Aerosol shortwave direct radiative effect and forcing based on MODIS Level 2 data in the Eastern Mediterranean (Crete)

N Benas,C Matsoukas,A Fotiadi,I Vardavas,N Mihalopoulos,N Hatzianastassiou

doi:10.5194/acp-11-12647-2011

Abstract

Abstract. The shortwave (SW) radiation budget was computed on a 10 km × 10 km resolution above FORTH-CRETE AERONET station in Crete, Greece, for the 11-year period from 2000 to 2010. The area is representative of the Eastern Mediterranean region, where air pollution and diminishing water resources are exacerbated by high aerosol loads and climate change. The present study aims to quantify the aerosol direct effect and forcing on the local surface and atmospheric energy budget. A radiative transfer model was used, with climatological data from the Moderate Resolution Imaging Spectroradiometer (MODIS), on board NASA's Terra and Aqua satellites. The instantaneous radiative fluxes were computed for satellite overpass times at the surface, within the atmosphere and at the top of atmosphere (TOA). Downward surface fluxes and aerosol input data were validated against ground measurements. Output fluxes reveal the direct radiative effects of dust events, with instantaneous values reaching up to −215, 139 and −46 Wm−2 at the surface (cooling), within the atmosphere (warming) and at TOA (cooling), respectively. Mean monthly values show a decreasing trend of the aerosol direct radiative effect, in agreement with a similar trend in AOT. The analysis of the contribution of anthropogenic and natural aerosol show major peaks of natural aerosol direct effect occurring mainly in spring, while a summer maximum is attributed to anthropogenic aerosol. During their peaks, anthropogenic aerosol forcing can reach values of −24 Wm−2 at the surface, 19 Wm−2 in the atmosphere and over −4 Wm−2 at TOA (monthly mean instantaneous values). The corresponding monthly peak values for natural aerosol are over −20 Wm−2, 12 Wm−2 and −9 Wm−2.

Highlights

Solar energy, the main source of energy for life on Earth, enters within the Earth-atmosphere system, where it is redistributed
The quantification of the aerosol effects is more complex than the quantification of radiative forcing by greenhouse gases, because aerosol mass, chemical composition and particle number concentrations are highly variable in space and time, due to their much shorter atmospheric lifetime compared with the important greenhouse gases (e.g. Kaufman et al, 2002), the huge number of aerosol sources and the modification processes which change their properties
We focus on the direct radiative effect (DRE) of aerosols, which is the overall effect of natural plus anthropogenic aerosols, and the direct radiative forcing (DRF), which is the effect of anthropogenic aerosols only, on the radiative energy budget

Summary

Introduction

The main source of energy for life on Earth, enters within the Earth-atmosphere system, where it is redistributed. The Earth’s Radiation Budget (ERB), the result of this procedure, regulates the climate and is an indicator of potential climatic changes. Knowledge and monitoring of the ERB is crucial for improving our understanding of the Earth’s climate and its possible changes (IPCC, 2007). Amongst the different factors that can cause climate change, both greenhouse gases and aerosols play an important role. The quantification of the aerosol effects is more complex than the quantification of radiative forcing by greenhouse gases, because aerosol mass, chemical composition and particle number concentrations are highly variable in space and time, due to their much shorter atmospheric lifetime compared with the important greenhouse gases Kaufman et al, 2002), the huge number of aerosol sources and the modification processes which change their properties. Better estimates of the aerosol radiative effects are required to reduce these uncertainties

Objectives

Results

Conclusion