Satellite observations of the impact of weak volcanic activity on marine clouds

Because emissions from weak volcanic eruptions tend to remain in the low troposphere, they may have a significant radiative impact through the indirect effect on clouds. However, this type of volcanic activity is underreported and its global impact has been assessed only by model simulations constrained with very limited observations. First observations of the impact of high- latitude active volcanoes on marine boundary layer clouds are reported here. These observations were made using a combination of standard derived products and visible images from the MODIS, AMSR- E and GOES detectors. Two distinctive effects are identified. When there is an existing boundary layer cloud deck, an increase in cloud brightness and a decrease in both cloud effective radius and liquid water content were observed immediately downwind of the volcanoes. The visible appearance of these "volcano tracks'' resembles the effect of man-made ship tracks. When synoptic conditions favor low cloudiness, the volcano plume ( or volcano cloud) increases significantly the cloud cover downwind. The volcano cloud can extend for hundreds of kilometers until mixing with background clouds. Unlike violent eruptions, the volcano clouds reported here ( the Aleutian Islands in the North Pacific and the South Sandwich Islands in the South Atlantic) have retrieved microphysical properties similar to those observed in ship tracks. However, when comparing the volcano clouds from these two regions, liquid water content can decrease, increase or remain unchanged with respect to nearby unperturbed clouds. These differences suggest that composition at the source, type of eruption and meteorological conditions influence the evolution of the cloud.