Using diurnal variation in water vapor HDO from satellite observations to trace evapotranspiration dynamics

Abstract:

The water isotopologue HDO is a tracer of water cycle processes including land surface evaporation and transpiration. We use satellite observations to quantify the diurnal variation of HDO and H2O in the atmospheric boundary layer (surface to 800 hPa), as this diurnal variation is associated with evaporation and transpiration (Ueta et al. 2013) and has not been extensively explored on a global scale. We characterize the spatial, seasonal, and interannual patterns of the diurnal variation of δD (a measure of HDO abundance) and we investigate the probable drivers of the diurnal variation in different regions and seasons, with particular focus on the Amazon basin. We analyze nine years (September 2004 to June 2013) of TES Lite data from the Aura satellite. We find that the average diurnal variation for each hemisphere changes seasonally from ~ 120 ‰ during each hemisphere’s respective winter to ~ -5 ‰ in the summer. In the northern hemisphere, observations over land follow a similar but damped cycle to the aggregate (ocean and land) data, with winter values of ~ 100 ‰ and summer values ~ 10‰. Average diurnal variation for land in the southern hemisphere is ~ 30‰, but does not follow a clear annual cycle. Diurnal variation of δD in the Amazon basin does not significantly correlate with southern hemisphere, or global trends. Surface temperature changes, vegetation type and amount, the rate of transpiration relative to evaporation, and transport of water vapor by lateral advection and convective entrainment all may contribute to observed diurnal variation. We assess these possible mechanisms using an atmospheric box model, and estimate the importance of each mechanism for different regions.