Radiative Effects Of Desert Dust On Weather Biology Essay

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Mineral dust aerosols interact strongly with solar and terrestrial radiation in several ways. By absorbing and scattering the solar radiation aerosols reduce the amount of energy reaching the surface. In addition aerosols enhance the greenhouse effect by absorbing and emitting outgoing longwave radiation. Desert dust forcing exhibits large regional and temporal variability due to its short lifetime and diverse optical properties, further complicating the quantification of the Direct Radiative Effect (DRE). The complexity of the links and feedbacks of dust on radiative transfer indicate the need of an integrated approach in order to examine these impacts.

In order to examine these feedbacks the SKIRON limited area model has been upgraded to include RRTMG radiative transfer model and was run for a 6 year long period. Two sets of simulations were completed, one without the effects of dust and the other including the radiative feedback. The results were first evaluated using aerosol optical depth data to examine the capabilities of the system in describing the desert dust cycle. The aerosol feedback on radiative transfer has then been quantified and the links between dust and radiation have been studied.

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The study has revealed a strong interaction between dust particles and solar and terrestrial radiation, with several implications on the energy budget of the atmosphere. The presence of aerosols decreases the incoming solar radiation while at the same time emits in the infrared towards the surface. Another profound effect is the increased absorption in the lower troposphere and the modification of the atmospheric temperature profile. These feedbacks depend strongly on the spatial distribution of dust, having more profound effects as the number of particles increases, like near the source areas.

2. Introduction

Mineral dust produced from arid and semi-arid areas of the world is injected into the atmosphere under favorable meteorological conditions. Aerosols interact strongly with solar and terrestrial radiation in several ways (Tegen et al., 1996; Sokolik et al., 2001; Haywood et al., 2003; Yoshioka et al., 2007; IPCC, 2007; Kallos et al., 2009a), also known as "Direct Aerosol Effect - DRE" (IPCC, 2007). By absorbing and scattering the solar radiation aerosols reduce the amount of energy reaching the surface (Kaufman et al., 2002; Tegen 2003; Kallos et al., 2009a; Spyrou et al., 2010). Moreover, aerosols enhance the greenhouse effect by absorbing and emitting outgoing longwave radiation (Dufrense, 2001; Tegen, 2003; Heinold et al., 2008; Pandithurai et al., 2008).

As a second type of effect, aerosols act as sites at which water vapor can accumulate during cloud droplet formation, serving as cloud condensation nuclei (Levin et al. 2005; Solomos et al., 2011). Any change in number concentration or hygroscopic properties of such particles has the potential to modify the physical and radiative properties of clouds, altering cloud brightness (Twomey, 1977) and the likelihood and intensity with which a cloud will precipitate (e.g. Liou and Ou, 1989; Albrecht, 1989; Solomos et al., 2011). Collectively changes in cloud processes due to aerosols are referred to as aerosol indirect effects. Finally, absorption of solar radiation by particles contributes to the reduction in cloudiness, a phenomenon referred to as the semi-direct effect. This occurs because absorbing aerosol warms the atmosphere, which changes the atmospheric stability, and reduces surface flux (U.S. Climate Change Science Program Synthesis and Assessment Product 2.3, 2009).

The magnitude of the feedback on the radiative transfer depends strongly on the optical properties of particles (single scattering albedo, asymmetry parameter, extinction efficiency), which in turn depend on the size, shape and refractive indexes of dust particles (Tegen, 2003; Helmert et al., 2007). The mineral composition of the dust source areas (Tegen, 2003), as well as the chemical composition and transformation of aerosols during their transportation, are all factors on the optical intensity of dust (Wang et al., 2005; Astitha et al., 2010). Furthermore the vertical distribution of dust, the presence of clouds and the albedo of the surface all contribute to the DRE (Sokolik and Toon, 1996; Tegen and Lacis 1996; Liao and Seinfeld 1998; Helmert et al., 2007).

Several studies have focused on calculating the radiative feedback of dust on a global scale: Liao et al. (2004) found a decrease in the incoming shortwave radiation of 0.21 W m-2, while the longwave radiation increased by 0.31 W m-2. Accordingly Reddy et al. (2005) simulated a decrease of 0.28 W m-2 on the shortwave and an increase of 0.14 W m-2 on the longwave. More studies and measurements revolved around the same numbers of global dust radiative effect (IPCC, 2007). However, little work is done in incorporating the dust radiative feedback on regional and mesoscale models (Stokowski, 2005).

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At a local scale, the DRE exhibits much stronger signals. Haywood et al. (2003) has measured a decrease of incoming shortwave radiation up to 130 W m-2 at the coast of West Africa during a dust storm on September 2000. Hsu et al. (2000) determined an increase in longwave radiation of 25 W m-2 by approximation over North Africa for July 1995. During a severe dust storm in Niger on 3 - 12 March 2006 the ARM (Atmospheric Radiation Measurement) mobile facility detected a shortwave decrease of 250 W m-2on the surface, where at the same time the reflected radiation at the top of the atmosphere increased by 100 W m-2 (Slingo et al., 2006). During the same episode, the emitted longwave radiation from the surface decreased by approximately 100 W m-2, corresponding to the "cooling" imposed due to dust shading.

The local effects of dust have been thoroughly examined in various other studies. However the main purpose of this work is to examine the long-term implications of the dust-radiation interactions. In order to model these diverse feedbacks on the radiation balance the SKIRON/Dust limited area model has been used (Kallos et al., 1997a,b; Papadopoulos et al., 2002; Kallos et al., 2006; Spyrou et al., 2010). The model has been further updated to include the Rapid Radiative Transfer Model - RRTMG (Mlawer et al., 1997; Oreopoulos et al., 1999; Iacono et al., 2003; Pincus et al., 2003; Baker et al., 2003; Clough et al., 2005; Morcette et al., 2008; Iacono et al., 2008). The addition of the RRTMG scheme has made it possible to model and study the effects of desert dust particles to the radiation balance of the atmosphere.

The following paragraph summarizes the main characteristics of the modelling system and the 3rd section outlines the new radiative transfer scheme that was incorporating in SKIRON/Dust system. The 4th paragraph depicts the optical properties of suspended particles and in particular dust aerosols that are discussed in the present study. The experimental setup that was designed for the present simulations is described in the 5th paragraph. The model evaluation and the sensitivity tests for the estimation of the energy impact are analysed in the 6th and 7th paragraphs, respectively. Finally, the last section summarizes the main conclusions of the study.

3. SKIRON/Dust modelling system

The SKIRON/Dust modeling system is based on the atmospheric model SKIRON. A dust module that simulates the production and removal of the desert dust aerosol is directly coupled with the host model. The SKIRON atmospheric model has been developed at the University of Athens from the Atmospheric Modelling and Weather Forecasting Group (Kallos et al., 1997; 2006; Spyrou et al., 2010) at the framework of the number of projects (SKIRON, MEDUSE, ADIOS, CIRCE and recently MARINA). The atmospheric model is based on the ETA/NCEP model, which was originally developed by Mesinger (1984; 1988) and Janjic (1990; 1994) at the University of Belgrade. The SKIRON atmospheric model is based on several sophisticated parameterization schemes, such as the OSU (Oregon State University) scheme for simulation of the surface processes, including a data assimilation scheme for soil temperature and soil wetness or possibility to choose among Betts-Miller-Janjic (Betts, 1986) and Kain-Fritsch (Kain and Fritsch, 1990) convective parameterization schemes for the representation of moisture processes. During the SKIRON/Dust runs, the prognostic atmospheric and hydrological conditions are used in order to calculate the effective rates of the injected dust concentration based on the viscous/turbulent mixing, shear-free convection, diffusion, soil composition and soil moisture. The dust module includes the effects of the particle size distribution in order to simulate more accurately the size-dependent processes. In the current form of the modelling system, the transport mode uses eight size bins (log-normally distributed) with effective radius 0.15, 0.25, 0.45, 0.78, 1.3, 2.2, 3.8, 7.1 μm (Table 1).

Recently, the modelling system was significantly upgraded in order to improve the model prediction efficiency according to the current needs for simulation of the mineral dust cycle and the interaction mechanisms with climate. In the frame of the upgrading, new features for the description of the bottom boundary (ground or sea surface) characteristics of the atmospheric model and the dust aerosol properties were incorporated (Spyrou et al., 2010). The new model version includes a 16-category soil characteristics dataset (Miller and White, 1998) that provides detailed information on soil physical properties, such as porosity, available water capacity. A high-resolution (30-second) global land use/cover database including urban areas and classified according to the 24-category USGS land use/land cover system (Anderson et al., 1976) is utilized. In the upgraded SKIRON/Dust model a new processor was developed that derives statistics for the slope steepness and orientation for more accurate description of the topographic variability that determines the incoming solar radiation reaching the ground surface. The dust aerosol is described by using the three-modal lognormal function of D'Almeida (1987) for the aerosol mass distribution at the source areas and the 8-size bin transport mode of Schulz et al. (1998) for the long-range transported particles. The dust particles are assumed to be mobilized through the process of saltation bombardment (Marticorena and Bergametti 1995) and deposited via dry (diffusion, impaction, gravitational settling) and wet (in-cloud and below-cloud removal) mechanisms. More details on the specific characteristics of the atmospheric model are provided in Spyrou et al. (2010).

4. Rapid Radiative Transfer Model - RRMTG

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Longwave and shortwave radiative transfer in the SKIRON/Dust model are parameterized with RRTMG (Mlawer et al., 1997; Iacono et al., 2003; Iacono et al., 2008), a broadband correlated k-distribution radiation model developed at AER, Inc. with support from the U.S. Department of Energy. Both RRTMG and the related single-column, reference radiation model, RRTM, were developed in the context of continual comparison to Line-by-line Radiative Transfer Model - LBLRTM, which is an accurate and highly flexible model that continues to be extensively validated with measured atmospheric radiance spectra from the sub-millimeter to the ultraviolet (Clough et al., 2005; Turner et al., 2004). This approach realizes the goal of providing an improved radiative transfer capability that is directly traceable to measurements. Molecular absorbers in RRTMG include water vapor, carbon dioxide, ozone, methane, nitrous oxide, oxygen, nitrogen and four halocarbons (CFC11, CFC12, CFC22, and CCL4) in the longwave and water vapor, carbon dioxide, ozone, methane and oxygen in the shortwave. The water vapor continuum is based on CKD_v2.4, and molecular line parameters are based on HITRAN 2000 for water vapor and HITRAN 1996 for all other molecules. RRTMG uses sixteen spectral bands to represent the longwave region, while the shortwave band is represented by fourteen spectral intervals. Absorption and emission from aerosols and clouds are included in the longwave, and the shortwave treatment includes extinction (absorption plus scattering) from aerosols, clouds and Rayleigh scattering. Aerosol radiative effects are treated in RRTMG through the specification of their optical properties within each spectral interval.

While RRTMG shares the same basic physics and absorption coefficients as RRTM, it incorporates several modifications to improve computational efficiency, to update the code formatting for easier application to global and limited area models, and to represent sub-grid scale cloud variations. The complexity of representing fractional cloudiness and cloud overlap in the presence of multiple scattering is addressed in RRTMG with the use of McICA, the Monte-Carlo Independent Column Approximation (Pincus et al., 2003), which is a statistical technique for representing sub-grid scale cloud variability including cloud overlap. Although this method introduces random noise to the cloudy calculation of radiance, the result has been shown to be unbiased. This approach provides the flexibility to represent the vertical correlation of clouds (i.e. cloud overlap) in some detail by imposing an assumed relation (such as random or maximum-random) among the stochastic cloud arrays across the vertical dimension. The maximum-random cloud overlap assumption, in which adjacent cloud layers in the vertical are presumed to overlap maximally and non-adjacent cloudy layers are assumed to overlap randomly, is applied within RRTMG in the SKIRON/Dust model.

5. Optical Properties of Dust

The absorption and scattering of light by a spherical particle is a classic problem in physics, the mathematical formulation of which is called Mie theory (Bohren and Huffman, 1983). The most important factors that govern these processes are the wavelength of the incident radiation, the size of the particle, the ratio of the circumference of the particle to the wavelength and the complex refractive index, expressed as. The real part represents the nonabsorbing and the imaginary is related to the absorption of light by the medium (Helmert et al., 2007).

The refractive indexes of various spectral ranges, for all the particle sizes, have been determined using the OPAC (Optical Properties of Aerosols and Clouds) software package (Hess et al., 1998), which in turn utilizes the Mie theory for its calculations (Quenzel and Muller, 1978). The same package was used for the definition of the aerosol single scattering albedo and the asymmetry parameter for the same wavelengths. Especially for the 550nm spectral window, where the extinction of the incoming solar radiation is most intense, a value of 0.95 was used instead of the smaller value (0.83) provided by OPAC, for transported mineral dust. Field measurements suggest this value is more realistic for desert dust in the mid-visible (Kalashnikova et al., 2005; Ralph Kahn personal contact).

For the calculation of the extinction efficiencies for each size bin , at each wavelength , the Wiscombe Mie algorithm was used (Wiscombe, 1980, Mishchenko et al., 2002). Although dust particles are know to be nonspherical (Nakajima et al., 1989; Meloni et al., 2004), Mie calculations can be used to compute the radiative parameters for equivalent volume spheres and provide very good representation for non-spherical scattering (Tegen and Lacis, 1996).

The aerosol optical thickness at each wavelength was calculated by the following formula:

(1)

where the number of particle size bins, g m-3 the particle density, the particle radius, the layer dust load and the extinction efficiency calculated from the Mie theory (Tegen and Lacis, 1996; Perez et al., 2006).

6. Experimental Design

In order to evaluate and quantify the effects of desert dust particles on the radiation budget in the atmosphere, a series of test runs were carried out. The model was integrated over an extended area that covers the European continent, the Mediterranean Sea and northern Africa, as well as a major part of Middle East and Turkey. The horizontal grid increment was 0.24°. In the vertical direction 38 levels were used stretching from the ground surface up to 20 km. The ECMWF reanalysis dataset (horizontal resolution of 0.25°) was used for the initial and lateral boundary conditions for the meteorological parameters.

The model runs were carried out under two different setups: a) by neglecting the effects of dust particles on the radiative parameters (NDE) and b) by including the dust-radiation interaction mechanisms (WDE). The differences in model simulations for various radiative magnitudes, as well as other atmospheric parameters, between the two setups are calculated and discussed in the following paragraphs.

7. Model evaluation - Qualitative study and statistical techniques

To estimate the impact in model performance with the consideration of dust-radiation interaction processes, a specific case of desert dust transport towards the SE Europe was examined. On the 5th of April 2006 strong winds originating from across the Southeastern side of the Atlas mountain range injected large quantities of mineral dust particles into the atmosphere. The particles were transported over the Mediterranean Sea and two days later on the 7th of April 2006 reached Crete on the Eastern Mediterranean (Figure 1). To evaluate the effects of dust on the radiative transfer, solar radiation data retrieved from a pyranometer located at the Technological University (TEI) of Crete were used. Model runs with the two different setups (NDE, WDE) were carried out for the simulation of the test case.

During the passage of the dust plume the incoming solar radiation decreased by approximately 200 Wm-2 (Figure 2). This abrupt change is depicted by the WDE simulation to a certain extent, but cannot be attributed solely on the shading effect of desert dust particles. One viable explanation is that the change in the radiation balance of the atmosphere, due to the presence of dust particles, led to cloud formation in the area that are blocking the incoming solar radiation observed at the station in Crete.

To further investigate this hypothesis the vertical profiles of the cooling rates have been derived. In Figure 3 the cooling rates together with the vertical extent of dust concentration are shown. The cooling rates of the WDE simulation show a sharp decrease of 4°K/day just above the dust plume at approximately 9 Km. The dust particles seem to create an albedo effect and thus they reflect the incoming solar radiation back towards the top of the atmosphere. As a side effect of this reflection, cooling of the adjacent atmospheric layer over the dust plume is observed. This temperature decrease drives the air temperature closer to the dew point temperature and in return to the formation of a cirrus cloud over the station in Crete, as shown in Figure 4. The SKIRON/Dust model (WDE run) appears to capture this feature and the formation of a high altitude cloud over the station in Crete simulation (Figure 4 right), which in turn leads to the decrease of the simulated incoming solar radiation.

For a more thorough evaluation of the model performance, a series of statistics were derived. At first in order to evaluate the radiation calculations, the model outputs were compared with the measurements obtained from three station of the AERONET network, namely Crete, Sede Boker and Moldova for the whole year period of 2006. The characteristics of the stations (geographical location and altitude), as well as the statistical parameters (regression trend lines, correlation coefficients) determined from the analysis are shown in Table 2. The flux comparisons were performed for the days of increased values of aerosol optical depth (AOD). Thus, AOD measurements at 500 nm from the AERONET stations were also obtained and the 95th, 99.5th percentiles were calculated for each dataset in order to detect the periods of increased particle suspension in the atmosphere. The 95th percentile of AOD was found at about 0.3, while the 99.5th percentile fluctuated around 0.6 for the three studied sites. In general, the SKIRON/Dust model produces accurate simulations of the incoming radiation, since the correlation coefficients calculated for the compared datasets have great values and the slopes of the trend lines are close to unity in general. However, the intercept of the regression lines that is indicative of the difference between the simulated and the observed values are increased exceeding 50 before considering the dust effects on radiation. This fact reveals the noticeable overestimation of the modeled fluxes by neglecting the decrease of the incoming radiation due to the presence of dust aerosols. On the other hand, the inclusion of dust-radiation interaction processes appears to diminish significantly this overestimation at Crete and Sede Boker, with a complete elimination noticed in Crete (-0.8) for AOD values greater than 0.6. The limitation of the model overestimation in Moldova (the most northern site out of the three) with the incorporation of dust effects appears to be negligible. The dust load that reaches the specific area is reduced comparing to the amounts that affect the more southern sites, thus, the radiative forcing due to the suspension of dust particles is expected to be reduced.

For the evaluation of the model calculations related to atmospheric parameters, namely air temperature, simulation runs for a 6-year period (2002 - 2007) were carried out with the implementation of the modeling system. The measured observations of air temperature close to the surface were collected from about 600 monitoring stations of the World Meteorological Organization network (WMO). For the two model setups (NDE, WDE), the Bias, the Root Mean Square Error (RMSE) and the Correlation Coefficient r were derived for each season of the year for the entire 6 year simulation period (Figure 5), as described in Wilks (1995). It appears that the correlation coefficient takes higher values with the new modifications incorporated in the model and exceeds 0.90 throughout the year. The Bias is reduced by taking into account the dust impacts on radiation. This reduction is larger during MAM and JJA, two seasons with the highest number of Saharan dust intrusions towards Europe and less profound during winter (DJF) and autumn (SON). However the improvement in the statistical scores for winter and autumn originate from the stations near the source areas, where dust clouds are always present and constantly modify radiative transfer. The RMSE values show respective decrease with the incorporation of the dust-radiation interaction mechanisms ranging from ~28% noticed in autumn (SON) to ~36% during the transitional period (MAM).

In general, the inclusion of dust-radiation interaction processes appears to upgrade the model performance as related to radiation simulations and temperature calculations.

8. Model sensitivity analysis - Estimation of the impact on energy budget

The dust cycle is determined through the accounting of emission, atmospheric loading and deposition of the desert dust particles. The quantification of the dust mass over the selected model domain appears to be essential for the determination of the modified amounts in the energy budget due to the suspension of desert dust particles. For the estimation of the desert dust impact on the energy budget over the studied area, the 6-year model runs covering the period 2002 - 2007 were extensively analyzed. After the model evaluation, the NDE model runs are assumed to determine the radiation transfer parameters in clean (free of dust particles) atmospheric conditions¸ while the WDE simulations represent the cases of dust transport events by taking into consideration the dust-radiation interaction processes.

Firstly, the areas that are mostly affected by the emission of desert dust particles were detected. In Figure 6a-d, the dust load (integrated column) values for the different seasons of the year are depicted across the studied area. Significant dust sources appear to be over Chad with dust load exceeding the value of 1 g m-2 and at the northwestern part of the African continent over Mauritania and Algeria. The suspension of desert dust particles is also evident across and around the Red Sea in spring and summer periods. As reported by many studies (e.g. Moulin et al., 1997; Nickovic et al., 2001), the Saharan dust transport events are characterized by a strong seasonal variability. In particular, maximum dust impact is observed in eastern-central Mediterranean in spring, while the maximum is shifted in summer (and early autumn) for the central-western Mediterranean (Querol et al., 2009).

The seasonal differences in the incoming short-wave and long-wave radiation fluxes reaching the surface are illustrated in Figures 7 and 8. The differences in the short-wave fluxes appear to be negative during all periods of the year and greater during the warm spring and summer periods approaching the value of 70 W m-2 over the extended desert areas of Chad. On the other hand, the depicted deviations over Europe and Mediterranean are negligible during the cold periods, where smaller radiation amounts are received by the ground surface. However, in the spring period, the discrepancies over European land areas could reach up to 10 W m-2 on the South and 5 W m-2 along the Central European countries. Another region of increased differences in the incoming short-wave radiation that maximize during summer is the northwestern African continent and in particular over the southern desert areas of Morocco, where deviations greater than 40 W m-2 can be observed (Figure 7c). As expected, the deviations between the two model setups (NDE and WDE) are more noticeable in the periods and areas of high particle concentrations. In all cases, the suspension of dust aerosols led to considerable reduction of the incoming short-wave radiation due to scattering and absorption of the solar radiation. This feedback could prove troublesome for photovoltaic and concentrating solar-thermal power plant installations in desert areas (e.g. DESERTEC, source: http://www.desertec.org), as the received amounts of solar energy are diminished from dust shading. As illustrated in Figure 8a the annual reduction of the incoming solar radiation can be as high as 500KW m-2, a significant portion of the available solar radiation (Figure 8b), that has to be taken into consideration.

The incoming long-wave radiation fluxes with the presence of dust particles are increased during all periods of the year (Figure 9) with greater differences noticed in the spring. The long-wave radiation emitted by the dust layer towards the ground surface during nighttime is superimposed with the solar radiation in the day; as a result the incoming flux that reaches the ground surface is enhanced. This increase could approach the value of 35 W m-2 over Chad and 25 W m-2 over southern Morocco during the warm periods. Smaller discrepancies are observed over the Mediterranean Sea.

In an attempt to estimate the net atmospheric forcing by the mineral dust particles, the difference in the amount of the radiative flux absorbed by the atmosphere along the model domain between the two model setups (WDE - NDE) was defined for the different seasons of the year, as illustrated in Figures 9a-d.

The radiative flux that is absorbed by the atmosphere can be determined through: . If we consider that the dust particles exert a negligible effect to the incoming radiation at the top of the atmosphere then can be expressed as . In effect negative values in Figures 10a-d denote an increase in the atmospheric absorption due to dust feedback and positive values a decrease.

The absorbed fluxes are increased over the desert areas of the northern African continent, while during the summer months differences can be also noticed over southern and central European areas. The energy amounts absorbed by the dust aerosols seem to reach 10 W m-2 over land areas, while over the Red Sea the increase in the absorbed energy exceed the 20 W m-2 during the summer period. The warm sea mass of Red Sea emits large amounts of long-wave radiation that are trapped between the sea surface and the dust layer when there are increased concentrations of dust particles (greenhouse effect). On the contrary the cold waters of the North Atlantic emit small amounts of long-wave radiation and the difference in atmospheric absorption detected is attributed to the emission of dust clouds (especially during the summer where dust particles move towards the Atlantic). The absorbed energy amounts by the dust particles may influence significantly the atmospheric stability by heating the lower tropospheric layer.

The analysis of the 3-D distributions of dust concentration reveals the vertical characteristics of the dust transport and suspension over the ground surface. Two vertical cross-sections of averaged dust concentration along the pathlines at 2°W for summer and 18°E for spring are illustrated in Figures 10a and 11a respectively.

The greatest dust concentration values are obtained over the main dust sources and in particular at about 20°-25°N over the western part (2°W) and over Chad in the central part (18°E) of the north-African region. The area of increased dust concentrations close to the surface is more extended in the western part of the African continent (Figure 11a), that is related to the existence of more wide-ranging dust source areas over this part. On the other hand, the high mountains expanding in the central part of northern Africa (Figure 12a) limit the extent of the dust source areas.

The vertical cross-sections of the temperature difference between the two model setups (WDE - NDE), as illustrated in Figure 11b for the summer months of the 6-year studied period, reveal the importance of dust radiative feedback (Figures 11b,12b): First of all a mid-tropospheric cooling of ~0.1°K at 300hPa, due to reflection from the dust particles below is observed. At the same time an increase in the air temperature profile (~0.2°K) at about 600 hPa (mid-tropospheric heating) due to the increased atmospheric absorption from the dust particles. The temperature decreases in the lower troposphere (925hPa up to 825 hPa) up to 0.3°K due to extinction of the incoming solar radiation, which is more pronounced in the western part of the African continent (Figure 11b). A really interesting feedback is that near source areas the temperature of the atmospheric layer near the ground increases by ~0.4 during the summer (Figure 11b) and even up to ~0.5°K during spring (Figure 12b), even though the extinction of the incoming solar radiation is more profound there. This can be attributed to two superimposed factors: 1) during the day the dust particles absorb significant amounts of incoming solar radiation (as seen also in Figure 9), raising the temperature of the near surface layer and 2) during the night this dust layer "traps" long-wave radiation emitted from the ground (greenhouse effect). As a result, the air temperature close to the surface increases, while a cooling of up to -0.2°K is observed inside the dust layer. This phenomenon indicates that the inclusion of dust feedback on climate studies will probably not wield the expected counter to the greenhouse effect.

Far from the desert areas over the Mediterranean Sea the expected temperature reduction is observed reaching -0.2°K attributed to the absorption and scattering by dust particles (Figures 11b, 12b).

9. Conclusions

In the present study, the effects of desert dust particles to the radiation balance of the atmosphere across the greater Mediterranean Region are investigated with the implementation of SKIRON/Dust modelling system. New radiative transfer mechanisms were incorporated in the atmospheric model and a thorough evaluation with the aid of radiation fluxes and temperature observational data was carried out. Using the updated system the effects of desert dust particles on radiative transfer have revealed important findings, as summarized below:

The presence of dust particles in the atmosphere has significant implications on the radiative transfer and energy distribution. Therefore, perturbations in dust particle production can have impacts on radiative properties, cloud formation and water budget. These links are not one way but there are feedbacks that are critical for both meteorological and climatological - scale phenomena.

The statistical evaluation with temperature data from more than 600 stations of the ECMWF and a 6 year simulation (2002 - 2007) has shown the improvement in the description of atmospheric processes when dust-radiation feedback is included. This improvement reaches 36% during the transitional period and depicts the importance of these phenomena.

The suspension of desert dust particles causes a decrease to the energy amounts reaching the surface. At the same time the atmospheric absorption is increased throughout the simulation domain, as the dust particles absorb incoming solar radiation and part of the longwave radiation emitted from the surface during the night.

On the vertical the phenomena are more complicated, as the redistribution of the energy has profound effects on the temperature profile: At the 300hPa layer dust particles cool the mid-troposphere due to reflection from the dust clouds below. At the same time the temperature of the layer below 600hPa is increased due to absorption and the near-surface layer temperature drops due to extinction from above.

A notable attribute of dust is the greenhouse effect in areas with high particle concentrations. Even thought the shading effect is dominant in these areas the long-wave trapping imposed by dust leads to an increase of the near surface temperature, rather than the expected reduction. This phenomenon is superimposed with the emission of infrared radiation by the dust cloud itself, further intensifying the heating effect. The later is an indication that the links and feedbacks between dust and climate are very complicated and not as linear as it was originally theorized.

In general, the presence of desert dust particles in the atmosphere plays a key role in the formation of the regional climate and the amounts of energy budget over the affected areas. In turn these effects should be included in the planning and predictions for using the Saharan solar potential. The dust influence on the energy parameters performs a seasonal variability that is in line with the seasonality of the dust transport events.