Observed Precipitation Vertical Structure to Support Assumptions used in Satellite Rainfall Retrieval Algorithms

Abstract:

Due to a limited number of measurements made on a single space-craft, satellite rainfall retrieval algorithms are under-constrained and often make assumptions about the vertical structure of precipitation. For example, an algorithm may assume the rain rate is constant with height below the freezing level. In order to help support or validate the assumptions used in NASA Global Precipitation Measurement (GPM) satellite rainfall retrieval algorithms, this study investigates the vertical structure of raindrop size distribution (DSD) parameters derived from vertically pointing ground based Doppler radars during GPM field campaigns MC3E (Mid-latitude Continental Convective Clouds Experiment), IFloodS (Iowa Flood Studies), and IPHEX (Integrated Precipitation and Hydrology EXperiment). The three estimated DSD parameters represent the scale and shape of the raindrop size distribution using the parameters: normalized number concentration N_w, mass spectrum mean diameter D_m, and mass spectrum effective variance ν_m = σ_m² / D_m²(mass spectrum variance / mean diameter squared).

A vertical pattern in the DSD parameters was often observed during stratiform rain. While the reflectivity was nearly uniform with height, the normalized number concentration (N_w) and mean diameter (D_m) had opposite vertical structures with N_w decreasing and D_m increasing from the melting layer down to the surface. Interestingly, the mass spectrum effective variance (ν_m) decreased as the raindrops fall indicating that the DSD was evolving into a narrower effective mass spectrum with a loss of small and/or large raindrops. This vertical structure of DSD parameters suggests breakup, coalescence, and evaporation were occurring in the vertical column.

In summary, the analysis of vertically pointing radar data during GPM ground validation field campaigns suggests that the net result of breakup, coalescence, and evaporation during stratiform rain appear in the vertical structure of DSD parameters N_w, D_m, and ν_m. Much work is still needed to verify and quantify the slopes of the DSD parameter vertical structures and whether these results can be used to support or validate the initial profiles used in satellite rainfall probabilistic retrieval algorithms.