Characteristics of the raindrop distributions in RICO shallow cumulus

Abstract. The physical properties of rain spectra are generally modeled using an analytical distribution. It is common for the gamma distribution and, to a lesser extent, the lognormal distribution to be used. The majority of studies in the literature focusing on the characterization of raindrop distribution are based on deep convective cloud observations, mostly at ground level. This study focuses on shallow-cumulus rain distributions throughout the depth of the cloud layer and subcloud layer using airborne in situ measurements made with both the Particle Measuring Systems (PMS) Optical Array Probe 260X (OAP-260-X) and the PMS two-Dimensional Precipitation (2DP) instruments during the Rain in Cumulus over the Ocean (RICO) field experiment. Sampled spectra analyzed on the scale of large-eddy simulation resolution (100 m) are found to be relatively broad, with values of the shape parameter – υ for the gamma law and σ_g for the lognormal law – on the order of 1–3 and 1.5–2, respectively. The dependence of the shape parameters on the main rain variables (number concentration, water content, mean volume diameter, sedimentation fluxes and radar reflectivity) is examined, and a parameterization of the shape parameters υ and σ_g as a function of a power law of the rainwater content and raindrop number concentration is proposed.