions I and III) did not match the values of 2π/T derived from wave period measurements obtained from the microbarograph stations at Vaduz and Altenrhein. Values of ω real indicate that KHW were quasi-stationary. Instead, it is believed that the advection of the KHW by the mean wind is responsible for periodicity observed in the microbarograph data. From a fixed point at the surface, the period of a wave of wavelength λ advected at 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 agreement with the values inferred from microbarographs (see Table 2). In conclusion, we believe that KHW triggered by wind shear at the top of the cold pool and advected with the mean wind had a pronounced influence on the structure of the cold pool. For example, water vapour mixing ratio fluctuations near the top of the cold pool were of the order of 3.4 g kg 1 in Region III (Table 2) as a result of the presence of waves. Also, as discussed below, these waves may also have affected the föhn flow above the cold pool.

15 FÖHN/COLD-POOL INTERACTIONS 3049 TABLE 2. VARIABLES RELEVANT TO THE ASSESSMENT OF THE EXISTENCE OF KELVIN HELMHOLTZ WAVES IN THE RHINE VALLEY, AS COMPUTED AROUND 1500 UTC ON 5NOVEMBER 1999 Variable Units Region I Region II Region III λ km V ms v ms ρ 1 kg m ρ 2 kg m T s π/T s ω real s q gkg σ q gkg q gkg Ri t q, σ q and q are the mean value, standard deviation and maximum peak-to-peak value of the water vapour mixing ratio derived from lidar near the top of the cold pool (600 m amsl). See text for definitions of the other variables and the three regions. Figure 9. Stability diagram (velocity difference, V, between the two fluids as a function of wavelength, λ): the three curves (I, II and III) are obtained using Eq. (1) for which parameters ρ 1 and ρ 2 are determined from balloon sounding measurements (see Table 2). The symbols indicate the position of the (λ, V ) pair determined from lidar and sounding measurements, respectively, in Region I (triangle), Region II (asterisk), and Region III (diamond). (d) Gravity waves above the cold pool As 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 the lowest overpass (880 m amsl). In Regions I and II, such a signature was not as obvious in the vertical velocity data. Note that the average flight level for the Merlin IV on the lower overpass was 450 m agl, hence the Merlin never flew into the cold pool. A FFT analysis of vertical velocity data in Region III (Region II) revealed that the dominant wavelength was 2km( 1km), the same wavelength as was obtained from the lidarderived cold-pool top heights (see Table 2).

16 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) between 1555 and 1607 UTC (880 m agl) and (b) between 1533 and 1553 UTC (450 m agl). The lower panels show measurements made by the Merlin along AB between 1533 and 1553 UTC (450 m agl, solid line) and between 1555 and 1607 UTC (880 m agl, dashed line): (c) water vapour mixing ratio R, (d) wind direction WD,(e)wind speed 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, temperature and relative humidity measurements made by the Merlin along the leg AB between 1533 and 1553 UTC at 450 m agl and between 1555 and 1607 UTC at 850 m agl. At the lower level, in Region III, wind direction, temperature and relative humidity exhibited well-marked wave-like fluctuations. The wavelength of these fluctuations was also of the order of 2 km, as derived from FFT analysis. At the higher level, wave-like fluctuations of smaller amplitude were also observed. The vertical velocity fluctuations at 850 and

17 FÖHN/COLD-POOL INTERACTIONS 3051 Figure 11. Scorer parameter profile derived from the Merlin IV soundings at Friedrichshafen. The horizontal dotted, dash-dotted and dash-triple dotted lines represent the altitude of the Merlin during the three Rhine valley overpasses, i.e. at 880, 1310 and 1815 m amsl, respectively. The vertical dashed line denotes the wave number of 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 the presence of trapped lee waves. However, given the time lag between the two Merlin legs (20 min on average) and the length of the legs (15 min on average), fluctuations at 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 region could support trapped lee waves, we computed the Scorer parameter profile in Region III using the Merlin IV sounding at Friedrichshafen (Fig. 11). The horizontal, k, and vertical, m, wave numbers of the wave over the cold pool are coupled through the Scorer parameter, l, given by: l 2 = N 2 U U U z 2, (3) where l 2 = k 2 + m 2, N is the Brunt Väisälä frequency and U the wind speed. If k<l, the wave is vertically propagating through a depth of the atmosphere where l remains greater than k.ifk>l, the wave is damped with height, as m is imaginary. If the vertical variations of N and U are such that l decreases significantly with height, then the flow is prone to favour wave trapping. In Region III, the wave number of the waves observed in situ with the Merlin IV above the cold pool and observed by lidar at the top of the cold pool (corresponding to a wavelength 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 pool and 880 m agl (altitude of the lower Merlin overpass). The 1385 m agl Merlin leg was located in an atmospheric layer also characterized by a significant decrease of the Scorer parameter. Interestingly, there exists a layer of constant Scorer parameter (of depth 200 m) between the lower two levels flown by the Merlin, which could explain the wave amplitude damped with height seen on temperature and relative humidity. Above 1.6 km amsl, the Scorer parameter is nearly constant and its value is close to the value of the

18 3052 C. FLAMANT et al. Figure 12. Horizontal wind field measured by the Merlin IV along leg AB at 450 m agl superimposed on the topography of the Rhine valley target area. wave number of the waves observed below. In this layer, the waves are damping with height, 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 cold pool at 450 m (880 m) agl, as shown in Fig. 10. The Scorer parameter was observed to decrease significantly with height between the top of the cold pool and 880 m agl, but was smaller than the wave number corresponding to the wavelength of 1 km observed by lidar at the top of the cold pool above 0.7 km agl (not shown), suggesting that the waves 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 at 450 m agl are believed to be connected to the presence of very weak flow splitting and wake 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 to the north (Region III), the flow at 450 m agl is no longer influenced by the topography.

19 FÖHN/COLD-POOL INTERACTIONS 3053 (e) On the relationship between KHW and gravity waves above Lidar measurements provide evidence of the existence of waves at the top of the cold pool. The wave characteristics verify the instability condition leading to the presence of KHW given by Eq. (1). It can then be argued that the deformation of the cold-pool top induced by the KHW is responsible for generation of gravity waves in the atmosphere above the cold pool. This argument can made by analogy with what is referred to as convection waves in the atmosphere (e.g. Hauf 1993), i.e. that in some conditions (most often daytime in the summer), strong thermals in the planetary boundary layer act as obstacles for the flow in the free troposphere above, just like a mountain. It follows that in this situation large fluctuations of the cold-pool depth induced by KHW would produce obstacle-like forcing likely to generate gravity waves in the föhn flow above. Based on a Scorer profile determined from a sounding in the vicinity of the Bodensee, we show that conditions are favourable for waves generated in the vicinity of the cold-pool top to actually propagate upwards and be trapped in a portion of the föhn layer. 4. COLD-POOL EROSION (a) Radiative and turbulent processes At 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 are favourable to the reformation or maintenance of the cold pool. High-resolution VERA analyses in the Rhine valley show that Lustenau was covered by the cold pool as far south at Vaduz, and the potential temperature field in the vicinity of the valley floor looked relatively homogeneous. Surface measurements along the Rhine valley also imply that, north of Balzers, the föhn 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 from the north (Bodensee area). Based on this, we may argue that the single-point flux measurement at Lustenau is representative of a larger area, i.e. the whole cold pool south of Vaduz. Hence, we may conclude that mechanism (i) (i.e. convection in the cold pool) 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 to the turbulent erosion of the cold pool, we have computed the profiles of the Richardson number in Regions I, II and III. Richardson number profiles have been derived from sounding data, even though it is widely recognized that radiosounding data collected along a skewed path within a few minutes provide a snapshot-like profile that can be a limiting factor in estimating planetary boundary layer characteristics (e.g. Parlange and Brutsaert 1989). We looked for the smallest Richardson number value, Ri t, in the wind speed shear region above the cold pool, turbulence generally being assumed to be produced below a critical value. This value is usually taken as 0.25, although suggestions in the literature range from 0.2 to 1.0 (see discussion by Jericevic and Grisogono 2006), and even larger when vigorous, intermittent turbulence occurs in stable conditions (e.g. Poulos and Burns 2003). There is also some suggestion of hysteresis, where laminar airflow must drop below Ri = 0.25 to become turbulent, but turbulent flow can exist up to Ri = 1.0 before becoming laminar. Hence, a single value of Ri at a given time cannot be used to assess whether mixing can occur. Rather, one has to monitor the temporal evolution of Ri associated with the

20 3054 C. FLAMANT et al. air mass of interest in order to reach a conclusion on the likeliness of mixing associated with KHW. Therefore, we have computed the temporal evolution of Ri t between 0500 and 2300 UTC (whenever there were adequate data) in Regions I and II. This was not possible in Region III as the calculation of Ri relies on a single aircraft sounding. Maximum mixing between the föhn layer and the cold pool is expectedwhere and when the föhn is strongest, i.e. the wind shear at the top of the cold pool is largest. Based on scintillometer and surface measurements (Figs. 4 and 5, respectively), it appears that the föhn is strongest around 2000 UTC in Regions I and II. In Region I, Ri t values were found to decrease with time from a value of 3 to a value of 0.3 between 0500 and 1700 UTC, and increase afterwards. (Note that the 2000 UTC profile did not enable the computation of Ri in the region of interest.) In Region II, Ri t values decreased from 8 to 1.3 between 0500 and 1500 UTC, and increased afterwards reaching a value of 3.9 at 2315 UTC. Hence it appears that the flow was laminar early in the day and since Ri t values did not drop below 0.25, it is likely that mixing between the föhn layer and the cold pool did not occur. (b) Dynamic displacement of the cold pool by föhn air In 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 1500 and 2200 UTC (Fig. 6). In the following, we investigate the evolution of the heat budget tendency terms in the lower Rhine valley from about 1100 to 2300 UTC. The evolution of the potential temperature results from an imbalance between the contribution of several processes, mainly advection, turbulence and radiation. The major sources of turbulence are wind shear and buoyancy. In the light of the above discussion, it appears that 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 is not expected to be a factor in this case. As the fog had dissipated early in the day, and clear-air conditions were experienced throughout the afternoon, the impact of radiation can be neglected as shown by Jaubert et al. (2005) using high-resolution numerical simulations. 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= θ, (4) t 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 coldpool nose in the early afternoon of 5 November 1999, and because the cold-pool nose advected over the station at Buchs-Grabs in the course of its displacement northwards in the early afternoon (as the result of the action of the föhn) and then southwards in the late afternoon (as the föhn weakened and cold air from the Bodensee area filled the lower 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 VERA analyses showing that the air behind the cold-pool edge (located south of Balzers) was increasingly cold with time until 1500 UTC. Between 1400 and 1700 UTC, Fig.13 shows cooling at the surface ( 1Kh 1 ) and pronounced warming at the top of the cold pool ( 2Kh 1 ) due to the action of the föhn. This is consistent with the surface measurements (Fig. 5(b)) which show that at Buchs-Grabs, at this time, the