Monitoring deep convection and convective overshooting with METEOSAT

Simultaneous observations of deep convective clouds in the infrared window (IR: 10.5 - 12.5 mu m) and the water vapour absorption band (WV: 5.7 - 7.1 mu m) from METEOSAT reveal that the equivalent brightness temperature in the WV channel can be larger than in the IR channel by as much as 6 - 8 K. Calibration errors, cloud microphysics and the effect of the Planck function over horizontally inhomogeneous areas cannot explain the observation. Simulations with a line-by-line radiative transfer model show that the larger brightness temperatures in the WV absorption band are due to stratospheric water vapour, which absorbs radiation from the cold cloud top and emits radiation at higher stratospheric temperatures. The brightness temperature difference depends on the amount of water vapour and on the temperature lapse rate in the stratosphere. The temperature difference is largest when the cloud top is at the tropopause temperature inversion. The tropopause temperature in legions of deep convective clouds can he estimated from the brightness temperature differences observed in consecutive images from geostationary satellites. The method also has the potential for monitoring areas of troposphere-stratosphere exchange. (C) 1997 COSPAR.