Sub-Millimeter-Diameter Raindrop Trajectories, Oscillations and Size Distributions Measured by the U.S. Navy’s High Resolution Mid-Course Radar

The U.S. Navy’s linear frequency modulated (LFM) pulse compression, Mid-Course Radar (MCR) has recently been the primary instrument used in weather data collection experiments at Cape Canaveral, Florida, designed to validate and improve the Navy’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS¹). The MCR was the first radar to detect individual raindrops falling in the free atmosphere. It is a 3-MW peak-power C-band dual-polarization radar that alternatively transmits two LFM wave forms with a 0.22° beamwidth at a pulse repetition frequency of either 160.1 or 320.2 Hz. The narrowband waveform has a 6-dB width range resolution of 37 m whereas the wideband waveform has a range resolution of 0.546 m. This combination of signal attributes leads to the remarkably small pulse volumes of the wideband, which enables the detection individual raindrops, seen as streaks of reflectivity segments (‘streak segments’ for short) superimposed on the background of unresolved smaller raindrops of comparatively weaker reflectivity.

A novel approach is presented here for utilizing the Doppler velocity power spectrum (DVPS) to simultaneously retrieve estimates of the raindrop size distribution and the vertical wind residing within the pulse volume at altitudes where individual raindrops are detected while the MCR is scanning vertically. A streak segment is identified in the wideband data and an FFT of the complex signal voltage components is performed across the midpoint of the streak segment. The drop size distribution of the background particles is derived from the resulting DVPS after correcting its radial velocities for the vertical wind. The vertical wind is approximated using two approaches that exploit the fact that the raindrop associated with the streak segment appears as a prominent narrow peak in the DVPS. One method appears to be more reliable than the other after consideration is given to the effects of reflectivity fluctuations along the streak segment. The methodology will be used on archived MCR data to glean new insights into the interplay between cloud microphysics and circulations at micro-gamma scales that ultimately could lead to improved numerical weather and climate prediction.