Abstract
Abstract. The aerosol direct radiative effect (ADRE) affecting the Po Valley and the adjacent North Adriatic Sea is studied using 10-year series of measurements collected at two AERONET sites located in the western part of the Valley (Ispra), and on a platform (AAOT) offshore Venice. This region is characterized by a high, mostly continental, aerosol load with comparable average aerosol optical thickness τa at both locations (0.21 at 500 nm) and more absorbing aerosols at Ispra. A dynamic aerosol model accounting for the changes in scattering phase function with τa is used for radiative transfer calculations, together with boundary conditions representative of terrestrial and marine surfaces. A sensitivity analysis allows the construction of an error budget for the daily ADRE estimates, found to be of the order of 20% and mostly due to uncertainties on aerosol single scattering albedo and τa. The daily radiative efficiencies, normalized by τa at 500 nm, increase from December to June, from −17 to −24 W m−2 τa−1 at top-of-atmosphere (TOA) and −33 to −72 W m−2 τa−1 at surface for the Po Valley, and from −15 to −32 (TOA) and −35 to −65 W m−2 τa−1 (surface) for the AAOT site. The average of log-transformed ADRE for TOA, surface and atmosphere are −5.2, −12.2 and +6.8 W m−2 for the Po Valley case, and −6.5, −13.0 and +6.5 W m−2 for the AAOT site but these values can be much higher for individual days. Concurrent clear-sky days give indications on the regional atmospheric heating spatial gradients. Differences between the atmospheric ADRE at the two locations average 6.3 W m−2 with a gradient positive towards the inner valley in 65% of the cases. This study confirms the importance of duly considering the radiative impact of aerosols on the regional climate.
Highlights
Aerosols have an impact on the radiative budget and hydrological cycle of the Earth through a variety of processes (Charlson et al, 1992; Ramanathan et al, 2001)
The direct radiative effect induced by aerosols is computed for the Po Valley region and the adjacent North Adriatic Sea, from long term time series of Aerosol Robotic Network (AERONET) measurements and MODIS albedo data
The different surface reflectivity over land and sea concurs to significant differences in radiative efficiencies fe at TOA (Fig. 7 and Table 2), varying for daily values along the year from −17 to −24 W m−2 τa−1 for the inner Po Valley conditions, and from −15 to −32 W m−2 τa−1 for the Alta Oceanographic Tower (AAOT) case
Summary
Aerosols have an impact on the radiative budget and hydrological cycle of the Earth through a variety of processes (Charlson et al, 1992; Ramanathan et al, 2001). By affecting cloud microphysical properties and lifetime, the aerosol particle distribution further affects the planetary albedo and precipitation patterns (Lohmann and Feichter, 2005; Rosenfeld, 2000). This overall impact is widely recognized for global studies but the uncertainties associated with the aerosol effects remain higher than those associated with greenhouse gases (IPCC, 2001). The main objective of the study is to quantify the aerosol direct radiative effect for the region at temporal scales from instantaneous to seasonal. The aerosol data are first described, and a comparative analysis of the direct radiative effect is performed for atmospheric and surface conditions characteristic of the two sites. The daily to seasonal estimates of the aerosol direct radiative effect are presented
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