The correct simulation of the atmospheric boundary layer (ABL) is crucial for reliable weather forecasts in truly complex terrain. However, common assumptions for model parametrizations are only ...valid for horizontally homogeneous and flat terrain. Here, we evaluate the turbulence parametrization of the numerical weather prediction model COSMO with a horizontal grid spacing of
Δ
x
=
1.1
km
for the Inn Valley, Austria. The long-term, high-resolution turbulence measurements of the i-Box measurement sites provide a useful data pool of the ABL structure in the valley and on slopes. We focus on days and nights when ABL processes dominate and a thermally-driven circulation is present. Simulations are performed for case studies with both a one-dimensional turbulence parametrization, which only considers the vertical turbulent exchange, and a hybrid turbulence parametrization, also including horizontal shear production and advection in the budget of turbulence kinetic energy (TKE). We find a general underestimation of TKE by the model with the one-dimensional turbulence parametrization. In the simulations with the hybrid turbulence parametrization, the modelled TKE has a more realistic structure, especially in situations when the TKE production is dominated by shear related to the afternoon up-valley flow, and during nights, when a stable ABL is present. The model performance also improves for stations on the slopes. An estimation of the horizontal shear production from the observation network suggests that three-dimensional effects are a relevant part of TKE production in the valley.
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.
The exchange of heat, momentum, and mass in the atmosphere over mountainous terrain is controlled by synoptic-scale dynamics, thermally driven mesoscale circulations, and turbulence. This article ...reviews the key challenges relevant to the understanding of exchange processes in the mountain boundary layer and outlines possible research priorities for the future. The review describes the limitations of the experimental study of turbulent exchange over complex terrain, the impact of slope and valley breezes on the structure of the convective boundary layer, and the role of intermittent mixing and wave–turbulence interaction in the stable boundary layer. The interplay between exchange processes at different spatial scales is discussed in depth, emphasizing the role of elevated and ground-based stable layers in controlling multi-scale interactions in the atmosphere over and near mountains. Implications of the current understanding of exchange processes over mountains towards the improvement of numerical weather prediction and climate models are discussed, considering in particular the representation of surface boundary conditions, the parameterization of sub-grid-scale exchange, and the development of stochastic perturbation schemes.
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.
The role of the atmospheric boundary layer (ABL) in the atmosphere-climate system is the exchange of heat, mass and momentum between ‘the earth’s surface’ and the atmosphere. Traditionally, it is ...understood that turbulent transport is responsible for this exchange and hence the understanding and physical description of the turbulence structure of the boundary layer is key to assess the effectiveness of earth-atmosphere exchange. This understanding is rooted in the (implicit) assumption of a scale separation or spectral gap between turbulence and mean atmospheric motions, which in turn leads to the assumption of a horizontally homogeneous and flat (HHF) surface as a reference, for which both physical understanding and model parameterizations have successfully been developed over the years. Over mountainous terrain, however, the ABL is generically inhomogeneous due to both thermal (radiative) and dynamic forcing. This inhomogeneity leads to meso-scale and even sub-meso-scale flows such as slope and valley winds or wake effects. It is argued here that these (sub)meso-scale motions can significantly contribute to the vertical structure of the boundary layer and hence vertical exchange of heat and mass between the surface and the atmosphere. If model grid resolution is not high enough the latter will have to be parameterized (in a similar fashion as gravity wave drag parameterizations take into account the momentum transport due to gravity waves in large-scale models). In this contribution we summarize the available evidence of the contribution of (sub)meso-scale motions to vertical exchange in mountainous terrain from observational and numerical modeling studies. In particular, a number of recent simulation studies using idealized topography will be summarized and put into perspective – so as to identify possible limitations and areas of necessary future research.
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.
Carbon dioxide exchange in an idealized valley Reif, Matthias; Rotach, Mathias W.; Gohm, Alexander ...
Environmental modelling & software : with environment data news,
01/2024, Volume:
171
Journal Article
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