Processes determining stratospheric water vapour

Lead Research Organisation: University of Cambridge
Department Name: Applied Maths and Theoretical Physics


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
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). In its later stages the project will consider the two-way coupling between tropical tropopause temperatures and water vapour concentrations in the UM and assess the possible implications for model predictions of long-term changes in these quantities. During visits to the Met Office the student will investigate these processes in long historical and scenario simulations of the new UKESM1 earth system model that will support future climate and ozone assessments. The work in the studentship project will provide an opportunity for the student to make a contribution in a scientific area that is both of
fundamental interest and of real practical interest to the Met Office earth-system modelling effort.


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Description Through extensive trajectory calculations, understanding of water vapour variability has been improved in various aspects. This work has clarified the role of key processes on timescales ranging from less than a month to the full 21st century. For example, Smith et al. 2021 reports on the primary processes on sub-monthly, annual and interannual time scales. These results, among others, are useful for climate model development to improve simulation of stratospheric water vapour. New methodologies have been applied with trajectory calculations, and a new research question is raised about the application of observed ice data from remote sensing in trajectory calculations. An outcome of the collaboration with the Met Office, beneficial to them and to the research field, is an in-depth assessment of their climate model's representation of stratospheric water vapour over the 21st century.
Exploitation Route Climate model developers, at the Met Office and elsewhere, might consider the award's results for further development. Researchers of this academic field may also seek to expand on the investigations, particularly the influences of 21st century projections which is a hot topic. Some researchers may also use and develop further the trajectory program developed as part of this award.
Sectors Environment

Description Results have been presented to the UK Met Office for consideration in development of their climate and weather forecasting model.
First Year Of Impact 2020
Sector Environment,Other
Impact Types Policy & public services

Title Lagrangian Dry Point data regarding the sensitivity of stratospheric water vapour to variability in tropical tropopause temperatures and large-scale transport 
Description This dataset contains results in support of a publication that investigates processes affecting water vapour entry to the stratosphere. The back trajectories were calculated using the OFFLINE trajectory model. Past publications have shown the key processes are temperatures in the tropical tropopause layer and large-scale transport into the stratosphere using trajectory methods. Lagrangian Dry Points (LDPs) are normally calculated as the minimum water vapour saturation mixing ratio experienced along a back-trajectory that has traversed from troposphere to tropical lower stratosphere in its recorded history. This study separated the two key processes by sampling alternative temperatures. These alternative temperatures are either time-shifted or averaged in time or longitude. This method is applied for two meteorological datasets: ERA-Interim (ERA-I) reanalysis for the period 1999-2009, and the UM-UKCA chemistry-climate model for eleven years of a repeat-year-2000 forcing scenario. The ERA-I trajectories were calculated by S Fueglistaler and S Liu for separate publications. The UM-UKCA climate model scenario was conducted by A Maycock. This dataset contains only the LDPs resulting from alternative-temperature sampling. The directory UM-UKCA/LDP-original-T/ provides a simple view of the original unmodified method to calculate LDPs. LDP-alt-T/ directories contain LDPs determined with time-shifted alternative temperature samplings. The time-shift is identified by the alternative initialisation date, denoted in the filename and file metadata. LDP-ave-T/ directories contain LDPs determine with averaged alternative temperature samplings. The averaging is identified by the variable name and metadata. In the variable names, shorthand and full-name identifiers include 6h (6 hourly instantaneous), ZM (zonal mean), 30DZM (30-day rolling window mean and zonal mean), 120DM, 90DM, 60DM, 30DM, 15DM, 14DM, 8DM, 7DM, 4DM, 2DM, 1DM (rolling window 120 day mean, 90 day mean, etc.). Note that various alternative temperatures are recorded at each LDP calculated from each alternative temperature. For more information on the directory structure, file naming conventions, variable naming conventions and attribute conventions please see the README.txt. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
Impact Publication: Smith, J. W., Haynes, P. H., Maycock, A. C., Butchart, N., and Bushell, A. C.: Sensitivity of stratospheric water vapour to variability in tropical tropopause temperatures and large-scale transport, Atmos. Chem. Phys., 21, 2469-2489,, 2021. 
Description Met Office iCASE studentship 
Organisation Meteorological Office UK
Country United Kingdom 
Sector Academic/University 
PI Contribution Further research of questions raised by collaborators, supervisors, and myself.
Collaborator Contribution Primarily, their expertise to act as additional supervisors when discussing relevant work. Secondly, provision of data and resources to access and process data.
Impact No specific outputs or outcomes yet; awaiting further work and publication of thesis.
Start Year 2015