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.

For further details see: http://www.nature.com/ngeo/journal/vaop/ncurrent/abs/ngeo822.html