Assimilation of ENVISAT data in an advanced data assimilation system

• Main Author: Peubey, Carole
• Published: University of Reading 2006
• Subjects: 551.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485367

i~to a stratosphere-troposphere version of the Met Office assimilation system, producing one of the first analyses to reproduce the September 2002 split of the Antarctic polar vortex. The aim of the project was to investigate the benefit of assimilating MIPAS retrievals and to assess the Met Office 3D-Var assimilation system by examining its different components. The ozone analysis was found to agree with independent ozone observations through most of the middle and upper stratosphere, biases above 60 hPa being within the range -20% to +10% and typically smaller. More significant positive biases were found in the lower stratosphere and inside the polar vortex. Although ozone amounts are shown to be slightly overestimated by MIPAS retrievals in these same regions, these biases are demonstrated to be caused by shortcomings in the model chemistry and transport. MIPAS data have been shown to have a limited impact on the Met Office temperature analysis, although a ' positive effect was identified at the mesopause. It is shown that MIPAScould bring larger benefits if more realistic background error statistics were used for ozone, especially in the lower stratosphere. Based on an evaluation of these statistics using independent datasets, it is suggested that background error variances should be decreased near the ozone maximum and increased below 70 hPa It is also recommended to introduce latitudedependence in vertical error correlations and height-dependence in horizontal error correlations. Improvements are also proposed to improve the ozone assimilation in the polar vortex region. Finally, analysed winds have been found to induce errOneous transport of ozone by increasing vertical diffusion of ozone and enhancing the mean zonal circulations. This especially affects the tropics, where ozone analyses reveal excessive exchanges of air parcels between the stratosphere and the troposphere.