A new METEOSAT "water vapor" archive for climate studies

Abstract

Water vapor plays an important role in the climate system through a number of mechanisms spanning a wide range of space and timescale. Since 1977, the METEOSAT satellites are equipped with a radiometer dedicated to the measurements of upper tropospheric humidity (UTH) which covers a relevant range of scales for a better understanding of the water vapor role in the climate. Due to the changes of the satellites and the calibration techniques over the last 20 years, this water vapor METEOSAT archive is not homogeneous and cannot be directly used for climatic studies. Hence the authors present in this paper a newly homogenized METEOSAT water vapor channel archive. Two main types of anomalies entail the original METEOSAT archive. The first one corresponds to the successive improvements of the calibration procedure. In this case, a statistical correction technique based on comparisons between ECMWF-simulated brightness temperature (BT) and water vapor METEOSAT-observed BT is developed. The second type of anomaly concerns the METEOSAT radiometer changes over the time. While still measuring the UTH, the details of the filter function, indeed, evolved over the last 20 years. In this second case, the correction is based on a physical method implying simulations of the same scene by different radiometer filter functions. Two major cases are documented in detail for September 1987 and for February 1994. Sensitivity analysis of the techniques is conducted and the methods are shown to be robust with respect to the details of their implementations. The efficiency of the two methods is then evaluated. The resulting archive reveals water vapor seasonal cycle features in better agreement with climatological estimates. The new homogenized METEOSAT archive consists of 3-hourly total sky radiance at the 0.625° × 0.625° resolution over the July 1983 to February 1994 period, offering the opportunity to investigate the variability of the regional UTH from synoptic scales to interannual and interdecadal scales.