Quantifying exchange processes over mountainous terrain

Weather and climate in mountainous regions are strongly influenced by thermally induced wind systems. These local winds control the exchange of heat, moisture, momentum and pollutants between the valley and the free atmosphere. As a result of valley and slope winds the magnitude of the net vertical flux of various physical quantities can be several times the net flux over flat terrain. With the relatively coarse mesh size of current numerical weather prediction and climate models, valleys and corresponding local wind systems are not adequately represented and, hence, exchange processes are quantitatively underestimated. Surface-layer parameterizations currently used in such models have been developed for flat terrain. Typically, they are based on some type of Monin-Obukhov similarity theory that only accounts for turbulent fluxes between the flat surface and the atmospheric near-surface layer. Since the spatial resolution of operational forecast models will not increase in the next years to a size that would enable explicit representation of vertical fluxes due to local circulations, a parameterization for surface-atmosphere exchange processes over complex terrain is urgently needed. In this project we will quantify the vertical heat and moisture flux from the valley to the free atmosphere by thermally induced circulations in an idealized but systematic way. The goal is to determine the sensitivity of these net fluxes to changes in various surface and atmospheric properties, including terrain geometry, atmospheric background state, land use type, soil moisture, and radiative forcing. For this purpose a comprehensive series of large-eddy simulations (LES) will be conducted by varying these properties over a range of observed values. The net flux will be computed by integrating the resolved flux over a horizontal area that defines the upper boundary of the valley atmosphere. The idealized three-dimensional valley topography used in the simulations will be based on an analytical function that allows for various geometries including curved and narrowing valleys. By continuously decreasing the model resolution to the one of typical weather prediction and climate models and by comparing the result of the coarse model run to the LES result, the model error due to unresolved fluxes will be quantified. Moreover, the simulations will be analyzed in a way to improve the understanding of mechanisms of thermally induced circulations. In a last step, the attempt will be made to develop a parameterization for unresolved fluxes due to valley and slope winds as a function of surface and atmospheric properties.

Local winds in mountainous regions, such as slope and valley winds, strongly control the exchange of physical properties (e.g., heat and air pollutants) between the valley and the free atmosphere. This exchange can be several times larger than over flat terrain. With the relatively coarse mesh size of current global numerical weather prediction and climate models, valleys and corresponding local wind systems are not adequately represented and, hence, exchange processes are quantitatively underestimated. In order to minimize corresponding forecast errors, unresolved exchange processes need to be parameterized by physical quantities which are resolved by coarse-resolution models. Results gained in this research project represent a first step towards the development of such a parameterization.They are based on a multitude of computer simulations with the Weather Research and Forecasting (WRF) model at a very high resolution for various idealized valley geometries. These simulations were performed in a systematic way for a broad range of influencing factors. We found that the exchange of mass and heat strongly depends on the terrain geometry, the vertical temperature structure in the valley (i.e., the static stability), and the heating of the valley atmosphere at the earth's surface during daytime. More specifically, the vertical exchange is larger for narrower and deeper valleys as well as for stronger inclined valley floors and narrowing valley cross-section. The exchange also increases with decreasing stability and increasing surface heating. Most of these dependencies can be described and parameterized by a so-called breakup parameter which is the ratio between the energy required to neutralize the initially stably stratified valley atmosphere and the total energy provided by surface heating. Since the breakup parameter can be estimated a priori without a sophisticated computer model, our results are useful for various meteorological applications in complex terrain, including environmental impact assessments.