C. Mari

Diurnal cycle of dust and cirrus over West Africa as seen from Meteosat Second Generation satellite and a regional forecast model.

A brightness temperature difference (BTD) technique is used to evaluate the dust and cirrus forecasts of a regional meteorological model. The technique based on a contrasted absorption property of dust and cirrus at two wavelengths within the atmospheric infrared window is applied to 3-hourly Meteosat Second Generation (MSG) observations in the 10.8- and 12-μm bands over West Africa. The satellite observation of dust coverage over the Sahara shows a well marked diurnal cycle associated with the boundary layer activity peaking at 15 UTC. A similar signature is obtained from the regional model when the dust scheme is activated. The cirrus cover over West Africa is maximum at 12 UTC as seen both from MSG and the model. The use of prognostic dust aerosol, instead of climatology, furthermore better captures the observed convective activity.

Coupled Atmosphere‐Wildland Fire Modelling

A tight interaction exists between the development of a wildfire and the local meteorology near the front. The convective effects induced by the fire heat release can modify the local wind circulation and consequently affect the fire propagation. In this study we use a meso-scale numerical model in a Large Eddy Simulation (LES) configuration coupled to a simplified physical front tracking wildfire model to investigate the differences induced by the atmospheric feedback in propagation speed and behaviour. Simulations of typical experimental configurations show a good response of the coupled fire-atmospheric model. Numerical results matches qualitatively observed values for fire induced winds and convection. Both numerical models already have operational usage and might ultimately be run to support decisions in wildfire management.

Export of Asian pollution during two cold front episodes of the TRACE-P experiment.

[1]xa0Two cold front episodes were sampled during the two flights out of Yokota, Japan, during the Transport and Chemical Evolution Over the Pacific (TRACE-P) experiment during March 2001. The data from these two flights are examined using a mesoscale three-dimensional model. We show how these cyclonic systems have impacted the export of pollution out of the Asian continent. We contrast the relative role of convection and ascent in the warm conveyor belts associated with the cyclone during these two episodes. Although the necessary meteorological conditions for an efficient export of pollution are met during flight 13 (i.e., the occurrences of the warm conveyor belt near the source regions), no significant pollution is simulated in the mid-Pacific in the lower and middle troposphere. The efficient ventilation of the WCB by convection near the coast, the advection by the anticyclonical flow above 700 hPa, and the downward motion associated with the Pacific high in the remote ocean significantly prevent any long-range transport of undiluted pollution in the WCB. During flight 15 the conveyor belts have already moved to the remote ocean. The polluted plume is split by the rising air in the warm conveyor belt which transports CO-poor air northward and by the oceanic convection which transports clean air masses upward. These mechanisms lead to the dilution of Asian pollution in WCB en route to North America and add to the episodic nature of the Asian outflow by fragmenting the pollution plume.

Lightning-produced NOx in an explicit electrical scheme tested in a Stratosphere-Troposphere Experiment : Radiation, Aerosols, and Ozone case study.

[1] An explicit lightning-produced nitrogen oxide (LNO x ) scheme has been implemented in a 3-D mesoscale model. The scheme is based on the simulation of the electrical state of the cloud and provides a prediction of the temporal and spatial distribution of the lightning flashes. The frequency and the 3-D morphology of the lightning flashes are captured realistically so fresh nitrogen oxide molecules can be added along the complex flash path as a function of the pressure, as suggested by results from laboratory experiments. The scheme is tested on the 10 July 1996 Stratosphere-Troposphere Experiment: Radiation, Aerosols, and Ozone (STERAO) storm. The model reproduces many features of the observed increase of electrical activity and LNO x flux density between the multicell and supercell stages. LNO x dominates the NO x budget in the upper part of the cells with instantaneous peak concentrations exceeding 4 ppbv, as observed. The computed flux of NO x across the anvil shows a mean value of 6 mol m -2 s -1 during the last 90 min of the simulation. This value is remarkably stable and compares favorably with the observations.