On the Influence of Air Mass Origin on Low‐Cloud Properties in the Southeast Atlantic

Abstract

This study investigates the impact of air mass origin and dynamics on cloud property changes in the Southeast Atlantic (SEA) during the biomass burning season. The understanding of clouds and their determinants at different scales is important for constraining the Earth's radiative budget and thus prominent in climate system research. In this study, the thermodynamically stable SEA stratocumulus cover is observed not only as the result of local environmental conditions but also as connected to large‐scale meteorology by the often neglected but important role of spatial origins of air masses entering this region. In order to assess to what extent cloud properties are impacted by aerosol concentration, air mass history, and meteorology, a Hybrid Single‐Particle Lagrangian Integrated Trajectory cluster analysis is conducted linking satellite observations of cloud properties (Spinning‐Enhanced Visible and Infrared Imager), information on aerosol species (Monitoring Atmospheric Composition and Climate), and meteorological context (ERA‐Interim reanalysis) to air mass clusters. It is found that a characteristic pattern of air mass origins connected to distinct synoptical conditions leads to marked cloud property changes in the southern part of the study area. Long‐distance air masses are related to midlatitude weather disturbances that affect the cloud microphysics, especially in the southwestern subdomain of the study area. Changes in cloud effective radius are consistent with a boundary layer deepening and changes in lower tropospheric stability (LTS). In the southeastern subdomain cloud cover is controlled by a generally higher LTS, while air mass origin plays a minor role. This study leads to a better understanding of the dynamical drivers behind observed stratocumulus cloud properties in the SEA and frames potentially interesting conditions for aerosol‐cloud interactions.

Spatiotemporal dynamics of fog and low clouds in the Namib unveiled with ground- and space-based observations

“…On seasonal scales, the spatiotemporal FLC occurrence indicates a connection to the southeastern Atlantic stratocumulus cloud deck. As such, process knowledge from studies on the heavily investigated stratocumulus clouds in this region (e.g., Adebiyi and Zuidema, 2018;Andersen and Cermak, 2015;Diamond et al, 2018;Fuchs et al, 2017Fuchs et al, , 2018Gordon et al, 2018;Painemal et al, 2014;Yuter et al, 2018) may be applicable to Namib-region FLCs, and vice versa.…”

Building a cloud in the southeast Atlantic: understanding low-cloud controls based on satellite observations with machine learning

“…While the aerosol index may be a better proxy for cloud condensation nuclei than AOD (Stier, 2016), its computation requires the Ångström exponent, which is not available in the 8-day MODIS L3 product (Levy et al, 2013). Studies that observed the bivariate relations between AOD and cloud properties are numerous (e.g., Kaufman, 2005Kaufman, , 2006Grandey et al, 2013), but spurious correlations exist. The strength of the relation between AOD and CF or REF is depending on satellite artifacts in the vicinity of clouds, e.g., cloud contamination and three-dimensional radiative effects (Várnai et al, 2013;Christensen et al, 2017) as well as on meteorological conditions, e.g., aerosol hygroscopic swelling with humidity Quaas et al, 2010).…”

Evidence of the complexity of aerosol transport in the lower troposphere on the Namibian coast during AEROCLO-sA

“…In this context, the coastal southern Africa region is arguably one of the regions where the aerosol-radiation-cloud interactions are strongest in the world (Adebiyi et al, 2015;Fuchs et al, 2017). However, state-of-the-art climate models diverge by several W m −2 when attempting to calculate the regional direct radiative effect over coastal southern region (Myhre et al, 2013;Stier et al, 2013) ranging from negative (−3 W m −2 ) to strong positive forcing (+5 W m −2 ) for mean seasonal averages.…”