Sub-Cloud Layer Mediates Interactions between the Ocean Surface and Trade Cumulus Clouds

Abstract:

Time-height series from 169 upper-air radiosondes released from research vessel Ronald H. Brown identify distinct and variable thermodynamic sub-cloud, trade cumulus, and free troposphere layers over the western tropical Atlantic Ocean during the Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) in January-February 2020. Changes in the vertical distribution of moisture in these layers is associated with synoptic waves, surface fluxes, and mesoscale convective cloud organization. Conserved atmospheric variables inform us about interactions between the clouds and the mixed sub-cloud layer. Wet-bulb potential temperature is nearly vertically uniform in the trade cumulus layer, indicating that the trade cumulus layer is approximately well mixed and moist adiabatic. Potential temperature and specific humidity in the sub-cloud mixed layer tell us the relative contribution to the sub-cloud mixed layer of surface flux, downdraft, and turbulent entrainment end members. We estimate heat and moisture fluxes due to downdrafts and turbulent entrainment at the level of cloud base using the ocean surface fluxes and budgets of potential temperature and humidity.