Hence, the technique has limited application for general DSD retrieval. (2002) apply the method only to situations in which K DP ≥ 0.3° km −1 (rain rates of greater than 20 mm h −1). Rather, K DP is used here for verifying the radar calibration ( Vivekanandan et al. Further, Illingworth and Blackman (2002) argue that the redundancy among Z H, Z DR, and K DP precludes the retrieval of the three DSD parameters in Eq. Retrievals of drop mean shape with K DP can produce large variations in space and time, which have not been independently verified. To reduce the uncertainty in K DP, the differential phase measurements are filtered in range, often over several kilometers (e.g., Ryzhkov and Zrnić 1996 Hubbert et al. There is some question with regard to the use of K DP for DSD parameter retrieval K DP is computed from measurements of differential propagation phase, which can be noisy, particularly at lower rain rates. The procedure yields the mean axis ratio of the drops, the DSD shape factor, a normalized number concentration, and the DSD median volume diameter. (2002) propose to retrieve the DSD from reflectivity, differential reflectivity, and specific differential phase ( K DP). (2003), who show that the μ–Λ relation is intrinsically different than the linear relation associated with measurement error and that retrieved μ and Λ values are not biased by statistical errors. Concern has been raised as to the influence of measurement errors on such relations ( Chandrasekar and Bringi 1987). The μ–Λ relation was derived from drop size distribution measurements. (2001), is based on measurements of radar reflectivity at horizontal polarization ( Z H) and differential reflectivity ( Z DR), and an empirical constraining relationship between the drop size distribution shape and slope parameters. The retrieval technique used here, an adaptation of that proposed by Zhang et al. Because the gamma DSD is described by three parameters, three measurements or relations are required. N D N 0 D μ D (1)where N 0 (mm − μ−1 m −3) is a number concentration parameter, μ is a distribution shape parameter, Λ (mm −1) is a slope term, and D (mm) is the drop equivalent volume diameter. DSD invariance is attributed to small total drop numbers, which result in few collisions. The radar measurements suggest that, although DSDs in stratiform rain were also broad and nearly constant in the rain layer, they were not at equilibrium but were merely steady. Median volume diameters at the ground were closely related to the intensity of an overlying bright band. DSDs for stratiform precipitation were dominated by relatively large drops. Rainy downdrafts exhibited what are believed to be equilibrium DSDs in which breakup and accretion are roughly in balance. Largest drop median volume diameters were at the leading edge of the storm core and were displaced slightly downwind from updrafts. Broad DSDs were determined for the core (high reflectivity) regions of thunderstorms. The method is applied to select storms to demonstrate utility. Retrieved physical characteristics of the drop size distribution (DSD) were generally well matched with disdrometer observations. The procedure assumes that drops are represented by a gamma distribution and retrieves the governing parameters from an empirical relation between the distribution shape and slope parameters and measurements of radar reflectivity and differential reflectivity. Polarimetric radar measurements are used to retrieve properties of raindrop distributions.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |