The impact of short timescale temperature fluctuations on dehydration in the TTL

The tropical tropopause temperatures are characterised by frequent bursts of anomalously low values lasting up to 10 days. Some extremely low temperatures have been linked with wave response to convection and others to the effects of stratospheric sudden warmings. The impact of these short-timescale temperature fluctuations, or "pulses" in the tropical tropopause layer (TTL) on tropical stratospheric water vapour distributions is investigated in this modelling study for DJF 2008-2012. We use the advection-condensation (A-C) approach to predict water vapour distribution from back trajectories calculated using ECMWF ERA-Interim reanalysis temperature and wind fields, employing a microphysical box model along trajectories to estimate mixing ratio at trajectory end-point. The microphysical model includes a nucleation threshold and gravitational removal of condensates based on particle size. Using this method, we produce a high vertical resolution picture of the water vapour distribution in the tropical upper troposphere/lower stratosphere, showing remarkable range of zonal variations of water vapour in the TTL. Model results are compared to Aura MLS H2O, taking into account the effect of MLS kernel averaging. We find model results show the effects of the "pulsed" temperature events on stratospheric water vapour are limited in time, and offer little evidence for these to control the overall water vapour distribution in NH winter. However, there are zonal discrepancies between model results and observations whose origins are investigated.