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 understood. For example, direct evaporation of rain contributes significantly to the heat and moisture budgets of clouds, but few observations of these processes are available. Similarly, the relative contributions to atmospheric moisture over land from local evaporation and humidity from oceanic sources are uncertain. Lighter isotopes of water vapour preferentially evaporate whereas heavier isotopes preferentially condense 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 spacecraft, 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 precipitation, 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.

The MUSICA MetOp/IASI H<sub>2</sub>O and δD products: characterisation and long-term comparison to NDACC/FTIR data

“…In the meanwhile ground-based FTIR (Fourier Transform InfraRed) instruments (Schneider et al, 2006 have been used for measuring δD in the lower and middle troposphere. Furthermore, there are space-borne scientific sensors that measure middle tropospheric δD (Worden et al, 2007) and δD at and above the upper troposphere and lower stratosphere (e.g. Steinwagner et al, 2007;Payne et al, 2007;Lossow et al, 2011).…”

“…But different effects on the energy budget are still not clear such as rainfall evaporation (e.g. Worden et al, 2007) or radiative impacts depending on the present water phase (e.g. Trenberth et al, 2009).…”

Characterizing moisture exchange between the Hawaiian convective boundary layer and free troposphere using stable isotopes in water

“…Because of saturation vapor pressure differences between heavy and light isotopologues, water vapor becomes depleted in isotopically heavy molecules during condensation and precipitation [Bigeleisen, 1961;Dansgaard, 1964]. As a result, stable isotope ratios in water help identify processes that moisten and dehydrate the atmosphere [Worden et al, 2007;Noone, 2012] and distinguish air masses by their integrated condensation histories [Gat, 1996].…”

Observation of tropospheric δD by IASI over western Siberia: comparison with a general circulation model

“…This method also has the large advantage of minimizing the error on the δD profile (e.g. Schneider et al, 2006) as demonstrated for example by Worden et al (2006Worden et al ( , 2007Worden et al ( , 2012 on TES measurements, on IASI measurements by Schneider and Hase (2011) and Lacour et al (2012), or for ground-based measurements by Schneider et al (2006Schneider et al ( , 2010Schneider et al ( , 2012.…”

“…Worden et al, 2006Worden et al, , 2007 whereas the SCanning Imaging Absorption spectrometer for Atmospheric CartograpHY (SCIAMACHY) instrument provided total column distributions, with sensitivity down to the boundary layer (Frankenberg et al, 2009). Both TES and SCIAMACHY distributions required significant time averaging to reach the global scale, which puts a limit on what can be expected for monitoring the short-term variability of water.…”