The TOA shortwave radiative forcing for unit AOT of volcanic ash aerosols at the time

5 [1] Using the Moderate Resolution Imaging Spectroradiometer (MODIS), Spinning 6 Enhanced Visible and Infrared Imager (SEVIRI), Clouds and the Earth’s Radiant Energy 7 System (CERES) instrument, and BaE146 aircraft data sets, we provide an overview of 8 volcanic ash spatial distribution for six days (4–18 May 2010) and assess their properties 9 and radiative impacts for 17 May primarily over the North Sea. We describe spectral 10 signatures of volcanic ash, compare the MODIS-retrieved 550 nm aerosol optical thickness 11 (AOT) and effective radii with the aircraft data, and then assess the change in radiative 12 fluxes at the top of atmosphere using CERES. Our results indicate that the MODIS 13 and SEVIRI thermal channels are adept at identifying volcanic ash near the source. 14 However, the volcanic ash far from the volcanic source, especially over land, is 15 contaminated by surface/atmospheric features. We assess the 17 May case in detail and 16 show that MODIS AOTs (0.23–0.86) are higher than the aircraft values (0.07–0.54), 17 probably due to different aerosol models used in the retrieval process. The MODIS 18 effective radii values are between 0.4 and 0.9 mm with fine mode fraction values between 19 0.4 and 0.7. The aircraft-derived effective radii values are between 0.82 and 1.2 mm. 20 The TOA shortwave radiative forcing for unit AOT of volcanic ash aerosols at the time 21 of the satellite overpass is 77 4.0 W m 2 and is larger than the longwave forcing 22 per unit optical depth (11 1.2 W m ) by seven times indicating that ash could 23 significantly impact radiative energy fluxes.