Momentum flux, energy flux and pressure drag associated with mountain wave across western ghat

An attempt has been made to parameterize the wave momentum flux wave energy flux and pressure drag associated with mountain wave across the Mumbai-Pune section of western ghat mountain in India. A two dimensional frictionless, adiabatic, hydrostatic, Boussinesq flow with constant basic flow (U) and constant Brunt Vaisala frequency (N) across a mesoscale mountain with infinite extension in the Cross wind direction, has been considered here. It has been shown that for a vertically propagating (or decaying) waves the wave momentum flux is downward (or upward) and the wave energy flux is upward (or downward). It has also been shown that both the fluxes are independent of the half width of the bell shaped part of the western ghat. The analytically derived formula have been used to compute the pressure drag and to find out the vertical profile of wave momentum flux and wave energy flux for different cases of mountain wave across western ghat, as reported by earlier workers.

An analytical model for mountain wave in stratified atmosphere

An analytical, two-dimensional computer model has been developed for real time prediction of 'mountain wave due to Principal mountains over Kashmir valley. Simulation of the L2 profile has been made with realistic, non-zero values at higher levels and exponentially decreasing values at lower levels. Unlike Doos (1961), present solution has no restriction on the value of wave number (k). Validity of the model has been tested with the satellite observed waves in seven cases and actual aircraft report in one case.

Observation and simulation of mountain wave trains in a tropical cyclone situation

This paper documents the observations by radar of wave trains downstream of mountains in a tropical cyclone situation. The wind disturbances associated with the wave trains together with the background strong southeasterly flow result in the occurrence of low-level wind shear as detected by the radar. So the case is not just scientifically interesting, but it also has practical application value. The wave trains can be simulated by using a computational fluid dynamics model initialized homogeneously by the upper air ascent data at the time close to that the occurrence of the wave trains. This points to the potential of using such a model in simple setup to forecast the occurrence of low-level wind shear.

Stereophotogrammetry of clouds observed during T-REX

Stereophotogrammetric images collected during the Terrain-induced Rotor Experiment (T-REX), which took place in Owens Valley, California, in the spring of 2006, were used to track clouds and cloud fragments in space and time. We explore how photogrammetric data complements other instruments deployed during T-REX, and how it supports T-REX objectives to study the structure and dynamics of atmospheric lee waves and rotors. Algorithms for camera calibration, automatic feature matching, and 3D positioning of clouds were developed which enabled the study of cloud motion in highly turbulent mountain wave scenarios.The dynamic properties obtained with photogrammetric tools compare well with data collected by other T-REX instruments. In a mild mountain wave event, the whole life cycle of clouds moving through a lee wave crest was tracked in space and time showing upward and downward motion at the upstream and downstream side of the wave crest, respectively. During strong mountain wave events the steepening of the first lee wave as it developed into a hydraulic jump was tracked and quantified. Vertical cloud motion increased from ~2 m/s to 4 m/s and horizontal cloud motion decreased from 20 m/s to 16 m/s with the development of the hydraulic jump. Clouds at distinct vertical layers were tracked in other mountain wave events: moderate southerly flow was observed in the valley (~8 m/s), westerly motion of the same magnitude at the Sierra Nevada mountain crest level, and westerlies with speeds of over 20 m/s at even higher altitudes.

Mountain-wave-induced polar stratospheric clouds and their representation in the global chemistry model ICON-ART

Polar stratospheric clouds (PSCs) are a driver for ozone depletion in the lower polar stratosphere. They provide surface for heterogeneous reactions activating chlorine and bromine reservoir species during the polar night. The large-scale effects of PSCs are represented by means of parameterisations in current global chemistry–climate models, but one process is still a challenge: the representation of PSCs formed locally in conjunction with unresolved mountain waves. In this study, we investigate direct simulations of PSCs formed by mountain waves with the ICOsahedral Nonhydrostatic modelling framework (ICON) with its extension for Aerosols and Reactive Trace gases (ART) including local grid refinements (nesting) with two-way interaction. Here, the nesting is set up around the Antarctic Peninsula, which is a well-known hot spot for the generation of mountain waves in the Southern Hemisphere. We compare our model results with satellite measurements of PSCs from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and gravity wave observations of the Atmospheric Infrared Sounder (AIRS). For a mountain wave event from 19 to 29 July 2008 we find similar structures of PSCs as well as a fairly realistic development of the mountain wave between the satellite data and the ICON-ART simulations in the Antarctic Peninsula nest. We compare a global simulation without nesting with the nested configuration to show the benefits of adding the nesting. Although the mountain waves cannot be resolved explicitly at the global resolution used (about 160 km), their effect from the nested regions (about 80 and 40 km) on the global domain is represented. Thus, we show in this study that the ICON-ART model has the potential to bridge the gap between directly resolved mountain-wave-induced PSCs and their representation and effect on chemistry at coarse global resolutions.

Observations of an elevated rotor and precipitation processes decoupled during a mountain wave event in the Eastern Pyrenees (Cerdanya-2017 Field Experiment)

<p>The study documents the formation of a rotor underneath the mountain waves generated the 15 January 2017 over the eastern Pyrenees (near the border between France, Spain and Andorra) during the Cerdanya-2017 field campaign. The event was characterized by strong winds, mountain waves and relevant snow accumulation over the Cerdanya valley and the eastern Pyrenees. The evolution and location of the mountain waves and precipitation structure were studied using high temporal resolution data from a UHF wind-profiler and a vertically pointing K-band Doppler radar, separated a few kilometres in horizontal distance.</p><p>A mountain wave was detected in the morning and shortened slightly in the afternoon when a transient rotor was formed disconnected from the surface flow (Udina et al. 2020). A strong turbulence zone was identified at the upper edge of the mountain wave, above the rotor, a feature observed in previous studies. The mountain wave and rotor induced circulation was favoured by the valley shape and the second mountain ridge location, in addition to the weak and variable winds, established during the sunset close to the valley surface. In addition, we find decoupling between precipitation processes and mountain wave induced circulations. During the studied event, mountain wave wind circulations and low-level turbulence do not affect neither the snow crystal riming or aggregation along the vertical column nor the surface particle size distribution of the snow. This study illustrates that precipitation profiles and mountain induced circulations may be decoupled which can be very relevant for either ground-based or spaceborne remote sensing of precipitation (Gonzalez et al 2019). This research is supported by CGL2015-65627-C3-1-R, CGL2015- 65627-C3-2-R (MINECO/FEDER), CGL2016-81828-REDT and RTI2018- 098693-B-C32 (AEI/FEDER).</p><p>References:</p><p>Gonzalez, S., Bech, J., Udina, M., Codina, B., Paci, A., & Trapero, L. (2019). Decoupling between precipitation processes and mountain wave induced circulations observed with a vertically pointing K-band doppler radar. <em>Remote Sensing</em>, <em>11</em>(9), 1034.</p><p>Udina, M., Bech, J., Gonzalez, S., Soler, M. R., Paci, A., Miró, J. R., Trapero, L., Donier, J.M., Douffet, T., Codina, B., Pineda, N. (2020). Multi-sensor observations of an elevated rotor during a mountain wave event in the Eastern Pyrenees. <em>Atmospheric Research</em>, <em>234</em>, 104698.</p>

Meso- to micro-scale modeling of atmospheric stability effects on wind turbine wake behavior in complex terrain

Most detailed modeling and simulation studies of wind turbine wakes have considered flat terrain scenarios. Wind turbines, however, are commonly sited in mountainous or hilly terrain to take advantage of accelerating flow over ridgelines. In addition to topographic acceleration, other turbulent flow phenomena commonly occur in complex terrain, and often depend upon the thermal stratification of the atmospheric boundary layer. Enhanced understanding of wind turbine wake interaction with these terrain-induced flow phenomena can significantly improve wind farm siting, optimization, and control. In this study, we simulate conditions observed during the Perdigão field campaign in 2017, consisting of flow over two parallel ridges with a wind turbine located on top of one of the ridges. We use the Weather Research and Forecasting model (WRF) nested down to micro-scale large-eddy simulation (LES) at 10 m resolution, with a generalized actuator disk (GAD) wind turbine parameterization to simulate turbine wakes. Two case studies are selected, a stable case where a mountain wave occurs and a convective case where a recirculation zone forms in the lee of the ridge with the turbine. The WRF-LES-GAD model is validated against data from meteorological towers, soundings, and a tethered lifting system, showing good agreement for both cases. Comparisons with scanning Doppler lidar data for the stable case show that the overall characteristics of the mountain wave are well-captured, although the wind speed is underestimated. For the convective case, the size of the recirculation zone within the valley shows good agreement. The wind turbine wake behavior shows dependence on atmospheric stability, with different amounts of vertical deflection from the terrain and persistence downstream for the stable and convective conditions. For the stable case, the wake follows the terrain along with the mountain wave and deflects downwards by nearly 100 m below hub-height at four rotor diameters downstream. For the convective case, the wake deflects above the recirculation zone over 50 m above hub-height at the same downstream distance. This study demonstrates the ability of the WRF-LES-GAD model to capture the expected behavior of wind turbine wakes in regions of complex terrain, and thereby to potentially improve wind turbine siting and operation in hilly landscapes.