Calculating Higher-Order Moments from Doppler Spectra Collected by an Airborne Cloud Radar

Radar Doppler spectra from vertically-pointing cloud radars have been successfully used to derive physical and microphysical properties of clouds and precipitation, such as air motion, particle fall speed, or cloud-drop and drizzle parameters. Past studies have frequently utilized spectral observations of stratiform clouds from ground-based radar systems, often augmented by observations from additional instrumentation, for example, radiometers or radars at different wavelengths. The use of Doppler spectra from airborne radars poses unique challenges, such as the broadening of the spectra caused by aircraft motion due to the finite beamwidth of the radar.

We use observations from the NSF NCAR HIAPER Cloud Radar (HCR), a 94 GHz radar which is deployed in an underwing pod on the NSF NCAR Gulfstream V aircraft. We show how the use of Doppler spectra can lead to an improvement in the quality of spectrum width estimates when compared to the pulse-pair technique. We further introduce a technique for filtering the HCR Doppler spectra and correcting them for broadening caused by aircraft motion. The corrected spectra are used to calculate higher-order moments, such as skew and kurtosis. These higher-order moments show promise for the retrieval of microphysical cloud parameters but are rarely used because of the difficulties in obtaining high-quality estimates. We will show examples of these quantities from convective and stratiform clouds observed in different climatic regions, ranging from the tropics to the Southern Ocean.