Vertical Variability of Rain Drop Size Distribution from Micro Rain Radar and Disdrometer Measurements Collected During Ifloods Campaign

Variability of the rain drop size distribution affects estimation of precipitation properties from weather radar data, collected both from space-borne or ground-based system. For space-borne radars, such as the Dual frequency Precipitation Radar on board the Core Observatory of the NASA/JAXA Global Precipitation Measurement (GPM) mission, it is linked to the well-known problem of the non-uniform beam filling that is one of the main sources of uncertainties in the precipitation estimates. One of the most important products obtained from ground-based radars is the quantitative estimation rainfall. Actually, rainfall is estimated at a certain altitude that depends on the radar elevation angle and on the distance from the radar. Therefore, depending on the vertical variability of rainfall, a time-height ambiguity between radar measurements and rainfall at the ground can affect the rainfall products. In addition, with increasing distance, the increase of the size of the radar sample volume and the likely increased DSD variability inside the volume may affect the performance of rain retrieval algorithms.

Vertically pointing radars are a valuable tool to support investigations of the vertical variability of precipitation near the ground, allowing both to investigate the vertical variability of DSD and ultimately, to fill the gap between the ground level and the first available radar elevation. Among them, the Micro Rain Radar (MRR), a 24 GHz system using the continuous-wave frequency-modulated scheme, has become popular both for fixed installations and for the use in field campaigns where it is often co-located with disdrometers of different kind. A method for determining vertical profiles of drop size distribution and of its moments is available, which, however, requires several assumptions.

From the GPM Ground Validation Iowa Flood Studies (IFloodS) field experiment, data collected by MRRs and co-located 2D video disdrometers (2DVD), and Autonomous OTT Parsivel2 Units (APU), were available from different sites within the coverage of the S-band NASA dual polarization Doppler radar (NPOL). In three different sites, clusters of co-located APU, 2DVD and MRR were almost aligned along a radar radial and at different distances from the radar. The IFloodS set-up has been used first to evaluate MRR drop size distribution estimates closer to the ground with respect to 2DVD and APU disdrometers. Then MRR data, resampled onto the NPOL resolution volumes are used to evaluate the uncertainties of MRR measurements along the vertical. Data collected by NPOL at 0.7° and 1.4° are considered. The vertical variability of DSD and integral rainfall parameters within the MRR bins (from ground to 1085 m each 35 m) is finally investigated in order to provide some insight on the variability of the rainfall microphysical characteristics within about 1 km above the ground in different precipitation regimes.