Code 613.1 Branch Seminar: Dr. David Atlas and Carlton W. Ulbrich

NASA GSFC / Clemson University
This work uses raindrop size spectra measured at the surface in tropical continental storms to determine the associated parameters of the best-fit gamma distributions. The physical processes responsible for those parameters and their relations to the measurable radar reflectivity, Z, and polarization differential reflectivity, Zdr, are then explored. So too are their relations to quantitative measurements of rain. Comparison is then made to corresponding features previously reported in tropical maritime regimes. The storms observed in Brazil and Arecibo, Puerto Rico have been divided into convective (C), transition (T), and stratiform (S) segments. The DSD parameters are clearly defined on a Gamma Parameter Diagram (GPD) that defines the relations between the median volume drop size (D0), the spectrum breadth parameter, and the slope parameter. The GPD provides strong evidence of the systematic behavior of the nature of the DSDs that is manifested in the integral parameters measured by polarimetric and profiler radars. Drop growth occurs predominantly below the 0°C level by collision, coalescence and breakup (CCB) in the C rains. In both maritime and continental storms the DSD in the convective portion of the storm approaches equilibrium. This is manifested by the well known observation that the coefficient 'A' in the Z=ARb relation increases with D0 while the exponent 'b' approaches unity. The D0 and 'A' pair increase with, and appear to be determined largely by the updraft strength, thus providing a possible means of determining the appropriate algorithms for rainfall measurement. There is a distinct climatological difference between maritime (M) convective and continental (C) convective storms: C is characterized by small concentrations of large drops while M behaves conversely due largely to the difference in updraft strength. The latter finding validates and extends the prior findings of Bringi et al.