Integration Time Influence on Rain Drop Size Distribution (DSD) and Radar Parameters from Disdrometer Data

Abstract:

The development of radar-rainfall algorithms is typically based on assumed radar and rain drop models, which include the fitting of observed drop size distribution (DSD) data to some assumed mathematical model such as a gamma function. The objective of this study is to determine if and how the integration time of continuous disdrometer raindrop measurements may influence gamma model parameters, rain DSD moments such as rain rate, radar variables, and their relations. Previous studies have often utilized 1-minute DSD samples based largely on empirical experience. However, if radar and disdrometer observations are to be inter-compared, the appropriate spatial and temporal scales of radar must be considered. To make an approximate connection between radar horizontal and vertical spatial scales and disdrometer integration times, it should be noted that disdrometers measure rainfall beneath a storm that is in horizontal motion and that drops have various vertical fall speeds. Finally, the type of precipitation (e.g., stratiform vs. convective) should also be taken into account in the selection of integration time so that the implied temporal and spatial scales are consistent with the dominant precipitation processes. Based on these simple principles, it can be shown that 1, 2 and 3-minute DSD integration times are all potentially consistent with typical radar spatial scales and physical processes. However, the effects of these different sample periods on gamma fits, DSD moments and radar parameters are not well characterized. Hence, in this study, 1, 2 and 3-minute integration times are utilized for the analysis of 2-dimensional video disdrometer (2DVD) DSD data from the NASA GPM (Global Precipitation Mission) Iowa Flood Studies (IFloodS) ground validation (GV) experiment. The truncated method of moments is employed to estimate gamma fit parameters from binned 2DVD data for each of the sample periods. The gamma fit parameters are then used to calculate DSD moments and as input into a T-matrix scattering model for the calculation of radar parameters. The impact of the various integration periods on the resulting gamma fit parameters and radar-rain relations will be assessed in the context of NPOL radar observed storm morphology and precipitation processes during IFloodS.