The ASSET intercomparison of ozone analyses: method and first results
1Data Assimilation Research Centre, University of Reading, Reading, UK
2CNRS Service Aeronomie, Université Pierre et Marie Curie, Paris, France
3European Centre for Medium-Range Weather Forecasts, Reading, UK
4Institut d’Aéronomie Spatiale de Belgique, Brussels, Belgium
5Royal Netherlands Meteorological Institute, De Bilt, The Netherlands
6Met Office, Exeter, UK
7British Atmospheric Data Centre, Rutherford Appleton Laboratory, Chilton, nr Didcot, UK
8CERFACS, Toulouse, France
9CNRM-GAME, Météo-France and CNRS URA 1357, Toulouse, France
Abstract. This paper examines 11 sets of ozone analyses from 7 different data assimilation systems. Two are numerical weather prediction (NWP) systems based on general circulation models (GCMs); the other five use chemistry transport models (CTMs). These systems contain either linearised or detailed ozone chemistry, or no chemistry at all. In most analyses, MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) ozone data are assimilated. Two examples assimilate SCIAMACHY (Scanning Imaging Absorption Spectrometer for Atmospheric Chartography) observations. The analyses are compared to independent ozone observations covering the troposphere, stratosphere and lower mesosphere during the period July to November 2003.
Through most of the stratosphere (50 hPa to 1 hPa), biases are usually within ±10% and standard deviations less than 10% compared to ozonesondes and HALOE (Halogen Occultation Experiment). Biases and standard deviations are larger in the upper-troposphere/lower-stratosphere, in the troposphere, the mesosphere, and the Antarctic ozone hole region. In these regions, some analyses do substantially better than others, and this is mostly due to differences in the models. At the tropical tropopause, many analyses show positive biases and excessive structure in the ozone fields, likely due to known deficiencies in assimilated tropical wind fields and a degradation in MIPAS data at these levels. In the southern hemisphere ozone hole, only the analyses which correctly model heterogeneous ozone depletion are able to reproduce the near-complete ozone destruction over the pole. In the upper-stratosphere and mesosphere (above 5 hPa), some ozone photochemistry schemes caused large but easily remedied biases. The diurnal cycle of ozone in the mesosphere is not captured, except by the one system that includes a detailed treatment of mesospheric chemistry.
In general, similarly good results are obtained no matter what the assimilation method (Kalman filter, three or four dimensional variational methods, direct inversion), or system (CTM or NWP system) and this in part reflects the generally good quality of the MIPAS ozone observations. Analyses based on SCIAMACHY total column are almost as good as the MIPAS analyses; analyses based on SCIAMACHY limb profiles are worse in some areas, due to problems in the SCIAMACHY retrievals.
Using the analyses as a transfer standard, and treating MIPAS observations as point retrievals, it is seen that MIPAS is ~5% higher than HALOE in the mid and upper stratosphere and mesosphere (above 30 hPa), and of order 10% higher than ozonesonde and HALOE in the lower stratosphere (100 hPa to 30 hPa).