Abstract
Abstract. This paper discusses the global analyses of stratospheric ozone (O3) and nitrogen dioxide (NO2) obtained by the Belgian Assimilation System for Chemical Observations from Envisat (BASCOE). Based on a chemistry transport model (CTM) and the 4-dimensional variational (4D-Var) method, BASCOE has assimilated chemical observations of O3, NO2, HNO3, N2O, CH4 and H2O, made between July 2002 and March 2004 by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) onboard the European Space Agency (ESA) Environment Satellite (ENVISAT). This corresponds to the entire period during which MIPAS was operating at its nominal resolution. Our analyses are evaluated against assimilated MIPAS data and independent HALOE (HALogen Occultation Experiment) and POAM-III (Polar Ozone and Aerosol Measurement) satellite data. A good agreement is generally found between the analyses and these datasets, in both cases within the estimated error bars of the observations. The benefit of data assimilation is also evaluated by comparing a BASCOE free model run with MIPAS observations. For O3, the gain from the assimilation is significant during ozone hole conditions, and in the lower stratosphere. Elsewhere, the assimilation does not provide significant improvement. For NO2, the gain from the assimilation is realized through most of the stratosphere. Using the BASCOE analyses, we estimate the differences between MIPAS data and independent data from HALOE and POAM-III, and find results close to those obtained by classical validation methods involving only direct measurement-to-measurement comparisons. Our results extend and reinforce previous MIPAS data validation efforts by taking into account a much larger variety of atmospheric states and measurement conditions. This study discusses possible further developments of the BASCOE data assimilation system; these concern the horizontal resolution, a better filtering of NO2 observations, and the photolysis calculation near the lid of the model. The ozone analyses are part of the PROMOTE project and are publicly available via the BASCOE website (http://www.bascoe.oma.be/promote/).
Highlights
Data assimilation is a set of well-known methods that are used to map observations onto a regular grid using the laws of the atmosphere embodied in a numerical model
What is new in this paper is: (1) the extended assimilation period which allows us to derive robust statistics valid for the widest range of atmospheric states and measurement conditions; (2) the monitoring procedure, which allows us to evaluate the datasets in an optimal manner; and (3) the use of BASCOE analyses to derive differences between Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and independent data from HALOE and POAM-III, even in the absence of direct collocation of the air masses measured by the different satellites
Three satellite datasets are used in this study: MIPAS data are assimilated by the BASCOE system to constrain its chemistry transport model (CTM) outputs, while HALOE and POAM-III data are monitored by the system and used for a posteriori evaluation of the BASCOE analyses
Summary
Data assimilation is a set of well-known methods that are used to map observations onto a regular grid using the laws of the atmosphere (or other system of interest) embodied in a numerical model. For the first time, an assimilation of the entire MIPAS level-2 data record available at nominal resolution has been done This record includes the vertical profiles of the six retrieved chemical species from July 2002 to March 2004 (i.e. 21 months). What is new in this paper is: (1) the extended assimilation period (especially for NO2) which allows us to derive robust statistics valid for the widest range of atmospheric states and measurement conditions; (2) the monitoring procedure, which allows us to evaluate the datasets in an optimal manner; and (3) the use of BASCOE analyses to derive differences between MIPAS and independent data from HALOE and POAM-III, even in the absence of direct collocation of the air masses measured by the different satellites.
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