The sensitivity of exchange processes in an idealized valley on solar forcing

The sensitivity of exchange processes in an idealized valley on solar forcing

Daniel Leukauf, Johannes Wagner, Christian Posch, Alexander Gohm, Mathias W. Rotach

April 23, 2014

Abstract

In recent years, the mechanisms of thermally-driven wind systems and the boundary layer over complex terrain have been investigated through real-case and idealized numerical simulations. However, these studies usually consider only one given latitude or one predefined surface forcing. The question remains how the exchange of the valley with the free atmosphere depends on solar forcing. The exchange is related to the break-up of the nocturnal cold pool and the boundary layer inside the valley. This question is fundamental if one aims at developing a parametrization of exchange processes from the valley to the free atmosphere, based on bulk fluxes of heat, mois- ture and other properties. In order to resolve small-scale turbulent processes, we conducted large eddy simulations with the Weather Research and Forecasting (WRF) model for an idealized valley. The valley consists of two sine-shaped mountain ridges which form a 20-km long and 20-km wide valley with a flat valley floor that is homogeneous in y-direction. The net short-wave radiation is predefined using a simple sine function during the day and a value of zero during the night. It is varied between 150 and 850 W m-2. The long-wave outgoing radiation is calculated using the Angstrom formula. This gives the advantage to have a single parameter, the amplitude, to vary the incoming solar radiation instead of tree parameters (albedo, latitude and date) when using a full radiation scheme. Parametrizations for surface-atmosphere exchange processes were used and the initial vertical profiles are characterized by a constant buoyancy frequency, a constant relative humidity, a dry soil and an atmosphere at rest. In our simulations, a slightly stable core remains inside the valley during the whole day for all simulations with a forcing below 400 W m-2 and even for the simulation with the strongest forc- ing, the stable core is not removed before 1100 UTC. However, slope winds are able to reach the top of the mountains even in presence of a stable valley core and, hence, they contribute to the exchange of air. The ratio of stability and buoyancy flux describes the effectivity of this transport mechanism. A relationship between this ratio and the export of air is found.