Boundary layer fluxes into trade cumulus clouds

Surface fluxes from the ocean moisten and warm the subcloud atmospheric boundary layer (SBL) as the surface winds approach the warm SST of the Intra-American Seas. The SBL supplies moisture that then enters the trade cumulus cloud base. Condensation of water in clouds generates buoyancy and stabilizes the trade inversion layer that surrounds the cumulus clouds. Cold pools generated by the evaporation of hydrometeors descend and cool the SBL.
The representation of trade wind clouds in climate models has important implications for climate. Shallow cumulus clouds reflect sunlight from the climate system, shade the ocean surface, generate wind gusts that increase surface evaporation, and moisten the lower free troposphere, building the conditions for deeper convection. Widespread shallow clouds contribute more to planetary albedo than narrow cumulus towers. Despite the shallower clouds’ stronger radiative effect, models with shallower clouds have a more positive cloud climate feedback. That is because vertical mixing dries shallower clouds more in a warmer climate, reducing the cloud albedo, and thus enhancing the warming.
The processes achieving this vertical mixing are not understood, either in models or in the observable atmosphere. Shallow cumulus clouds occur at the grey scales that are too small to resolve with weather models, and whose mesoscale circulations are too large to represent in large eddy simulations. Observations of turbulence, conserved thermodynamic variables (potential temperature, humidity, and stable isotope ratios of water vapor), and clouds from the US Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) will measure the turbulence hypothesized to affect the mixing and humidity of climatically important shallow cumulus clouds.
This project is focused on analysis of measurements of the shallow cumulus clouds, and their related thermodynamic fluxes. These fluxes tie the cumulus clouds to the subcloud boundary layer and its moisture budget. We will observe the moisture transport by clouds, by clear air, and by turbulence, that are hypothesized to be responsible for the difference of shallow cumulus cloud feedbacks in models. We will make rawinsonde observations of the wind and thermodynamic structure of the atmosphere and stable isotope ratio observations to characterize surface fluxes and evaporative downdrafts. Vertical fluxes of moisture and heat from the surface through the cumulus layer will be estimated from thermodynamic budgets and from direct measurements of turbulent and cloud vertical velocities. We will integrate the sounding and isotope measurements with observations of the turbulence at the ocean surface, in the subcloud boundary layer, and in the shallow cumulus cloud layer.
Relevance to the CVP competition “Observing and Understanding Upper-Ocean Processes and Shallow Convection in the Tropical Atlantic Ocean”: Proposed soundings observations support the ATOMIC program on observing and understanding the interactions of upper ocean, atmospheric boundary layer, and shallow trade cumulus clouds. Stable isotope ratios combined with humidity data will place strong constraints on the transport of water between the ocean surface and the troposphere. The isotopic measurements contribute to the international effort with partners coordinated as the EUREC4A-iso project. Analysis of soundings and turbulent vertical velocity observations will be used to quantify mixing process in the atmosphere, differences of which have been used to explain differences in clouds and climate sensitivity among models.