1Remote Sensing Division, Swedish Meteorological and Hydrological Institute, Norrköping, Sweden
2Air Quality Division, Swedish Meteorological and Hydrological Institute, Norrköping, Sweden
Abstract. The presence of aerosols over highly reflective liquid water cloud tops poses a big challenge in simulating their radiative impacts. Particularly, absorbing aerosols, such as smoke, may have significant impact in such situations and even change the sign of net radiative forcing. Until now, it was not possible to obtain information on such overlap events realistically from the existing passive satellite sensors. However, the CALIOP instrument onboard NASA's CALIPSO satellite allows us to examine these events with an unprecedented accuracy.
Using four years of collocated CALIPSO 5 km Aerosol and Cloud Layer Version 3 Products (June 2006–May 2010), we quantify, for the first time, the macrophysical characteristics of overlapping aerosol and water cloud layers globally. We investigate seasonal variability in these characteristics over six latitude bands to understand the hemispheric differences. We compute a) the percentage cases when such overlap is seen globally and seasonally when all aerosol types are included (AAO case) in the analysis, b) the joint histograms of aerosol layer base height and cloud layer top height, and c) the joint histograms of aerosol and cloud geometrical thicknesses in such overlap cases. We also investigate frequency of smoke aerosol-cloud overlap (SAO case).
The results show a distinct seasonality in overlap frequency in both AAO and SAO cases. Globally, the frequency is highest during JJA months in AAO case, while for the SAO case, it is highest in SON months. The seasonal mean overlap frequency can regionally exceed 20% in AAO case and 10% in SAO case. There is a tendency that the vertical separation between aerosol and cloud layers increases from high to low latitude regions in the both hemispheres. In about 5–10% cases the vertical distance between aerosol and cloud layers is less than 100 m, while about in 45–60% cases it less than a kilometer in the annual means for different latitudinal bands. The frequency of occurrence of thicker aerosol layers gradually increases from poles to tropics. In about 70–80% cases, aerosol layers are less than a kilometer thick, while in about 18–22% cases they are 1–2 km thick. The frequency of aerosol layers 2–3 km thick is about 4–5% in the tropical belts during overlap events. The results further highlight spatial and temporal variations in aerosol-liquid water cloud overlap and suggest that the frequency of occurrence of such overlap events is far from being negligible globally.