| The measurement of rainfall using weather radar technologies is an important application. Rainfall measurement is important in hydrology and flash flood prediction. Physically-based methods of rainfall estimation rely on the precipitation model, namely, models for the drop size distribution (dsd), drop shape, and drop orientation (or, canting). Traditionally, surface disdrometer, aircraft imaging probes and wind profilers are used in studying the dsd variability. These instruments have limited spatial and temporal resolution. On the other hand, scanning radar can offer measurements with high spatial and temporal resolution. Dual-polarization radar technologies, which use the backward-scatter and forward-scatter based measurements at two polarization states (horizontal and vertical), have progressed dramatically in recent decades. However, the accuracy of the precipitation model has not been systematically addressed; this thesis seeks to more fully address the basic model assumptions and the related issue of accuracy.; In this thesis, we propose an areal rainfall algorithm using Fdp . We show that the random noise and measurement fluctuations can be reduced significantly by averaging rainfall over an area. Since Kdp (or Fdp) is a function of the mean axis ratio of drops, we apply a “b-correction” as proposed by Gorgucci et al. (1999, 2000). The “b-correction” can significantly reduce the bias in accumulation over an area. Moreover, we use b along with Zh and Zdr to retrieve dsd parameters of a normalized gamma model following Bringi et al. (2002). After retrieving the gamma dsd parameters, we propose a polarimetrically-based Z-R algorithm of the form Z = aR1.5, where the coefficient can be continuously adjusted. In chapter 4, we study the drop orientation (or, canting angle) distribution. Based on the natural behavior of drop orientation (i.e., azimuthal symmetry), the canting angle should have zero mean. So we only estimate the standard deviation of canting angle (sb) from the polarimetric data. We study two existing sb estimators, the r4 method and the Ryzhkov method (2001). A new algorithm, “simplified r4 method”, is also proposed. We test the stability of these algorithms and apply them to three rain events. |
