Diurnal variation of upper tropospheric humidity and its relations to convective activities over tropical Africa

Diurnal variations of upper tropospheric humidity (UTH) as well as middle tropospheric humidity (MTH) were examined in conjunction with the diurnal cycle of convection over tropical Africa and the adjacent tropical Atlantic Ocean using Meteosat-8 measurements. Cloud and humidity features were also tracked to document the diurnal variations of humidity and clouds in the Lagrangian framework. <br><br> A distinct diurnal variation of UTH (and MTH) is noted over regions where tropical deep convective cloud systems are commonly observed. The amplitude of the UTH diurnal variation is larger over land, while its variations over convectively inactive subtropical regions are much smaller. The diurnal variation of UTH tends to reach a maximum during nighttime over land, lagging deep convection and high cloud whose maxima occurred in the late afternoon and evening, respectively. It was revealed that these diurnal variations over the African continent are likely associated with continental-scale daytime solar heating and topography, in which topographically-induced signals develop earlier around the mid-afternoon and merge into stronger and broader continental-scale convection clusters later, forming a precipitation maximum in the late afternoon. It was also revealed that advection effect on the diurnal variation appears to be insignificant

Mesoscale convective complexes over southern Africa

Includes bibliographical references.A combination of numerous factors, including geographic position, regional orography and local sea surface temperatures, ensures that subtropical southern Africa experiences considerable spatial and temporal variability in rainfall and is prone to both frequent flooding and drought events

Frequency of extreme Sahelian storms tripled since 1982 in satellite observations

The hydrological cycle is expected to intensify under global warming, with studies reporting more frequent extreme rain events in many regions of the world, and predicting increases in future flood frequency. Such early, predominantly mid-latitude observations are essential because of shortcomings within climate models in their depiction of convective rainfall. A globally important group of intense storms—mesoscale convective systems (MCSs)—poses a particular challenge, because they organize dynamically on spatial scales that cannot be resolved by conventional climate models. Here, we use 35 years of satellite observations from the West African Sahel to reveal a persistent increase in the frequency of the most intense MCSs. Sahelian storms are some of the most powerful on the planet, and rain gauges in this region have recorded a rise in ‘extreme’ daily rainfall totals. We find that intense MCS frequency is only weakly related to the multidecadal recovery of Sahel annual rainfall, but is highly correlated with global land temperatures. Analysis of trends across Africa reveals that MCS intensification is limited to a narrow band south of the Sahara desert. During this period, wet-season Sahelian temperatures have not risen, ruling out the possibility that rainfall has intensified in response to locally warmer conditions. On the other hand, the meridional temperature gradient spanning the Sahel has increased in recent decades, consistent with anthropogenic forcing driving enhanced Saharan warming. We argue that Saharan warming intensifies convection within Sahelian MCSs through increased wind shear and changes to the Saharan air layer. The meridional gradient is projected to strengthen throughout the twenty-first century, suggesting that the Sahel will experience particularly marked increases in extreme rain. The remarkably rapid intensification of Sahelian MCSs since the 1980s sheds new light on the response of organized tropical convection to global warming, and challenges conventional projections made by general circulation models

The diurnal cycle of cloud cover over southern and central Africa

Includes abstract.Includes bibliographical references (leaves 113-119).The current understanding of the temporal and spatial distribution of clouds over southern and central Africa is poor and the regional processes governing cloud occurrence is only weakly understood. This study seeks to improve the current understanding of cloud diurnal variability over this region by providing a base-line diurnal climatology of lowlevel, mid-level and high-level cloud cover. Diurnal variations of cloudiness are examined using ten years of cloud data from latest version of the International Satellite Cloud Climatology Project (ISCCP-D1). The broad seasonal average diurnal variability is explored across the region. Thereafter a more detailed analysis of regionally specific variability is made using a Self-Organising Map. The findings of this study are in broad agreement with previous work. Cloud over the southern and central African region shows clear spatial organisation, most significantly associated with the location of the Intertropical Convergence Zone. The diurnal variation of high-level cloud is large, closely correlated to its mean and is enhanced by orographic features. Minimum high-level cloud occurs at 1100 LST and maximum extent is reached during the evening around 1800 LST, except in locations experiencing deep convection which displayed a redevelopment of cloud in the early morning (0300 LST). This redevelopment of HLCA is hypothesised to be due to the destabilization of the upper troposphere through nighttime cloud radiative cooling. Mid-level cloud exhibits smaller diurnal variations, reaching maximum coverage at approximately 0300 LST. Clouds at this level are severely obscured by higher clouds and therefore the detected diurnal variation is due to both real and artificial signals and care needs to be taken in interpreting the results. Low-level cloud shows strong diurnal variations when not obscured by higher clouds, reaching a maximum just after midday. The results of this study are interpreted in terms of the life-cycle of deep convective cloudiness. A number of mechanisms are suggested to explain the regional differences in diurnal variations with land surface heating being the primary mechanism

Evaluation of the model representation of the evolution of convective systems using satellite observations of outgoing longwave radiation

We introduce a technique for assessing the diurnal development of convective storm systems based on outgoing longwave radiation fields. Using the size distribution of the storms measured from a series of images, we generate an array in the lengthscale-time domain based on the standard score statistic. It demonstrates succinctly the size evolution of storms as well as the dissipation kinematics. It also provides evidence related to the temperature evolution of the cloud tops. We apply this approach to a test case comparing observations made by the Geostationary Earth Radiation Budget instrument to output from the Met Office Unified Model run at two resolutions. The 12km resolution model produces peak convective activity on all lengthscales significantly earlier in the day than shown by the observations and no evidence for storms growing in size. The 4km resolution model shows realistic timing and growth evolution although the dissipation mechanism still differs from the observed data

Characterising Saharan Dust Sources and Export using Remote Sensing and Regional Modelling

The PhD-thesis aims to characterise the Saharan dust cycle at diffent seasons using satellite remote sensing techniques and regional modelling studies. A dust index based on 15-minute infrared satellite measurements provided by the SEVIRI instrument onboard the Meteosat Second Generation (MSG) satellite is used to infer spatio-temporal charcteristics of dust sources north of 5°N over Africa since March 2006. The spatial distribution of dust sources points towards the importance of endorehic drainage systems in mountain areas. The temporal distribution of the time-of-day when dust mobilisation starts shows maximum activity during local morning hours, pointing towards the role of the breakdown of the nocturnal low-level jet. Details of the role and ability of the low-level jet breakdown for dust entrainment were studied using regional modelling. Furthermore, the seasonal dust export towards the tropical North Atlantic is considered using regional modelling

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

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

Globally Gridded Satellite (GridSat) Observations for Climate Studies

Geostationary satellites have provided routine, high temporal resolution Earth observations since the 1970s. Despite the long period of record, use of these data in climate studies has been limited for numerous reasons, among them: there is no central archive of geostationary data for all international satellites, full temporal and spatial resolution data are voluminous, and diverse calibration and navigation formats encumber the uniform processing needed for multi-satellite climate studies. The International Satellite Cloud Climatology Project set the stage for overcoming these issues by archiving a subset of the full resolution geostationary data at approx.10 km resolution at 3 hourly intervals since 1983. Recent efforts at NOAA s National Climatic Data Center to provide convenient access to these data include remapping the data to a standard map projection, recalibrating the data to optimize temporal homogeneity, extending the record of observations back to 1980, and reformatting the data for broad public distribution. The Gridded Satellite (GridSat) dataset includes observations from the visible, infrared window, and infrared water vapor channels. Data are stored in the netCDF format using standards that permit a wide variety of tools and libraries to quickly and easily process the data. A novel data layering approach, together with appropriate satellite and file metadata, allows users to access GridSat data at varying levels of complexity based on their needs. The result is a climate data record already in use by the meteorological community. Examples include reanalysis of tropical cyclones, studies of global precipitation, and detection and tracking of the intertropical convergence zone