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An Algorithm to Generate Deep-Layer Temperatures from Microwave Satellite Observations for the Purpose of Monitoring Climate Change

Mitchell D. GoldbergNOAA/National Environmental Satellite, Data, and Information Service, Satellite Research Laboratory, Washington, DC

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Henry E. FlemingNOAA/National Environmental Satellite, Data, and Information Service, Satellite Research Laboratory, Washington, DC

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Abstract

An algorithm for generating deep-layer mean temperatures from satellite-observed microwave observations is presented. Unlike traditional temperature retrieval methods, this algorithm does not require a first guess temperature of the ambient atmosphere. By eliminating the first guess a potentially systematic source of error has been removed. The algorithm is expected to yield long-term records that are suitable for detecting small changes in climate.

The atmospheric contribution to the deep-layer mean temperature is given by the averaging kernel. The algorithm computes the coefficients that will best approximate a desired averaging kernel from a linear combination of the satellite radiometer's weighting functions. The coefficients are then applied to the measurements to yield the deep-layer mean temperature. Three constraints were used in deriving the algorithm: 1) the sum of the coefficients must be one, 2) the noise of the product is minimized, and 3) the shape of the approximated averaging kernel is well behaved. Note that a trade-off between constraints 2 and 3 is unavoidable.

The algorithm can also be used to combine measurements from a future sensor [i.e., the 20-channel Advanced Microwave Sounding Unit (AMSU)] to yield the same averaging kernel as that based on an earlier sensor [i.e., the 4-channel Microwave Sounding Unit (MSU)]. This will allow a time series of deep-layer mean temperatures based on MSU measurements to be continued with AMSU measurements. The AMSU is expected to replace the MSU in 1996.

Abstract

An algorithm for generating deep-layer mean temperatures from satellite-observed microwave observations is presented. Unlike traditional temperature retrieval methods, this algorithm does not require a first guess temperature of the ambient atmosphere. By eliminating the first guess a potentially systematic source of error has been removed. The algorithm is expected to yield long-term records that are suitable for detecting small changes in climate.

The atmospheric contribution to the deep-layer mean temperature is given by the averaging kernel. The algorithm computes the coefficients that will best approximate a desired averaging kernel from a linear combination of the satellite radiometer's weighting functions. The coefficients are then applied to the measurements to yield the deep-layer mean temperature. Three constraints were used in deriving the algorithm: 1) the sum of the coefficients must be one, 2) the noise of the product is minimized, and 3) the shape of the approximated averaging kernel is well behaved. Note that a trade-off between constraints 2 and 3 is unavoidable.

The algorithm can also be used to combine measurements from a future sensor [i.e., the 20-channel Advanced Microwave Sounding Unit (AMSU)] to yield the same averaging kernel as that based on an earlier sensor [i.e., the 4-channel Microwave Sounding Unit (MSU)]. This will allow a time series of deep-layer mean temperatures based on MSU measurements to be continued with AMSU measurements. The AMSU is expected to replace the MSU in 1996.

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