Continental Stratus Clouds: A Case Study Using Coordinated Remote Sensing and Aircraft Measurements

Abstract

A continental stratus cloud layer was studied by advanced ground-based remote sensing instruments and aircraft probes on 30 April 1994 from the Cloud and Radiation Testbed site in north-central Oklahoma. The boundary layer structure clearly resembled that of a cloud-topped mixed layer, and the cloud content is shown to be near adiabatic up to the cloud-top entrainment zone. A cloud retrieval algorithm using the radar reflectivity and cloud droplet concentration (either measured in situ or deduced using dual-channel microwave radiometer data) is applied to construct uniquely high-resolution cross sections of liquid water content and mean droplet radius. The combined evidence indicates that the 350?600 m deep, slightly supercooled (2.0° to ?2.0°C) cloud, which failed to produce any detectable ice or drizzle particles, contained an average droplet concentration of 347 cm?3, and a maximum liquid water content of 0.8 g m?3 and mean droplet radius of 9 ?m near cloud top. Lidar data indicate that the Ka-band radar usually detected the cloud-base height to within ?50 m, such that the radar insensitivity to small cloud droplets had a small impact on the findings. Radar-derived liquid water paths ranged from 71 to 259 g m?2 as the stratus deck varied, which is in excellent agreement with dual-channel microwave radiometer data, but ?20% higher than that measured in situ. This difference appears to be due to the undersampling of the few largest cloud droplets by the aircraft probes. This combination of approaches yields a unique image of the content of a continental stratus cloud, as well as illustrating the utility of modern remote sensing systems for probing nonprecipitating water clouds.