Processes leading to inversion buildup in small enclosed basins

Meteorologists have been taught that basin temperature inversions build up overnight due to the continuous flow of cold air down the sidewalls and its convergence over the valley center. Recent observations and some selected literature references from small basins and sinkholes suggest a very different form of inversion buildup in which downslope advection plays an important role only at the beginning of the night. The surrounding topography and the initial static stability built up by cold air drainage increasingly protect the basin atmosphere from the generally stronger winds aloft. After this initial stage of buildup, the winds die within the basin and over the slopes and radiative processes take on an increasingly important role. In these basins, it is the radiative processes rather than the advective processes that are ultimately responsible for the total nighttime cooling and inversion strength in the basin . A key determinant of the rate of cooling and growth of inversion strength in the basin is the sky view factor - the exposure of the basin to the sky hemisphere. In this paper, we will present the observational evidence for this new interpretation and a simple 3-layer analytical model that illustrates the role of the sky view factor in nighttime basin energetics. The model will be compared to temperature data from three small basins of different size in the eastern Alps and general conclusions will be drawn regarding the effect of basin size on the physical processes that lead to the buildup of temperature inversions in valleys and basins.