Identification of Valley-Wind Days

The identification of valley-wind days is an important issue in mountain meteorology. The term valley-wind days refers to days with synoptically undisturbed conditions, when the valley atmosphere is usually characterized by clear sky conditions and the wind field is dominated by distinct diurnal cycles of valley- and slope-wind circulations. We will present an objective criterion for the identification of these valley-wind days in the Inn Valley, Austria, based on ERA-Interim reanalysis data and local surface meteorological data, including longwave radiation measurements in the valley. Horizontal gradients of geopotential height above the top of the surrounding mountains are used to preselect synoptically undisturbed conditions. Clear-sky conditions are identified based on an index that relates longwave incoming radiation observed at an i-Box measurement site on the valley floor to the clear-sky emissivity calculated from different diagnostic models. Based on an optimized version of this classification scheme, approximately 15 valley-wind days are identified for the Inn Valley per year. The classification scheme is evaluated against wind measurements at the valley floor.