Regional and seasonal variations of the clear sky atmospheric longwave cooling over tropical oceans

The vertical distribution of the clear sky longwave cooling of the atmosphe re over tropical oceans is inferred from three different datasets. Two of t he datasets refer to the TIROS-N Operational Vertical Sounder (TOVS) NOAA/N ASA Pathfinder project, PathA and PathB, and the last one refers to the ECM WF reanalysis (ERA-15). Differences are identified originating from the tem perature and water Vapor fields. They affect the geographical distribution of the longwave fields to various degrees. However, the three datasets lead to similar conclusions concerning the sensitivity of the clear sky total l ongwave cooling to SST variations. For the highest values of the SST (great er than 27 degrees C), positively correlated to the increased efficiency of the longwave trapping (super-greenhouse effect), the atmosphere shows a le sser efficiency to cool radiatively. The atmosphere does reradiate the long wave radiation toward the surface as efficiently as it traps it. This is ve rified on regional as well as on seasonal scales. Such longwave cooling beh avior is due to an increased mid- and upper-tropospheric humidity resulting from convective transports. The three datasets agree with the vertical dis tribution of the radiative cooling variations from normal to favorable to s uper-greenhouse effect conditions, except in the boundary layer, where the coarse resolution of the TOVS-retrieved data makes them not reliable in it. In "normal" conditions the cooling uniformly increases over the vertical w ith the SST. Over 27 degrees C, the cooling is intensified above 400 hPa an d reduced between 900 and 400 hPa.