Quentin Errera

Four-dimensional variational chemical assimilation of CRISTA stratospheric measurements

This paper presents a chemical assimilation system based on the variational method. It has been applied to an off-line stratospheric three-dimensional chemical trans- port model. This technique determines the best model initial conditions that minimize the model errors when compared to a set of observations for a predefined time window. The system is applied to the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) measurements for the November 5-11, 1994, mission. Observed species are O3 ,C H4, HNO3 ,C lONO2 ,N 2O5 ,N 2O, and CFC-11. For an assimilation pe- riod of 12 hours, CRISTA observations are well estimated by the assimilation system. These results vary according to the species considered and depend on the CRISTAs ob- servational error. In particular, assimilated ozone differ from CRISTA by less than 5%. Analyses have been compared with independent observations in order to validate the as- similation system. It was found that observations by the Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment and the Halogen Occultation Experiment (HALOE) agree well with the analyses. These comparisons suggested systematic differences between CRISTA and HALOE or ATMOS. Unconstrained constituents like NOx and HCl are in- fluenced through chemical coupling and show small discrepancies with ATMOS and HALOE data.

Evaluation of Ozone Analyses From UARS MLS Assimilation by BASCOE Between 1992 and 1997

We present the analyses of UARS MLS ozone data obtained by the Belgian Assimilation System for Chemical ObsErvations (BASCOE). This system, based on the 4D-var method, is dedicated to the assimilation of stratospheric chemistry observations. It uses a 3-D Chemical Transport Model (3D-CTM) including 57 chemical species with explicit calculation of stratospheric chemistry. The CTM is driven by ECMWF ERA-40 analyses of winds and temperature, with a horizontal grid of 3.75 in latitude by 5 in longitude, and with 37 pressure levels from the surface to 0.1 hPa. BASCOE has assimilated UARS MLS observations acquired during the period 1992-1997. We discuss how BASCOE is able to reproduce MLS data, and we evaluate the BASCOE analyses with respect to independent observations from UARS HALOE, ozonesondes, and ground-based lidars. An excellent agreement is found with independent observations (bias usually less than 10%), except in the lowermost stratosphere and in the Antarctic ozone hole. The performances of BASCOE ozone analyses are also compared to those of two other long-term ozone reanalyses; namely, ERA-40 and ERA-Interim, both from ECMWF. Finally, sensitivity test based on BASCOE free model runs suggest that ozone analyses during the ozone hole period would be greatly improved by driving BASCOE with the dynamical fields of the new ECMWF reanalyses ERA-Interim. This work is part of the Stratospheric Ozone Profile Record service raised by the GMES Service Element PROMOTE.

A global OClO stratospheric layer discovered in GOMOS stellar occultation measurements

The stratospheric ozone depletion observed in polar regions is caused by several catalytic cycles induced by reactive chlorine and bromine species. By reacting with BrO, ClO causes the formation of OClO which is considered as a proxy of the halogen activation. We present the first global determination of the stratospheric OClO distribution measured during the year 2003 by the stellar occultation spectrometer GOMOS. Besides its expected polar abundance, we discovered the presence of a worldwide OClO layer in the upper stratosphere. At lower altitudes, OClO seems also to be present beyond the limit of the polar vortices, an unreported feature.

4D-var assimilation of stratospheric aerosol satellite data

Abstract A 4D Variational assimilation system of stratospheric aerosols has been developed. The aerosol assimilation system is applied to the 1.02 μm optical extinction data as measured by SAGE—II (Stratospheric Aerosol and Gas Experiment). A very simple model, a minimal standard aerosol model, was used for both the evolution of aerosols and for the inverse observational operator to map observations into model values. Since this assimilation was set up to provide aerosol analyses for isolated cases, the implementation of a straightforward choice of the first-guess field and corresponding background error covariances is discussed. The results of the aerosol analysis for January 1989 are discussed focussing on the added value of aerosol data assimilation. With a minimal aerosol model, the results show that the assimilation analyses offer the possibility to obtain direct global dynamically consistent information of the 1.02—μm aerosol extinction and aerosol surface area densities. The aerosol analyses can also be used to study dynamical features in greater detail.