Variability and trend of mean cloud parameters and outgoing longwave radiation determined by satellite sounders

Global energy balance of the Earth-atmosphere system may change due to natural and man-made climate variations. For example, changes in the outgoing longwave radiation (OLR) can be regarded as a crucial indicator of climate variations. Clouds play an important role -still insufficiently assessed-, in the global energy balance on all spatial and temporal scales, and satellites can provide an ideal platform to measure cloud and large-scale atmospheric variables simultaneously. This is extremely important for developing more reliable cloud models, which could help to improve the representation of cloud-climate feedbacks (probably the least known ones, still hindering global warming predictions, for example) in global general circulation models. The TOVS series of satellites were the first to provide this type of information since 1979 on. OLR [Mehta and Susskind, 1999], cloud cover and cloud top pressure [Susskind et al., 1997] are among the key climatic parameters computed by the TOVS Path A algorithm using mainly the retrieved temperature and moisture profiles. Here we present validation efforts and preliminary trend analyses of TOVS-retrieved cloud top pressures [Pc] and "effective" (Aeff, a product of infrared emissivity at 11 μm and physical cloud cover or Ac) cloud fractions. For example, the TOVS and ISCCP [available since 1983] cloud top pressures correlate strongly. Decadal trends in Pc and Aeff/Ac are also similar. However, validation of the TOVS Aeff time series requires further effort, since the ISCCP provides the Ac time series only. We are also presenting encouraging agreements between MODIS and AIRS [which can be regarded as the "new and improved TOVS"] Pc and Aeff interannual variabilities for selected Months. We also present TOVS and AIRS OLR validation effort results and (for the longer-term TOVS) trend analyses. OLR interannual spatial variabilities from the available state-of-the-art CERES measurements and both from the AIRS [Susskind et al., 2003] and TOVS OLR computations are in remarkably good agreement. Global mean Monthly CERES and TOVS OLR time series show very good agreement in absolute values also. References Mehta, A., and J. Susskind, Outgoing Longwave Radiation from the TOVS Pathfinder Path A Data Set, J. Geophys. Res., {104, NO. D10, 12193-12212. Susskind, J., P. Piraino, L. Rokke, L. Iredell, and A. Mehta, 1997: Characteristics of the TOVS Pathfinder Path A Dataset. Bull. Am. Met Soc., 78, 1449-1472. Susskind, J., C, D. Barnet, and J. M. Blaisdell, Retrieval of Atmospheric and Surface Parameters from AIRS/AMSU/HSB Data in the Presence of Clouds, IEEE Transactions on Geoscience and Remote Sensing, Vol. 41, No. 2, 2003.