Effects of Data

Precipitation Data
Factors Influencing MAPX Calculations

Objectives

This section covers the evaluation of multisensor precipitation estimates to be used in the computation of basin average precipitation (using a procedure known as MAPX). After completing this section, the reader should know:

the basic principles of radar-based precipitation estimation or multisensor precipitation estimation (MPE).
the sources of error affecting quality and accuracy of radar-based precipitation estimates.
the aspects of multisensor precipitation estimation impacting the accuracy and representativeness of MAPX calculations.

This section is NOT intended to be an all encompassing lesson on the WSR-88D system. In general, it is fair to say that radar gives a good picture of where it is raining. Calibration issues and other phenomena make the quantity estimates somewhat more problematic. This does not imply that radar estimates are incorrect; however, the precipitation estimates derived purely from radar often differ considerably from gage values. Fortunately, multisensor precipitation estimation (MPE) tools are available to produce data sets which capitalize on the strengths of both radar- and gage-based information.

Relatively speaking, radar is one of the newest tools in measuring precipitation. The current radar systems are “Weather Surveillance Radars – 1988 Doppler” (WSR-88D). There are currently more than 160 active WSR-88D sites, as illustrated in Figure 1.

Figure 1 – WSR-88D Locations