The Evolution of Raindrop Spectra in Warm-Based Convective Storms as Observed and Numerically Modeled

Abstract

Airborne optical spectrometer measurements of raindrop spectra were performed in Texas together with 10 and 3 cm radar observations. The observations reveal large drop spectra emanating from cloud base with low concentrations of small drops compared to the Marshall and Palmer (1948) distributions. Systematic patterns of exponential spectrum parameters (N0 and ?) are revealed to constitute a ?parametric cycle? in time and space. Drop spectrum form is attributed principally to updraft sorting of smaller drops. Subsequent to updraft sorting, the importance of the collision breakup process with respect to spontaneous breakup appears to be greatly diminished. Recent theoretical studies by Srivastava (1971, 1978) partially support conclusions drawn with respect to the relative importance of spontaneous versus collisional breakup. A one-dimensional, time-dependent numerical model was employed for initial conditions simulating the observations. The resultant spectra emanating from the model were quantitatively similar to the measurements and exhibited temporal evolution consistent with the observed parametric cycle. Implications for estimation of spectral moments such as liquid water content and radar reflectivity factor as well as rainfall rate are discussed briefly.