Dispersion of Droplet Size Distributions in Supercooled Non-precipitating Stratocumulus from Aircraft Observations Obtained during the Southern Ocean Cloud Radiation Aerosol Transport Experimental Study

The characteristics of cloud droplet size distributions and statistical relations of the relative dispersion (ε) with the vertical velocity (w) and with the interstitial aerosol concentration (N_ia) are investigated for ubiquitous supercooled shallow stratocumulus observed over the Southern Ocean (SO) using aircraft measurements obtained during the Southern Ocean Cloud Radiation Aerosol Transport Experimental Study. Distinct vertical variations have been found using 36 non-precipitating cloud profiles. The cloud droplet effective radius (r_e) increases nearly monotonically from 5.3 ± 1.9 μm at cloud base to 9.4 ± 2.2 μm at cloud top. The ε decreases rapidly from cloud base (0.42 ± 0.13) and then remains relatively constant in the upper cloud layer (0.27 ± 0.09). This study also shows robust dependence of ε on both N_ia and w. The ε increases (decreases) with increasing N_ia (w) at a 95% confidence level when values of w (low N_ia) are restricted to a small range. The important roles of aerosols and dynamics on ε are demonstrated and are crucial to estimating aerosol indirect radiative forcing, especially for pristine SO regions where models almost universally underestimate reflected radiation.