COTTEE NERC project

Principal Investigator: Prof Martyn Chipperfield, Beatriz Monge-Sanz, Adrian Simmons (ECMWF)

Start Date: Jan 2008
End Date: Mar 2011

Abstract

Meteorological analyses of winds and temperature play an important and pervasive role in studies of chemistry-transport in the stratosphere and troposphere. Essentially all trajectory and off-line 3D model studies (e.g. to quantify the impact of stratospheric ozone transport on surface air quality, studies of tropical dehydration, etc.) depend critically on the accuracy of the analyses being used to force the models. While atmospheric chemical transport modellers thus rely strongly on these analyses, realistic (longer term) tracer transport is not a main priority of the meteorological centres who produce them. Their primary concerns are current weather and short-term forecasts. However, these centres now understand that tracer transport diagnostics are important additional tests of their analyses.

Long-term reanalysis datasets (e.g. ECMWF ERA-40) have a huge scientific potential to help understand atmospheric processes over the past few decades. Their availability has initiated many studies but has also highlighted serious problems when used for tracer transport. These include a too-fast stratospheric (Brewer-Dobson) circulation and too rapid stratosphere-troposphere transport. (Re)analyses such as these therefore need to be further tested and improved.

This project will use an off-line 3D chemical transport/trajectory model (TOMCAT/SLIMCAT), in close cooperation with three meteorological centres, to test and improve tracer transport by (re)analyses and to derive key transport rates. Our specific objectives are to:

• Test the tracer transport performance of currently available (re)analyses from ECMWF and project partners UK Met Office and NASA/GEOS. Perform a systematic intercomparison for studies of (i) stratospheric transport, (ii) strat-trop transport, and (iii) transport in the TTL.
• Determine the cause of improvements between different assimilated datasets. Perform new analysis experiments with the ECMWF system to test (and isolate) the impact of specific changes in the assimilation. Feed information on optimum configuration of assimilation system for tracer studies into discussions on future reanalyses (e.g. successor to ERA-40/ERA-Interim).
• Quantify the rate and destination of air transiting the TTL using the better performing meteorological analyses and different methods of applying them in CTM/trajectory model.

This project will result in a better understanding of the quality of tracer transport by various meteorological analyses.

Links
http://www.ecmwf.int