Relationship between low-cloud presence and the amount of overlying aerosols

Abstract

Abstract. Aerosols are often advected above cloud decks, and the amount of aerosols over cloud has been assumed to be similar to that at the same heights in nearby clear sky. In this assumption, cloud and aerosol above cloud-top height are considered randomly located with respect to each other. The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data are analyzed here to investigate this assumption on global scales.The CALIPSO data reveal that the aerosol optical depth (AOD) above low cloud tends to be smaller than in nearby clear sky during the daytime, and the opposite is true during the nighttime. In particular, over oceanic regions with widespread low cloud, such as the tropical southeastern Atlantic Ocean and northeastern Pacific Ocean, the daytime AOD above low cloud is often 40 % smaller than in surrounding clear skies.

Daytime aerosol optical depth above low-level clouds is similar to that in adjacent clear skies at the same heights: airborne observation above the southeast Atlantic

“…The difference is up to 0.04 over the southeastern Atlantic Ocean (see their Fig. 2) As Chung et al (2016) point out, it is conceivable that aerosol amounts over cloud can be different from those in nearby clear sky. There are two sets of potential reasons.…”

“…Since AAC are difficult to see from the ground or a ship, previous studies have relied on satellite observations (see Table 2 of Kacenelenbogen et al, 2019). Among them is Chung et al (2016), which used the level 2 products of the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) (Winker et al, 2009) to calculate the AOD above the maximum low-cloud-top-height in each grid cell in clear sky as well as the AOD above low clouds on a global 2 • × 5 • latitudelongitude grid. Their results indicate that daytime 532 nm AOD above low clouds is generally lower than that in clear sky at the same heights.…”

“…The proximity has been variously defined, for example less than 100 m in the vertical direction (Costantino and Bréon, 2013) and less than 20 km in the horizontal direction (Várnai and Marshak, 2018). Chung et al (2016) note that aerosols were shown to influence underlying cloud by indirect effects and semidirect effects Bréon, 2010, 2013;Johnson et al, 2004;Wilcox, 2010) and that these aerosolcloud interactions and possibly more (e.g., a pronounced if unlikely aerosol entrainment (Diamond et al, 2018)) might somehow affect the aerosol amount over cloud. A bias in the CALIOP standard retrieval was also raised as a possible explanation for the Chung et al (2016) results.…”

“…Chung et al (2016) note that aerosols were shown to influence underlying cloud by indirect effects and semidirect effects Bréon, 2010, 2013;Johnson et al, 2004;Wilcox, 2010) and that these aerosolcloud interactions and possibly more (e.g., a pronounced if unlikely aerosol entrainment (Diamond et al, 2018)) might somehow affect the aerosol amount over cloud. A bias in the CALIOP standard retrieval was also raised as a possible explanation for the Chung et al (2016) results. The detection threshold in the feature detection algorithm varies depending on the background lighting conditions, the atmospheric features (e.g., aerosols, high altitude cirrus or boundary layer clouds) and the horizontal averaging required by CALIOP for detection (see Fig.…”

“…Liu et al (2015) show a clear AOD underestimate of the CALIOP level 2 retrieval in comparison to a separate retrieval after Hu et al (2007) for smoke above opaque water clouds over the southeast Atlantic, and explain this by the CALIOP layer detection scheme prematurely assigning layer base altitudes and thus underestimating the geometric thickness of smoke layers. According to Chung et al (2016), the negative daytime AOD differences between cloudy and cloud-free conditions "might simply be a result of systematic differences between the detection thresholds in clear sky and above low bright clouds". The authors add that the bias may be enhanced over the ocean due to the lower albedo compared to that of land.…”