Optimal Convolution of AMSU-B to AMSU-A

Abstract

In order to find an optimal convolution of the Advanced Microwave Sounding Unit (AMSU) -B to AMSU-A resolution the scan characteristics of AMSU-A and AMSU-B on board NOAA-15 are examined. A set of coefficients for this degradation is derived using the Backus?Gilbert technique. A 7 ? 7 set of adjacent AMSU-B pixels is used where the center pixel is the one closest to a given AMSU-A observation. The error characteristics of the convolution are investigated and except for the two outermost footprints a good reproduction of the spatial sensitivity of the AMSU-A by the convolved AMSU-B is obtained. For a NOAA-15 overpass over inhomogeneous terrain AMSU-A data at 89 GHz were compared to convolved AMSU-B data at the same frequency. The root-mean-square deviation between the so-convolved AMSU-B data and the AMSU-A data was on average 1.7 K, including a systematic deviation of ?1 K of AMSU-B to AMSU-A. In comparison, simple, equally weighted averages of AMSU-B data produce rms errors in the order of 4 K and large deviations in regions where gradients in the brightness temperatures occur. To apply the Backus?Gilbert technique the AMSU?s effective field of view (EFOV) as a function of the scan position was determined. For the continuously scanning AMSU-B the integration time of 18 ms per observation in conjunction with the sensors rotation leads to a considerable broadening of the antenna pattern in cross-track direction and thus to an increase of the EFOV as compared to the instantaneous field of view (IFOV). This does not occur for the stepwise scanning AMSU-A where the IFOV equals the EFOV (neglecting the second-order effects induced by the ?1-km movement of the subsatellite point during AMSU-A integration). Analytical expressions to calculate the AMSU-A and AMSU-B footprint sizes as functions of their respective scan positions were derived. These expressions exhibit rms deviations to the actual footprint size of 0.5 km with maximum deviations of less than 1 km.