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Cloud‐Scale Droplet Number Sensitivity to Liquid Water Path in Marine Stratocumulus
Author(s) -
Szoeke Simon P.,
Verlinden Kathryn L.,
Covert David
Publication year - 2018
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1029/2017jd027508
Subject(s) - drizzle , marine stratocumulus , liquid water path , cloud condensation nuclei , liquid water content , atmospheric sciences , environmental science , aerosol , effective radius , coalescence (physics) , cloud base , precipitation , chemistry , meteorology , cloud computing , physics , quantum mechanics , computer science , galaxy , astrobiology , operating system
Cloud droplet number concentration ( N c ) is retrieved from remotely observed marine stratocumulus cloud liquid water path (LWP), cloud optical depth ( τ 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 τ 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 6 hr following N c . Consistent with coalescence and drizzle scavenging cloud droplets, lag correlations show that N c decreases for 1 hr 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.