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Turbulence Structure and Exchange Processes in an Alpine Valley: The Riviera Project. Rotach, Mathias W. 2011

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1367SEPTEMBER 2004AMERICAN METEOROLOGICAL SOCIETY | L ocal circulation systems and average temperatureand stratification conditions in valleys have beeninvestigated in some detail in the past, and are fairly well understood at present (e.g., Whiteman 1990, 2000). In contrast, little has been done with re- spect to the associated turbulence structure and tur- bulent exchange processes in mountainous terrain (Rotach et al. 2000). Even from relatively recent large field programs, such as the Atmospheric Studies over Complex Terrain (ASCOT) program (Clements et al. 1989), only a few “representative” turbulence obser- vations are available. Often, the emphasis of atmospheric studies in mountainous terrain is focused on pollutant transport [e.g., Anfossi et al. (1998) for the Trans-Alpine At- mospheric Transport experiment (TRANSALP); Cionco et al. (1999) for the Meteorology and Diffu- sion over Non-Uniform Areas (MADONA) study] or AFFILIATIONS: ROTACH, CALANCA, GURTZ, ANDRETTA, WEBER, WEISS, AND ZAPPA—Institute for Atmospheric and Climate Science, Swiss Federal Institute of Technology, Zurich, Switzerland; VOGT, CHRISTEN, AND VAN GORSEL—MCR Laboratory, University of Basel, Basel, Switzerland; STEYN, DE WEKKER, AND RUCKER—Atmospheric Science Program, The University of British Columbia, Vancouver, British Columbia, Canada; GRAZIANI, CIESLIK, CONNOLLY, AND GALMARINI—Joint Research Center, Ispra, Italy; E. N. KADYGROV, V. KADYGROV, AND MILLER—Central Aerological Observatory, Moscow, Russia; NEININGER—MetAir, Illnau, Switzerland *Current affiliation: Swiss Federal Office for Meteorology and TURBULENCE STRUCTURE AND EXCHANGE PROCESSES IN AN ALPINE VALLEY The Riviera Project BY MATHIAS W. ROTACH,* PIERLUIGI CALANCA, GIOVANNI GRAZIANI, JOACHIM GURTZ, D. G. STEYN, ROLAND VOGT, MARCO ANDRETTA, ANDREAS CHRISTEN, STANISLAW CIESLIK, RICHARD CONNOLLY, STEPHAN F. J. DE WEKKER,+ STEFANO GALMARINI, EVGENY N. KADYGROV, VLADISLAV KADYGROV, EVGENY MILLER, BRUNO NEININGER, MAGDALENA RUCKER, EVA VAN GORSEL, HEIDI WEBER, ALEXANDRA WEISS,# AND MASSIMILIANO ZAPPA The new detailed data set resulting from this field campaign, set in highly complex terrain, is helping improve turbulence schemes in meteorological and hydrological numerical models. Climatology, MeteoSwiss, Zurich, Switzerland +Current affiliation: Pacific Northwest National Laboratory, Richland, Washington #Present affiliation: MPI, Hamburg, Germany CORRESPONDING AUTHOR: Mathias W. Rotach, Swiss Federal Office for Meteorology and Climatology, MeteoSwiss, Kraebuehlstr. 44, P.O. Box 514, CH-8044 Zurich E-mail: mathias.rotach@meteoswiss.ch DOI:10.1175/BAMS-85-9-1367 In final form 12 January 2004 ©2004 American Meteorological Society 1368 SEPTEMBER 2004| atmospheric chemistry. For example, in the Vertical Ozone Transport in the Alps (VOTALP) campaign surface turbulence information was available at only two selected sites within the Mesolcina valley (Furger et al. 2000). Because pollutant distribution over any type of terrain is largely determined by turbulent ex- change processes and their interaction with the mean flow, it is clear that there is a need to understand tur- bulence in complex terrain. What we do know, con- cerning turbulence structure in an alpine valley, is that commonly used approaches in boundary layer meteo- rology (such as the validity of scaling regimes; Holtslag and Nieuwstadt 1986) cannot be expected to hold, due to the violation of many underlying assump- tions. For flows over gentle hills linearized models and scaling considerations based thereupon are available, which identify different scaling regimes (e.g., Jackson and Hunt 1975; Belcher and Hunt 1998), but these cannot be expected to apply over steep mountains. Most theoretical work concerning valleys is devoted to the thermodynamics of local circulation due to dif- ferential heating (e.g., Vergeiner 1987). In these stud- ies the contribution of turbulent exchange is usually treated in a very simplistic manner. Numerical models of all scales—numerical weather prediction, climate, mesoscale, and large eddy simulation models—use parameterizations, which are based on scaling approaches for flat and horizontally homogeneous terrain in their surface ex- change and boundary layer schemes (Randall 2001; Beljaars and Viterbo 1998; Emeis and Rotach 1997). Large-scale models having a horizontal resolution of 100 km or more will not resolve many of the oro- graphic features of complex topography. In their highly smoothed topography the first model level at typically 30 m above “ground” would need a param- eterization to account for bulk turbulent exchange from a series of valleys and ridges. Rather, they have at present a surface layer description based on the Monin–Obukhov similarity theory (e.g., Louis 1979). This leads to sometimes unrealistically high values of the effective roughness length (e.g., Georgelin et al. 1994). Similarly, nonhydrostatic models with high horizontal resolution use the same surface exchange schemes over the valley slope, valley floor, and nearby ridges irrespective of their having been developed for flat, horizontally homogeneous terrain. Hydrological models for the prediction of runoff in entire catchments typically use even more simpli- fied surface layer schemes to obtain surface fluxes of sensible and latent heat from energy balance consid- erations (e.g., Gurtz et al. 1999). For such simulations, where mountainous terrain is not the exception but the rule, it is clear that these approaches may some- times be problematic (e.g., Plüss and Mazzoni 1994), but, little data are presently available to improve these schemes. The use of these surface exchange and boundary layer schemes in numerical models can easily be criti- cized based on first principles, but they are employed only due to the lack of better knowledge (Rotach 1995). As a step toward filling this gap, the Riviera project was planned and executed in conjunction with the Mesoscale Alpine Programme (MAP), an international research effort to improve our knowledge of meteoro- logical and hydrological processes over the Alps (Binder and Schär 1996; Bougeault et al. 2001). It was decided to investigate one valley in detail, rather than a num- ber of topographical features with different sizes, ori- entation, and other characteristics (Emeis and Rotach 1997). Therefore, a wide range of instrumentation was deployed in a single valley in the southern Alps. SITE SELECTION. To link this research with hy- drological components of the project, a test valley was sought in the Lago Maggiore target area of MAP (Bougeault et al. 2001). The following valley selection criteria were applied: a) valley structure: as unobstructed, straight, and symmetric as possible with equal height and slopes on both sides, and few side valleys; b) valley size: a small valley, with respect to optimal spatial resolution, given the limited instrumenta- tion available [However, it was felt that a valley with a width comparable to the resolution of modern operational models (e.g., the highest reso- lution Swiss operational model: 7 km) was pref- erable]; and c) infrastructure: availability of suitable, easily acces- sible experimental sites on the slope(s). Based on these criteria the Riviera valley in south- ern Switzerland was chosen. It constitutes a part of the Ticino valley (Fig. 1) between Bellinzona and Biasca that discharges from the Saint Gotthard mas- sif in the central Alps to Lago Maggiore. It is approxi- mately straight and U-shaped, with a floor ranging from about 250 m above sea level (ASL) to ridge tops at about 2500 m ASL on both sides. The valley floor is 1.5 km wide, and the slopes are roughly 30∞ and 35∞ on the eastern and western sides, respectively. The Riviera valley contains some minor tributary valleys on both sides. The valley floor consists of agricultural land and a number of villages and isolated farmhouses. A high- 1369SEPTEMBER 2004AMERICAN METEOROLOGICAL SOCIETY | way, a railroad, and the Ticino River run along the valley axis. The slopes are mainly forest covered. Above roughly 1200 m ASL, meadows are inter- spersed with rocky rubble. One special feature of the Riviera valley is the fact that its outflow leads to an- other valley rather than the “classical” plain. This clearly influences the flow (Weigel and Rotach 2003) and will have to be analyzed in detail in the future. OBSERVATIONAL STRATEGY. Some of the “continuous observations,” as described in the follow- ing similarly titled section, started as early as 10 July 1999 and were operating until 13 October 1999. The majority of instrumentation at the continuous sites was active from the beginning of August to the be- ginning of October 1999. Due to power outages at some of the remote sites, not all instruments were continuously recording in this period. Two intensive observation periods (IOPs) were scheduled to coin- cide with the availability of the research aircraft (Research flights section). These periods lasted from 15 to 31 August and from 20 September to 8 October 1999 and are referred to as R-IOP1 and R-IOP2, re- spectively. A total of 14 flights of 2–4 h duration each were performed on eight flight days, with all of the “additional instrumen- tation” (Table 2, see sections on Scintillometry through Sodar for details) then be- ing available. Figure 2 de- picts the topography of the Riviera valley and indicates the main cross section of observations near Claro (close to sites A1 and A2), a small village at the foot of the eastern slope. Shown are the continuous sites and the distribution of addi- tional instrumentation dur- ing the IOPs. Full detail concerning the observa- tions (instrument levels, sampling frequencies, in- strument types, and cali- brations, etc.) can be found on the project’s homepage (online at www.iac.ethz.ch/ en/research/map_riviera/index.html), under “metadata report.” Continuous observations. Eleven towers were estab- lished along a southeast–northwest cross section in the Riviera valley through the village of Claro (Fig. 2) FIG. 1. Topography of southern Switzerland with the Riviera valley between the towns of Biasca and Bellinzona. Cross section of surface stations (see text) runs through the valley approximately at the height of the little village of Claro. FIG. 2. Topography of the Riviera valley overlaid with land use types and the obser- vational sites with indications of the instrumentation. The color codes for the land use are green: deciduous forest, blue-green: coniferous forest, yellow: alpine vegetation, gray: rock, white: snow, light green: meadows, light gray: agricultural land, red: settlement, light red: industrial area, blue: water, black: roads. The village of Claro can be recognized below the symbol for site A2. 1370 SEPTEMBER 2004| in order to obtain months-long observations of near- surface turbulence and meteorological/hydrological characteristics. This setup was chosen because the cross-valley variability of flow characteristics was an- ticipated to be more prominent than that along the valley. Towers were located on the valley floor and the eastern slope (Fig. 2). At the principal sites, high towers with turbulence probes at several levels were erected (Table 1) and typically accommodated obser- vations of the full radiation balance, precipitation, and standard meteorological parameters (pressure, profiles of temperature, humidity, and wind speed). Smaller towers with only one sonic anemometer level were operated mainly in regions of problematic logistics. Altogether, 20 sonic anemometers and six fast-response hygrometers continuously recorded turbulence statistics in the lowest 30 m of this valley’s cross section for about 2 months. A field intercom- parison of different sonic anemometers will briefly be introduced in the section titled Instrument intercomparison. Detailed hydrological observations were per- formed at two of the principal sites (A1 and B, see Table 1). These observations included profiles of soil moisture and temperature (down to a 1.6-m depth), as well as soil heat flux, leaf wetness, and canopy pre- A1 28 3 levels 6 levels Full Yes Detailed MTP-5, Valley (2) balance RS, floor, sodar mixed agriculture A2 5 — 4 levels — — Scint., As A1 T¢ TB B 30 3 levels 1 Net Yes Detailed — Slope, (1) reference forest C 6 1 level — Net Yes — — Slope, vineyard D 6 1 level — — — — MTP-5, Foot of (1) scint. slope E1 12 2 levels — Full Yes Reduced Sodar Slope, (1) balance meadow E2 23 6 levels 8 levels Full — — 18 levels of T¢ Slope, (1) balance forest F1 6 m 1 level 3 levels Net — Some — Slope, sparse vegetation F2 11 2 levels 3 levels Full Yes Some — Slope, balance shrub F3 2 — 1 level Net — — — Slope, grass G 5 1 level 2 levels — — — 1 level of T¢ Slope, forest TABLE 1. Sites and observations in the Riviera valley: “No. hygr.” stands for the number of fast-response hygrometers, Standard meteo refers to standard meteorological observations. Precip denotes precipita- tion, Hydro denotes hydrological observations (see text for details), and Additional obs. refers to additional observations at the respective site as described in sections Scintillometry to Sodar MTP-5: microwave temperature profiler; RS: radio sounding; TB: tethered balloon, Scint.: scintillometric measurements; T¢: fast response temperature measurements. Site Height Turbulence Standard Radiation Precip. Hydro. Additional Surface of tower (m) (No. hygr.) meteo  obs. character 1371SEPTEMBER 2004AMERICAN METEOROLOGICAL SOCIETY | cipitation. At A1, profiles into the ground were ob- tained at various subsites with different surface cover. Together with simultaneously recorded meteorologi- cal parameters, these observations yielded an excel- lent opportunity to evaluate hydrological and coupled meteorological/hydrological models (Zappa et al. 2000; Carlaw et al. 2000). Additionally, soil moisture observations were obtained episodically at site E1 and in three other valleys of the region, in order to better cover the variety of surface types. The following sections describe the additional ob- servations that were performed during the IOPs. Scintillometry. Due to the expected spatial variability of turbulent fluxes, it was desirable to obtain, in ad- dition to the detailed array of point measurements from sonic anemometers, spatially averaged informa- tion on turbulence characteristics. Two small-aper- ture, displaced-beam scintillometers (Scintec, SLS 20) were, therefore, operated at the valley floor (site A2) and at site D on a gentle slope. These scintillometers yield path-weighted sensible heat flux, momentum flux, and the dissipation rate of turbulent kinetic en- ergy over a distance between 50 and 200 m. There is some uncertainty surrounding the mea- surement principle of scintillometers in complex ter- rain (see Weiss et al. 2001). Large efforts were, there- fore, undertaken to investigate the performance of the instruments prior to taking actual observations (Weiss et al. 1999 and our Instrument intercomparison section). Figure 3 shows a typical example of the daily cycle of the sensible heat flux as measured simultaneously by a scintillometer at the valley floor (site A2), and a scintillometer and a sonic anemometer at the foot of the slope (site D). Generally, the difference between path-averaged and point observations at site D is within the respective instrument uncertainties (sec- tion on Instrument intercomparison). Daytime val- ues of sensible heat flux on the slope (site D) are con- sistently larger than those observed on the valley floor (A2). Also, the valley floor site shows an earlier tran- sition to positive heat fluxes in the morning and to negative fluxes in the afternoon. These differences re- flect the variability in net radiation (Matzinger et al. 2003) due to differences in exposure and slope angle. At other sites with steeper slopes and more favorable exposure, these differences become even more pro- 21 Aug 1999 1 ± Convective 0721–1031, T-B-A-B-T, Full, no. H2O; 1434–1700 T-B-A-B-T full, no. H2O 22 Aug 1999 2 ± Convective 0736–1010, T-A-B-T, No wind and H2O 1136–1554 T-B-A-B-A-B-T full, no H2O 25 Aug 1999 3 Convective 0649–0942, T-A-B-A-B-T, Full, 1112–1541 T-B-A-B-A-B-T full 21 Sep 1999 4 Mixed 0716–0905, T-B-A-B, Full, 1117–1510 T-B-A-B-A-B-A-T no wind 22 Sep 1999 5 Mechanical 0658–0925, T-A-B-A-T-B, Full, 1115–1302 T-B-A-B-A-B low resolution wind 28 Sep 6 Mechanical, 1202–1614 T-B-A-B-A-B-T Full, H2O low transition from quality rainy period 29 Sep 1999 7 Convective 0722–0945, T-B-A-B-T, Full, morning, 1156–1553 T-B-A-B-A-B-T full mixed afternoon 1 Oct 1999 8 Convective 1033–1409 T-B-A-B-A-B-T Full TABLE 2. Summary information on flight days. Flight patterns are described in section titled Research flights. Date Flight day Type of day Flight times Flight Data coverage, no. (UTC) patterns failures 1372 SEPTEMBER 2004| nounced (De Wekker et al. 2004, see also our section on Surface energy balance on a slope). Radio soundings. Radiosondes were launched from the valley floor (site A1) at 3-hourly intervals during flight days, starting at 0600 and ending at 2400 UTC. In ad- dition, on the day before a flight day, two soundings at 1200 and 1800 UTC were launched to obtain some detail of the developing situation. For all the R-IOP2 soundings, release times were 1 h earlier (so that the sonde would reach the tropopause around 0000, 0600, 1200, and 1800 UTC). A Vaisala MW11 receiving sys- tem was employed using RS80 sondes, which are equipped with standard pan-tilt unit (PTU) sensors and an 8-channel digital GPS receiver. The accuracy of the raw data is 0.5 hPa (pressure), 0.2 K (tempera- ture), 3% (relative humidity), and 0.5 m s-1 (wind speed). The sounding system has a vertical range of over 20 km. However, in about 50% of the soundings the radio signal was lost be- fore the sonde reached the tropopause. Passive microwave tempera- ture profiler. A meteorologi- cal temperature profiler (Kipp & Zonen, MTP-5) was operated at the valley floor (site A1) during R-IOP1, and closer to the eastern slope of the Riviera valley (site D) during R-IOP2. The MTP-5 is a passive micro- wave sensor, which allows determination of air tem- perature profiles from the ground up to 600 m, at a vertical resolution of 50 m and a temporal resolution of 5 min. The instrument senses a cone of about 500 m in length, yielding a “profile” of the air tempera- ture with some spatial av- eraging (Kadygrov and Pick 1998). This instrument sensed the daily evolution of static stability in the lower part of the valley atmosphere at high temporal resolution. Because the MTP-5 is a rela- tively recently developed FIG. 4. Temperature information derived from MTP-5 data for the week 17– 24 Aug 1999, at site A1, MAP-Riviera. (top) Time–height cross section: color coding from dark blue (12∞–13.5∞C) to orange (25.5∞–27∞C) in intervals of 1.5∞C. (bottom) Temperature inversion parameters. The temperature time series are shown for various heights (dark red = 0 m to dark blue = 600 m) in 50-m intervals. Also, the inversion height (blue line, right scale) and its maximum strength (purple columns, left scale) are shown. FIG. 3. Time series of turbulent sensible heat flux (H) as measured by scintillometry on the valley floor (site A2, dotted line) and on lightly sloping terrain (site D, solid line) on 1 Oct 1999. Dashed line: Sonic anemom- eter observation at site D. 1373SEPTEMBER 2004AMERICAN METEOROLOGICAL SOCIETY | instrument, its collocation with the radio sounding system at site A1 dur- ing R-IOP1 was exploited for a sys- tematic intercomparison. As an example of data from the MTP-5, a time–height cross section of temperature in the lowest 600 m at site A1 in the Riviera valley is presented (Fig. 4). The daily cycle is very pronounced with the expected well-mixed layer within the ob- served lowest 600 m (see also Fig. 9) and a stable near-surface layer dur- ing the night. The maximum tempera- ture difference across the inversion (9.8 K) was observed at 0555 LST (Local Standard Time, corresponds to UTC + 2) 6 October 1999 at a height of 350 m. Continuous mea- surements at 5-min temporal resolution yield infor- mation on the very quick transition between near- surface nocturnal and daytime temperature regimes. Within about 20 min of sunrise or sunset (Weber 2000, unpublished Msc Thesis) the stable (unstable) layer close to the surface is eroded and the lowest 50 m of the valley atmosphere changes stability. Figure 5 shows an example of the sunrise transition on 24 September. The newly developed morning un- stable layer near the surface is topped by an approxi- mately neutral region, while in the evening the tran- sition directly leads to an elevated inversion. Tethered balloon sounding. Profiles of temperature, hu- midity, wind speed, and direction were taken at site A2 on the valley floor by operating an Atmospheric Instrumentation Research, Inc. (AIR), tethersonde (type TS-3A-SPH). Due to restrictions imposed by lo- cal air traffic control authorities, these ascents could only be performed on flight days during R-IOP2. A typical ascent–descent cycle took about 30 min and reached a maximum height of 800 m above the valley floor. In combination with other “profiling systems,” the tethersonde data allow for a detailed investigation of the valley atmosphere’s spatial structure. Sodar. Two monostatic flat-array sodars (Scintec, FAS64) were situated at sites A1 and E1. Due to noise pollution these instruments could not be operated continuously for day-and-night periods during flight days. In addition, the sodar at site A1 exhibited some technical problems, which inhibited operation most of the time. The instrument at site E1 was operated with a height-dependent vertical resolution (10 m near the ground, and coarser with increasing height) and a temporal resolution of 20–30 min. Research flights. Research flights were performed with an instrumented light aircraft (Neininger et al. 2001) operated by MetAir for a total of eight flight days. The aircraft is typically operated at a cruising speed of 200 km h-1 and has an endurance of 4–5 h. The aircraft measured standard meteorological variables with a high enough sampling rate to allow for the derivation of tur- bulence statistics (for details on the instrumentation see the MAP-Riviera Web site). Precise locations and height were continuously recorded for later mapping. In ad- dition to meteorological parameters, aerosol concen- trations and some trace gas concentrations were mea- sured as parameters that can be used to identify the origin of air masses and transport processes. During some flights a downward-looking IR camera was em- ployed to map the surface (radiation) temperature field. A flight day consisted of one of the following: a morning flight, an afternoon flight, or both. Generally, a flight started and ended with a profile flown up to about 4000 m or to the ceiling height (pattern T). In between, a succession of two different flight patterns was flown: i) pattern A: valley traverses at different heights (Fig. 6a) yielding a quasi-stationary valley cross section of mean flow and turbulence characteris- tics; and ii) pattern B: along-valley flight legs close to the slopes and in the center of the valley at different heights yielding a three-dimensional picture of the valley atmosphere (Fig. 6b). FIG. 5. Temperature profiles from the MTP-5 on 24 Sep 1999 during the morning transition period. 1374 SEPTEMBER 2004| A typical flight then consisted of a succession of flight patterns such as T-A-B-A-B -T, with care taken to ensure that flight legs were flown in similar loca- tions on the various days. Depending on the weather conditions and other considerations, longer and shorter sequences were flown during the eight flight days. Flight days were selected such that days with distinctly different boundary layer characteristics were captured, exhibiting the following: a) “fully convective” condi- tions (weak synoptic forc- ing, clear sky, develop- ment of a valley wind system), b) “fully mechanical” condi- tions (strong synoptic forcing, overcast), and c) “mixed” conditions or, in boundary layer terminol- ogy, conditions of forced convection. A summary of the eight flight days is given (Table 2), where the number of differ- ent day types can be found. The majority (usually more than 95%) of continuous observations (see similarly titled section) were active during a flight day. Further- more, all the “additional observations systems” (Table 3) were operating at least during the periods of ac- tual flights. In total, 14 flights were made, with ap- proximately 45 h of data. Tracer releases. The simulation of pollutant dispersion in complex terrain is notoriously difficult due to com- plicated flow and turbulence fields in such environ- ments. The detailed meteorological and turbulence data, as obtained in the present project, yielded an excellent background for testing and possibly improv- ing currently available dispersion models for complex terrain. Two release experiments (on 29 September and 6 October) were carried out during the field phase of the project. Both of these days were dominated by clear skies with a valley wind developing, thereby sim- plifying planning of sampler locations. The tracer was released from a point located near the village of Claro (point “R” in Fig. 7) at 1400 LST for both cases. The release height was 5 m AGL. The actual release lasted for 25 min and was made at a con- stant rate of 2 g s-1. The tracer, belonging to the fam- ily of cyclic perfluorocarbons (PFCs), was per- fluorodymethylcyclohexane (PP3; see Girardi et al. 1998). These substances are environmentally benign, very stable, nontoxic, insensitive to rain washout, and are detectable by chemical analysis at extremely low concentrations (10-16 v/v, i.e., 1.5 pg m-3). Ground-level samplers were deployed with positions dictated by accessibility, with 6 along the valley axis over Scintillometer, A2 and D H, M, e (path- All flight days + SLS20 weighted averages) additional days during R-IOPs Temperature profiler, A1 or D Profile of T R-IOP1 and R-IOP2 MTP-5 (continuously) Radio sounding, A1 Profiles of T, RH, All flight days MW11 WS, and WD Tethered balloon A2 Profiles of T, RH, All flight days WS, and WD during R-IOP2 Sodar (A1) and E1 Profile of WS Some periods during selected flight days TABLE 3. Details of the additional observations, H and M denote the turbulent fluxes of sensible heat and momentum, respectively, “e” = dissipation rate of turbulent kinetic energy, T = temperature, RH = relative humidity, WS = wind speed, and WD = wind direction. Instrument Site(s) Variables Duration FIG. 6. Flight patterns of the research flight during the MAP Riviera field campaign (a) Cross-valley flight legs (pattern A) and (b) along—valley flight legs (pattern B). 1375SEPTEMBER 2004AMERICAN METEOROLOGICAL SOCIETY | a distance of roughly 10 km (Fig. 7) and 10 along the east- ern slope. Figure 8 shows that the sampler positions allowed for the investiga- tion of plume spread along the valley slope. Samplers along the valley floor were equipped with eight bags per sampler, sampling air over 30-min intervals. Samplers distributed on the slope were equipped with tubes. Each sampler contains 12 tubes programed to sequentially sample for 20 min (4-h sam- pling period). The three panels of Fig. 8 show the time evolution of the tracer mixing ratio (10–15l/l = fl/l) as measured during the two tracer releases at selected sampler sites. The two upper panels give the tracer mixing ratio measured by samplers on the eastern slope of the val- ley. The lower panel gives the time evolution of the tracer mixing ratio measured along the valley during the first release. No tracer was detected at the valley floor during the second release. The three panels re- veal that the sampling network nicely detected the plume’s passage. During the first release (Fig. 8, top) the tracer cloud is confined below 1000 m ASL and arrives at different times at the lower samplers. During the second release, on the other hand, the tracer peak is simultaneously detected by all samplers within the first three 20-min sampling periods (Fig. 8, middle). The mixing ratio then decreases at a constant rate at all altitudes. Along the valley axis (Fig. 8, bot- tom), the arrival of the peak mixing ratio is delayed for larger travel distances. The first three samplers ex- hibit a clear peak, while those at some kilometers dis- tance (cf. positions in Fig. 7) show an approximately constant concentration after the arrival of the plume. This suggests a relatively well-mixed lower portion of the valley atmosphere as was found on other days with a well-developed valley wind system (see Fig. 9). INSTRUMENT INTERCOMPARISON. The MAP Riviera project was mainly devoted to the in- vestigation of spatial variability of turbulence statis- tics in highly complex terrain and its relation to mean flow. This was achieved by using a large number of instruments of a similar type, such as sonic anemom- eters, and by combining different observational tech- FIG. 7. Tracer release point (R) and the sampler locations for the release of 6 Oct 1999. The sampling locations along the valley axis are indicated with an S, those on the slope with simple numbers. niques at different sites. It was, therefore, of major im- portance to assess the relative performance of various instruments prior to or during the field campaign. It is not within the scope of this overview to present all of the corresponding efforts in detail, but a brief sum- mary of some of the most important aspects is pro- vided as follows. Sonic anemometer intercomparison. During a week in early July 1999, 19 sonic anemometers of five differ- ent types were deployed on a small airfield with ho- mogeneous surface conditions. Details of the setup and procedures can be found in Christen et al. (2000, 2001). The sonic anemometers were found to com- pare reasonably well in the derived turbulence statis- tics (Table 4). Those statistics that include the tem- perature are generally less reliable than those with only wind components. Furthermore, the relative error of the wind statistics (and, more so, those in- cluding temperature) increases with smaller absolute wind speed. Overall, it was concluded that the rela- tive accuracy of the instruments was good enough to detect significant differences in turbulence statistics in the actual field study. Scintillometers. Two displaced-beam scintillometers (Scintec, SLS 20) were compared to sonic anemom- eters, and with each other, in the same prefield campaign described above (Weiss 2002). Excellent correspondence was found between the two scintil- lometers with correlation coefficients of rM = 0.94 (momentum flux M) and of rH = 0.99 (sensible heat 1376 SEPTEMBER 2004| flux H), and rms differences of about 0.02 N m-2 (momentum flux) and less than 10 W m-2 (heat flux). This is considerably less than the scatter among the various sonic an- emometers. Also, the comparison between sonic anemometers on the one hand and scintillometers on the other was satisfying: rmsH = 12 W m -2 and rmsM = 0.04 N m -2, respectively. A more detailed analysis of the dataset revealed that the correspon- dence of turbulent fluxes was very good for clearly stable and unstable conditions, but was less so for the momentum flux under near-neutral conditions (Weiss 2002). An addi- tional experiment that addressed the question of measurement height in complex topography in connection with scintillometers is summarized in the Sidebar. MTP-5 versus radio soundings. Data from the passive microwave tem- perature profiler MTP-5 were com- pared to temperature profiles from simultaneously launched radio- sondes at the same site (A1), yield- ing a total of 52 profiles for compari- son. Rms differences between data from the two systems were smaller than 1.0 K everywhere (Kadygrov et al. 2001), regardless of the shape of the profile. When selecting only closely linear profiles, the rms differ- ence was 0.39 K, while those profiles with a distinct inversion exhibited an rms difference of 0.47 K. The largest differences between the two systems were observed for “mixed profiles.” In most of the compared profiles the temperature gradients, that is, the static stability, were closely compa- rable. The favorable correspondence between the data from the two sys- tems allows a combination of their respective advantages for investiga- tion of temperature structure in the Riviera valley. While the MTP-5 has a limited height range (600 m) but a high temporal resolution (5 min), the radiosondes can reach higher al- FIG. 8. Tracer concentrations at the various sampling points (as indi- cated in the inlets, cf. Fig. 7). The sampling time interval of the sam- plers along the valley (bottom panel) was 30 min., that on the slope amounted to 20 min. 1377SEPTEMBER 2004AMERICAN METEOROLOGICAL SOCIETY | titudes but were launched only every 3 h during the flight days. RESEARCH OBJECTIVES AND PRELIMI- NARY RESULTS. Here, we summarize specific research objectives of the Riviera project. Rather than simply listing research objectives we will illustrate them using data from the field observations. Turbulence structure in an alpine valley. Due to the sparse (essentially nonexistent) observational evi- dence concerning the turbulence structure in an alpine valley, a first goal of the MAP-Riviera project must consist of establishing a phenomenological picture of turbulence in such an environment. This provides a basis for exploring specific exchange mechanisms and also serves as a basic picture, on which results from numerical models can be examined. The approach consists in first defining characteristic flow regimes similar to those in Table 2 (Andretta et al. 2001). For each of these flow regimes background boundary layer structure in the valley is established with refer- ence to earlier work (e.g., Whiteman 2000). Figure 9 depicts the profiles of wind direction and potential temperature on a clear summer day with weak synoptic forcing. Wind direction exhibits val- ley wind characteristics with down-valley wind (about 340∞) during the night and up-valley wind (160∞) dur- ing the day. Valley wind depth reaches about 1600 m ASL during night with a transition to geostrophic Scintillometers use information on the spread of a laser beam over a distance of some 100 m to derive spatially averaged turbulence charac- teristics. In this algorithm the height of observation plays an important role, which is, obviously, difficult to determine in highly complex terrain. To address this problem, an experiment was performed on the occasion of the instrument intercomparison (see similarly titled section). Thereby, a scintillometer was set up with an inclined path, with the transmitter at a height of 1.35 m and the receiver at a height of 2.15 m, with the resulting mean height of the laser beam at 1.75 m. This latter height corresponded to that of a set of sonic anemometers (1.8 m). Figure SB1 shows the resulting scatterplots of turbulent fluxes from this experiment. The root-mean-square differences for sensible heat and momentum fluxes amounted to 16 W m-2 and 0.02 N m-2, respectively, and were, thus, comparable to those for experiments with the leveled scintillom- eters (section titled Instrument intercomparison) under similar atmo- spheric conditions. These results indicate that over uneven terrain an average height of the laser beam determined over the path of the scintillometer may be a useful choice for the “measuring height.” MEASUREMENT HEIGHT FOR A SCINTILLOMETER BEAM FIG. SB1. Comparison of (top) momentum flux and (bottom) sensible heat flux from an inclined scintillometer (see text) to an average of five collocated sonic anemom- eter measurements. Maximum error of 2 4 8 15 11 15 good instruments (%) Outliers (%) 14 14 18 27 27 89 TABLE 4. Typical instrument-to-instrument uncertainty (rms differences) for turbulence statistics as derived from the field intercomparison on an airfield in southern Switzerland in Jul 1999. “Outliers” refers to three instruments that were found to have problems in one or more of the measured vari- ables. Statistics are based on about 40 half-hourly averages under optimal conditions (fetch, threshold requirements). ū (m s-1) su,v (m s -1) sw (m s -1) sq (K) u* (m s -1) æ w¢q ¢ æ  (K m s-1) 1378 SEPTEMBER 2004| wind direction (300∞) higher up. This latter is reached above the mean crest height (roughly 2000 m ASL). During the day valley wind direction extends up to about 1600 m ASL. In the transition layer toward the geostrophic wind direction some indications of return flow can be observed. Potential temperature profiles show a distinct three- or four-layer structure. During nighttime, a strongly stable layer a few hundred meters deep is topped by a less stable layer up to 2000 m ASL. A sharp transition then occurs toward the modestly stable lower free troposphere. For some ascents dur- ing the night (not shown) this transition occurs over a layer of a few hundred meters depth. The daytime valley atmosphere is well mixed in the lowest 700 m. Higher up there is a deep stable layer (dQ/dz ª 0.006 K m-1) up to about 1600 m ASL where a sharp transition occurs. There is some indication of a fourth layer between 1600 and 2000 m ASL, that is, the mean crest height. The observed daytime potential temperature pro- files do not support the inversion breakup hypothesis of Whiteman (1990), which leads to a well-mixed layer in the entire valley atmosphere for larger valleys in the afternoon. However, it does bear some similar- ity to a multilayer structure observed in a neighbor- ing valley during VOTALP (Furger et al. 2000) or to the observations of Kuwagata and Kimura (1995) who describe a two-layer structure related to a cross-valley circulation. From these observations it appears that dynamic and thermodynamic structures of the valley flow do not necessarily correspond. During nighttime the layer between approximately 1600 m ASL and crest height is characterized by a transition in wind direc- tion, but no clear signal can be seen in the stratifica- tion. During daytime, on the other hand, the valley wind layer (up to about 1600 m ASL) is well mixed up to a few hundred meters and stably stratified in its upper part. Calanca et al. (2000) presented a preliminary analysis of turbulence structure in the afternoon val- ley atmosphere of 25 August 1999. As an example, we show the kinematic turbulent momentum flux as ob- tained from airborne observations on a cross section through the Riviera valley collocated with the surface towers (Fig. 10). In a relatively shallow band a few hundred meters deep, the turbulent flux is observed to be substantially different from zero. Comparison to the noon sounding of the same day (Fig. 9) indi- cates that this turbulent layer roughly corresponds to the well-mixed lowest portion of the valley atmo- sphere. Nonzero turbulent momentum flux is con- fined to the layer of the valley wind (Fig. 9). The larg- est momentum fluxes are found near the eastern slope, and near the center of the valley atmosphere, that is, somewhat west of the observed “core” of the valley wind on that day (Calanca et al. 2000). Over all, the turbulence structure is found to ex- hibit large spatial variability that is largely driven by the differences in the surface energy balance (section titled Surface energy balance on a slope) at the valley floor and the two slopes. Other flight days (not shown) exhibit a quite different structure in the valley atmosphere’s turbulence structure, and its relation to the mean flow field remains to be evaluated in detail. It is one of the primary objectives of the Riviera project to investigate to what extent high-resolution numerical models are capable of reproducing not only observed mean flow fields, but also the turbulence structure of an alpine valley. First simulations using the Regional Atmospheric Modeling System (RAMS; De Wekker et al. 2002) show a quite favorable corre- spondence in the mean variables (potential tempera- ture, wind speed components) for one case study. The turbulent surface fluxes, on the other hand, exhibit FIG. 9. Profiles of (top) potential temperature and (bot- tom) wind direction at site A1 in the Riviera valley on 25 Aug 1999. Release times are indicated in the inlets. 1379SEPTEMBER 2004AMERICAN METEOROLOGICAL SOCIETY | substantial differences between the model and observation (De Wekker et al. 2002). The investi- gation of turbulent exchange pro- cesses, as outlined in the next sec- tion, will allow us to evaluate the corresponding parameterizations in numerical models in the future. Turbulent exchange processes of heat, moisture, and momentum. To investigate, and possibly improve on, turbulent exchange param- eterizations in numerical models the underlying processes must first be known and understood. Of primary importance in this re- spect is the surface exchange of heat, momentum, and moisture for the various characteristic sur- faces (valley floor, sunlit and shaded slopes). One major chal- lenge in this respect is the simul- taneous occurrence of slope ef- fects, surface inhomogeneity, and the presence of a roughness sublayer due to the variable na- ture of the slope surface (Rotach 1995). However, this is the normal situation in a real valley and the present dataset provides an excel- lent basis for addressing these questions. Mechanisms of turbu- lent exchange within the canopy and the roughness sublayer on the slope have been investigated in detail by Van Gorsel et al. (2003, 2001, 2000). Andretta et al. (2001, 2000) have investigated the interaction between the valley wind and slope wind systems on clear summer days with weak synoptic forcing. While turbulent moment trans- port at the valley floor (site A1) is dominated by the along-wind component of Reynolds stress (æu¢w¢æ), as is the case over flat, hori- zontally homogeneous terrain, a substantial contribu- tion of lateral shear stress (æv¢w¢æ) is observed at the slope site B and at other slope sites (van Gorsel et al. 2003). This is due to directional shear introduced through the transition from the near-surface up-slope wind to a valley wind regime (up valley) at the higher levels of observation. This raises the question of whether the conventional definition of a friction ve- locity u* = ( æu¢w¢æ2 + æv¢w¢æ2)1/4 can be retained for such a complicated flow. Figure 11 shows some preliminary FIG. 10. Kinematic turbulent momentum fluxes on a cross section through the Riviera valley. The data are 2-h averages from the after- noon flight of 25 Aug 1999 (1200–1400 UTC, flight day 3, Table 2). The light blue diamonds indicate the position of data points from which the fields are interpolated (if below the terrain: irregularity of the terrain mask). The surface towers are indicated by only one square even if three or more levels are available. Units are m2 s-2. FIG. 11. Scaled vertical velocity fluctuations at site B (see Fig. 2). Obser- vations from the upper two levels at 24 and 30 m ASL, respectively. The scaling velocity is derived from the local observations of longitudinal and lateral Reynolds stress components (see text). 1380 SEPTEMBER 2004| evidence that indeed the sum of (vertical) frictional and directional shear stresses determine a character- istic local velocity scale. Andretta et al. (2001) show that using the longitudinal stress component alone to derive a “friction velocity” (what makes sense only over homogeneous surfaces, when æv¢w¢æ vanishes) leads to markedly inferior results (not shown). In Fig. 11, a distinction is made between “morning” and “after- noon” periods because Andretta et al. (2000) found these periods to exhibit distinct vertical profiles of Reynolds stress components. However, the locally scaled vertical velocity seems to be insensitive to this difference. Conventional surface exchange schemes employed in numerical models rely on the Monin–Obukhov similarity theory to determine momentum fluxes from the mean wind profile and, thus, do not incor- porate the contribution of directional shear. They can, therefore, be expected to underestimate the friction velocity and, hence, mechanical turbulence. A more detailed analysis is needed in future investigations of this problem. Surface energy balance on a slope. Clearly, the radia- tion balance in a valley and particularly on a slope is different from that on a flat surface (Whiteman et al. 1989a). This is due to both shortwave and longwave contributions, which are influenced by surrounding topography (Matzinger et al. 2003). While the former exhibits temporal variation according to obstruction of direct solar radiation, incoming longwave radiation is not only determined by temperature and density profiles in the atmosphere, but is also modified by emission from nearby surfaces. In consequence, the energy balance on a slope is different from that on a flat surface and a position-specific energy partition- ing can be expected at various valley locations. Figure 12 compares components of the energy balance at the valley floor site (A1) to that on the slope for a selection of 15 “valley wind days,” that is, clear days with weak synoptic forcing (see Andretta et al. 2001 for a definition). At site A1 the sum of sensible, latent, and ground heat fluxes makes up about half the available radiative energy, and this is observed on each of the individual days as well as in the average over all valley wind days. Similar failure of closure of the energy balance has been observed at other, even less complicated, sites (e.g., Vogt et al. 1996) provided that all of the components were di- rectly measured. Clearly, if the energy balance is de- termined using, for example, the Bowen ratio method, it will be closed by definition even if the measurements are taken over highly complex terrain (e.g., Whiteman et al. 1989b). The consistent gap between available radiative energy and turbulent and ground heat fluxes cannot be attributed to measurement errors alone in the present case. Although the various contributions to FIG. 12. Mean daily cycles of the components of the near-surface energy balance at (left) site A1 and (right) B. Data are averages over 15 valley wind days (see text). The turbulent fluxes are measured at the respective low- est level at each of the sites (3.5 m at A1 and 15 at B). Net radiation is observed at 2 m at A1 and at 15 m at B. 1381SEPTEMBER 2004AMERICAN METEOROLOGICAL SOCIETY | the energy balance are observed within a few meters and are relatively close to the surface, it is obvious from Fig. 12 that the idealized energy balance ap- proach (i.e., Rn = H + LE + G) for a flat and horizon- tally homogeneous surface cannot be applied in the type of environment investigated here. The full en- ergy balance for a near-surface layer, including diver- gence of turbulent fluxes and advective processes, has to be assessed in detail in order to devise appropriate approaches for modeling purposes. Rotach et al. (2003) have demonstrated that vertical advection into an idealized box between the height of observation and the surface is likely to make up the energy gap, as is evident from Fig. 12. However, small magnitudes of the vertical velocity make it extremely difficult to determine the vertical advection term to reliable accuracy. The situation at the slope site B is similar, albeit less extreme (Fig. 12). Due to the inclination of the slope net radiation reaches larger absolute values, and the maximum is obtained later in the day than at the valley floor (Matzinger et al. 2003). The turbulent fluxes, H in particular, at the canopy top are larger in magnitude than at site A1, but their sum is still much too small to close the simple energy balance equation. Again, while Fig. 12 shows an average over 15 valley wind days, the individual days exhibit very similar daily cycles of the contributions to the energy balance. The present dataset yields the possibility to inves- tigate in detail a proper description of the surface en- ergy balance in complex terrain. In particular, it will be interesting to assess to what extent local circula- tions driven by variations in the surface energy bal- ance itself are important in explaining the observed energy fluxes. Boundary layer height in a valley. The exchange of air between the valley boundary layer and atmosphere aloft, and the exchange between neighboring valleys are key processes in the investigation of air pollutant dispersion in valley atmospheres in which a major proportion of the population in mountainous terrain resides. Characteristics of this exchange are largely determined by the behavior of the boundary layer height and thermally driven circulation in a valley. The discussion in connection with Fig. 9 has shown that the inversion height may not, in general, be regarded as the boundary layer height. The depth of the well-mixed portion in the valley atmosphere (if significant at all) corresponds neither to that of the valley wind layer nor to the depth of a layer of nonzero turbulence (Fig. 10). De Wekker (2002) and De Wekker et al. (2004) have, therefore, investigated in a case study the valley atmosphere using the vari- ous observations and a mesoscale model at high spa- tial resolution. The numerical model used was the RAMS (Pielke et al. 1992) with 4 two-way interac- tive nested grids down to a horizontal grid spacing of 0.333 km. Model-simulated boundary layer height was determined using a Richardson number criterion. For this case study the model was found to repro- duce broad flow features in the Riviera valley. However, the simulated spatial variability was considerably larger than that observed, especially for the dynamic fields and also for the near-surface turbulence structure, that is the surface heat flux (De Wekker et al. 2004). Model simulations indicated the boundary layer height to be relatively uniform along the valley axis and across the valley. For a clear sunny summer day with strong radiative heating and significant surface heat fluxes a modest “convective boundary layer height” of about 1300 m was reached in the afternoon. This corresponds to the height of observed nonnegligible turbulence on that day (Fig. 10). Note, however, that it does not correspond at all to the height of a “well mixed” regime for potential tempera- ture (Fig. 9) or other scalars. Presently, efforts in numerical simulation of the observations and numerical experimentation are in- tensified using the Advanced Regional Prediction System (ARPS) modeling system. Results from these efforts will be reported in forthcoming publications. Hydrological processes. Hydrometeorological observa- tions conducted at MAP-Riviera sites include soil moisture, soil temperature, soil heat flux, precipita- tion, and leaf interception, with all of these as time series having a high temporal resolution (Zappa et al. 2000). These data, which are available concurrently with the meteorological observations at towers in the Riviera valley, comprise a complete dataset for test- ing and validating soil–vegetation–atmosphere trans- fer schemes (SVATSs) of various degrees of complex- ity. SVATs are key features in both atmospheric and hydrological modeling systems, although important differences exist in the representation of the complex- ity of energetic and hydrological processes. SVATSs constitute the model interface for describing inter- actions between hydrological processes at the soil surface and subsurface, and atmospheric processes within the planetary boundary layer. They deal with spatial and temporal variations of evaporation, tran- spiration, interception, soil moisture, soil tempera- ture, and the soil energy balance. The respective interactions generate continuous feedback on meteo- 1382 SEPTEMBER 2004| rological processes at various scales, on the site’s energy and water balance, and on runoff generation processes. This highlights the importance of research leading to a consistent coupling of atmospheric and hydrological processes and models in complex terrain. Figure 13 depicts a time series of some of the mea- sured and modeled hydrological components at site A1 for the beginning of September 1999. The year 1999 was rather wet when compared to the climato- logical average resulting in relatively large soil moisture. The soil did not completely dry out, even during the 2-week period of fine weather (5–18 Sep- tember). The decrease in soil moisture and subse- quent recovery after the precipitation events are gen- erally well captured by the Precipitation Runoff Evapotranspiration Hydrotope model (PREVAH) (Gurtz et al. 1999), though the depletion of soil mois- ture is somewhat underestimated by the model dur- ing the dry period. Both methods to determine the latent heat flux that use the energy balance equation (i.e., the Bowen ratio approach and the PREVAH model, which uses the Penman–Monteith approach) overestimate, as compared to the eddy correlation measurements. This is most likely related to the nonclosure of the near-surface energy balance at site A1 (section titled Surface energy balance on a slope) during periods of fine weather. Figure 12 reveals that the sum of observed turbulent sensible and latent heat fluxes is substantially smaller than the observed avail- able energy Rn – G. Thus, if a model assumes energy balance closure in its simplest form, it will overesti- mate the contribution of turbulent fluxes. If it should turn out that the missing energy is due to local ad- vection and is a consequence of the valley wind sys- tem itself, this would have to give rise to a modifica- tion of these methods in complex terrain. This example nicely shows the mutual benefits in differ- ent research areas that can be gained through the col- laborative nature of the MAP-Riviera project. SUMMARY AND OUTLOOK. In the present paper an overview is provided of the scientific back- ground and experimental arrangements of the MAP-Riviera project. This project aims at investigat- ing the near-surface and boundary layer turbulent ex- change processes in the presence of highly complex topography because it is the rule rather than the ex- ception in the Alps and other major mountain ranges. During the experimental phase (July– October 1999), a high-quality dataset was collected in the Riviera valley in southern Switzerland. It com- FIG. 13. (top) Time series of hydrological parameters for 1–23 Sep 1999 at site A1. (bottom) Observed soil mois- ture integrated over a 0.65-m-deep layer (dark line) and simulated with the PREVAH hydrological model (labeled “Penman–Monteith”); dark columns indicate precipitation events (right scale). (top) Latent heat flux observed using eddy correlation (diamonds), calculated from temperature and humidity profiles using the Bowen ratio approach (dark line) and determined with the PREVAH model (gray lines), which uses the Penman–Monteith equation. 1383SEPTEMBER 2004AMERICAN METEOROLOGICAL SOCIETY | prises detailed near-surface turbulence measure- ments, profiles of mean meteorological variables throughout the valley atmosphere, and airborne ob- servations of the mean and turbulence structure in the bulk of the valley. Also, two tracer release experi- ments were conducted. Detailed hydrological mea- surements at various sites within the valley complete the experimental efforts. The Riviera dataset is presently being exploited to characterize and understand turbulent exchange pro- cesses near the surface, and between the valley atmo- sphere and free troposphere. Emphasis is given to the interaction of the local dynamic and thermodynamic fields (e.g., a thermally driven valley wind system) and the associated turbulence structure. The overall goal of the project lies in the evaluation and assessment of turbulence exchange parameterizations in meteoro- logical and hydrological numerical models. Not only do we have a dataset at hand, which contains all the necessary variables to critically assess such param- eterizations, but we also hope to use the results of the Riviera project to improve our understanding of the underlying processes. This, in turn will potentially lead to modified turbulence parameterizations for numerical models in complex terrain. First simula- tions with the mesoscale atmospheric model RAMS with simpler diagnostic models (not shown), and the hydrological runoff model PREVAH, are promising in the sense that they reveal some overall skill in re- producing the observed mean flow features. This is especially noteworthy because these models were used beyond their range of applicability. We mention here only the efforts that were necessary in order to obtain a stable integration for RAMS at the given high spa- tial resolution in the very steep orography of the Riviera valley. A careful analysis of both the observa- tions and the model results clearly bears the poten- tial to identify possible weaknesses in the numerics and physical parameterizations for mesoscale meteoro- logical modeling in highly complex terrain. We an- ticipate that other models of various degrees of complex- ity will be involved in this process in the near future. ACKNOWLEDGMENTS. Many individuals and insti- tutions have contributed to the success of the MAP-Riviera project. Funds were available from the Swiss National Sci- ence Foundation (Grants 21-54060.98, 21-55874.98, and 20-63820.01), grants from the Natural Sciences and Engi- neering Research Council of Canada to D. G. Steyn and the European Joint Research Center (JRC) in Ispra (I). Support before and during the observational campaign of the local authorities of Claro, Switzerland, and in particular Mr. Pellegrini, is greatly appreciated. The passive micro- wave temperature profiler (MTP-5) was made available for the project by Kipp & Zonen B.V. (Delft, the Netherlands) through the kind intervention of markasub AG (Basel, Switzerland) and Meteodat GmbH (Zürich, Switzerland). We are indebted to the Forschungszentrum Karlsruhe (Dr. N. Kalthoff) and to the Paul Scherrer Institute (Dr. M. Furger) for lending us a number of sonic anemometers. Karl Schroff and Hansjürg Frei from ETHZ, as well as Valentino Badà and N. Cao from JRC were key people in the preparation, construction, and maintenance of all of the countless surface towers, the instrumentation, and the necessary facilities. 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