Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 3 2004 10.5194/acpd-4-2727-2004 http://www.atmos-chem-phys-discuss.net/4/2727/2004/ http://www.atmos-chem-phys-discuss.net/4/2727/2004/acpd-4-2727-2004.html http://www.atmos-chem-phys-discuss.net/4/2727/2004/acpd-4-2727-2004.pdf 2727 2745 2004-05-17 On the decadal increase in the tropical mean outgoing longwave radiation for the period 1984–2000 D. Hatzidimitriou I. Vardavas vardavas@iesl.forth.gr K. G. Pavlakis N. Hatzianastassiou C. Matsoukas E. Drakakis Department of Physics, University of Crete, Heraklion, Crete, Greece Foundation for Research and Technology-Hellas, Heraklion, Crete, Greece Department of Physics, University of Ioannina, Greece Department of Electrical Engineering, Technological Educational Institute of Crete, Greece In the present paper, we have calculated the outgoing longwave radiation at the top of the atmosphere (OLR at TOA) using a deterministic radiation transfer model, cloud data from ISCCP-D, and atmospheric temperature and humidity data from NCEP/NCAR reanalysis, for the seventeen-year period 1984–2000. We constructed anomaly time-series of the OLR at TOA, as well as of all of the key input climatological data, averaged in the tropical region between 20° N and 20° S. We compared the anomaly time-series of the model calculated OLR at TOA with that obtained from the ERBE S-10N (WFOV NF edition 2) non-scanner measurements. The model results display very similar seasonal and inter-annual variability as the ERBS data, and indicate a decadal increase of OLR at TOA of 1.9±0.2 Wm^−2/decade, which is lower than that displayed by the ERBS time-series (3.5±0.3 Wm^−2). Analysis of the inter-annual and long-term variability of the various parameters determining the OLR at TOA, showed that the most important contribution to the observed trend comes from a decrease in high-level cloud cover over the period 1984–2000, followed by an apparent drying of the upper troposphere and a decrease in low-level cloudiness. Opposite but small trends are introduced by a decrease in low-level cloud top pressure, an apparent cooling of the lower stratosphere (at the 50 mbar level) and a small decadal increase in mid-level cloud cover.