Stratocumulus Liquid Water Content from Dual-Wavelength Radar
Journal of Atmospheric and Oceanic Technology
A technique is described to retrieve stratocumulus liquid water content (LWC) using the differential attenuation measured by vertically pointing radars at 35 GHz and 94 GHz. Millimeter-wave attenuation is proportional to LWC and increases with frequency, so LWC can be derived without the need to make any assumptions on the nature of the droplet size distribution. There is also no need for the radars to be well calibrated. A significant advantage over many radar techniques in stratocumulus is
... stratocumulus is that the presence of drizzle drops (those with a diameter larger than around 50 µm) does not affect the retrieval, even though such drops may dominate the radar signal. It is important, however, that there are not significant numbers of drops larger than 600 µm which scatter outside the Rayleigh regime at 94 GHz. We use a lidar ceilometer to locate cloud base in the presence of drizzle falling below the cloud. An accuracy of around 0.04 g m −3 is achievable with averaging over one minute and 150 m (two range gates), but for the previously suggested frequency pair 10 GHz and 35 GHz, the corresponding accuracy would be considerably worse at 0.34 g m −3 . We first simulate the retrieval of LWC using aircraft-measured size spectra taken from a profile through marine stratocumulus. Results are then presented from two case studies, one using two cloud radars at Chilbolton in Southern England, and another using the Cloud Profiling Radar System at the Atmospheric Radiation Measurement site in Oklahoma. Liquid water path from the technique was found to be in good agreement with the values obtained from microwave radiometers, with the difference between the two being close to the accuracy of the radiometer retrieval. In the case of wellmixed stratocumulus the profiles were close to adiabatic.