Understanding the Variability of Z-R Relationships Caused by Natural Variations in Raindrop Size Distributions (DSD): Implication of Drop Size and Number

Abé D Ochou,Manlandon Koffi,Eric-Pascal Zahiri,Bakary Bamba

doi:10.4236/acs.2011.13017

Abstract

In the issue of rainfall estimation by radar through the necessary relationship between radar reflectivity Z and rain rate R (Z-R), the main limitation is attributed to the variability of this relationship. Indeed, several pre-vious studies have shown the great variability of this relationship in space and time, from a rainfall event to another and even within a single rainfall event. Recent studies have shown that the variability of raindrop size distributions and thereby Z-R relationships is therefore, more the result of complex dynamic, thermody-namic and microphysical processes within rainfall systems than a convective/stratiform classification of the ground rainfall signature. The raindrop number and size at ground being the resultant of various processes mentioned above, a suitable approach would be to analyze their variability in relation to that of Z-R relation-ship. In this study, we investigated the total raindrop concentration number NT and the median volume di-ameter D0 used in numerous studies, and have shown that the combination of these two ‘observed’ parame-ters appears to be an interesting approach to better understand the variability of the Z-R relationships in the rainfall events, without assuming a certain analytical raindrop size distribution model (exponential, gamma, or log-normal). The present study is based on the analysis of disdrometer data collected at different seasons and places in Africa, and aims to show the degree of the raindrop size and number implication in regard to the Z-R relationships variability.

Highlights

In the study of rainfall, one parameter of interest to estimate with regard to rain drop size distributions (DSD) is the rain rate R
It can be measured from the ground using rain gauges and weather radar.The nature of space-time radar measurements represented by the reflectivity factor Z has generated so much interest that many studies have focused on finding connections to bring it in intensity [1,2,3,4,5,6,7,8,9,10,11,12].numerous studies based on the measurement of DSD in precipitation around the world continue to show that empirical expression of the form Z ARb is a suitable relation to describe the relationship between these two parameters
Without assuming a certain drop size distribution model, we propose in this study, to use two observed parameters namely the total raindrop number per unit volume NT and the median volume diameter D0 to show that the Z and rain rate R (Z-R) relationship variability in samples taken throughout rainy event scale and at different climatic sites, depends only on the combined effect of the raindrop size and number, an effect taken into account by the ratio of these two DSD integrated variables

Summary

Introduction

In the study of rainfall, one parameter of interest to estimate with regard to rain drop size distributions (DSD) is the rain rate R It can be measured from the ground using rain gauges and weather radar.The nature of space-time radar measurements represented by the reflectivity factor Z has generated so much interest that many studies have focused on finding connections to bring it in intensity [1,2,3,4,5,6,7,8,9,10,11,12].numerous studies based on the measurement of DSD in precipitation around the world continue to show that empirical expression of the form Z ARb is a suitable relation to describe the relationship between these two parameters. Since Z-R relations proliferate in literature, it is difficult to know if an instantaneous rain rate calculated with a fixed Z-R relation is necessarily correct, even if rainfall accumulations should be reasonable

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Conclusion