Martin C. Todd

Dust-Storm Source Areas Determined by the Total Ozone Monitoring Spectrometer and Surface Observations

Abstract Dust storms are recognized as having a very wide range of environmental impacts. Their geomorphological interest lies in the amount of deflation and wind erosion they indicate and their role in loess formation. Atmospheric mineral-dust loading is one of the largest uncertainties in global climate-change modeling and is known to have an important impact on the radiation budget and atmospheric instability. Major gaps remain in our understanding of the geomorphological context of terrestrial sources and the transport mechanisms responsible for the production and distribution of atmospheric dust, all of which are important in reducing uncertainties in the modeling of past and future climate. Using meteorological data from ground stations, from the space-borne Total Ozone Monitoring Spectrometer (TOMS), and from the National Center for Environmental Prediction–National Center for Atmospheric Research reanalysis project, we illustrate the key source regions of dust and demonstrate the primacy of the Sahara. Objectively defined source regions for the Sahara are determined from eigenvector techniques applied to the TOMS data. Other key regions include the Middle East, Taklamakan, southwest Asia, central Australia, the Etosha and Mkgadikgadi basins of southern Africa, the Salar de Uyuni (Bolivia), and the Great Basin (United States). In most of these regions, large basins of internal drainage, as defined from a digital elevation model, are dust sources where the near-surface atmospheric circulation (determined by calculated means of potential sand flux) is favorable for dust mobilization. Surface observations indicate some regions as being important that do not appear on the TOMS maps. Possible reasons for these discrepancies are explored.

The Bodélé depression: a single spot in the Sahara that provides most of the mineral dust to the Amazon forest

About 40 million tons of dust are transported annually from the Sahara to the Amazon basin. Saharan dust has been proposed to be the main mineral source that fertilizes the Amazon basin, generating a dependence of the health and productivity of the rain forest on dust supply from the Sahara. Here we show that about half of the annual dust supply to the Amazon basin is emitted from a single source: the Bodele depression located northeast of Lake Chad, approximately 0.5% of the size of the Amazon or 0.2% of the Sahara. Placed in a narrow path between two mountain chains that direct and accelerate the surface winds over the depression, the Bodele emits dust on 40% of the winter days, averaging more than 0.7 million tons of dust per day

Satellite Rainfall Estimation Using Combined Passive Microwave and Infrared Algorithms

Abstract The development of a combined infrared and passive microwave satellite rainfall estimation technique is outlined. Infrared data from geostationary satellites are combined with polar-orbiting passive microwave estimates to provide 30-min rainfall estimates. Collocated infrared and passive microwave values are used to generate a database, which is accessed by a cumulative histogram matching approach to generate an infrared temperature–rain-rate relationship. The technique produces initial estimates at 30-min and 12-km resolution ready to be aggregated to the user requirements. A 4-month case study over Africa has been chosen to compare the results from this technique with those of some existing rainfall techniques. The results indicate that the technique outlined here has statistical scores that are similar to other infrared/passive microwave combined algorithms. Comparison with the Geostationary Operational Environmental Satellite (GOES) precipitation index shows that while these algorithms result...

Dust and the low‐level circulation over the Bodélé Depression, Chad: Observations from BoDEx 2005

Dust plays an important role in climate, recognition of which has led to a concentrated research effort in field campaigns, development and analysis of remotely sensed data, and modeling to better understand dust. There have, however, been very few direct surface-based field measurements from key dust source regions. The Bodele, Chad, has been shown to be one of the premier sources of dust in the world. This paper reports on the Bodele Field Experiment (BoDEx 2005) which took place during February and March 2005 and presents the first surface-based measurements of the circulation over the Bodele. On the basis of Pilot Balloon and AWS data, we confirm the existence of the Bodele Low Level Jet (LLJ) and show that winds undergo a strong diurnal cycle such that strongest surface winds typically occur in the midmorning when momentum is mixed downward in turbulence induced by radiative heating. In contrast, the core of the LLJ, near 500 m, peaks during the evening and is weakest during the day. The LLJ was present on all days during BoDEx 2005, but winds at the surface reached speeds necessary for large-scale dust entrainment on only a few days. The winds strength during the main dust plume event of BoDEx (10–12 March 2005) was in the bottom third of March plume events of the last 4 years. Pathways of dust transport from the Bodele using a trajectory model show potential advection of dust over the west African coastline within 5 days.

Meteorological processes forcing Saharan dust emission inferred from MSG-SEVIRI observations of subdaily dust source activation and numerical models

Fifteen-minute Meteosat Second Generation (MSG) Spinning Enhanced Visible and Infrared Imager (SEVIRI) infrared dust index images are used to identify dust source areas. The observations of dust source activation (DSA) are compiled in a 1° × 1° map for the Sahara and Sahel, including temporal information at 3-hourly resolution. Here we use this data set to identify the most active dust source areas and the time of day when dust source activation occurs most frequently. In the Sahara desert 65% of DSA (March 2006 to February 2008) occurs during 0600-0900 UTC, pointing toward an important role of the breakdown of the nocturnal low-level jet (LLJ) for dust mobilization. Other meteorological mechanisms may lead to dust mobilization including density currents initiated by deep convective systems which mobilize dust fronts (haboobs) occurring preferentially in the afternoon hours and cyclonic activities. The role of the nocturnal LLJ for dust mobilization in the Sahara is corroborated by regional model studies and analysis of meteorological station data.

Mineral dust aerosols over the Sahara: Meteorological controls on emission and transport and implications for modeling

Atmospheric mineral dust has recently become an important research field in Earth system science because of its impacts on radiation, clouds, atmospheric dynamics and chemistry, air quality, and biogeochemical cycles. Studying and modeling dust emission and transport over the worlds largest source region, the Sahara, is particularly challenging because of the complex meteorology and a very sparse observational network. Recent advances in satellite retrievals together with ground- and aircraft-based field campaigns have fostered our understanding of the spatiotemporal variability of the dust aerosol and its atmospheric drivers. We now have a more complete picture of the key processes in the atmosphere associated with dust emission. These cover a range of scales from (1) synoptic scale cyclones in the northern sector of the Sahara, harmattan surges and African easterly waves, through (2) low-level jets and cold pools of mesoscale convective systems (particularly over the Sahel), to (3) microscale dust devils and dusty plumes, each with its own pronounced diurnal and seasonal characteristics. This paper summarizes recent progress on monitoring and analyzing the dust distribution over the Sahara and discusses implications for numerical modeling. Among the key challenges for the future are a better quantification of the relative importance of single processes and a more realistic representation of the effects of the smaller-scale meteorological features in dust models. In particular, moist convection has been recognized as a major limitation to our understanding because of the inability of satellites to observe dust under clouds and the difficulties of numerical models to capture convective organization.

Links between topography, wind, deflation, lakes and dust: The case of the Bodélé Depression, Chad

The Bodele Depression, Chad is the planets largest single source of dust. Deflation from the Bodele could be seen as a simple coincidence of two key prerequisites: strong surface winds and a large source of suitable sediment. But here we hypothesise that long term links between topography, winds, deflation and dust ensure the maintenance of the dust source such that these two apparently coincidental key ingredients are connected by land-atmosphere processes with topography acting as the overall controlling agent. We use a variety of observational and numerical techniques, including a regional climate model, to show that: 1) contemporary deflation from the Bodele is delineated by topography and a surface wind stress maximum; 2) the Tibesti and Ennedi mountains play a key role in the generation of the erosive winds in the form of the Bodele Low Level Jet (LLJ); 3) enhanced deflation from a stronger Bodele LLJ during drier phases, for example, the Last Glacial Maximum, was probably sufficient to create the shallow lake in which diatoms lived during wetter phases, such as the Holocene pluvial. Winds may therefore have helped to create the depression in which erodible diatom material accumulated. Instead of a simple coincidence of nature, dust from the worlds largest source may result from the operation of long term processes on paleo timescales which have led to ideal conditions for dust generation in the worlds largest dust source. Similar processes plausibly operate in other dust hotspots in topographic depressions.

Uncertainty in the estimation of potential evapotranspiration under climate change

21st century climate change is projected to result in an intensification of the global hydrological cycle, but there is substantial uncertainty in how this will impact freshwater availability. A relatively overlooked aspect of this uncertainty pertains to how different methods of estimating potential evapotranspiration (PET) respond to changing climate. Here we investigate the global response of six different PET methods to a 2 degrees C rise in global mean temperature. All methods suggest an increase in PET associated with a warming climate. However, differences in PET climate change signal of over 100% are found between methods. Analysis of a precipitation/PET aridity index and regional water surplus indicates that for certain regions and GCMs, choice of PET method can actually determine the direction of projections of future water resources. As such, method dependence of the PET climate change signal is an important source of uncertainty in projections of future freshwater availability. Citation: Kingston, D. G., M. C. Todd, R. G. Taylor, J. R. Thompson, and N. W. Arnell (2009), Uncertainty in the estimation of potential evapotranspiration under climate change, Geophys. Res. Lett., 36, L20403, doi: 10.1029/2009GL040267.

A Combined Satellite Infrared and Passive Microwave Technique for Estimation of Small-Scale Rainfall

Abstract There are numerous applications in climatology and hydrology where accurate information at scales smaller than the existing monthly/2.5° products would be invaluable. Here, a new microwave/infrared rainfall algorithm is introduced that combines satellite passive microwave (PMW) and infrared (IR) data to account for limitations in both data types. Rainfall estimates are produced at the high spatial resolution and temporal frequency of the IR data using rainfall information from the PMW data. An IRTb–rain rate relationship, variable in space and time, is derived from coincident observations of IRTb and PMW rain rate (accumulated over a calibration domain) using the probability matching method. The IRTb–rain rate relationship is then applied to IR imagery at full temporal resolution. MIRA estimates of rainfall are evaluated over a range of spatial and temporal scales. Over the global Tropics and subtropics, optimum IR thresholds and IRTb–rain rate relationships are highly variable, reflecting the co...

Mineral dust emission from the Bodélé Depression, northern Chad, during BoDEx 2005

Mineral dust in the atmosphere is an important component of the climate system but is poorly quantified. The Bodele depression of Northern Chad stands out as the world’s greatest source region of mineral dust into the atmosphere. Frequent dust plumes are a distinguishing feature of the region’s climate. There is a need for more detailed information on processes of dust emission/transport and dust optical properties to inform model simulations of this source. During the Bodele Dust Experiment (BoDEx) in 2005 instrumentation was deployed to measure dust properties and boundary layer meteorology. Observations indicate that dust emission events are triggered when near surface wind speeds exceed 10ms-1, associated with synoptic scale variability in the large-scale atmospheric circulation. Dust emission pulses in phase with the diurnal cycle of near surface winds. Analysis of dust samples shows that the dust consists predominantly of fragments of diatomite sediment. The particle size distribution of this diatomite dust estimated from sun photometer data, using a modified Aeronet retrieval algorithm, indicate a dominant coarse mode (radius centred on 1-2?m) similar to other Saharan dust observations. Single scattering albedo values are high, broadly in line with other Saharan dust even though the diatomite composition of dust from the Bodele is likely to be unusual. The radiative impact of high dust loadings results in a reduction in surface daytime maximum temperature of around 7°C in the Bodele region. Using optical and physical properties of dust obtained in the field we estimate the total dust flux emitted from the Bodele to be 1.18±0.45Tg per day during a substantial dust event. We speculate that the Bodele depression (~10,800km2) may be responsible for between 6-18% of global dust emission, although the uncertainty in both the Bodele and global estimates remain high.