Simon Chabrillat

Simple parameterization of the absorption of the solar Lyman‐alpha line

The absorption of the solar Lyman-alpha line by the terrestrial atmosphere is calculated, taking in account the wavelength variations of the emission line and of the O2 cross-section, as well as the temperature dependence of the cross-section. A new parameterization is developed to reproduce in atmospheric models the results of this high-resolution calculation, up to an attenuation of 1010 for the incident solar radiation. The error made in most of existing models when computing the Lyman-alpha contribution to photo dissociation rates in the middle atmosphere, using a constant O2 cross-section of 10-20 cm2, is shown to be important and this can affect the loss rate of mesospheric constituents such as H2O or CH4. Copyright 1997 by the American Geophysical Union.

The University of Bern Atmospheric Ion Model: Time-dependent modeling of the ions in the mesosphere and lower thermosphere

In this paper the first time-dependent model of ion chemistry in the mesosphere/lower thermosphere (MLT) region acting within a global, time-dependent, two-dimensional neutral atmosphere model is described. Selected diurnal results are presented for undisturbed solar minimum conditions. The University of Bern Atmospheric Ion Model (UBAIM) is a time-dependent, pseudo-two-dimensional model of the ion chemistry in the Earth atmosphere. It covers latitudes from 85°S to 85°N and (log-pressure) altitudes from 20 to 120 km. On this grid a system of differential equations describing the ion chemistry is integrated numerically until a periodical solution, governed by the diurnal changes in the incident radiation, is reached; this solution constitutes a model for quiet or undisturbed conditions. The basic ion chemistry of the UBAIM contains 311 reactions for 71 charged species. Ionization sources are solar X-ray and EUV radiation, resonantly scattered Lyman α and β photons, and galactic cosmic rays. Densities of main and trace neutral atmospheric constituents are taken from a new version of the bidimensional NCAR model SOCRATES, which has been specifically optimized for mesospheric and lower thermospheric processes with upper boundary conditions set using the empirical MSIS thermosphere model. Direct solar flux inputs are computed by the SOLAR2000 model; scattered Lyman α and β fluxes are calculated using geocoronal hydrogen density profiles consistent with the adopted MSIS density distributions.

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.

Ozone profiles from 30 to 110 km Measured by the Occultation Radiometer Instrument during the period Aug. 1992–Apr. 1993

We present ozone volume mixing ratio profiles obtained by the ORA instrument during the period Aug 1992-Apr 1993. They have been retrieved by applying a specific inversion algorithm to a radiometric UV channel contaminated by Rayleigh scattering. The results compare reasonably well with other instruments up to the mesopause and are probably the first extended ozone data in the lower thermosphere (90-110 km).

The NOy budget above Eureka, Nunavut from ground-based FTIR measurements, space-based ACE-FTS measurements, and the CMAM-DAS, GEM-BACH, and SLIMCAT models

Reactive nitrogen species, NOy, play an important role in stratospheric chemistry. Using a Bruker 125HR FTIR installed at Eureka, Nunavut, ACE-FTS satellite data, and model simulations, we study the NOy budget for this Arctic site.