Improved W-band Doppler Radar Spectrum Width Estimates during the VOCALS 2008 Cruise

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 a few randomly distributed large values. During the post-cruise processing, it was determined that the large spectrum widths occurred when the recorded Doppler velocity spectra contained multiple peaks. The multiple peaks were symmetric about the dominant peak and were at least 30 dB smaller than the dominant peak. The few randomly distributed large spectrum widths were due to the real-time spectra processing routine estimating the spectrum moments across multiple peaks. Since the secondary peaks are so small relative to the dominant peak, there was very little change in the estimated reflectivity as the spectrum width varied from observation to observation. This presentation will describe how a multi-peak picking routine was used to identify the dominant peak in the Doppler velocity reflectivity spectra and how the spectrum moments were recalculated using just the dominant peak. Examples of the multiple peaks will be shown to clarify that the multiple peaks were not due to multi-peaked drizzle droplet size distributions and not due to Mie scattering effects. Improvements in the spectral moments during drizzle will be shown and will show consistent spectrum widths with time and height during drizzle.