Talk, 7th COPS Workshop, Collège Doctoral Européen, 46 Boulevard de la Victoire, 67000 Strasbourg, France: 2008-10-27 - 2008-10-29
Abstract:
Surface energy exchange measurements of two COPS energy balance stations in the Kinzig valley are used to investigate the generation of free convection events initiated by a change of the local circulation system. These free convection events are assumed to have an essential impact on ABL thermodynamics and structure. In the Kinzig valley, a clear diurnal thermally-induced valley wind system was frequently observed to be generated in high-pressure situations with weak synoptic forcing characterized by down-valley winds – prevailing at night – changing to up-valley winds in the early morning hours. At COPS IOP8b, at station Fußbach this wind direction change occurred at about 8:30 UTC. During this transition period, a strong collapse of the horizontal wind speed through the whole vertical extension of the valley atmosphere lasting from 6:30 until 8:50 UTC in the morning hours, with values smaller than 1.5 ms-1, is evident from Sodar measurements. Simultaneously, a free convection situation lasting from 7:35 to 8:40 UTC was detected by ground based measurements of the stability parameter z/L indicated for z/L<-1. Free convection situations will be facilitated for small values of the friction velocity and high buoyancy fluxes, as buoyant forces then dominate over shear forces within turbulence production. Thus, the wind speed collapse in the valley induced by the wind direction change provides a powerful trigger mechanism for the generation of free convection events emphasized by its occurrence on about half of the days during the COPS field campaign at both stations under investigation.