A Conceptual and Scaling Evaluation of the Surface Wetness Effect on Daytime Moisture Convergence along a Surface Cold Front with Differential Cloud Cover

Abstract

A conceptual evaluation and scaling of the potential impact of surface wetness on spring/summer midlatitude daytime surface cold front moisture convergence is presented. First, a simplified expression is derived, evaluating the effect of surface wetness on frontal moisture convergence due to a differential cloud-cover-induced thermal gradient perturbation. It indicates that wet surfaces may be conducive to enhanced moisture convergence compared with dry surfaces only for very high values of both the cross-frontal relative wind component and the frontal background vertical velocity. With increased background specific humidity in the warm sector, decreased cross-frontal relative wind speed, and a less stable early morning temperature lapse rate, dry surface conditions are significantly more conducive to enhanced frontal moisture convergence. When the daytime boundary layer thermal destabilization effects on the frontal updraft are considered, generally insignificant modifications of the above patterns of frontal moisture convergence are indicated. Overall, the evaluation suggests that typically dry surfaces better promote daytime frontal moisture convergence than wet surfaces, a result that is counterintuitive.