Institute of Meteorology and Climate Research

MIPAS measurements of deuterated water provide constraints on the role of convection for dehydration processes near the tropical tropopause

MIPAS observations provide global distributions of both H2O and HDO. In the tropics, the well-known 'tape recorder' signal of water vapor (top) is also re-produced by HDO (bottom) which hints, similar as for H2O, towards the seasonally varying tropopause temperature as the controlling parameter for HDO transport from the troposphere into the stratosphere. Hence, the HDO-to-H2O ratio is determined by cirrus formation processes in thermodynamic equilibrium which are described by the so-called Rayleigh fractionation. However, closer examination of the seasonally varying HDO-to-H2O ratios shows that in the stratosphere, they deviate little but with statistical significance from the slope which would be expected from pure Rayleigh fractionation. A seasonally varying contribution from the evaporation of convectively lofted ice can explain this deviation.

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