Most air enters the stratosphere in the tropical tropopause region, where temperatures are low, and the resulting dehydration through freeze-drying reduces water vapour concentrations to very small values. Notwithstanding the very low concentrations, stratospheric water vapour is important in the chemistry-climate system through its role in stratospheric ozone chemistry and also through its effects on the radiative balance of the troposphere. Model simulation of past and future changes depend on correct simulation of both the temperature distribution in the tropical tropopause region and the pathways taken by air parcels as they sample this distribution in moving from troposphere to stratosphere. Important aspects of this include both the annual cycle and the longitudinal variation in tropical tropopause temperatures and perhaps variation on intraseasonal and shorter timescales.

The co-operating partners in this project will be the University of Cambridge and the Met Office. Improving simulation of stratospheric water vapour remains a challenge for Met Office Earth System Models that are used for climate prediction. There are strong links between the water vapour distribution in the lower stratosphere and the tropopause temperatures which in turn determine water vapour, so positive feedbacks are possible that may significantly enhance the effects of modest errors in model representation of other relevant processes. The project will build on recent work in Cambridge and elsewhere that (a) has exploited trajectory techniques to examine the annual, interannual and longer-term links between tropopause temperatures and stratospheric water vapour and (b) has investigated the radiative coupling between water vapour and temperatures in the tropical tropopause region using a combination of offline radiative calculations and simple dynamical models. The focus of the project will be to analyse the variations of water vapour on monthly, annual, interannual and longer timescales simulated by the Met Office Unified Model (UM) and link these to the corresponding temperature and transport variations. (One component of this analysis would be
the use of a trajectory code which is already available for the UM.) The results will be compared against corresponding analysis of the recent history of the real atmosphere (some of which is already on record in scientific publications).