Intercomparison of satellite-derived aerosol optical depths with ship-based microtops observations over the Red Sea from the AEGAEO campaign in 2011 and insights into dust influence over the region

Among its many influences on the climate system, dust aerosol can substantially modify the Earth's radiation balance via its direct impact on both the amount of reflected shortwave and emitted longwave energy escaping to space. Over ocean, instantaneous aircraft and satellite observations of dust events have shown that the reflected shortwave component can be enhanced by over 100Wm-2. However, while there have been a number of observational and modelling campaigns designed to probe dust-climate interactions over North Africa, the North Atlantic, Asia, the Americas and the Mediterranean, relatively little attention has been paid to the Arabian Peninsula and the Red Sea. Here we describe a novel set of ship-based observations aimed at characterising aerosol loading over the Red Sea via a series of five cruises through September-December 2011. Typically ship-based aerosol loadings were indicative of background, hazy conditions, with 0.6 micron optical depths of the order 0.2. However, somewhat elevated loadings were also captured during the earliest of the ship cruises in September, optical depths reaching ~ 0.5. In all cases, comparisons of these data with co-located satellite retrievals from the SEVIRI and MODIS sensors showed good overall agreement with correlation coefficients exceeding 0.9 for all five cruises, giving confidence in the general performance of the retrieval algorithms employed. To probe the general behaviour of the region in more detail, aerosol optical depths from each satellite sensor were then used to create a record of aerosol loading over the Red Sea for the full September-December 2011 time period. The fifteen minute time resolution data available from SEVIRI provides insight into the timing and duration of specific dust events, while the enhanced spatial resolution of MODIS can allow greater detail to be discerned at the instrument overpass times. Initial results show the expected pattern of behaviour, with enhanced aerosol optical depths in the southern part of the Red Sea (south of ~20 N) compared to the north. Over the Sea as a whole, monthly mean aerosol optical depths are highest in September, declining with time to November before increasing slightly in December. Contributing to the elevated loading in September are a number of major dust outbreaks originating from southern Sudan. Here dust uplift is typically initiated by outflow from meso-scale convective systems, beginning during the late evening or night. In December, a number of more isolated events are also evident, in this case originating from the Arabian Peninsula itself. Comparison of SEVIRI and MODIS imagery suggests that the cloud detection algorithm used in the standard processing of the former data may be too restrictive and efforts to correct this issue are also presented here.