Cloud-Scale Droplet Number Sensitivity to Liquid Water Path in Marine Stratocumulus

Abstract

Cloud droplet number concentration (Nc) is retrieved from remotely observed marine stratocumulus cloud liquid water path (LWP), cloud optical depth (τc), and cloud thickness, using an optical model that assumes liquid water content (LWC) increases linearly from cloud base. Assuming that LWC is vertically uniform would underestimate τc by 5% and Nc by 14%. Individual retrievals of Nc from 10-min averages vary by orders of magnitude from long-term averages. Surface cloud condensation nuclei (CCN) number concentration NCCN is weakly but significantly correlated with Nc (R = 0.3) for the day leading and 6 hr following Nc. Consistent with coalescence and drizzle scavenging cloud droplets, lag correlations show that Nc decreases for 1 hr after the peak area-average rain rate. Greater observed LWP for lower Nc [d(log Nc)/d(log LWP) = −2.3] is consistent with enhanced entrainment drying of clouds with greater Nc and consistent with removal of Nc by thicker clouds with more coalescence and drizzle. Stronger precipitation in clouds with greater Nc is the opposite sensitivity as expected were LWP to be controlled by the “cloud lifetime” indirect aerosol effect. The strong sensitivity of Nc to LWP suggests that cloud dynamic and thermodynamic forcings drive macrophysical variability that controls Nc in southeastern tropical Pacific stratocumulus clouds. Regressions are relatively insensitive to assumptions about the covariance of errors among the sensors.