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Lukas Umek
and
Alexander Gohm

Abstract

This is one of the first case studies of a snowstorm at Lake Constance, located between Austria, Germany, and Switzerland, which assesses the influence of the lake and the orography on the generation of heavy precipitation. The analysis is based on surface and radar observations and numerical simulations with the Weather Research and Forecasting (WRF) Model. On 8 February 2013, a rather stationary and banded radar reflectivity pattern was observed during postfrontal conditions with northwesterly flow. The associated snowband affected the downstream shore and the adjacent mountainous region with 36 mm of precipitation within 5 h at the shore. Surface observations show a convergence in the wind field over the lake during the period of banded precipitation. The control simulation captures the formation of a convergence line and a snowband near the shoreline and over the downstream orography. A lake-induced, low-level conditionally unstable layer is essential for the snowband formation. Orographically and thermally induced convergence provides the lifting to release conditional instability and to trigger convection. Orographic enhancement of precipitation occurs downstream of the lake. Sensitivity experiments with modified orography, land use, and lake surface temperature show that the lake is a crucial factor controlling the amount and distribution of snowfall. However, neither the lake nor the orography alone would have been able to form a snowband. This study highlights the complex interaction between lake and orographic effects and shows that Lake Constance is large enough to impact the formation of precipitation.

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Simon K. Siedersleben
and
Alexander Gohm

Abstract

On 1 February 2014, the southern side of the Alps was affected by a severe snowstorm that forced authorities to issue the highest level of avalanche danger in southern parts of Austria. The northern side of the Alps was mostly dry. Nevertheless, radar imagery captured the evolution of quasi-steady convective cloud bands over the northern Alpine foreland with a remarkable length of up to 300 km. This study illuminates the processes that generated these cloud bands based on numerical simulations. The storm was associated with a deep large-scale trough that caused strong southwesterly cross-Alpine flow, orographic precipitation on the southern side, and foehnlike subsidence on the northern side of the Alps. Orographic potential vorticity (PV) banners developed at small-scale topographic features embedded in the Alps and extended downstream over the northern Alpine foreland. Convective cloud bands were aligned parallel to these PV banners. They formed in an environment of inertial instability (negative absolute vorticity) and conditional instability. Sensitivity experiments reveal that the structure and size of these cloud bands are strongly sensitive to the small-scale terrain roughness. Removing small-scale topographic features suppresses the formation of orographic vorticity banners, which in turn suppresses the development of cloud bands. These results suggest that the release of inertial instability at negative orographic vorticity banners was crucial for establishing circulations and associated uplift that triggered conditional instability. To summarize, inertial instability was most likely responsible for the banded structure and conditional instability for the convective nature of these cloud bands.

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Daniel Leukauf
,
Alexander Gohm
, and
Mathias W. Rotach

Abstract

The convective export of heat from different types of idealized valleys for fair-weather daytime conditions is studied with the Weather Research and Forecasting (WRF) Model. The goal is to test the hypothesis that the total export of heat over the course of the day depends on a so-called breakup parameter B. The breakup parameter is the ratio between the energy required to neutralize the initially stably stratified valley atmosphere and the total energy provided by the surface sensible heat flux. To achieve this goal, simulations with different surface heating, initial stability, and terrain geometry are performed. The fraction of the sensible heat provided at the surface that is exported at crest height over the course of the day depends exponentially on B. The effects of variations of the valley width, crest height, forcing amplitude, and initial stratification on the total export of heat can be described by this function. The complete neutralization of the stratification in the valley is never reached if B exceeds a critical value of about 0.65 for an initially constant stratification. For a valley geometry with linear slopes and sharp crests, up to 60% of the provided heat is exported for the strongest forcing and the weakest stability (i.e., B ≈ 0.1), whereas less than 5% is exported for B > 0.65. The minimum heat export for larger B is higher for rounded crests (10%) and for a deep residual layer that extends to above crest height (17%).

Open access
Georg J. Mayr
,
Johannes Vergeiner
, and
Alexander Gohm

Abstract

An instrument package to measure temperature, pressure, humidity, and position was designed to be quickly deployable on any automobile to be used for the study of gap and other orographically influenced flows. Differential GPS (global positioning system) measurements together with a distance counter gave the submeter accuracy of vertical position that was needed for observation of changes in the horizontal pressure field, which is an integral measure of the flow field aloft. A slantwise pressure reduction method was tailored for this application and verified with data from radio soundings. The automobile platform was successfully used during the field phase of the Mesoscale Alpine Programme (MAP) to classify flow states and observe hydraulic jumps in gap flows and to extend aircraft measurements to the ground.

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Alexander Gohm
,
Günther Zängl
, and
Georg J. Mayr

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 values are high during the early stage of the event (∼7 m s−1), have a transient peak for about 1 h at 1400 UTC, and are minimal at the fully developed foehn stage near 1500 UTC (∼5 m s−1). The discrepancies at the beginning are likely to be related to deficiencies in the model profile on the upstream side of the pass, exhibiting a too low inversion and a too shallow southerly flow. The transient error peak at 1400 UTC is related to a mismatch in the timing of the enhancement of the upper-level winds. Moreover, evidence is found for an overestimation of the mass flux through the lower Brenner gap, which is the narrowest and deepest part of the incision in the main Alpine crest, and a subsequent underestimation of the flow descent into the Wipp Valley on the leeward side of the Brenner Pass. Considering mass continuity, the latter effect is probably a result of the former. Nevertheless, the model captures most of the striking foehn features: Simulated isentropes and aerosol backscatter measurements consistently indicate regions of flow descent, across-valley asymmetries, and hydraulic jump–like features. The across-valley asymmetry of the foehn strength near the Wipp Valley exit is particularly well reproduced by the model. The primary reason for the stronger winds on the eastern sidewall is the asymmetry in the position of the mountain ridges protruding into the valley together with the westward bending of the valley axis.

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Daniel Leukauf
,
Alexander Gohm
,
Mathias W. Rotach
, and
Johannes S. Wagner

Abstract

The breakup of a nocturnal temperature inversion during daytime is studied in an idealized valley by means of high-resolution numerical simulations. Vertical fluxes of heat and mass are strongly reduced as long as an inversion is present; hence it is important to understand the mechanisms leading to its removal. In this study breakup times are determined as a function of the radiative forcing. Further, the effect of the nocturnal inversion on the vertical exchange of heat and mass is quantified. The Weather Research and Forecasting Model is applied to an idealized quasi-two-dimensional valley. The net shortwave radiation is specified by a sine function with amplitudes between 150 and 850 W m−2 during daytime and at zero during the night. The valley inversion is eroded within 5 h for the strongest forcing. A minimal amplitude of 450 W m−2 is required to reach the breakup, in which case the inversion is removed after 11 h. Depending on the forcing amplitude, between 10% and 57% of the energy provided by the surface sensible heat flux is exported out of the valley during the whole day. The ratio of exported energy to provided energy is approximately 1.6 times as large after the inversion is removed as before. More than 5 times the valley air mass is turned over in 12 h for the strongest forcing, whereas the mass is turned over only 1.3 times for 400 W m−2.

Open access
Johannes S. Wagner
,
Alexander Gohm
, and
Mathias W. Rotach

Abstract

The role of horizontal model grid resolution on the development of the daytime boundary layer over mountainous terrain is studied. A simple idealized valley topography with a cross-valley width of 20 km, a valley depth of 1.5 km, and a constant surface heat flux forcing is used to generate upslope flows in a warming valley boundary layer. The goal of this study is to investigate differences in the boundary layer structure of the valley when its topography is either fully resolved, smoothed, or not resolved by the numerical model. This is done by performing both large-eddy (LES) and kilometer-scale simulations with horizontal mesh sizes of 50, 1000, 2000, 4000, 5000, and 10 000 m. In LES mode a valley inversion layer develops, which separates two vertically stacked circulation cells in an upper and lower boundary layer. These structures weaken with decreasing horizontal model grid resolution and change to a convective boundary layer over an elevated plain when the valley is no longer resolved. Mean profiles of the LES run, which are obtained by horizontal averaging over the valley show a three-layer thermal structure and a secondary heat flux maximum at ridge height. Strong smoothing of the valley topography prevents the development of a valley inversion layer with stacked circulation cells and leads to higher valley temperatures due to smaller valley volumes. Additional LES and “1 km” runs over corresponding smoothed valleys reveal that differences occur mainly because of unresolved topography and not because of unresolved turbulence processes. Furthermore, the deactivation of horizontal diffusion improved simulations with 1- and 2-km horizontal resolution.

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Brigitta Goger
,
Mathias W. Rotach
,
Alexander Gohm
,
Ivana Stiperski
,
Oliver Fuhrer
, and
Guy de Morsier

Abstract

The correct simulation of the atmospheric boundary layer (ABL) in highly complex terrain is a challenge for mesoscale numerical weather prediction models. An improvement in model performance is possible if horizontal contributions to turbulence kinetic energy (TKE) production, such as horizontal shear production, are implemented in the model’s turbulence parameterization. However, 3D turbulence parameterizations often only have a constant horizontal length scale that depends on the horizontal grid spacing. This is unphysical for mesoscale applications, because such parameterizations were initially developed for much smaller model grid spacings (e.g., for large-eddy simulations). In this study, we develop a new physically based horizontal length scale for the high-resolution mesoscale model COSMO. We analyze days dominated by thermally driven circulations (valley wind days) in the Inn Valley, Austria. Results show that the new horizontal length scale improves TKE simulations in the valley, when horizontal shear processes contribute to the overall TKE budget. Vertical profiles of TKE and transects across the valley indicate that the model simulates the ABL in a more realistic way than standard turbulence schemes, because the new scheme is able to account for terrain inhomogeneities. A model validation with 88 stations in Austria for four case study days indicates no change in the mean surface fields of temperature, relative humidity, and wind speed by the new turbulence parameterization.

Open access
Mathias W. Rotach
,
Georg Wohlfahrt
,
Armin Hansel
,
Matthias Reif
,
Johannes Wagner
, and
Alexander Gohm

Among the processes contributing to the global CO2 budget, net uptake by the land surface bears the largest uncertainty. Therefore, the land sink is often estimated as the residual from the other terms that are known with greater certainty. On average over the last decades, the difference between modeled land surface uptake and this residual is negative, thus suggesting that the different modeling approaches miss an important part in land–atmosphere exchange. Based on experience with atmospheric modeling at high resolution, it is argued that this discrepancy is likely due to missed mesoscale (thermally or dynamically forced) circulations in complex terrain. Noting that more than 50% of the land surface qualifies as complex terrain, the contribution of mesoscale circulations is hypothesized to alleviate at least partly the uncertainty in the modeled land surface uptake. While atmospheric models at coarse resolution (e.g., for numerical weather prediction; also climate simulations) use a parameterization to account for momentum exchange due to subgrid-scale topography, no such additional exchange is presently taken into account for energy or mass. It is thus suggested that a corresponding parameterization should be developed in order to reduce the uncertainties in the global carbon budget.

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Thomas Karl
,
Alexander Gohm
,
Mathias W. Rotach
,
Helen C. Ward
,
Martin Graus
,
Alexander Cede
,
Georg Wohlfahrt
,
Albin Hammerle
,
Maren Haid
,
Martin Tiefengraber
,
Christian Lamprecht
,
Johannes Vergeiner
,
Axel Kreuter
,
Jochen Wagner
, and
Michael Staudinger
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