The GPM Core Spacecraft will fly two precipitation instruments: the GPM Microwave Imager (GMI) and the Dual-frequency Precipitation Radar (DPR). Together, these instruments will provide a unique capability for measuring precipitation falling as light rain or snow—conditions that have been difficult to detect using previous instruments. Compared to the earlier generation of instruments, the new capabilities of the GMI and DPR are enabled by the addition of high frequency channels (165.6 and 183.3 GHz) on the GMI, and the inclusion of a Ka-band (35.5 GHz) radar on the DPR.

While the new capabilities of the GMI and DPR provide unique opportunities for observing the atmosphere, they also present new challenges for turning these observations into accurate estimates of precipitation. The GPM Ground Validation (GV) team is helping to meet this challenge by using ground observations and numerical simulations.

Investigators supported by GPM developed computer models to simulate both space-based GPM GMI and DPR observations and coincident ground-based observations by ground-based radar and radiometer observations. Figure 1 illustrates one of the DPR model simulations. In this case, the input fields for the model were provided by a high-resolution cloud resolving model. The DPR simulator calculated radar reflectivity and other parameters (e.g., rain and rain drop size) at range gates through the model atmosphere. The figure compares simulated ground-based S-band radar observations to simulated space-based DPR Ku-band observations. S-band ground radars are commonly used in the United Sates as part of the national meteorological network. During the GPM mission, it is expected that data from ground and space-based radars will often be combined for the purpose of validation, for weather forecasting, and for other scientific and operational applications.

Figure 1. This figure compares simulated radar observations from the DPR Ku-band instrument (top) to simulated ground-based S-band radar observations (bottom). Note that the DPR provides good resolution in the vertical dimension, but that the DPR observations are “smeared” in the horizontal dimension compared to the S-band radar. Both raw and attenuation corrected observations were simulated (left and right panels, respectively). Note: click on image panels above to view larger version of observation figures.

Other model runs (not illustrated here) were also performed comparing simulated GMI radiometer data to simulated ground and aircraft-based observations. Dr. Robert Meneghini and coworkers at NASA’s Goddard Space Flight Center (GSFC) performed the radar simulations, while Dr. William Olson and coworkers (also resident at NASA GSFC) performed the radiometer simulations.

In the near term, these simulations will be augmented with additional capabilities, including surface scattering and randomizing features to model the effects of potential error sources. The introduction of simulated errors will help make the model’s data more realistic—i.e., increasingly similar to the type of measurements expected from GPM. "Adjoint" models that determine how sensitive measurements are to precipitation and other components of the atmosphere will also be implemented. In time, data from the simulations will be used to evaluate the algorithms that scientists will use to retrieve precipitation information from GPM GMI and DPR measurements. The simulations will also be used to evaluate the effectiveness of ground validation instruments and data processing methods prior to their deployment in field campaigns.

By Mathew Schwaller/GPM Project

For further information on this topic, please contact the author (mathew.r.schwaller@nasa.gov).

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