Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol

Abstract

Abstract. Marine stratocumulus cloud properties, and the free-tropospheric environment above them, are examined in NASA A-Train satellite data for cases where smoke from seasonal burning of the West African savannah overlay the persistent southeast Atlantic stratocumulus cloud deck. CALIPSO space-borne lidar observations show that features identified as layers of aerosol occur predominantly between 2 km and 4 km. Layers identified as cloud features occur predominantly below 1.5 km altitude and beneath the layer of elevated smoke aerosol. The diurnal mean shortwave heating rates attributable to the absorption of solar energy in the aerosol layer is nearly 1.5 K d −1 for an aerosol optical thickness value of 1, and increases to 1.8 K d −1 when the smoke resides above clouds owing to the additional component of upward solar radiation reflected by the cloud. As a consequence of this heating, the 700 hPa air temperature above the cloud deck is warmer by approximately 1 K on average for cases where smoke is present above the cloud compared to cases without smoke above cloud. The warmer conditions in the free-troposphere above the cloud during smoke events coincide with cloud liquid water path values that are greater by 20 g m −2 and cloud tops that are lower for overcast conditions compared to periods with low amounts of smoke. The observed thickening and subsidence of the cloud layer are consistent with published results of large-eddy simulations showing that solar absorption by smoke above stratocumulus clouds increases the buoyancy of free-tropospheric air above the temperature inversion capping the boundary layer. Increased buoyancy inhibits the entrainment of dry air through the cloud-top, thereby helping to preserve humidity and cloud cover in the boundary layer. The direct radiative effect of absorbing aerosols residing over a bright cloud deck is a positive radiative forcing (warming) at the top of the atCorrespondence to: E. M. Wilcox (eric.wilcox@dri.edu) mosphere. However, the greater liquid water path for cases of smoke overlaying cloud contributes an additional negative semi-direct radiative forcing (cooling) of climate in locations such as the southeast Atlantic Ocean owing to the enhanced albedo of the thicker cloud.

Black carbon solar absorption suppresses turbulence in the atmospheric boundary layer

“…Thus, the response of low clouds to BC aerosol absorption may depend on the amount of aerosol present. Enhancement of cloud development and lowering of the boundary layer top owing to aerosol heating above clouds has been observed over the southeastern Atlantic Ocean (10). Although the consequences of BC absorption for the coverage of clouds was not measured during CARDEX, the enhanced humidity and thicker saturated cloud layer observed during the more polluted conditions of CARDEX suggest that the same process may impact trade cumulus clouds within a layer of absorbing aerosol, as was observed for southeastern Atlantic clouds beneath absorbing aerosols, at least for the conditions seen during CARDEX.…”

“…A reduction in low cloudiness is not always the response to lowertropospheric heating by absorbing soot and smoke aerosols, however. If the heating reduces turbulent entrainment of dry air from above the humid marine boundary layer, then a thicker layer of clouds can be supported by the greater relative humidity in the surface mixed layer (9,10). The result is an enhancement of cloud albedo and a reduction in absorbed solar radiation that partly compensates for the increase in net radiative forcing attributable to the dark aerosol residing above a bright cloudy scene (11).…”

Occurrence of pristine aerosol environments on a polluted planet

“…Given the different behaviors of single clouds and regional-scale cloud systems (44), it is unlikely that local processes will be informative about regional PI to PD changes that we present here. It is also important to recognize that pristine CCN environments could still be perturbed by light-absorbing aerosols either within or above the clouds (45,46). Such effects, and associated fast adjustments of the cloud system, may alter the extent of PI-like aerosol−cloud environments shown in this study.…”

Cloud albedo changes in response to anthropogenic sulfate and non-sulfate aerosol forcings in CMIP5 models

“…Aerosols that are not efficient as CCN contribute less to cloud brightening but may still affect cloud properties. Absorbing aerosols can cause local heating and a reduction of cloud cover, as suggested by Ackerman et al (2000), or reduced turbulence and entrainment and an increase in cloudiness, as suggested by Wilcox (2010) and Wilcox et al (2016). These, and the many additional possible pathways for aerosol influence cloud properties, are difficult to disentangle, but the relative strength of the individual processes and their net effect are dependent on the properties of the underlying aerosol distribution.…”