The Contribution of Large Drop Sizes to Rainfall within the Stratocumulus-to-Cumulus Transition from CSET Observations

The effect of Mie scattering upon conventional Z-R radar-inferred rainfall rates is analyzed in the stratocumulus-to-cumulus transition region using data from the G-V aircraft gathered during the Cloud System Evolution in the Trades (CSET) campaign held in the northeast Pacific in July of 2015. Radars traditionally use Rayleigh approximation to estimate a rain rate from the radar reflectivity. This is appropriate for drops with diameters less than 200µm at a wavelength of 3.2 mm, corresponding to that of the Hiaper Cloud Radar. For larger drops the observed reflectivity can be significantly lowered by reduced scattering, leading to underestimated radar-derived rainfall rates if unaccounted for. This in turn may confuse understanding of the role of precipitation within the stratocumulus-to-cumulus transition, particularly when the measured radar reflectivity is near its upper limit of approximately 20 dBZ. The contribution to rainfall from large drizzle drops is analyzed from one California-Hawaii-California round trip, using one-second data from the in-situ 2D-C probes (measuring between 75 to 3000 µm in diameter) from below-cloud and near-surface 10-minute legs. Z-R relationships are initially determined from the in-situ data. Mie scattering will generate noticeable underestimates once rain rates exceed 3 mm/hour (calculated from in-situ data). For an in-situ rain rate of 4mm/hr, 10mm/hr and 30mm/hr, the rain rate is underestimated by 10%, 50% and ~90% respectively. Rain rates exceeding 4 mm/hr are found to occur 0.8% and 13% percent of the total and raining (>0.01 mm/hour) samples in the cumulus regime (SST>293 K). Even in the stratocumulus regime (SST<293K), most of the rain is from drops with diameters>500µm and rainfall rates are underestimated by Z-R relationships. In contrast, comparison of below-cloud to near-surface rainrate-dropsize distributions indicate the sub-cloud evaporation is from drops with diameters less than 300 µm, consistent with a simple evaporation model. Thus shallow cumulus precipitation acts to both deplete the atmosphere of liquid water and decouple the boundary layer below the cloud. Future work will evaluate the ability of the radar to retrieve rain rate through incorporating information from the Doppler velocity along with the radar reflectivity.