4D-Var assimilation of MIPAS chemical observations: ozone and nitrogen dioxide analyses

[摘要] This paper discusses the global analyses of stratospheric ozone (O₃) andnitrogen dioxide (NO₂) obtained by the Belgian Assimilation System forChemical Observations from Envisat (BASCOE). Based on a chemistry transportmodel (CTM) and the 4-dimensional variational (4D-Var) method, BASCOE hasassimilated chemical observations of O₃, NO₂, HNO₃, N₂O, CH₄and H₂O, made between July 2002 and March 2004 by the MichelsonInterferometer for Passive Atmospheric Sounding (MIPAS) onboard the EuropeanSpace Agency (ESA) Environment Satellite (ENVISAT). This corresponds to theentire period during which MIPAS was operating at its nominal resolution.

Our analyses are evaluated against assimilated MIPAS data and independentHALOE (HALogen Occultation Experiment) and POAM-III (Polar Ozone and AerosolMeasurement) satellite data. A good agreement is generally found between theanalyses and these datasets, in both cases within the estimated error bars ofthe observations. The benefit of data assimilation is also evaluated bycomparing a BASCOE free model run with MIPAS observations. For O₃, thegain from the assimilation is significant during ozone hole conditions, andin the lower stratosphere. Elsewhere, the assimilation does not providesignificant improvement. For NO₂, the gain from the assimilation isrealized through most of the stratosphere. Using the BASCOE analyses, weestimate the differences between MIPAS data and independent data from HALOEand POAM-III, and find results close to those obtained by classicalvalidation methods involving only direct measurement-to-measurementcomparisons. Our results extend and reinforce previous MIPAS data validationefforts by taking into account a much larger variety of atmospheric statesand measurement conditions.

This study discusses possible further developments of the BASCOE dataassimilation system; these concern the horizontal resolution, a betterfiltering of NO₂ observations, and the photolysis calculation near the lidof the model. The ozone analyses are part of the PROMOTE project and arepublicly available via the BASCOE website (http://www.bascoe.oma.be/promote/).

[学科分类] 大气科学