Quantifying cloud liquid water content in cumulus and stratiform clouds using dual wavelength scanning ARM cloud radar observations

Description

Accurate observations of Liquid Water Content (LWC) in clouds are necessary to improve our understanding of microphysical processes such as entrainment and cloud radiative cooling. Existing LWC retrieval techniques remain limited because of their need to rely on assumptions about either the shape of the LWC profile with height or the cloud droplet particle size distribution and/or require absolute radar reflectivity calibration. Using the difference in reflectivity measured at two wavelengths within the same scattering regime, a technique referred to as dual wavelength ratio (DWR), can alleviate these issues. The SACR2 configuration, with its collocated beam-matched Ka-band and W-band radars, is optimum for decreasing DWR-based LWC retrieval errors. The current work describes a new technique to reduce radar random reflectivity errors responsible for LWC retrieval uncertainty and presents its application to 20hrs of SACR2 observations collected at the Eastern North Atlantic facility. It is shown that the technique, which fits 30-s averaged DWR observations collected in 187.5m vertical sliding windows using a second order polynomial, can capture the shape of various theoretical LWC profiles even nonlinear ones, which paves the way for retrievals within cumulus cloud, where complicated LWC distribution are believed to exist. It was estimated that LWC retrieved using this technique agrees within 10% with microwave radiometer liquid water path estimates and has an average uncertainty of 0.1gm-3. The effect of cloud top dynamics and drizzle production on the vertical distribution of LWC in stratiform and cumulus clouds is also illustrated in composites normalized by cloud depth.