The use of saphir on Megha-Tropiques for intercalibration of polar-orbiting microwave water vapor sounders

Abstract : The Megha-Tropiques satellite (MT), a joint program between France and India, was launched on October 12, 2011 into a low inclination (20°) orbit. The payload includes two microwave sensors, Madras, a conically-scanning window channel radiometer and the cross-track-scanning SAPHIR, a water-vapor sounder with 6 channels centered on the 183 GHz water vapor line. Here we are concerned with the use of SAPHIR for intercalibration of sensors within the context of the Global Precipitation Measurement mission (GPM). Some precipitation retrieval algorithms, most notably those concerned with separating falling snow from snow already on the ground use 183 GHz water vapor sounding channels. The GPM Microwave Imager (GMI) includes channels near 183 GHz to be used directly for these retrievals. Also GPM plans to use the water vapor sounders on the polar-orbiting operational satellites, NOAA and METOP for precipitaton retrievals. Thus, GPM will be used to intercalibrate the water vapor sounders. Before the launch of Megha-Tropiques, there were no 183 GHz radiometers in low-inclination orbits to facilitate intercomparison of the operational microwave water vapor sounders. Thus, SAPHIR represents an opportunity to preview the use of GMI for intercalibration of water vapor sounders. Because of the low inclination of MT, there are a great many co-located, nearly-simultaneous observations between MT and the various satellites spread over a range of roughly 25S to 25N. For the sub-set of the data where the incidence angles and frequencies of SAPHIR and the sensor with which it is being compared are close enough to identical, direct comparisons are possible; in other cases, an algorithm is needed. Typically an intercalibration effort begins with examination of each single sensor for reasonableness and self consistency. Images of the data are the most obvious test. One and two dimensional histograms allow us to examine a large volume of data for wild-points. Any sensor particularly one in the early stages of analysis will have some issues and SAPHIR is no exception. The data are examined for scan position dependant biases. Such biases in SAPHIR are very small, well below any reasonable limit for intercalibration work. When matched up with another sensor for intercalibration even more sensitivity is available for detection of cross-track biases. Two primary algorithms are applied for intercalibration: One uses atmospheric profiles derived from a weather forecast analysis and the other retrieves a water vapor profile from SAPHIR. Fundamentally, the two methods only differ in the choice of the water vapor data base. In both cases each member of the database is used to compute brightness temperatures for both SAPHIR and the target sensor (eg. AMSU-B, MHS, ATMS or SSM/IS). Some of the passbands of SAPHIR channels include ozone lines. Analysis of the impacts of these lines requires detailed knowledge of the passbands. Typically the impacts are of the order of a few tenths of a Kelvin. The use of multiple approaches provides confidence and quantitative uncertainty estimates for the intercalibration. These methods are shown to provide sufficient accuracy for precipitation retrieval purposes.
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Submitted on : Saturday, June 14, 2014 - 2:01:00 PM
Last modification on : Wednesday, June 12, 2019 - 11:00:12 AM

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  • HAL Id : hal-01006237, version 1

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Thomas Wilheit, Hélène Brogniez, Saswati Datta, W. Liinwood Jones, Vivienne Payne, et al.. The use of saphir on Megha-Tropiques for intercalibration of polar-orbiting microwave water vapor sounders. 2013 EUMETSAT Meteorological Satellite Conference 19th American Meteorological Society (AMS) Satellite Meteorology, Oceanography, and Climatology Conference: Understanding the past, observing the present and protecting the future, Vienna 2013, Sep 2013, Vienna, Austria. ⟨hal-01006237⟩

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