Abstract. By applying four-dimensional variational dataassimilation (4-D-Var) to a combined ozone and dynamics Numerical Weather Prediction model (NWP), ozone observations generate wind increments through the ozonedynamics coupling. The dynamical impact of Aura/MLS satellite ozone profiles is investigated using Météo-France operational ARPEGE NWP 4-D-Var assimilation system for a period of 3 months. A data-assimilation procedure has been designed and run on 6-h windows. The procedure includes: (1) 4-D-Var assimilating both ozone and operational NWP standard observations, (2) ARPEGE transporting ozone as a passive-tracer, (3) MOCAGE, the Météo-France chemistry and transport model re-initializing the ARPEGE ozone background at the beginning time of the assimilation window. Using observation minus forecast statistics, it is found that the ozone assimilation reduces the wind bias in the lower stratosphere. Moreover, the Degrees of Freedom for Signal diagnostics show that the MLS data covering the 68.1-31.6 hPa vertical pressure range are the most informative and their information content is nearly of the same order as tropospheric humidity-sensitive radiances. Furthermore, with the help of error variance reduction diagnostics, the ozone contribution to the reduction of the horizontal divergence background-error variance is shown to be better than tropospheric humidity-sensitive radiances.
This paper presents analyses of data collected with three ground‐based Doppler radars during the Mesoscale Alpine Programme (MAP) Intensive Observation Period (IOP) 2b from 1900 UTC 19 September until 1100 UTC 20 September 1999. During this period, the synoptic situation was characterized by the propagation of a deep upper‐tropospheric trough towards the Alps, as often observed when heavy precipitation occurs on the southern slopes of the Alpine massif. A frontal cloud system with embedded convective cells passed over northern Italy in association with the trough moving rapidly eastwards. Ahead of the advancing cold front, a strong south to south‐easterly low‐level flow impinged on the mountains near Lago Maggiore and produced copious amounts of rain (<200mm at several locations). An analysis of three‐dimensional radar‐derived wind and precipitation fields shows that the most intense precipitation occurred where and when the easterly component of the low‐ to mid‐level flow was the strongest with, however, a preferential location on the southern slopes of the Alps. This phenomenon can be explained by enhanced confluence between the main southerly flow and an intensifying easterly flow, and/or more favourable orientation of the moist inflow perpendicular to the mountain slopes. In addition, the propagation of convective cells from the Po Valley toward the Alps led to enhanced precipitation over the mountainous area. Taking advantage of the approximately two‐dimensional character of the event, the different terms of the water budget are calculated for a series of 25 successive vertical cross‐sections to analyse the transformation of atmospheric moisture into surface rainfall. Copyright © 2003 Royal Meteorological Society.
The global chemistry and transport model MOCAGE (Modèle de Chimie Atmosphérique à Grande Echelle) is used to investigate the contribution of transport to the carbon monoxide (CO) distribution over West Africa during spring 2001. It is constrained with the CO profiles provided by the Measurements Of Pollution In The Troposphere (MOPITT) instrument through a sequential assimilation technique based on a suboptimal Kalman filter. The improvement of tropospheric CO distribution from MOCAGE is evaluated by comparing the model results (with and without assimilation) with the MOPITT CO concentrations observed during the analysed period (between 2001 March 15 to 2001 April 30), and also with independent in situ CMDL and TRACE‐P observations. The initial overestimation in high CO emissions areas (Africa, SE Asia and NW coast of South America) is considerably reduced by using the MOPITT CO assimilation. We analysed the assimilated CO for a period of three successive 15 d periods in terms of average fields over West Africa and contributions to the CO budget of transport and chemical sources. It is found that the horizontal and vertical CO distributions are strongly dependent on the characteristics of the large‐scale flows during spring, marked by the onset of the low‐level southerly monsoon flow and the gradual increase of the well‐known African and tropical easterly jets at middle and upper levels, respectively. Total transport by the mean flow (horizontal plus vertical advection) is important in the CO budget since it mostly compensates the local sink or source generated by chemical reactions and small‐scale processes. The major source of CO is concentrated in the lower troposphere (1000–800 hPa) mainly due to convergent low‐level flow advecting CO from surrounding regions and surface emissions (biomass burning). Vertical transport removes 70% of this low‐level CO and redistributes it in the middle troposphere (800–400 hPa) where chemical reactions and horizontal exports contribute to the loss of CO. A lesser proportion is transported upwards into upper troposphere, and then horizontally, out of the considered domain.
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