Stratus Cloud Processing of CCN and the Radiative Impacts on Subsequent Stratus Clouds

Marine stratus clouds cover vast areas and are susceptible to aerosol-cloud interactions (ACI).  Changes in cloud condensation nuclei (CCN) concentrations make changes to cloud droplet number concentrations (N_c) and droplet mean diameters (MD).  An additional ACI is cloud processing of CCN.  Hudson et al. (2015; H15) found bimodal CCN distributions below polluted status clouds during the MArine Stratus/stratocumulus Experiment (MASE).  They categorized CCN distributions on a 1-8 scale and attributed this bimodality in MASE to chemical cloud processing.  Cloud processing improves CCN by adding soluble material.  Upon evaporation the residual CCN then have lower critical supersaturations (S_c).  In low updrafts typical of stratus clouds, these processed CCN from bimodal distributions more easily activate thus creating competition for condensate.  Consequently, greater N_c with smaller MD and less drizzle ensue.  MASE bimodal CCN distributions were related to greater stratus N_c, indicating this chemical cloud processing. 

Bulk chemistry measurements further support H15 findings of chemical cloud processing in MASE.  Greater amounts of sulfate (Fig. 1a, black) and nitrate (Fig. 1b, green) were related to bimodal CCN distributions (low modal rating).  Unimodal distributions (higher modal rating) had lower amounts.  Likewise, bimodal distributions were associated with less sulfur dioxide (Fig. 1a, red) and ozone (Fig. 1b, blue), suggesting uptake and removal of these gases by cloud droplets and aqueous chemistry.  This can be observed by changes in solubility (kappa) between the unprocessed and processed peaks.  Kappa was determined by comparison of peaks in the CCN and aerosol (from a differential mobility analyzer) distributions.  Often when kappa of the unprocessed peak was high (>0.7), the processed peak was reduced towards kappa of ammonium sulfate.  Similarly, when kappa was low (<0.6) for the unprocessed peak, kappa for the processed peak was increased.  These measurements support H15 findings of chemical cloud processing in MASE.

Measured uni- and bimodal MASE CCN distributions were selected as input to an adiabatic droplet growth model to observe the impacts of cloud altered bimodal distributions on subsequent clouds.  Various vertical wind speeds (W) to simulate the variety within stratus clouds and cumulus clouds.  Bimodal CCN produced greater N_c (Fig. 2a) and smaller MD (Fig. 2b) at lower W typical of stratus clouds (<70 cm/s).  Improved CCN (low S_c) were more easily activated at the lower supersaturations of stratus from low W, thus, creating greater N_c. Competition for condensate thus reduced MD and drizzle.  At greater W, typical of cumulus clouds (>70 cm/s), bimodal CCN made lower N_c with larger MD thus enhancing drizzle whereas the reverse was true for unimodal CCN; greater N_c, smaller MD, and reduced drizzle.  These theoretical predictions of N_c and MD for uni- and bimodal CCN agree with observations reported by H15.  Radiative effects of bimodal CCN were determined using a cloud grown to a 250-meter thickness. Ratios of cloud effective radius (r_e), cloud optical thickness (COT), and cloud albedo from bimodal and unimodal CCN are shown in Fig. 2c.  At low W, similar to stratus, bimodal CCN reduced r_e, made greater COT, and made greater cloud albedo. At very low W (<15 cm/s) changes were as much as +9% for albedo, +17% for COT, and -12% for r_e.  Therefore, because stratus clouds typically have low W and cover large areas, these changes in cloud radiative properties impact climate. Stratus clouds improve CCN through cloud processing that further impact subsequent clouds.  Shallow clouds are therefore susceptible to CCN distribution shape changes from chemical processing and further investigation is required to determine their impact on ACI.

Hudson et al. (2015), JGRA, 120, 3436-3452.