A Z-R Relationship for the GATE B-Scale Array

Abstract

A better understanding of how the precipitation budget operates in tropical convective systems is a prime objective of the GATE research effort. Measurement of rainfall rate with shipboard radar is the principal method by which precipitation from tropical clouds that develop within the GATE B-scale array will be determined. Knowledge of the relationship between radar reflectivity (Z) and rainfall rate (R) is essential for an accurate interpretation of precipitation data derived through the use of radar technology. The Z-R relationship is determined through application of a least-squares linear regression to data points derived by appropriate integration of the third and sixth moments of a series of raindrop size spectra. Drop spectra measurements were obtained during GATE by means of a foil impactor operated at cloud-base level on board the NOAA DC-6 aircraft. A total of 107 Z-R data points are available, representing showers occurring on 12 days. The best-fit Z-R relationship for the cloud-base aircraft foil data showed little variability from day to day or on the basis of stratification by rain rate. For all foil data combined, the best-fit Z-R relationship was found to have the form Z=170 R1.52, which gives, for example, rain rates of 66, 15 and 3 mm h?1 for Z values of 50, 40 and 30 dBZ, respectively.