Cooperative Institute for Research in Environmental Sciences

Identifying Multiple Peaks in W-band Radar Doppler Spectra during Drizzle Events in VOCALS 2008

Christopher R. Williams (1, 2), Chris Fairall (2), and Ken Moran (1, 2)

(1) CIRES, (2) NOAA ESRL PSD

During the VOCALS 2008 cruise, a NOAA W-band (94 GHz) radar was mounted on a stabilized platform and pointed vertically to observe the marine boundary layer cloud structure. Occasionally during drizzle events, while the reflectivity was consistent with time and height, the estimated spectrum width had large values. Examination of the Doppler velocity power spectra reveled that many of the spectra with large spectrum widths actually contained multiple peaked spectra. These multiple peak spectrum invalidates the assumption that drizzle can be modeled as a single peaked spectrum of droplets described by three parameters (e.g., intensity, mode, and spread). The observed multiple peaked spectra suggest that multiple physical processes are generating different sized drizzle droplets that are being observed within the same radar pulse volume.

To identify and study the origin of the different sized drizzle droplets, a multiple peak picking routine was developed that identifies multiple peaks in each spectrum. The spectrum moments (reflectivity, mean velocity, and spectrum width) are estimated for each peak. Time-height cross-sections of Doppler velocity spectra show evolution of drizzle droplet distributions with time and height.

This work was presented in Poster number OS43A-1520 at the December 2011 AGU Fall Meeting.