Importance of rain evaporation and continental convection in the tropical water cycle

Abstract

Atmospheric moisture cycling is an important aspect of the Earth’s climate system, yet the processes determining atmospheric humidity are poorly understood1,2,3,4. For example, direct evaporation of rain contributes significantly to the heat and moisture budgets of clouds5, but few observations of these processes are available6. Similarly, the relative contributions to atmospheric moisture over land from local evaporation and humidity from oceanic sources are uncertain3,7. Lighter isotopes of water vapour preferentially evaporate whereas heavier isotopes preferentially condense8,9,10 and the isotopic composition of ocean water is known. Here we use this information combined with global measurements of the isotopic composition of tropospheric water vapour from the Tropospheric Emission Spectrometer (TES) aboard the Aura spacecraft11,12, to investigate aspects of the atmospheric hydrological cycle that are not well constrained by observations of precipitation or atmospheric vapour content. Our measurements of the isotopic composition of water vapour near tropical clouds suggest that rainfall evaporation contributes significantly to lower troposphere humidity, with typically 20% and up to 50% of rainfall evaporating near convective clouds. Over the tropical continents the isotopic signature of tropospheric water vapour differs significantly from that of precipitation8,10,13, suggesting that convection of vapour from both oceanic sources and evapotranspiration are the dominant moisture sources. Our measurements allow an assessment of the intensity of the present hydrological cycle and will help identify any future changes as they occur.

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Figure 1: Global distribution of TES observations averaged vertically between 550 and 800 hPa.
Figure 2: Scatter plots of δD versus q H 2 O reveal underlying hydrologic processes.
Figure 3: Contrast between cloudy and clear sky ocean, and continental observations.

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Acknowledgements

We thank W. Read, D. Waliser, H. Su, F. Li, E. Fetzer and B. Kahn for discussions on this work, and C. Still, J. Rial and W. Riley for comments on earlier versions of this manuscript. The research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration, and at the University of Colorado.

Author Contributions J.W. and K.B. were responsible for the spectroscopic retrievals of the HDO and H2O profiles and data quality assurance. D.N. developed the isotopic models and led interpretation of the data. The TES team (see below) helped with the development, analysis and validation of the TES data.

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Correspondence to David Noone.

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Worden, J., Noone, D., Bowman, K. et al. Importance of rain evaporation and continental convection in the tropical water cycle. Nature 445, 528–532 (2007) doi:10.1038/nature05508

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