Drizzle onset detection using skewness of the cloud radar Doppler spectrum

Accurate knowledge of the presence of drizzle particles in liquid stratiform clouds is of paramount importance to the observation of precipitation onset in shallow clouds, and to the correct application of cloud property retrieval algorithms. Past studies have focused on identifying a radar reflectivity threshold beyond which drizzle particles dominate the radar observables. However, the initial generation and growth of drizzle particles and their subsequent impact on radar observables occur at a much lower radar reflectivity dominated by the cloud component. We introduce the use of the radar Doppler spectrum skewness as a very sensitive parameter for the detection of early drizzle that is preferred over radar reflectivity or Doppler velocity, which are sensitive only when the drizzle contribution to the total observed reflectivity is significantly larger than the cloud contribution.

Using 95 GHz cloud radar Doppler spectra collected by the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) during its recent 21-month deployment in the Azores, we show that drizzle can be routinely observed in marine stratocumulus clouds having reflectivity as low as -40 dBZ. We also show the use of Doppler spectrum skewness for differentiating cloud regions where drizzle growth is dominated by autoconversion from those where accretion dominates.