Speaker

Xuanyu Chen, Ph.D. ’20, Research Associate, NOAA Physical Sciences Laboratory

Mesoscale sea surface temperature warm anomalies increase trade cumulus generation in the North Atlantic trade-wind boundary layer

Abstract

Trade-wind cumuli play a crucial role in Earth’s energy budget due to their prevalence and net cooling effect. How
these shallow clouds respond to a warming climate remains a key uncertainty for climate projections. The Atlantic
Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) provided a unique opportunity to
investigate how relatively weak yet ubiquitous mesoscale sea surface temperature (SST) features impact trade
cumulus cloudiness. This talk will present complementary investigations using ATOMIC-validated satellite
observations and cloud-resolving Large Eddy Simulations (LES). Composite analysis of the satellite observations
shows in-phase modulations of daily mean cloudiness over weak sea surface temperature anomalies (0.25 K on
average). These daily cloudiness anomalies are positively correlated with the 10-m neutral surface wind speed but are
offset from the near-surface wind convergence. Idealized LES experiments conducted with the mean large-scale
environmental conditions during ATOMIC reproduce these satellite composite results. LES results further suggest
that the enhanced cloudiness signal over SST warm patches in the satellite composite mainly reflects enhanced
cloudiness below 1 km. LES results also show that the increased low-level cloudiness is driven by locally enhanced
turbulence updraft, a result of enhanced surface buoyancy flux, instead of surface convergence-induced upward
motions. Mechanism denial experiments further clarify that the enhanced near cloud-base cloudiness is a nonlinear
response to the locally enhanced surface sensible and latent heat fluxes.