Diurnal Patterns in the Observed Cloud Liquid Water Path Response to
Droplet Number Perturbations
Abstract
A key uncertainty in Aerosol-cloud interactions is the cloud liquid
water path (LWP) response to increased aerosols (λ). LWP can either
increase due to precipitation suppression or decrease due to
entrainment-drying. Previous research suggests that precipitation
suppression dominates in thick clouds, while entrainment-drying prevails
in thin clouds. The time scales of the two competing effects are vastly
different, requiring temporally resolved observations. We analyze 3-day
Lagrangian trajectories of stratocumulus clouds over the southeast
Pacific using geostationary data. We find that clouds with a LWP
exceeding 200 g m-2 exhibit a positive response, while clouds with lower
LWP show a negative response. We observe a significant diurnal cycle in
λ, indicating a more strongly negative daytime adjustment driven by
entrainment-drying. In contrast, at night, precipitation suppression can
occasionally fully counteract the entrainment-drying mechanism. The
time-integrated adjustment appears weaker than previously suggested in
studies that do not account for the diurnal cycle.