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

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