Slobodan Nickovic

Asian dust events of April 1998

On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low-pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface-based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the surface along the mountain ranges between British Columbia and California, and impacted severely the optical and the concentration environments of the region. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10–20%, but it reduced the near-UV cloud reflectance, causing a yellow coloration of all surfaces. The yellow colored backscattering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30–40%, doubled the diffuse radiation, and caused a whitish discoloration of the blue sky. On April 29 the average excess surface-level dust aerosol concentration over the valleys of the West Coast was about 20–50 μg/m3 with local peaks >100 μg/m3. The dust mass mean diameter was 2–3 μm, and the dust chemical fingerprints were evident throughout the West Coast and extended to Minnesota. The April 1998 dust event has impacted the surface aerosol concentration 2–4 times more than any other dust event since 1988. The dust events were observed and interpreted by an ad hoc international web-based virtual community. It would be useful to set up a community-supported web-based infrastructure to monitor the global aerosol pattern for such extreme aerosol events, to alert and to inform the interested communities, and to facilitate collaborative analysis for improved air quality and disaster management.

A model for prediction of desert dust cycle in the atmosphere

An integrated modeling system has been developed to accurately describe the dust cycle in the atmosphere. It is based on the SKIRON/Eta modeling system and the Eta/NCEP regional atmospheric model. The dust modules of the entire system incorporate the state of the art parameterizations of all the major phases of the atmospheric dust life such as production, diffusion, advection, and removal. These modules also include effects of the particle size distribution on aerosol dispersion. The dust production mechanism is based on the viscous/turbulent mixing, shear-free convection diffusion, and soil moisture. In addition to these sophisticated mechanisms, very high resolution databases, including elevation, soil properties, and vegetation cover are utilized. The entire system is easily configurable and transferable to any place on the Earth, it can cover domains on almost any size, and its horizontal resolution can vary from about 100 km up to approximately 4 km. It can run on one-way-nested form if necessary. The performance of the system has been tested for various dust storm episodes, in various places and resolution using gridded analysis or forecasting fields from various sources (ECMWF and NCEP) for initial and boundary conditions. The system is in operational use during the last two years, providing 72 hour forecasts for the Mediterranean region. The results are available on the internet (http://www.icod.org.mt and http://forecast.uoa.gr).

Long‐range transport of Saharan dust to northern Europe: The 11–16 October 2001 outbreak observed with EARLINET

The spread of mineral particles over southwestern, western, and central Europe resulting from a strong Saharan dust outbreak in October 2001 was observed at 10 stations of the European Aerosol Research Lidar Network (EARLINET). For the first time, an optically dense desert dust plume over Europe was characterized coherently with high vertical resolution on a continental scale. The main layer was located above the boundary layer (above 1-km height above sea level (asl)) up to 3–5-km height, and traces of dust particles reached heights of 7–8 km. The particle optical depth typically ranged from 0.1 to 0.5 above 1-km height asl at the wavelength of 532 nm, and maximum values close to 0.8 were found over northern Germany. The lidar observations are in qualitative agreement with values of optical depth derived from Total Ozone Mapping Spectrometer (TOMS) data. Ten-day backward trajectories clearly indicated the Sahara as the source region of the particles and revealed that the dust layer observed, e.g., over Belsk, Poland, crossed the EARLINET site Aberystwyth, UK, and southern Scandinavia 24–48 hours before. Lidar-derived particle depolarization ratios, backscatter- and extinction-related Angstrom exponents, and extinction-to-backscatter ratios mainly ranged from 15 to 25%, −0.5 to 0.5, and 40–80 sr, respectively, within the lofted dust plumes. A few atmospheric model calculations are presented showing the dust concentration over Europe. The simulations were found to be consistent with the network observations.

An illustration of the transport and deposition of mineral dust onto the eastern Mediterranean

Abstract The analyses of aerosol samples and deposition (wet) measurements during August 1991–December 1992 at Erdemli (36°N, 34°E) located on the Turkish coast of the eastern Mediterranean has shown higher dust concentration and total deposition during transitional seasons (spring and autumn) compared to summer and winter seasons. The data, complemented by three-dimensional (3D) air mass back trajectories and satellite observations suggest that North African and Middle East desert derived dust particles are transported to the region during transitional seasons. Transport events in the last part of March 1992 and early October 1992 are studied through combined analyses of ground based and satellite observations and modelling results. It is shown that dust transport constitutes a large fraction of the annual atmospheric deposition in the eastern Mediterranean, with two deposition events of short duration accounting up to 30% of the total annual flux. Therefore, the dissolved and particulate species associated with dust could be extremely variable in the mixing layer during large deposition events and could easily be missed in a short-term sampling program. The possible impact of large pulses on biological productivity of the sea also warrants consideration.

Optical properties of Saharan dust layers as detected by a Raman lidar at Thessaloniki, Greece

[1]xa0Measurements during Saharan dust transport events were performed at several stations of the European Aerosol Research Lidar Network. During the period 2001–2002, 12 cases were captured at Thessaloniki, Greece (40.5°N, 22.9°E) with a 355 nm Raman lidar. For these cases the vertical profiles of the extinction and backscatter coefficients were determined and examined, as well as profiles of extinction-to-backscatter ratio (lidar ratio) and the backscatter-related Angstrom exponent between 355 and 532 nm (color index). Model calculations from the DREAM model confirmed the existence of Saharan dust in all cases examined. Within the dust plumes the lidar ratios ranged from 20 to 100 sr and the color index ranged from −1 to 3. An anticorrelation was found between the lidar ratio and the color index during the Saharan dust events. In certain cases the results suggest that the mixing of dust particles with boundary layer aerosols reduces the dust ‘signature’ and thus, makes difficult the separation of the dust particles from those of mostly urban origin.

The Weather Forecasting System for Poseidon - an Overview

A weather forecasting system has been developed in the framework of the POSEIDON project Monitoring, forecasting and information system for the Greeks seas. The system is currently running operationally at the National Centre for Marine Research (NCMR) providing detailed and accurate weather forecasts. It also forces a general circulation ocean model and an offshore wave model with the surface fluxes of momentum and heat and with the precipitation rates. Because of the specific applications for marine studies and operations, emphasis was given to the surface processes especially over the water body. The system contains a viscous sublayer model, which is considered as appropriate for a flux description. The system is evaluated systematically with the aid of the POSEIDON buoy network and other meteorological information available. A description of the system and its capabilities are presented below.

Observations and model simulations of a winter sub-synoptic vortex over the central Mediterranean

A sub-synoptic vortex with characteristics of a tropical storm developed over the area between Italy and Greece, during the period 14–18 January 1995. Satellite imagery revealed that this vortex was associated with spirally distributed bands and a clearly defined ‘eye’. Ship reports and SSM/I retrieved winds show the existence of strong surface winds around the vortex. Simulation of the genesis and life-cycle of the vortex using the Regional Atmospheric Modelling System and the Eta model successfully reproduced the vortex formation, its strength and its path towards the northern African coast. The model results showed the importance of the surface fluxes (sensible and latent heat) in the development of the vortex on the rear side of a parent low-pressure system which was advected towards the east. During the mature stage of the vortex, latent-heat release within the convective motions was the dominant mechanism which sustained the vortex until its landfall. Copyright

A hemispheric dust storm affecting the Atlantic and Mediterranean in April 1994: Analyses, modeling, ground-based measurements and satellite observations

One of the largest recorded dust transport events originating from the great Sahara desert during April 1994 affected the entire region extending from the Caribbean to the Eurasian continent. This hemispherical transport of airborne dust took place during a series of storms that developed during the first three weeks of April in a background of low-index circulation. These repeated events are studied through the combined analyses and interpretation of atmospheric data, ground-based aerosol measurements, visibility observations, AVHRR and Meteosat visible band satellite data, and the results of Eta model simulations, including an aerosol transport component. The observations produce a consistent picture of the temporal and spatial development of the dust events, whose main features are used in parts to verify the model results. The rate of dust suspension from some areas of the western Sahara desert exceeded 1.5 mg m−2 h−1 and the maximum column integrated dust load reached 2 g m−2 during April 3–5 1994, when the first major suspension event produced two simultaneous pulses of dust moving in opposite directions across the subtropical Atlantic Ocean and the eastern Mediterranean Sea. These dust suspensions were created by surface winds resulting from subsidence on the northeastern side of a blocking anticyclone in the Atlantic region and subsequent winds of an intense developing cyclone in the Mediterranean-African region. In the following period, maximum dust loads of 4.5 and 2.5 g m−2 occurred on April 12 and 17, respectively, when new cyclones transported dust across the Mediterranean from Africa to Europe. The generation of the two dust pulses during the first even and the recurrent cyclone transport in the following period is shown to be the result of a large-scale, anomalous atmospheric circulation connected with blocking in the Atlantic Ocean and the interactions of upper air jets downstream of the blocking. The particular state of the hemispheric circulation during the studied period corresponded to the positive phase of the North Atlantic Oscillation (NAO). While previous statistical evidence has consistently linked dust transport in the region with the NAO signatures, we show the same connection on the basis of this case study.

Production and Long-Range Transport of Desert Dust in the Mediterranean Region: ETA Model Simulations

Saharan dust storms are the main source of the atmospheric dust in the Mediterranean region. Once injected to the atmosphere, dust may pass long distances under favourable meteorological conditions before it deposits to the ground or sea surfaces. Typically, several hundreds millions of tonnes of dust is transported away from sources annually (D’Almeida, 1986). Continuous presence of dust in the atmosphere causes diverse climatic and environmental effects. For example, dust modifies radiation properties of the air through absorption and scattering of the solar energy on dust particles (e. g. Chen et al., 1995). Recent estimate of Tegen and Fung (1994) shows that mineral dust may decrease the net radiation for about 1 Wm−2, revealing thus the fact that it could be a significant climate forcing factor. Another environmental effect of the dust process is dust deposition on the sea surface, which may significantly change the marine biochemical properties (e. g. Martin and Fitzwater, 1988; Kubilay and Saydam, 1995). Also, the atmospheric dust may significantly influence human activities: for example, it reduces the visibility, causing thus problems in the air and ground traffic; during dust storms, increased number of eye and respiratory organs infections is recorded, too.

Contribution of Desert Dust Transport to Air Quality Degradation of Urban Environments Recent Model Developments

Mineral dust, produced by wind erosion over arid and semi-arid areas of North Africa, may transport away to the Middle East, Mediterranean, Europe, even into and across the Atlantic Ocean (Kallos et al., 2002). This material transported away from its origin is considered as an important climate and environment modifier. Dust particles by absorbing and backscattering both the incoming solar radiation and the infrared outgoing radiation modify the Earth’s radiation budget (Andreae, 1996). In addition, they alter the cloud microphysics processes acting as cloud condensation nuclei and having pH < 7.0 play a role in neutralization of the acid rains (Hedin and Likens, 1996). Also with the long-range dust transport, important nutrients are transported from their sources to other regions and may significantly modify the biogeochemistry of these marine and terrestrial ecosystems (Swap et al., 1996). For example, the deposition of the North African dust material on the Mediterranean Sea provides important nutrients, such as nitrogen species, phosphorus and iron, which may enhance the marine productivity. Some summer algal blooms in the Mediterranean Sea may be explained by such Saharan dust deposition (Dulac et al., 1996). Guerzoni et al. (1999) have estimated the amount of the atmospheric dust mass deposited on Mediterranean region to be ∼40×106 tons. Even though, they turned out to this magnitude by measured atmospheric mass flux at 9 coastal sites (which is considered as a small number of sites for such a work), this is considered as a valuable estimation since it is the only one found so far.