SUMMARYA data-assimilation scheme to assimilate the Global Ozone Monitoring Experiment (GOME) total-ozone data is described. The corresponding software (called TM3DAM) has been operational since early 2000 and is used to produce daily ozone analyses and ve-day ozone forecasts. The model is a tracer-transport model with a parametrized description of stratospheric gas-phase and heterogeneous ozone chemistry. It is driven by operational meteorological elds from the ECMWF numerical weather-prediction model. TM3DAM analyses near-real-time level-2 ozone data from the GOME instrument on the ESA ERS-2 satellite. The focus of this paper is on the dataassimilation aspects and the analysis results. The assimilation approach is based on the Kalman-lter equations and provides detailed and realistic maps of the forecast error. The analysis scheme is nevertheless computationally ef cient. The forecast-minus-observation statistics, accumulated over a two-year period, are described in detail. A comparison with TOMS and Brewer observations shows good agreement.

[1] This paper presents one of the first extensive intercomparisons of models and methods used for estimating stratosphere-troposphere exchange (STE). The study is part of the European Union project Influence of Stratosphere Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO). Nine different models and methods, including three trajectory methods, one Eulerian method, two Lagrangian and one Eulerian transport model, and two general circulation models applied the same initialization. Stratospheric and tropospheric tracers have been simulated, and the tracer mass fluxes have been calculated through the tropopause and the 700 hPa surface. For a 12-day case study over Europe and the northeast Atlantic the simulated tracer mass fluxes have been intercompared. For this case the STE simulations show the same temporal evolution and the same geographical pattern of STE for most models and methods, but with generally different amplitudes (up to a factor of 4). On the other hand, for some simulations also the amplitudes are very similar.

[1] The 40-year reanalysis data set ERA-40 from the European Centre for Medium-Range Weather Forecasts includes, unlike ERA-15, archived convective mass fluxes. These convective fluxes are useful for off-line chemistry transport modeling. The impact of using these archived convective mass fluxes (based on a convective parameterization described in Gregory et al. [2000]) instead of off-line diagnosed mass fluxes (based on a convective parameterization described in Tiedtke [1989]) was investigated with the chemistry transport model TM3. At first sight the two types of mass fluxes look similar. However, some differences can be noted: the archived updrafts extend higher than the off-line diagnosed ones; they are also less intense below 500 hPa over sea. The archived downdrafts are much weaker than the off-line diagnosed downdrafts. With archived convective mass fluxes, we found slightly higher 222 Rn concentrations in the boundary layer, lower Rn or another tracer of convective transport will be needed to unambiguously identify either of the convective data sets as optimal for use in chemistry transport models.

SUMMARYThe propagation of inertia-gravity waves (IGWs) through a dynamical transport barrier, such as the Antarctic polar vortex edge is investigated using a linear wave model. The model is based on the linearized, inviscid hydrostatic equations on an f -plane. Typical values for the parameters that are appropriate to the Antarctic polar vortex are given. The background ow U is assumed to be barotropic and its horizontal shear is represented by a hyperbolic tangent background wind pro le. The wave equation that describes the latitudinal structure of a monochromatic disturbance contains two singularities. The rst corresponds to the occurrence of a critical level where the intrinsic wave frequency Ä D ! ¡ kU becomes zero. ! is the absolute wave frequency and k its longitudinal wave number in the direction of U . The second is an apparent singularity and does not give rise to singular wave behaviour. It becomes zero whenever the square of the intrinsic wave frequency Ä 2 D f .f ¡ U y /, f being the Coriolis frequency and U y the horizontal shear of the ow. The wave equation is solved numerically for different values of the angles of incidence of the wave upon the background ow, of the wave frequency, of the horizontal wave number and of the Rossby number. Re ection (jRj) and transmission (jT j) coef cients are determined as a function of these parameters. The results depend on whether the ow is inertially stable or not. They also depend on the presence and location of the turning levels, where the wave becomes evanescent, with respect to the location of the Ä-critical levels. For inertially stable ows, the wave totally re ects at the turning level and never reaches the critical level. If the background ow is inertially unstable, turning levels can disappear and the wave can now reach the critical level. Then over-re ection, over-transmission and absorption can occur.

No abstract