Analysis of the influence of a lake on the lower convective boundary layer from airborne observations

The influence of an intermediate-scale lake, with a dimension of approximately 2 kmx10 km, on a convective boundary layer has been analysed. Data were collected by the airborne platform Helipod during the STINHO 2002 and LITFASS 2003 campaigns in eastern Germany, during early summer months, when the lake was much colder than the surrounding surface. The objective was to assess which atmospheric parameters show influence from the lake by the airborne observations. While spatial variability for mean quantities is not significant at the observation height of 70m and above, the second-order statistics related to potential temperature exhibit a clear decrease in the vicinity of the lake for measurements taken below 100m above ground level. Second-order statistics of humidity and vertical wind velocity are not suited to identify the foot print of the lake in our study. Several length scales of surface heterogeneity were calculated following previous studies. Only the scale that considers vertical velocity is compatible with our airborne observations. In addition, the application of a convective scale indicates that the lake could affect the lower convective boundary layer above the lake and above the surrounding land downstream of the flow for low wind speeds (below 4 m s(-1)). Finally, the downstream propagation of the lake influence has been addressed by calculating the cross-correlation function between the surface radiative temperature and the variance of potential temperature. A clear relationship between the spatial lag of the maximum correlation and the horizontal advectivon could be identified.