Search

The regional climate model RegCM3 coupled with a radiatively active aerosol model with online feedback is used to investigate direct and semi-direct radiative aerosol effects over the Sahara and Europe in a test case of July 2003. The aerosol model includes dust particles in addition to sulfates, hydrophobic and hydrophilic black carbon and organic carbon. The role of the aerosol online feedback on the radiation budget and the direct radiative forcing (short-wave and long-wave) by dust particles are investigated by intercomparing results from three experiments: REF, including all interactive aerosol components, Exp1, not accounting for the aerosol radiative feedback, and Exp2 not accounting for desert dust particles. The comparison of results in the REF experiment with satellite observations, sun/sky radiometer measurements, and lidar profiles at selected Central Mediterranean sites reveals that the spatio-temporal evolution of the aerosol optical depth is reasonably well reproduced by the model during the entire month of July. Results for the dust outbreaks of 17 and 24 July, averaged over the simulation domain, show that the daily-mean SW direct radiative forcing by all particles is −24 Wm−2 and −3.4 Wm−2 on 17 July and −25 Wm−2 and −3.5 Wm−2 on 24 July at the surface and top of the atmosphere, respectively. This is partially offset by the LW direct radiative forcing, which is 7.6 Wm−2 and 1.9 Wm−2 on 17 July and 8.4 Wm−2 and 1.9 Wm−2 on 24 July at the surface and top of the atmosphere, respectively. Hence, the daily-mean SW forcing is offset by the LW forcing of ~30% at the surface and of ~50% at the ToA. It is also shown that atmospheric dynamics and hence dust production and advection processes are dependent on the simulation assumptions and may significantly change within few tens of kilometers. The comparison of REF and Exp1 shows that the aerosol online feedback on the radiation budget decreases the domain-average daily-mean value of the 2 m-temperature, aerosol column burden (CB), and short-wave (SW) atmospheric forcing by −0.52 °C, 14%, and 0.9%, respectively on 17 July and by −0.39 °C, 12% and 12%, respectively on 24 July. The comparison of REF and Exp2 reveals that on 17 July, radiatively-active dust particles decrease the daily-mean 2 m-temperature averaged over the whole simulation domain by 0.4% even if are responsible for 99.8% and 97% of the daily-mean aerosol column burden and SW atmospheric forcing, respectively.

Daily averaged retrievals of AERONET sun photometer measurements from March 2003 to March 2004 are used to provide preliminary results on the characterization of aerosol properties and changes over south–east Italy (40°20′N, 18°6′E). It is shown that aerosol optical and microphysical properties and the dominating aerosol types depend on seasons. Aerosol-parameter frequency distributions reveal the presence of individual modes that lead to the assumption that moderately absorbing urban–industrial and marine-polluted aerosols dominate in spring–summer and autumn–winter, respectively. It is shown that aerosol optical depths (AODs), single scattering albedos (SSAs), and Angstrom coefficients (A) of urban–industrial (spring–summer) aerosols are characterized by lognormal distributions with peak values of 0.20±0.03, 0.94±0.01, and 1.58±0.03, respectively. On the contrary AOD, SSA and A values of maritime-polluted (autumn–winter) aerosols are characterized by lognormal distributions with peak values of 0.049±0.008, 0.974±0.003, and 0.7±0.1, respectively. It is also shown that the frequency distribution of real n and imaginary k refractive indices permits inference of the dominant aerosol constituents: sea-salt, water soluble, soot, and mineral particles. Finally, it is shown that dust outbreaks do not significantly affect the seasonal evolution of aerosol parameters, and that sunphotometry retrievals along dust events are in satisfactory accord with experimental findings indicating that moderately-absorbing (0.005≤ k ≤0.05) dust particles with a high content of illite are mainly advected over the Mediterranean basin during Sahara dust storms.

This paper describes the main features of a compact transmitter in W band. It is also highlighted the high voltage modulator designed to satisfy the strict requirements for the transmitter itself. The philosophy of the design does not cover only the hardware architecture but it also takes a look to the radar maintenance concept.

Ground-based particulate matter (PM) samplers, an XeF Raman lidar operating in the framework of the European Aerosol Research Lidar Network (EARLINET), and a sun/sky radiometer operating in the framework of the Aerosol Robotic Network (AERONET) have been used to characterize vertical profiles, optical and microphysical properties, and chemical composition of aerosols during the 29 June–1 July 2005 dust outbreak that occurred over the central-eastern Mediterranean. Aerosol backscatter coefficient, total depolarization, and lidar ratio vertical profiles revealed that a well-mixed dust layer extending from ∼0.5 to 6 km was present over southeastern Italy on 30 June. Sun/sky radiometer measurements revealed a bimodal lognormal size distribution during all measurement days. The particle volume distribution was found to be well correlated either to the PM mass distribution measured at ground by a seven-stage cascade impactor and to the fine to total suspended PM mass ratio measured by ground-based PM samplers. Scanning electron microscopy and ion chromatography analyses on PM samples revealed that coarse-mode aerosols were mainly made of carbonate, aluminum-silicate, and sea salt particles. Carbon, sulfate, and nitrate particles were the main components of fine-mode aerosols representing more than 50% of the total aerosol load; the significant role of fine-mode anthropogenic particles during a dust event is highlighted. Finally, the potential capabilities of complementary measurements by passive and active remote sensing techniques and in situ observations to retrieve the vertical distribution of the particle number and mass concentration are analyzed and discussed.

Aerosol products by AERONET sun-sky radiometer measurements combined with air-mass backtrajectories were analyzed to identify source regions and pathways of air masses carrying aerosols to south-east Italy, and to determine the dependence of aerosol mean optical properties on advection patterns. Aerosol optical depth (AOD), fine mode fraction (η ), single scattering albedo (SSA), asymmetry factor (g), and lidar ratio (Lr) at 440 nm were used to characterize aerosol properties. The analysis of 5-day-backtrajectories ending in Lecce on south-east Italy and referring to 240 measurement days of the 2003–2004 years revealed that 32% of the measurement days were characterized by air masses coming from all continental European sources with the exception of Spain. 3% of the measurement days were characterized by air masses coming from both the Southern Mediterranean Sea and the Africa continent, and the Western Mediterranean, the Iberian Peninsula, and the Atlantic Ocean. 62% of the measurement days were characterized by mixed advection patterns. We found that AOD, SSA and g average values were not significantly dependent on air mass source regions. In contrast, η and Lr average values were quite affected by the air mass source region. AOD, &eta, SSA, g, and Lr average values, which were equal to 0.29±0.15, 0.93±0.03, 0.93±0.03, 0.67±0.03, and 72±20 sr, respectively indicated that the aerosol advected from all continental European sources with the exception of Spain, could be considered representative of "continental average aerosol", mostly made of water soluble and a small amount of soot and insoluble components. Polluted-desert dust particles characterized by AOD=0.29±0.05, η=0.72±0.05, SSA=0.94±0.03, g=0.69±0.02, Lr=56±13 sr, were advected over south-east Italy from the Southern Mediterranean Sea and the Africa continent. The Western Mediterranean, the Iberian Peninsula, and the Atlantic Ocean were instead responsible of the advection of maritime-polluted particles, which were characterized by AOD=0.27±0.17, η=0.8±0.1, SSA=0.94±0.03, g=0.67±0.03, Lr=58±24 sr. Hence, we found that the aerosol load over south-east Italy was dominated by moderately-absorbing, fine-mode particles even if it was also affected by the minor contribution of desert and maritime type aerosol. The application of an aerosol mask to the data points retrieved on measurement days characterized by mixed advection patterns, supported last comment

Moderate Resolution Imaging Spectroradiometer (MODIS)–ocean aerosol products and corresponding Aerosol Robotic Network (AERONET) data retrieved during Sahara dust outbreaks that occurred over the central Mediterranean basin have been analyzed, to contribute to the validation of MODIS retrievals during dust-loading conditions. It is shown that MODIS aerosol optical depths (AODM) retrieved at 550 nm for window sizes of 50 × 50 km2 centered at different Mediterranean AERONET sites are in satisfactory accordance to corresponding AERONET optical depths (AODA) colocated in space and time: Correlation factor and slope of the linear regression are 0.86 and 1.04, respectively, and 42% of data points are within the prespecified accuracy range. However, the plot of the differences (AODM – AOD A ) versus AOD A has revealed that the differences (AODM – AOD A ) are almost positive and increase with AOD A , and has led assuming that MODIS overestimates AODs mainly during high dust load conditions. The comparison of MODIS-ocean and corresponding AERONET fine-mode fractions (η M and η A, respectively) colocated in space has revealed that the MODIS-ocean inversion algorithm overestimates the contribution of fine-mode particles for dust-dominated aerosols: (η M − η A) difference values vary within the 0.11–0.24 range. In addition, the comparison of MODIS-ocean and corresponding AERONET volume size distributions has revealed that the latter results are mainly due to the poor sensitivity of the MODIS-ocean inversion algorithm to the concentration of coarse-mode particles during dust outbreaks. The frequency distributions of MODIS fine and coarse volume modal radii are found to be rather close to corresponding AERONET frequency distributions. Received 31 January 2007; accepted 30 May 2007

[1] Aerosol parameters retrieved by Aerosol Robotic Network (AERONET) Sun photometer measurements at the Physics Department of Lecce's University (40°20′N, 18°6′E) are compared to similar Moderate Resolution Imaging Spectroradiometer (MODIS) data retrieved at different spatial resolutions colocated in space and time to contribute to the validation of MODIS aerosol products over southeast Italy and to investigate the correlation dependence on spatial resolution and identify regional biases of Lecce's AERONET data. In particular, MODIS aerosol optical depths retrieved at 550 nm over ocean and over land-ocean for window sizes of 50 × 50, 100 × 100, and 300 × 300 km2 centered on the AERONET monitoring site are correlated to AERONET aerosol optical depths colocated in time. It is shown that correlation factors of linear regressions span the 0.88–0.83 range and weakly tend to reduce with window size. In addition, MODIS aerosol optical depths meet expected uncertainties, and the percentage of data points within expected uncertainties is not affected by the window size. Slope and intercept values of linear regressions fitting over ocean aerosol optical depths are instead different than those fitting over land-ocean aerosol optical depths and are dependent on window size. The observed dependence is analyzed and discussed in the paper. Finally, it is shown that the paper's results can allow inferring that the tested AERONET aerosol parameters can be considered representative, at least, of a ∼300 × 300 km2 southeast Italy area centered on the Lecce's AERONET site.