Spaceborne Radiometry Intercalibration Variability and Dependence on Geophysical Parameter

Abstract:

Intercalibrating spaceborne microwave radiometers is critical for integrating data from multiple instruments to provide additional spatial coverage, improved temporal resolution, and increased record length for the study of weather and climate relative to data from a single instrument. In order to make proper use of the data, it is important to quantify intercalibration temporal and spatial variability and identify root causes. In this study, we present useful methods for characterizing intercalibration variability and investigating calibration dependence on geophysical parameters. We apply the methods to intercalibrate the constellation radiometers of the Global Precipitation Measurement (GPM) mission.

We discovered a pronounced signal with ~40-day period in GPM intercalibration. This 40-day signal is related to the variability of geophysical parameters including water vapor, surface wind speed (SWS), and sea surface temperature (SST). GPM intercalibration makes use of collocations between each radiometer in the constellation and the GPM Microwave Imager (GMI) to compute offsets of each with respect to GMI. The position of these collocations varies cyclically based on the orbit of the constellation radiometer and GMI. Geophysical parameters vary accordingly, with this variability propagating into intercalibration in a manner that is not removed with the double difference method. The computed intercalibration offsets with respect to GMI show nonlinear and non-monotonic dependence on geophysical parameters even for the calm conditions (low water vapor, SWS) used for intercalibration. These results imply issues with the radiative transfer models and ancillary data representing geophysical fields.