A Microphysical Retrieval Scheme for Continental Low-Level Stratiform Clouds: Impacts of the Subadiabatic Character on Microphysical Properties and Radiation Budgets

Abstract

Using measurements from the Department of Energy?s Atmospheric Radiation Measurement Program, a modified ground-based remote sensing technique is developed and evaluated to study the impacts of the subadiabatic character of continental low-level stratiform clouds on microphysical properties and radiation budgets. Airborne measurements and millimeter-wavelength cloud radar data are used to validate retrieved microphysical properties of three stratus cloud systems occurring in the April 1994 and 1997 intensive observation periods at the Southern Great Plains site. The addition of the observed cloud-top height into the Han and Westwater retrieval scheme eliminates the need to invoke the adiabatic assumption. Thus, the retrieved liquid water content (LWC) profile is represented as the product of an adiabatic LWC profile and a weighting function. Based on in situ measurements, two types of weighting functions are considered in this study: one is associated with a subadiabatic condition involving cloud-top entrainment mixing alone (type I) and the other accounts for both cloud-top entrainment mixing and drizzle effects (type II). The adiabatic cloud depth ratio (ACDR), defined as the ratio of the actual cloud depth to the one derived from the adiabatic assumption, is found to be a useful parameter for classifying the subadiabatic character of low-level stratiform clouds. The type I weighting function only exists in the lower ACDR regime, while the type II profile can appear for any adiabatic cloud depth ratio. Results indicate that the subadiabatic character of low-level stratiform clouds has substantial impacts on radiative energy budgets, especially those in the shortwave, via the retrieved LWC distribution and its related effective radius profile of liquid water. Results also show that this subadiabatic character can act to stabilize the cloud deck by reducing the in-cloud radiative heating/cooling contrast. As a whole, these impacts strengthen as the subadiabatic character of low-level stratiform clouds increases.