Chairpersons: Chantal Staquet, Mathias Rotach, Dino Zardi
Local thermal circulations developing over heated valley slopes strongly influence the convective boundary layer (CBL) over mountainous terrain. For this study, large-eddy simulations are carried out both over idealized valleys and semi-idealized complex terrain. The flow is decomposed into a turbulent part, a local mean circulation capturing the slope winds, and a large-scale (upper-level) wind. This allows a detailed budget analysis for heat and moisture. The temperature distribution is horizontally fairly uniform inside each valley due to the homogenizing effect of the thermally-induced circulations. In contrast to that, the slope winds contribute strongly to the transport of moisture up to the ridges. The entrainment of dry air by the recirculation leads to a horizontally non-uniform moisture distribution. Consequently, a large-scale, upper-level wind hardly affects the horizontally homogeneous temperature distribution while it can considerably reduce the vertical moisture transport: a horizontal wind mixes the moisture from the slope-wind layer into the dryer regions of each valley. Single updrafts mark the small-scale end of coherent motions in the CBL over complex terrain. A conditional sampling method is applied in order to identify the thermal plumes using a passive tracer. In the mixed layer, the plumes are moving upslope with the slope wind. In order to quantify the contribution of the plumes to the vertical transport of heat and moisture, the joint probability density functions of the turbulent fluxes are calculated and decomposed into a local and a coherent part. From this perspective, the turbulence statistics is analyzed at different heights in the CBL and compared to the statistics over flat terrain. In general, the plumes turn out to dominate the vertical fluxes in the valleys and in the lower part of the boundary layer. Especially at ridge height, where the updrafts are few but almost stationary, the statistics are fairly different.
How to cite: Weinkaemmerer, J., Göbel, M., Bastak Duran, I., Serafin, S., and Schmidli, J.: Turbulent slope winds in complex terrain: from heat and moisture transport to the sampling of single plumes, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-621, https://doi.org/10.5194/ems2022-621, 2022.
Processes on the meso-gamma scale play an essential role in triggering deep moist convection. In mountainous terrain, thermally-driven circulations–such as slope, valley, and plain-to-mountain winds–can provide the necessary trigger mechanism to lift air parcels above the level of free convection. Despite its relevance, the impact of thermally-driven circulations on convection initiation has so far not been systematically quantified.
We study the effect of the cross-valley circulation on convection initiation with idealized large-eddy simulations with the WRF model, considering quasi-2D mountain ranges of different heights and widths and different instability profiles. Despite being idealized by using simplified initial profiles, an idealized mountain geometry, and periodic lateral boundary conditions, the simulations employ a complete suite of physics parametrizations to achieve an adequate representation of the essential processes.
The vertical and horizontal redistribution of heat and moisture is quantified using the newly developed budget analysis tool WRFlux, and an attempt is made to determine how the initial stratification and terrain characteristics affect the time scale of convective destabilization. One distinctive finding is that steeper mountain ranges feature a later onset and lower intensity of deep moist convection due to a weaker thermal circulation.
We present different scaling arguments for the vertical profiles of vertical velocity and horizontal convergence above the mountain ridge. Surprisingly, classical velocity scales for the boundary layer over flat and horizontally homogeneous terrain work well also in our idealized mountainous terrain. In contrast, a framework that was specifically designed for estimating the strength of thermal circulations, the heat-engine framework, shows the poorest performance.
How to cite: Göbel, M., Serafin, S., and Rotach, M. W.: Idealized simulations of orographically-induced thermal circulations triggering deep moist convection, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-478, https://doi.org/10.5194/ems2022-478, 2022.
The response of the lower atmosphere to resolved versus parametrized orographic drag over moderately complex terrain is investigated. The larger terrain scales may trigger propagating gravity waves and generate flow blocking, while the smaller scales (smaller than 5 km) may modify the turbulent atmospheric boundary layer leading to turbulent orographic form drag (TOFD). We perform high-resolution numerical simulations to evaluate the ability of a TOFD parametrization to reproduce the impact of small-scale orographic features on the flow over complex terrain. The tool selected to perform the simulations is the Icosahedral Nonhydrostatic (ICON) numerical model, a unified modelling system for global numerical weather prediction (NWP) and climate studies. In the present study, the model is used in its limited-area mode. In the TOFD parametrization, the surface stress and its vertical distribution are formulated in terms of the orography spectrum, meaning that it only depends on the orography characteristics in the box (more specifically, only on the variance of the sub-grid scale orography).
As a first step, simulations using different grid spacings, from the km scale (NWP mode) to the O100 m scale (large-eddy simulations, LES), are carried out to reproduce the intensive observational period (IOP) of the Perdigão field experiment. The high-resolution LES is used to assess the performance of the TOFD parametrization (used in NWP mode) in simulating the surface stress, the near-surface atmosphere and to highlight possible issues resulting in a miss-representation of the flow over moderately complex terrain. The km-scale simulations are run continuously for the complete 49-day IOP using the ERA5 reanalysis dataset for initial and boundary conditions. LES at 130 m grid spacing, are run for the whole IOP nested into the NWP runs (offline nesting). Initial results of the NWP control simulation show good performance compared to the tower wind observations for the whole IOP. The model performance was especially good for the second part of the IOP when a high-pressure regime and weak synoptic forcing were observed.
Larger-scale thermally driven flows from the mountain ranges surrounding the Perdigão site impact the wind flow system at the double ridge, and when decomposed into along- and cross-valley, the performance of the model in the cross-valley direction tends to be very good. However, the model failed to reproduce the transition between anabatic- and katabatic-slope flow in the Serra da Estrela mountain range (NE of Perdigão). The latter negatively impacted the reproduction of the along-valley flow during the afternoon hours at the Perdigão site, when the most important deviations from observations were observed.
How to cite: Quimbayo-Duarte, J., Schmidli, J., Köhler, M., and Schlemmer, L.: Analysis of the Impacts of Small-Scale Orography on the Atmospheric Boundary Layer: Developing ICON-LES for the Perdigão Field Experiment, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-409, https://doi.org/10.5194/ems2022-409, 2022.
The Pyrenees is a west to east oriented mountain range in southwest Europe along the border between France, Spain and Andorra. In the eastern part there are two relatively high populated valleys oriented ENE to WSW: (i) the Cerdanya basin, a wide valley (35 km long, 9 km wide) with the bottom around 1000 m asl at the centre and (ii) the Andorra Central valley, a more closed close valley (5 km long, 0.5 km wide) with the bottom about 1013 m asl on average.
In Cerdanya, northern synoptic flows favour mountain waves formation and associated rotors over the valley, with strong turbulence zones at the upper edge of the mountain wave crest (Udina et al. 2020). For specific precipitation events during the Cerdanya-2017 campaign there was no evidence of modification of precipitation profiles due to mountain-induced circulations (Gonzalez et al. 2019, 2021).
A decoupling is frequently observed between the stalled air of the valley and the air of the free atmosphere above the mountain crest level, at around 1000-1500 m agl. Circulations in the first hundreds of meters above the surface are dependent on multi-scale interactions and can be described as a function of thermal and dynamical stability. A remarkable feature in the valley is that nocturnal strong temperature inversions with cold-air pools formation occur more than 50% of the nights mainly during winter (Conangla et al. 2018, Miró et al. 2018), which lead to very low minimum temperatures (-22.8 °C, 12th February 2018).
In Andorra central valley, terrain-induced circulations dominate the mountain boundary layer structure. Winter temperature inversions and cold pools formation are one of the key factors that determine the thermal stability conditions and limit the pollutant dispersion. Persistent temperature inversions are identified, and selected case studies are explored using pseudo-profiles of observations and mesoscale models.
The study and comprehension of the aforementioned phenomena in mountainous terrain are fundamental for improving their representation in models and to assess the model limitations in resolving them.
References
Conangla, L., et al. (2018). Cold‐air pool evolution in a wide Pyrenean valley. International Journal of Climatology, 38(6), 2852-2865.
Gonzalez, S., et al. (2019): Decoupling between precipitation processes and mountain wave induced circulations observed with a vertically pointing K-band Doppler radar. Remote Sens., 11,
Gonzalez, S., et al. (2021): Vertical structure and microphysical observations of winter precipitation in an inner valley during the Cerdanya-2017 field campaign. Atmos. Res., 264, 10586,
Miró, J. R., et al. (2018). Key features of cold‐air pool episodes in the northeast of the Iberian Peninsula (Cerdanya, eastern Pyrenees). International Journal of Climatology, 38(3), 1105-1115.
Udina, M., et al. (2020): Multi-sensor observations of an elevated rotor during a mountain wave event in the Eastern Pyrenees. Atmos. Res., 234, 104698
How to cite: Udina, M., Trapero, L., Bech, J., González, S., Paci, A., Beaufils, S., and Sarasua, M.: Untangling mountain boundary layer processes in the eastern Pyrenees: the case of the Cerdanya Basin and Andorra central valley, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-675, https://doi.org/10.5194/ems2022-675, 2022.
Along-valley winds in four major valleys in the southern slope of Nepal Himalayas are studied by means of a 5-day long high-resolution Weather Research and Forecasting model simulation. Model evaluation against observations from three automatic weather stations in the Khumbu valley showed a good agreement in the simulated diurnal cycle of wind direction and strength. The characteristics of the daytime up-valley winds are identified in the four valleys and compared to each other. Since all four valleys are under similar large-scale forcing, the differences in the along-valley winds are assumed to be mainly due to differences in the valley topographies. These valleys are separated by their topographic characteristics in two groups: the valleys in the west have a continuous inclination (2–5 degrees) in the valley floor and there is an 1-km high perpendicular mountain barrier between the valleys and the plain. The two valleys in the east have a 40 km portion with a nearly flat valley floor (<1 degree inclination) from the open valley entrance into the valley.
Daytime up-valley winds develop in all of the four valleys and they vary between the valleys and their parts in strength (2–10 m/s) and flow depth (600–1500 m). The night-time along-valley winds are weak and flow mostly in the up-valley direction. During large-scale northerlies, the daily cycle of the along-valley winds is interrupted more compared to the days with large-scale westerlies especially in the heads of the valleys that reach up to 4000 masl. The night-time down-valley winds are found more during the large-scale northerlies, which is most likely due to channelling of above-valley winds into the valley atmosphere.
The daytime up-valley winds are shallower and weaker in the parts of the valleys where the floor inclination exceeds 2 degrees compared to the parts where the valley floor is almost flat. The depth of the surface-based heated layer within the valleys is correlated with the flow depth and is lower in the steeply inclined parts of the valleys. The steep inclination of the valley floor and ridges in along-valley direction may shift the dominant driving mechanism of the along-valley winds from the valley volume effect to the buoyancy mechanism which would explain the shallower and weaker along-valley winds.
The winds at the valley entrances are weaker in the two valleys with the 1-km high barrier between the valleys and the plain, compared to the valleys with open valley entrances. A shallow layer with strong along-valley winds is found on the lee-side slope of the barrier and 20 km after this (i.e. towards the head of the valley), weaker winds are evident. This spatial distribution of the along-valley wind speed resembles the typical structure of a hydraulic jump related to down-slope windstorms.
How to cite: Mikkola, J., Sinclair, V., Bister, M., and Bianchi, F.: Along-valley winds in the Himalayas as simulated by the WRF-model, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-20, https://doi.org/10.5194/ems2022-20, 2022.
When anticyclonic conditions persist over mountainous regions, inversion layers can develop in the valleys and persist from a few days to a few weeks, especially in winter. During inversion episodes, the atmosphere inside the valley is stable and vertical mixing is prevented, promoting the accumulation of pollutants below the inversion layer and affecting the air quality of the valley.
Mountainous areas are experiencing a warming rate twice stronger than the global atmospheric temperature, thus, they are particularly sensitive to climate change. In the 21st century, the valley circulation will undergo unknown modifications due to climate change, and this concerns also the fate of wintertime persistent inversions, which could be reinforced or weakened with consequences on pollutant trapping.
This work addresses the issue of climate change impact on persistent inversions in the Grenoble valley, which is the most populated city in the Alps. The long-term projections of the regional climate model MAR (Modéle Atmosphérique Régional) forced by the global climate model MPI (developed by the Max Planck Institute) are first used to perform a statistical study of the inversions over the 21st century. The main characteristics of the inversions, e.g. duration, frequency, intensity, and their trends, are investigated for two different scenarios (SSP2-4.5 and SSP5-8.5). The intensity and the frequency of the inversions show a statistically significant decreasing trend in the 21st century for the worst-case scenario.
The detailed structure of the inversion (atmospheric circulation, vertical temperature profiles, and inversion top) is next investigated by comparing two persistent episodes in the past and around 2050. For this purpose, the WRF (Weather Research and Forecasting) model, forced by MAR, is used at high resolution (111 m). In order to perform a fair comparison of the episodes, and given the influence of the orography on the valley circulation, the episodes are selected in such a way to satisfy common large-scale characteristics of a wintertime anticyclonic regime over Grenoble.
How to cite: Bacer, S., Beaumet, J., Le Bouëdec, E., Ménégoz, M., Gallée, H., and Staquet, C.: Impact of climate change on wintertime persistent inversions in the Grenoble valley during the 21st century, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-358, https://doi.org/10.5194/ems2022-358, 2022.
Up-slope and down-slope winds are basic components of a variety of flows composing the system of thermally driven circulations over complex terrain. They commonly develop over inclines under daytime heating and nighttime cooling, respectively. The basic mechanisms of these flows are nowadays quite understood, after many research efforts performed in the last decades, including both intensive field measurements and high-resolution numerical model simulations. However, differently from the case of flat horizontal terrain, where Monin-Obukhov similarity theory provides a well-established conceptual framework, for the above winds the scientific community has not yet reached a consensus on unifying theories explaining the properties and structure of turbulence associated with them, e.g. in terms of generally applicable scaling laws. The latter would be beneficial for providing appropriate parameterizations in numerical models for weather and climate prediction, as well as for simulations of other atmospheric processes, such as pollutant transport, or for the evaluation of surface energy and water budgets for hydrological, glaciological, and ecological purposes.
Here we concentrate on the evaluation of the mixing properties of the surface layer associated with slope winds. In this layer strong slope-normal gradients are known to occur, and K-closures are expected to be reasonable closures for turbulent fluxes of momentum, heat and mass, provided appropriate expressions are found for eddy viscosity and eddy diffusivities. In the present contribution some solutions for K coefficients are proposed, based on the analysis of data from various field campaigns and similarity arguments. The resulting formulations are tested on the basis of their success in reproducing observed structures of first and second order moments.
How to cite: Farina, S. and Zardi, D.: On the definition of appropriate eddy viscosity and eddy diffusivities for the surface layer of thermally driven slope winds , EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-689, https://doi.org/10.5194/ems2022-689, 2022.
An accurate representation of atmospheric boundary layer (ABL) processes is critical for trace gas simulations and inverse modelling on the regional scale. This is particularly challenging over complex terrain such as the Swiss Midlands. Thus, it is not surprising that current operational numerical weather prediction models still provided limited accuracy for the simulation of vertical mixing processes over such regions. Vertical mixing in the ABL induced by local circulations directly impacts the vertical profiles of trace gases emitted at the surface. A bias in these processes may introduce significant errors in the estimate of trace gas concentrations. For example, the accumulation of greenhouse gases (GHG) during nighttime stable boundary layers (SBLs) is significantly underestimated compared to GHG observation sites such as Payerne or tall tower site Beromünster. Therefore, in the present work, high-resolution simulations using Cloud Model 1 (CM1) with idealized topography, representative of the Swiss Midlands, are performed, with the aim to contribute to an improved understanding of the relevant storage and transport processes, e.g. accumulation of passive tracer in nocturnal cold pools, separation of turbulence and local circulation effects, morning depletion and export of tracers to higher levels, entrainment of free-tropospheric air into the boundary layer, and their sensitivity to different environmental conditions, such as atmospheric stratification, upper-level winds, surface forcings and to the properties of underlying topography. The model setup also includes virtual towers located at valley floor, over the slope and the mountain ridge. First results will be presented, and the current state of the research will be discussed.
How to cite: Bašić, I., Schmidli, J., and Singh, S.: A preliminary study of storage and transport processes in an idealized valley, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-268, https://doi.org/10.5194/ems2022-268, 2022.
Previous studies have found a pronounced nocturnal low-level jet at the exit of the Inn Valley north of the valley contraction near Schwaigen which reaches into the Alpine foreland for several tenth of kilometers (e.g. Pamperin and Stilke, 1985 as part of the MERKUR experiment or a model study by Zängl, 2004). The exit jet forms under nocturnal stably stratified atmospheric conditions and is interpreted as a transition from subcritical to supercritical hydraulic flow.
As part of the pre-experiment of the TEAMx programme in June to August 2022, we will conduct measurements to corroborate and extend the previous findings on the formation and maintenance of the Inn Valley exit jet. For this, a wind lidar will be deployed north of the valley contraction. Further, during selected days, radiosondes will be launched at the site of the wind lidar, accompanied by drone measurements. Additionally, the operational network of the German Meteorological Service (DWD) will be supplemented by ground station measurements of wind, temperature and humidity along the valley and at one higher elevated station between Kufstein and the wind lidar site. At selected stations, pressure, radiation and precipitation measurements will be conducted additionally. These will run until the completion of the main TEAMx campaign in 2024/2025.
This contribution will show some preliminary results of the measurement campaign.
References:
Zängl, G. "A reexamination of the valley wind system in the Alpine Inn Valley with numerical simulations." Meteorology and Atmospheric Physics 87.4 (2004): 241-256.
Pamperin, H., and G. Stilke. "Nächtliche Grenzschicht und LLJ im Alpenvorland nahe dem Inntalausgang." Meteorologische Rundschau 38.5 (1985): 145-156.
How to cite: Sedlmeier, K., Kossmann, M., Paunovic, I., Bock, L., Nitsche, O., and Mühlbacher, G.: An observational study of the Inn Valley exit jet, EMS Annual Meeting 2022, Bonn, Germany, 5–9 Sep 2022, EMS2022-449, https://doi.org/10.5194/ems2022-449, 2022.
