Microwave Radiometer Inter-Calibration: Algorithm Development and Application.

Abstract

Microwave radiometer inter-calibration is an essential component of any effort to combine measurements from two or more radiometers into one dataset for scientific studies. One spaceborne instrument in low Earth orbit is not sufficient to perform long-term climate studies or to provide measurements more than twice per day at any given location on Earth. Measurements from several radiometers are necessary for analyses over extended temporal and spatial ranges. In order to combine the measurements, the radiometers need to be inter-calibrated due to the instruments having unique instrument designs and calibrations. Inter-calibration ensures that consistent scientific parameters are retrieved from the radiometers. The development of a cold end inter-calibration algorithm is presented. The algorithm makes use of vicarious cold calibration, along with the double difference method, to calculate calibration differences between radiometers. The performance of the algorithm is characterized using data from current conical scanning microwave radiometers. The vicarious cold calibration double difference is able to sufficiently account for design differences between two radiometers including frequency, earth incidence angle, and orbital characteristics. An estimate of the uncertainty in the inter-calibration algorithm is given as a result of potential errors in the geophysical inputs and improper accounting of seasonal and diurnal variability. The vicarious cold calibration double difference method is shown to be a valid and accurate inter-calibration algorithm. Results are compared with calibration differences calculated using alternate algorithms and sufficient agreement is attained. Inter-calibration is shown to be necessary for achieving consistency in retrieved scientific parameters by using the vicarious cold calibration double difference method to inter-calibrate two radiometers that are then used to derive rain accumulations. Inter-calibration results in a significant improvement in the rain accumulation agreement between the radiometers. This validates inter-calibration algorithm development and shows that it has a positive impact on achieving consistency in scientific parameter retrievals.