Stratocumulus cloud thickening beneath layers of absorbing smoke aerosol

E M Wilcox

doi:10.5194/acp-10-11769-2010

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 atmosphere. 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.

Highlights

The impact of aerosols upon the climate of Earth has received intense scrutiny because of the uncertain role aerosols play in present-day anthropogenic radiative forcing of climate (IPCC, 2007), and the prospect for large present and future impacts of aerosol forcing on regional and global hydrological cycles (e.g. Ramanathan et al, 2001; Liepert et al, 2004)
The stratocumulus clouds capping the marine boundary layer over the Southeast Atlantic Ocean frequently reside below an elevated layer of smoke aerosol transported offshore from the regions of African Savannah burning
This study has investigated the radiative impact of the smoke on the atmospheric temperature in the free-troposphere above the marine boundary layer, and the properties of overcast samples of the clouds capping the marine boundary layer

Summary

Introduction

The impact of aerosols upon the climate of Earth has received intense scrutiny because of the uncertain role aerosols play in present-day anthropogenic radiative forcing of climate (IPCC, 2007), and the prospect for large present and future impacts of aerosol forcing on regional and global hydrological cycles (e.g. Ramanathan et al, 2001; Liepert et al, 2004). In cases where absorbing aerosols (e.g. smoke and soot) coincide with clouds in the same column, the radiative heating of the troposphere by aerosol solar absorption may modify the thickness and coverage of the cloud layer depending on the radiative properties of the aerosol, the meteorology driving the cloud dynamics, and the vertical distribution of the aerosol relative to the cloud in the column This socalled semi-direct effect of aerosols (Hansen et al, 1997) is often assumed to yield a positive radiative forcing (warming) of climate. When the aerosol layer occurs entirely above the boundary layer the temperature inversion above the boundary layer is enhanced, LWP increases, and the semi-direct radiative forcing is negative (a cooling) Evidence for both increases and decreases in cloud fraction associated with biomass burning aerosols offshore of California were reported by Brioude et al (2009) depending on whether simulated smoke in a chemical transport model had mixed into the boundary layer or not. The observations confirm that the smoke layer is often distinct from the cloud layer below and that a positive correlation of smoke loading and cloud liquid water path can be explained by warming of the cloud-capping temperature inversion attributable to solar absorption by smoke

Data and methods

The radiative effect of smoke aerosol

Smoke absorption and lower tropospheric air temperature

Lower tropospheric warming and cloud thickening

Findings

Summary