The Early Evening Surface-Layer Transition: Temporal and Spatial Variability

On clear nights with appreciable radiative cooling, rates of change of mean quantities observed in the first 1 or 2 h after sunset are many times larger than they are subsequently until sunrise. These variations include large temperature drops, specific humidity increases, and abrupt wind speed decay. The early evening transition (EET) is dominated by vertical surface flux convergence as the turbulent mixing layer becomes confined to a shallow stable layer near the surface. Effects of surface heterogeneities are enhanced by the subsequent small eddy size, so that spatial variability of mean variables peaks during the EET. Hilltops do not experience such large variations at EET, as turbulence persists longer at those locations. Wind speed decays faster at obstructed sites, which show earlier transitions. The Richardson number increases exponentially at the EET, and the rate of exponential increase is proportional to the rate of wind decay. Qualitative aspects of the EET at a single point are resolved by a two-layer model, similar to those currently used in parameterizations of the surface exchange in mesoscale simulations. However, in order to simulate the different behaviors observed in an area as big as one grid cell in a mesoscale model, very different values of the geostrophic wind need to be imposed. A large eddy model is used to verify that the specific humidity jump and maximum cooling rate during the EET are primarily the consequence of enhanced vertical flux divergence, though in later stages of the EET, advective effects become more important.