The Humidity and Temperature Sensitivity of Clear-Air Radars in the Convective Boundary Layer

Abstract

A top-down/bottom-up diffusion Model is used to evaluate the relative contributions of humidity temperature, and their cross correlation to the radar refractive index structure function paramer Cn2 in the cloud-free, convective planetary boundary layer (PBL). Profiles of Cn2 can be measured continuously in the clear air with Doppler radars. Extraction of information about PBL dynamical parameters is more straightforward if Cn2 is dominated by either moisture (Cq2) or temperature (CT2). In the lower part of the PBL the surface Bowen ratio ?0 determines this dominance. In the upper part of the PBL the inversion Bowen ratio ?1 is the primary determining factor. The model suggests that humidity accounts for at least 75% of Cn2 over tropical and midlatitude oceans. Polar oceans, where ?0 often exceeds 0.5, may not be dominated by either temperature or moisture. Over land ?0 can easily vary from 0.1 to 10. Geographical regions with ?0<0.3 will be dominated by Cq2 in the lower PBL; regions with ?0>5 will be dominated by CT2. Even for large values of ?0, the upper part of the PBL will often be dominated by humidity.