To develop realistic real-time and historical precipitation estimates from the WSR-88D, a number of complex meteorological issues must be resolved. Toward that end, Computational Geosciences Inc. has developed an enhanced Radar Precipitation Interface (RPI) for hydrological modeling. The purpose of the RPI is to provide high spatial resolution estimates of meteorological fields (including precipitation, air temperature, humidity, wind speed and direction, solar radiation, and atmospheric pressure) for hydrological monitoring in both real-time and historical settings. It is designed to be a stand-alone product, without requiring user intervention, that can operate unattended for real-time applications or can be enhanced with user-supplied inputs for reassessments of significant climatological or hydrological events.
To develop the precipitation estimates, the RPI incorporates Level III Digital Precipitation Array (DPA) data from the network of WSR-88D radars that provide coverage of the area of interest. These estimates are combined with gage data from the radar coverage umbrella to produce a calibrated radar-gage precipitation product. This calibrated precipitation estimate incorporates adjustments for measurement biases in the gage data and allows inclusion of user-defined, virtual gage information so that the field may be manually-adjusted by end-users, if necessary. In particular, these virtual gages allow for the incorporation of ancillary precipitation data which may not be transmitted electronically.
In summary, the radar DPA data provide a spatial "footprint" of the precipitation extent while the gage data allow for a real-time reassessment of the reflectivity-to-rainfall (Z/R) conversion equation. It is assumed that the functional relationship is still exponential although the coefficients are calibrated using gage/radar pairs. Distance also plays a key role in the attenuation of the radar beam and so it too is included in the calibration. Results of the gage-radar composite precipitation product are thus consistent with the gage observations and provide the high spatial resolution character of the radar data.
The RPI incorporates a sophisticated Graphical User Interface for setup, execution, and maintenance of the system and an Application Programming Interface for models to acquire precipitation and other meteorological data. This presentation, however, will focus on the meteorological issues that must be addressed to generate real-time and historical precipitation estimates exploiting the advantages of Doppler radar- and gage-based precipitation data and alleviating their disadvantages