New insights into the hydrological cycle from IASI δD distributions across the globe
Abstract
Because they have different vapour pressures, the different isotopologues of water (H2
16O and HDO)
preferentially evaporate or condense during phase changes, leading to a fractionation of their ratio
(δD). Numerous key processes of the hydrological cycle - such as convection, re-evaporation of
hydrometeors, evapotranspiration, mixing - mark distinctly the isotopic ratio of the air masses.
Observations of δD constitute therefore a useful constraint on the moistening/drying processes. IASI
provide observations of δD in the free troposphere, after inversion of the measured radiances, at an
unprecedented resolution.
In this presentation, we illustrate the IASI capabilities to retrieve δD in the troposphere and show first
distributions across the globe. The interest of these distributions is presented by analyzing the
information contained in δD. For example, we show that the height of convection gives the water
vapour a particular isotopic fingerprint which is of great interest to evaluate the representation of
tropospheric mixing in climate models. We also present long term (6 years) time-series in the Northern
Atlantic. The seasonal and inter-annual variations are interpreted with backward trajectory analyses
and we show that the Saharan dynamic strongly influence the isotopic composition above the Atlantic
Ocean through the formation of the Saharan Heat Low.