Consistency in <i>Z</i>-<i>R</i> Relationship Variability Regardless Precipitating Systems, Climatic Zones Observed from Two Types of Disdrometer

Abstract

Data from rain Drop Size Distributions gathered on five sites in Africa as well as those of the pilot site in Kourou (French Guyana, South America), located in different climatic zones, and collected by two types of disdrometer (the impact JW RD-69 disdrometer and the Optical Spectro-Pluviometer, OSP) are used to study the consistency of the reflectivity factor-rain rate at the ground (Z-R) relationship variability. The results clearly confirm that the relationship Z-R knows a large spatial variability, from a type of precipitation to another and within the same precipitation regardless the type of disdrometer used for DSD measurements. Base on the similarity of the relations reflectivity factor-rain rate and ratio median volume diameter over the total number of drops-rain rate, the variability of the Z-R coefficients (A, b) through the simultaneously implication of the size and number of drops which characterize the DSD was exhibited. It was shown that the relationships A-α and b-β designed to understand the involvement of parameters D0 and NT of DSD in the variability of the relationship Z-R are similar regardless the types of disdrometer used. However, the relations A-α in the Sahelian region appear to deviate from those of Guinean, equatorial and Soudanian zones. The plausible reasons were discussed.

Modeling the Interdependence Structure between Rain and Radar Variables Using Copulas: Applications to Heavy Rainfall Estimation by Weather Radar

“…However, the results were disappointing as they barely differed from those obtained by using a unique relationship Z − R [13,19]. Steiner et al [4], using simulations, and Ochou et al [9] and Bamba et al [10], based on DSD observations in West Africa, emphasized that understanding the variability of the Z − R relation includes taking into account the simultaneous variability of the size and number of raindrops. Although the implementation of such approaches is difficult in an operational way, these studies suggest, because of numerous combinations of size and the number of drops, that additional radar parameters are necessary to better characterize the rain media by describing the microstructure of clouds.…”

“…This relationship is often formulated based on measurements of radar reflectivity and ground rain gauge rainfall or from raindrop size distributions (DSD). However, the conversion of Z in R presents numerous errors, including the variability of a reflectivity vertical profile [1], the error in measuring radar reflectivity [2,3], the variability of rainfall drop size distribution [4][5][6][7][8][9][10], the nature of rainfall [11][12][13][14], the data analysis method [15] and the use of point rainfall measurements on the ground as corresponding to radar pixel-averaged values [16][17][18] (mismatch of sampling volume between radar and rain gauge observations). Because the major source of error in the Z − R precipitation algorithm is the variability in the drop size distribution, error reduction attempts in radar rainfall estimation have been suggested by several authors with a distinction between the convective or stratiform nature of DSD-based algorithms.…”