Surface turbulent heat flux partitioning and warm-season boundary layer clouds

 

Authors

Margaret S. Torn — Lawrence Berkeley National Laboratory
Ian N. Williams — Lawrence Berkeley National Laboratory

Category

General Topics – Cloud

Description

The partitioning between surface sensible and latent heat flux (evaporative fraction) is thought to influence the base height, fraction, and diurnal timing of shallow convective clouds. Supporting studies have focused on a few days or intensive observing periods, which may not be representative of warm-season clouds in general. We found substantial scatter (weak correlations) in the relationships between evaporative fraction and boundary layer (PBL) cloud properties observed over a 10-year period using measurements from the U.S. Southern Great Plains (SGP) Atmospheric Radiation Measurement (ARM) central facility site. However, mixed-layer theory predicts that the sensitivity of cloud properties to evaporative fraction can be confounded by the covariation between evaporative fraction and PBL entrainment rates (heat and water vapor flux). Depending on PBL entrainment, a decrease in evaporative fraction can either increase or decrease the tendency of relative humidity at PBL-top (at cloud base), thus contributing to scatter in the relationship between evaporative fraction and cloud properties. We will present an analysis of the PBL-top relative humidity tendency, to explore how the sensitivity of PBL clouds to evaporative fraction depends on PBL entrainment and large-scale forcing.