Global ship track distribution and radiative forcing from 1 year of AATSR data

Abstract

One year of ENVISAT-AATSR (Advanced Along Track Scanning Radiometer) satellite data is analyzed to derive a global distribution of ship tracks and their radiative forcing (RF). Ship tracks are changes in cloud reflectance, visible in satellite data, and result from the emission of aerosols and their precursors by ships into the clean marine boundary layer. An algorithm is presented that extracts scenes dominated by low clouds over the oceans that are susceptible to be affected by ship emissions. These selected cloud scenes are used to examine ship tracks on a global scale via visual analysis. The results show a high temporal variability of ship track occurrence with peak values in July. They also show a high spatial variability with peak values in the North Pacific Ocean and on the west coast of southern Africa, correlated with high ship traffic and frequent low cloud occurrence in regions of cold upwelling ocean currents. The analysis of backscattered radiation at top of the atmosphere (TOA) compared to the surrounding area reveals enhanced backscattering with values between 0 and 100 Wm-2. For particular regions on the west coast of North America, the annual mean RF due to ship tracks estimated by the changes in backscattered radiation at TOA can be up to 50 mWm-2. The global annual mean RF due to ship tracks is small (0.4 to 0.6 mWm-2) and negligible compared to previous global model estimates on the total indirect aerosol effect and RF contributions of other ship emissions.