Abstract
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 σg 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 σg as a function of a power law of the rainwater content and raindrop number concentration is proposed.
Highlights
Raindrops play a role in the lower troposphere water and energy budgets by carrying water and latent energy from the cloud layer to the subcloud layer and to the surface
The observations used in this study are derived from in situ shallow precipitating cumulus cloud measurements collected during the Rain in Cumulus over the Ocean (RICO) field experiment (Rauber et al, 2007; Snodgrass, 2008; Nujiens et al, 2009)
For data processed at 1 Hz, that is, a resolution of about 100 m along the flight track, the sampled volume is on the order of 1–4 L and 100–200 L for the Particle Measuring Systems (PMS) OAP-260X and the PMS 2DP, respectively
Summary
Raindrops play a role in the lower troposphere water and energy budgets by carrying water and latent energy from the cloud layer to the subcloud layer and to the surface. Since only a limited number of rainfall integral variables are generally known (e.g., M0 and M3 in two-moment bulk schemes, M6 in remote-sensing measurement), a hypothesis on the shape of the distribution is necessary in order to derive the other microphysical properties. Since the work of Marshall and Palmer (1949) and Best (1950), a large number of studies have been dedicated to the retrieval of the value of these parameters characteristic of deep convective events Most of these studies suggest that rain spectra are narrower than the MP distribution (ν = 1), with ν values roughly in the range of 5–10 (Nzeukou et al, 2003; Uijlenhoet et al, 2003) or more (Tokay and Short, 1996) or σg values on the order of 1.4 (Feingold and Levin, 1986).
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