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Projects

Improving the representation of the stratosphere in meteorological reanalyses

The stratosphere (15 – 50 km altitude) contains the ozone (O3) layer which, by filtering out harmful ultraviolet radiation, is essential for the support of life on Earth. In addition, the ozone layer, and stratosphere in general through water vapour and other greenhouse gases, plays an important role in regulating the Earth’s climate. Moreover, an accurate representation of the stratosphere is now known to improve the accuracy of medium-range and seasonal weather forecasts (e.g. Scaife et al., 2012). For this reason, major weather centres have found it necessary to include a representation of the stratosphere in their forecast models. Climate reanalyses use the same forecast models with data assimilation systems to represent the evolution of the atmosphere over multi-decadal timescales. However, there are known shortcomings in these current stratospheric schemes.

The European Centre for Medium-Range Weather Forecasts (ECMWF) has just completed a major reanalysis project (ERA5; Hersbach et al., 2020). This has produced a world-leading meteorological dataset from 1950 to the present day which is produced by assimilating observations into a weather prediction model with simplified O3 and H2O chemistry. Satellite data are critically important in this by constraining stratospheric O3 from the start of observations in the 1980s. The ERA5 reanalysis is very widely used for scientific studies, including for simulations of ozone layer depletion (e.g. Chipperfield et al., 2018). However, the ERA5 reanalyses have demonstrated a number of shortcomings in the representation of stratospheric temperatures related to: (i) excess transport of water vapour across the tropical tropopause, (ii) deficiencies in the parametrisation of methane oxidation, and (iii) approximations in the linear ozone parameterisation. These issues can be addressed by evaluating the model using a wealth of height-resolved satellite observations of the stratosphere, such as those from the NASA Microwave Limb Sounder (MLS) and Atmospheric Chemistry Experiment (ACE), for which there is expertise in Edinburgh, and by transferring knowledge from state-of-the-art detailed chemical models developed in Leeds.

Aims and Objectives

The overall aim of the PhD is to use satellite EO, modelling and data analysis to resolve known shortcomings in the latest ECMWF ERA5 reanalysis, thereby helping to prepare for the next-generation ERA6. Specifically the student will:

  • Evaluate the performance of the ECMWF stratospheric water vapour and ozone reanalyses using independent satellite data, such as MLS and ACE.
  • Evaluate and improve the parameterisation of water vapour production from methane.
  • Develop an initialisation scheme for water vapour for the pre-satellite era using machine learning techniques applied to recent data.
  • Use the updated parameterisations of H2O and O3, implemented into global 3-D models (TOMCAT and UK Earth System Model (UKESM1)) at Leeds, to study drivers of long-term changes in the stratosphere over the past decades.

The student will be based at the University of Leeds but will make regular visits to ECMWF in Reading, including for a 3-month placement. This will provide excellent training and first-hand experience of working in a world-leading weather agency.

References

Chipperfield, M.P., et al., On the cause of recent variations in lower stratospheric ozone, Geophys. Res. Lett., 45, 5718-5726, doi:10.1029/2018GL078071, 2018.

Hersbach, H, Bell, B, Berrisford, P, et al., The ERA5 global reanalysis. Q J R Meteorol Soc., 146, 1999– 2049. https://doi.org/10.1002/qj.3803, 2020.

Scaife, A., et al., Climate change projections and stratosphere-troposphere interaction, Climate Dynamics, 38, 9/10, 2089-2097, 10.1007/s00382-011-1080-7, 2012.