Abstract
Abstract. Water vapor in the tropical tropopause layer (TTL) has a local radiative cooling effect. As a source for ice in cirrus clouds, however, it can also indirectly produce infrared heating. Using NASA A-Train satellite measurements of CALIPSO and Aura/MLS we calculated the correlation of water vapor, ice water content and temperature in the TTL. We find that temperature strongly controls water vapor (correlation r=0.94) and cirrus clouds at 100 hPa (r=−0.9). Moreover we observe that the cirrus seasonal cycle is highly (r=−0.9) anticorrelated with the water vapor variation in the TTL, showing higher cloud occurrence during December-January-February. We further investigate the anticorrelation on a regional scale and find that the strong anticorrelation occurs generally in the ITCZ (Intertropical Convergence Zone). The seasonal cycle of the cirrus ice water content is also highly anticorrelated to water vapor (r=−0.91) and our results support the hypothesis that the total water at 100 hPa is roughly constant. Temperature acts as a main regulator for balancing the partition between water vapor and cirrus clouds. Thus, to a large extent, the depleting water vapor in the TTL during DJF is a manifestation of cirrus formation.
Highlights
Water vapor (H2 O) is a key constituent of the Earth atmosphere and climate system
In order to determine to what extent tropical 100 hPa temperatures control tropical and subtropical water vapor we calculated the correlation of the daily zonal mean water vapor and the daily 100 hPa tropical zonal mean temperature
Our results suggest that the tropical 100 hPa tropopause temperature determines the balance between cirrus cloud ice and water vapor at 100 hPa so that total water stays roughly constant
Summary
Water vapor (H2 O) is a key constituent of the Earth atmosphere and climate system. It is the dominant greenhouse gas and has important impacts on atmospheric circulations through latent heat exchanges and redistribution of energy (Schneider et al, 2010). As a source of clouds, water vapor has an indirect effect on the surface longwave and shortwave radiation budget. Water vapor is important for the radiative balance and chemistry in the stratosphere An increase in stratospheric water vapor tends to cool the stratosphere due to more longwave emission to space
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