Studio della convezione durante il monsone africano: osservazioni e modellazione della precipitazione e del ruolo della circolazione regionale sulla composizione atmosferica

Abstract

The improvement of knowledge and understanding of the West African Monsoon (WAM) is a fundamental scientific issue with implications on economy, health, water resources and food security in West African countries. In a region where agriculture is mainly rain fed, a delay in the rainy season onset or a dry year could compromise food and water security and lead to dearth conditions. The natural interannual and interseasonal variability of the WAM and the dramatic change from wet conditions (1950s-1960s) to much drier ones (1960s-today) over West Africa represents one of the world strongest variation in the 20th century. Mesoscale convective systems (MCS) are responsible for the 80% of the rain production in the sub- Saharan region during the rainy season (June-September), playing a key role in the rainfall variability over a large domain of spatial and temporal scales. The annual variability of MCSs in West Africa is driven by monsoon circulation, which provides favourable conditions for convection formation in the Sahelian area. An increase in the vulnerability of West African societies to climate variability is expected for the next decades as demands on resources increase in association with growing population and for those reasons predictions at various scales of the WAM is a key issue for West African countries. Further motivation comes from the need to quantify the role of the WAM on the global climate. Africa is one of the largest and less known sources of dust and aerosols, which plays a major role in radiative forcing and in cloud microphysics. Furthermore Africa emits the largest amount of biomass burning emissions with a strong interhemispheric transition between West Africa in boreal winter to Central and Southern Africa in boreal summer following the location of the dry season in each hemisphere. Moreover emissions due to urban pollution of large African cities are poorly known due to a lack of in-situ measurements. Satellite observations indicates that large areas in West Africa appear to be characterized by industrial activity, traffic, biomass burning from house fires that can have a great impact on air quality and possibly damage vegetation growth and agricultural production. In this work we have approached three issues: (1) how mesoscale models describe the dynamics of MCS (2) how to improve them in order to improve water cycle, precipitation and deep convection analysis and (3) which dynamical mechanisms drive the chemical composition of the atmosphere in Africa Firstly, the ability of the mesoscale meteorological model BOLAM (BOlogna Limited Area Model) in reproducing convection in West Africa has been tested against other meteorological models and rainfall measurements. Models performed simulations of the propagation of a MCS observed to cross part of West Africa in August 2005. An evaluation of precipitation simulated by mesoscale models is carried out. It has been found that the BOLAM model is capable to reproduce the structure and the associated precipitation of the observed squall line even if it overestimates precipitation amount with respect to the reference satellite estimations and produces a eastward shifted rain band. The models intercomparison showed that convective precipitation forecast in West Africa is a difficult issue to be addressed. To address this issue we have developed and implemented into the BOLAM model a nudging scheme based on the use of satellite observations of cloud top brightness temperature to correct the model humidity profiles. The nudging approach is based on the continuous assimilation of METEOSAT infrared brightness temperatures within the model in order to trigger convection, where observations show the presence of large convective systems. The nudging also inhibits convection, when the model reproduces unrealistic convective precipitation and coherently modifies the dynamical fields. It is shown that the assimilation scheme improves the geographical distribution and time evolution of the MCSs reproduced by the model; the impact of assimilation is positive up to 13 hours after the end of the nudging period. It is also shown that the nudging improves the simulated amount and spatial distribution of precipitation. In order to upscale the results obtained on a single event, we performed a seasonal mesoscale simulation covering west Africa during the whole monsoon season. Therein the nudging scheme is used throughout the period to obtain a reanalysis for the June-August 2006 period. The assimilation of cloud top brightness temperature greatly improves the spatial patterns and the amount of rainfall generated by the BOLAM model over a seasonal time-scale. The last issue studied in the present work regards the transport of pollutants and greenhouse gases in the African continent. We used BOLAM mesoscale model simulations, nudged with infrared radiance temperature, to estimate the convective impact in the upper troposphere and to assess the fraction of air processed by convection. Comparison between simulated convective transport and aircraft measurements shows that BOLAM model correctly reproduces the location and the vertical structure of convective outflow. Model-aided analysis indicates that convection can influence the composition of the upper troposphere above the level of main outflow for an event of deep convection close to the observation site and that deep convection occurring in the central Sahelian region has a likely role in convective transport in the upper troposphere. Then we focused on the long-range transport of biomass burning gases out of West Africa, which has been recognised to have important implications for the global oxidizing capacity of the atmosphere and global climate change. Mid and upper-tropospheric pollutant plumes with enhanced levels of trace gases and aerosols were observed over the southern coast of West Africa during August 2006 as part of the AMMA wet season field campaign. Runs using the BOLAM mesoscale model including biomass burning CO tracers were used to confirm an origin from central African fires. Modelled tracer results showed that pollutants resided for between 9 and 12 days over Central Africa before being transported for 4 days, in the case of the mid-troposphere plume, and 2 days in the case of the upper tropospheric plume to the measurement location over the southern part of West Africa