A sub-basin scale dust plume source frequency inventory for southern Africa, 2005-2008

We present a dust plume source inventory for southern Africa. In order to locate and track the local, short-lived plume events, source and frequency data have been derived from Meteosat Second Generation (MSG) thermal infrared composite data (4 km data using 8.7, 10.8, and 12.0 µm) and Moderate Resolution Imaging Spectroradiometer (MODIS) visible composite data (0.25 km data utilizing 0.620 – 0.670 µm, 0.545 – 0.565 µm, and 0.459 – 0.479 µm). Between January 2005 and December 2008, a total of 328 distinct daytime dust plumes more than 10 km in length were detected. These plumes were attributed to 101 distinct point sources, consisting largely of ephemeral inland lakes, coastal pans as well as dry river valleys in Namibia, Botswana, and South Africa. These data also provided sub-basin scale source observations for large basins such as Etosha and Makgadikgadi Pans

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

Synergy of multispectral and multisensors satellite observations to evaluate desert aerosol transport and impact of dust deposition on inland waters: study case of Lake Garda

The capabilities of different Earth Observation multispectral satellites are employed for detecting and tracking of desert dust coming from North Africa toward the Northern Italy area and for evaluating the impact of Saharan dust deposition in inland waters, such as those of Lake Garda. Absorbing and scattering spectral optical properties of desert aerosol in the atmospheric windows in the ultraviolet, visible-near-infrared, and infrared spectral ranges are exploited in the dust retrieval performed by OMI/Aura, MODIS/Terra-Aqua, and SEVIRI/MSG satellite sensors. Therefore, the direct link between dust deposition and increase in phytoplankton abundance has been assessed retrieving MERIS-based chlorophyll-a (chl-a) concentration for the desert dust events. Estimates of the increased chl-a in the lake have been derived with values in concentration from 30% to 170%. AERONET sun-photometer measurements, gravimetric particulate matter samplings

New fire diurnal cycle characterizations to improve fire radiative energy assessments made from low-Earth orbit satellites sampling

Accurate near real time fire emissions estimates are required for air quality forecasts. To date, most approaches are based on satellite-derived estimates of fire radiative power (FRP), which can be converted to fire radiative energy (FRE) which is directly related to fire emissions. Uncertainties in these FRE estimations are often substantial. This is for a large part because the most often used low-Earth orbit satellite-based instruments like the MODerate-resolution Imaging Spectroradiometer (MODIS) have a relatively poor sampling of the usually pronounced fire diurnal cycle. In this paper we explore the spatial variation of this fire diurnal cycle and its drivers. Specifically, we assess how representing the fire diurnal cycle affects FRP and FRE estimations when using data collected at MODIS overpasses. Using data assimilation we explored three different methods to estimate hourly FRE, based on an incremental sophistication of parameterizing the fire diurnal cycle. We sampled data from the geostationary Meteosat Spinning Enhanced Visible and Infrared Imager (SEVIRI) at MODIS detection opportunities to drive the three approaches. The full SEVIRI time-series, providing full coverage of the diurnal cycle, were used to evaluate the results. Our study period comprised three years (2010–2012), and we focussed on Africa and the Mediterranean basin to avoid the use of potentially lower quality SEVIRI data obtained at very far off-nadir view angles. We found that the fire diurnal cycle varies substantially over the study region, and depends on both fuel and weather conditions. For example, more "intense" fires characterized by a fire diurnal cycle with high peak fire activity, long duration over the day, and with nighttime fire activity are most common in areas of large fire size (i.e., large burned area per fire event). These areas are most prevalent in relatively arid regions. Ignoring the fire diurnal cycle as done currently in some approaches caused structural errors, while generally overestimating FRE. Including information on the climatology of the fire diurnal cycle provided the most promising avenue to improve FRE estimations. This approach also improved the performance on relatively high spatiotemporal resolutions, although only when aggregating model results to coarser spatial and/or temporal scale good correlation was found with the full SEVIRI hourly reference dataset. In general model performance was best in areas of frequent fire and low errors of omission. We recommend the use of regionally varying fire diurnal cycle information within the Global Fire Assimilation System (GFAS) used in the Copernicus Atmosphere Monitoring Services, which will improve FRE estimates and may allow for further reconciliation of biomass burning emission estimates from different inventories

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

Numerical model simulation of the Saharan dust event of 6–11 March 2006 using the Regional Climate Model version 3 (RegCM3)

The Sahara desert is the world's primary source of mineral dust aerosols and is known to be an important but poorly understood component of the climate system. Climate models which incorporate dust modules have the potential to improve our understanding of the climate impacts of dust. In this study, the performance of the Regional Climate Model version 3 (RegCM3) with an active dust scheme is evaluated, using a major dust event of 6-11 March 2006 as a test case. To account for the distribution of preferential dust source regions, soil texture characteristics were modified in dust source regions identified from long-term SEVIRI satellite data. The dust event was associated with a pronounced cold outbreak of midlatitude air over the northern Sahara which produced anomalously strong northerly winds, which propagated from west to east over the Sahara during the study period. This resulted in dust mobilization from multiple dust sources across the domain. RegCM3 represents the space/time structure of near-surface meteorology well, although surface winds are underestimated in absolute terms. The experiment in which soils are modified provides a better representation of local dust sources and emission and resulting atmospheric optical thickness (AOT). In this experiment, model simulated dust flux exported from the Sahara to the Sahel and the tropical east Atlantic is estimated as 1.9 Tg d(-1). The dust event had a profound impact on the surface solar radiation budget of similar to-140 W m(-2) per unit AOT (domain average). The shortwave radiative effect at the top of the atmosphere is similar to-10 W m(-2) per unit AOT over the study domain. However, this is strongly dependent on surface albedo. The results also highlight how errors in model simulated circulation lead to errors in the position of the dust plume

Biomass Burning in the Conterminous United States: A Comparison and Fusion of Active Fire Observations from Polar-Orbiting and Geostationary Satellites for Emissions Estimation

Biomass burning is an important source of atmospheric greenhouse gases and aerosol emissions that significantly influence climate and air quality. Estimation of biomassburning emissions (BBE) has been limited to the conventional method in which parameters (i.e., burned area and fuel load) can be challenging to quantify accurately. Recent studies have demonstrated that the rate of biomass combustion is a linear function of fire radiative power (FRP), the instantaneous radiative energy released from actively burning fires, which provides a novel pathway to estimate BBE. To obtain accurate and timely BBE estimates for near real-time applications (i.e., air quality forecast), the satellite FRP-based method first requires a reliable biomass combustion coefficient that converts fire radiative energy (FRE), the temporal integration of FRP, to biomass consumption. The combustion coefficient is often derived in controlled small-scale fire experiments and is assumed a constant, whereas the coefficient based on satellite retrievals of FRP and atmospheric optical depth is suggested varying in a wide range. Undoubtedly, highly variable combustion coefficient results in large uncertainty of BBE estimates. Further, the FRP-based method also depends on high-spatiotemporalresolution FRP retrievals that, however, are not available in any active fire products from current polar-orbiting and geostationary satellites due to their sampling limitations. To address these challenges, this study first investigates the combustion coefficient for landscape-scale wildfires in the Conterminous United States (CONUS) by comparing FRE from the polar-orbiting Moderate Resolution Imaging Spectroradiometer (MODIS) and the Geostationary Operational Environmental Satellite system (GOES) with the Landsat-based biomass consumption. The results confirms that biomass consumption is a linear function of FRE for wildfires. The derived combustion coefficient is 0.374 kg · MJ- 1 for GOES FRE, 0.266 kg · MJ-1 for MODIS FRE, and 0.320 kg · MJ-1 considering both GOES and MODIS FRE in the CONUS. Limited sensitivity analyses indicate that the combustion coefficient varies from 0.301 to 0.458 kg · MJ-1, which is similar to the reported values in small fire experiments. Then, this study reconstructs diurnal FRP cycle to derive high-spatiotemporal-resolution FRP by fusing MODIS and GOES FRP retrievals and estimates hourly BBE at a 0.25°×0.3125° grid across the CONUS. The results indicate that the reconstructed diurnal FRP cycle varies significantly in magnitude and shape among 45 CONUS ecosystems. In the CONUS, the biomass burning annually releases approximately 690 Gg particulate matter (smaller than 2.5 μm in diameter, PM2.5). The diurnal-FRP-cycle-based BBE estimates compare well with BBE derived from Landsat burned areas in the western CONUS and with the hourly carbon monoxide emissions simulated using a biogeochemical model over the Rim Fire in California. Moreover, the BBE estimates show a similar seasonal variation to six existing BBE inventories but with variable magnitude. Finally, this study examines potential improvements in fires characterization capability of the Visible Infrared Imaging Radiometer Suite (VIIRS), which is the follow-on sensor of the MODIS sensor, for integrating VIIRS FRP retrievals into the FRP-based method for BBE estimation in future work. The results indicate that the VIIRS fire characterization capability is similar across swath, whereas MODIS is strongly dependent on satellite view zenith angle. VIIRS FRP is generally comparable with contemporaneous MODIS FRP at continental scales and in most fire clusters. At 1-degree grid cells, the FRP difference between the two sensors is, on average, approximately 20% in fire-prone regions but varies significantly in fire-limited regions. In summary, this study attempts to enhance the capability of the FRP-based method by addressing challenges in its two parameters (combustion coefficient and FRP), which should help to improve estimation of BBE and advance our understanding of the effects of BBE on climate and air quality. This research has resulted in two published papers and one paper to be submitted to a peer-reviewed journal so far

An case of extreme particulate matter concentrations over Central Europe caused by dust emitted over the southern Ukraine

On 24 March 2007, an extraordinary dust plume was observed in the Central European troposphere. Satellite observations revealed its origins in a dust storm in Southern Ukraine, where large amounts of soil were resuspended from dried-out farmlands at wind gusts up to 30 m s?1. Along the pathway of the plume, maximum particulate matter (PM10) mass concentrations between 200 and 1400 ?g m?3 occurred in Slovakia, the Czech Republic, Poland, and Germany. Over Germany, the dust plume was characterised by a volume extinction coefficient up to 400 Mm?1 and a particle optical depth of 0.71 at wavelength 0.532 ?m. In-situ size distribution measurements as well as the wavelength dependence of light extinction from lidar and Sun photometer measurements confirmed the presence of a coarse particle mode with diameters around 2?3 ?m. Chemical particle analyses suggested a fraction of 75% crustal material in daily average PM10 and up to 85% in the coarser fraction PM10?2.5. Based on the particle characteristics as well as a lack of increased CO and CO2 levels, a significant impact of biomass burning was ruled out. The reasons for the high particle concentrations in the dust plume were twofold: First, dust was transported very rapidly into Central Europe in a boundary layer jet under dry conditions. Second, the dust plume was confined to a relatively stable boundary layer of 1.4?1.8 km height, and could therefore neither expand nor dilute efficiently. Our findings illustrate the capacity of combined in situ and remote sensing measurements to characterise large-scale dust plumes with a variety of aerosol parameters. Although such plumes from Southern Eurasia seem to occur rather infrequently in Central Europe, its unexpected features highlights the need to improve the description of dust emission, transport and transformation processes needs, particularly when facing the possible effects of further anthropogenic desertification and climate change