ZR Relationships derived from Drop Size Distributions for the Canadian Radar Network

There have been numerous studies of the relationship between the radar reflectivity factor (Z) and rainrate (R) beginning with the original Marshall-Palmer study. Radar reflectivity factor is defined as the sum of the sixth power of the raindrop diameter in a unit volume and depends on the drop size distribution (DSD). The studies have shown a diverse set of ZR relations.

Canada is a geographically large and climatologically diverse country. The strategy of implementing any ZR strategy, in the Canadian Radar Network, depends on the climatology of the precipitation systems and therefore on actual measured drop size distribution's (DSD's) and ZR's. The critical questions are: Is a single ZR relationship appropriate for a vast geographically and climatologically diverse country such as Canada? What is it? Is it regional or universal? What are the errors? Is there justification for multiple ZR relationships for different weather situations in different parts of Canada?

A data set of 1 minute drop size distribution's was collected at 10 climatologically and geographically diverse locations. There is data for an entire year at seven sites, data for another site for the summer months and several years of data at two other sites. The data was collected using the POSS sensor. It was part of a performance evaluation of an automated surface observation suite of sensors. In addition, regular manual human observations were also made. The sensors were well maintained, producing a consistent and high quality data set. While short and isolated DSD's measurements have been made in Canada, this is the first comprehensive data set of this nature allowing for extensive statistical analysis. Comparisons of the accumulated amounts derived from DSDs measured by POSS with conventional raingauges were consistent and comparable for periods when the wind speed is less than 5 knots (measured at sensor height).

Looking at the data set in its entirety, a single relationship appears to be valid for a single site. However, the relationship appears to be best represented by two lines - one for low and one for high rainrates. The relationships appear to be universal across the network. On a rain event basis, there is a correlation between the coefficients 'a' and 'b' in the Z=a R^b relationship. Also there is a correlation between N_o coefficient in the two parameter exponential DSD model and the coefficient 'a'. In addition, the POSS is able to detect drizzle and the drizzle ZR relationship appears to be quite distinct from the rain ZR relationship.