Georg J. Mayr

The Alpine Mountain–Plain Circulation: Airborne Doppler Lidar Measurements and Numerical Simulations

On summer days radiative heating of the Alps produces rising air above the mountains and a resulting inflow of air from the foreland. This leads to a horizontal transport of air from the foreland to the Alps, and a vertical transport from the boundary layer into the free troposphere above the mountains. The structure and the transports of this mountain–plain circulation in southern Germany (“Alpine pumping”) were investigated using an airborne 2-m scanning Doppler lidar, a wind-temperature radar, dropsondes, rawinsondes, and numerical models. The measurements were part of the Vertical Transport and Orography (VERTIKATOR) campaign in summer 2002. Comparisons of dropsonde and lidar data proved that the lidar is capable of measuring the wind direction and wind speed of this weak flow toward the Alps (1–4 ms 1 ). The flow was up to 1500 m deep, and it extended 80 km into the Alpine foreland. Lidar data are volume measurements (horizontal resolution 5 km, vertical resolution 100 m). Therefore, they are ideal for the investigation of the flow structure and the comparison to numerical models. Even the vertical velocities measured by the lidar agreed with the mass budget calculations in terms of both sign and magnitude. The numerical simulations with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) (mesh size 2 and 6 km) and the Local Model (LM) of the German Weather Service (mesh size 2.8 and 7 km) reproduced the general flow structure and the mass fluxes toward the Alps within 86%–144% of the observations.

Strongly Nonlinear Flow over and around a Three-Dimensional Mountain as a Function of the Horizontal Aspect Ratio

Abstract The influence of the obstacle shape, expressed through the ratio of spanwise to streamwise extension β, on flow over and around a mesoscale mountain is examined numerically. The initial wind U as well as the buoyancy frequency N are constant; the earth’s rotation and surface friction are neglected. In these conditions the flow response depends primarily on the nondimensional mountain height Hm = hmN/U (where hm is the maximum mountain height) and the horizontal aspect ratio β. A regime diagram for the onset of wave breaking, lee vortex formation, and windward stagnation is compiled. When β is increased, smaller Hm are required for the occurrence of all three features. It is demonstrated that lee vortices can form with neither wave breaking above the lee slope nor upstream stagnation. For β ⩽ 0.5 a vortex pair can appear although the isentropes above the lee slope do not overturn for any Hm. For β > 1, on the other hand, lee vortex formation is triggered by wave breaking. On the windward side two ...

South Foehn in the Wipp Valley on 24 October 1999 (MAP IOP 10): Verification of High-Resolution Numerical Simulations with Observations

Abstract A case study of a south foehn windstorm observed across the Brenner Pass in the Wipp Valley near the Austrian–Italian border is presented based on a detailed comparison and verification of high-resolution numerical simulations with observations. The event of 24 through 25 October 1999 was part of the Intensive Observing Period 10 of the Mesoscale Alpine Programme (MAP). The simulations were performed with the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The observations were collected with a ground-based scanning Doppler lidar, an airborne aerosol backscatter lidar, a Doppler sodar, several weather stations, and two radiosounding systems. The study provides a synoptic-scale and mesoscale overview of the event and focuses on a comparison of simulated and observed fields for a 9-h period on 24 October 1999. The quantitative agreement between the numerical results and the observations is discussed in terms of root-mean-square error (rmse) and mean error (ME). Rmse value...

Coplanar Doppler Lidar Retrieval of Rotors from T-REX

Abstract Dual-Doppler analysis of data from two coherent lidars during the Terrain-Induced Rotor Experiment (T-REX) allows the retrieval of flow structures, such as vortices, during mountain-wave events. The spatial and temporal resolution of this approach is sufficient to identify and track vortical motions on an elevated, cross-barrier plane in clear air. Assimilation routines or additional constraints such as two-dimensional continuity are not required. A relatively simple and quick least squares method forms the basis of the retrieval. Vortices are shown to evolve and advect in the flow field, allowing analysis of their behavior in the mountain–wave–boundary layer system. The locations, magnitudes, and evolution of the vortices can be studied through calculated fields of velocity, vorticity, streamlines, and swirl. Generally, observations suggest two classes of vortical motions: rotors and small-scale vortical structures. These two structures differ in scale and behavior. The level of coordination of ...

Heteroscedastic Extended Logistic Regression for Postprocessing of Ensemble Guidance

AbstractTo achieve well-calibrated probabilistic forecasts, ensemble forecasts are often statistically postprocessed. One recent ensemble-calibration method is extended logistic regression, which extends the popular logistic regression to yield full probability distribution forecasts. Although the purpose of this method is to postprocess ensemble forecasts, usually only the ensemble mean is used as the predictor variable, whereas the ensemble spread is neglected because it does not improve the forecasts. In this study it is shown that when simply used as an ordinary predictor variable in extended logistic regression, the ensemble spread affects the location but not the variance of the predictive distribution. Uncertainty information contained in the ensemble spread is therefore not utilized appropriately. To solve this drawback a new approach is proposed where the ensemble spread is directly used to predict the dispersion of the predictive distribution. With wind speed data and ensemble forecasts from the...

Wind Speeds at Heights Crucial for Wind Energy: Measurements and Verification of Forecasts

AbstractWind speed measurements from one year from meteorological towers and wind turbines at heights between 20 and 250 m for various European sites are analyzed and are compared with operational short-term forecasts of the global ECMWF model. The measurement sites encompass a variety of terrain: offshore, coastal, flat, hilly, and mountainous regions, with low and high vegetation and also urban influences. The strongly differing site characteristics modulate the relative contribution of synoptic-scale and smaller-scale forcing to local wind conditions and thus the performance of the NWP model. The goal of this study was to determine the best-verifying model wind among various standard wind outputs and interpolation methods as well as to reveal its skill relative to the different site characteristics. Highest skill is reached by wind from a neighboring model level, as well as by linearly interpolated wind from neighboring model levels, whereas the frequently applied 10-m wind logarithmically extrapolated...

Three-Dimensional Wind Retrieval: Application of MUSCAT to Dual-Doppler Lidar

Abstract During the field campaign of the Terrain-induced Rotor Experiment (T-REX) in the spring of 2006, Doppler lidar measurements were taken in the complex terrain of the Californian Owens Valley for six weeks. While fast three-dimensional (3D) wind analysis from measured radial wind components is well established for dual weather radars, only the feasibility was shown for dual-Doppler lidars. A computationally inexpensive, variational analysis method developed for multiple-Doppler radar measurements over complex terrain was applied. The general flow pattern of the 19 derived 3D wind fields is slightly smoothed in time and space because of lidar scan duration and analysis algorithm. The comparison of extracted wind profiles to profiles from radiosondes and wind profiler reveals differences of wind speed and direction of less than 1.1 m s−1 and 3°, on average, with standard deviations not exceeding 2.7 m s−1 and 27°, respectively. Standard velocity–azimuth display (VAD) retrieval method provided higher ...

Automatic and Probabilistic Foehn Diagnosis with a Statistical Mixture Model

Diagnosing foehn winds from weather station data downwind of topographic obstacles requires distinguishing them from other downslope winds, particularly nocturnal ones driven by radiative cooling. We present an automatic classification scheme to obtain reproducible results that include information about the (un)certainty of the diagnosis. A statistical mixture model separates foehn and no-foehn winds in a measured time series of wind. In addition to wind speed and direction, it accommodates other physically meaningful classifiers such as relative humidity or the (potential) temperature difference to an upwind station (e.g., near the crest). The algorithm was tested for the central Alpine Wipp Valley against human expert classification and a previous objective method (Drechsel and Mayr 2008), which the new method outperforms. Climatologically, using only wind information gives nearly identical foehn frequencies as when using additional covariables, making the method suitable for comparable foehn climatologies all over the world where station data are available for at least one year.

Dynamically-Driven Winds

This chapter is concerned with dynamically-forced atmospheric flow phenomena which occur when the wind encounters mountains. The range of effects is wide and therefore attention is restricted to arguably the most important phenomena in terms of weather forecasting. These are mountain waves, rotors, downslope windstorms, gap winds and barrier jets. The essence of many of these phenomena is described by mountain wave theory. Recent advances in observation technologies and their application in field programs, as well as in numerical modeling, have led to new understanding, including the incorporation of complicating factors like boundary-layer processes. This chapter describes current theory for each of these phenomena, along with recent observational studies and the latest forecast techniques and models.

Observations of the Temporal Evolution and Spatial Structure of the Gap Flow in the Wipp Valley on 2 and 3 October 1999

Abstract The investigation of gap flow in the Wipp Valley (GAP project) is one of the objectives of the Mesoscale Alpine Programme (MAP). The valley runs south–north across the Brenner Pass, from Italy to Austria. The pass is the lowest one of the main Alpine ridge and is therefore a favorable location for a gap flow, which is called foehn. Based on the extensive dataset of MAP, this study gives a detailed analysis of foehn on 2 and 3 October 1999 [intensive observation period 5 (IOP 5)]. The foehn event began as a gap flow that was separated from midlevel winds by a strong temperature inversion during the night of 1–2 October 1999. On the next night (2– 3 October) the inversion dissipated, and the gap flow was combined with strong midlevel cross-ridge flow on 3 October 1999. This study shows that the existence of a temperature inversion above the gap flow has a crucial impact on the flow structure. Another emphasis of the study was the investigation of the small-scale flow structure downstream of the gap...