Role of Sub-Cloud Rain Evaporation on Boundary Layer Decoupling over Barbados Island

Abstract

This study investigates the influence of sub-cloud rain evaporation on the decoupling of sub-tropical marine cumulus-topped raining boundary layers. Using 24-hour wind lidar and Ka-band radar observations on February 9, 2020 from the Barbados Cloud Observatory (BCO), along with in-situ rain microphysical observations from the ATR aircraft during the EUREC4A field campaign, we extract rain microphysical parameters - raindrop number concentration (N0) and geometric mean diameter (Dg). These parameters, alongside surface relative humidity measurements, serve as inputs to initialize a single-column rain evaporation model, allowing us to derive vertical profiles of rain evaporation fluxes and evaporation cooling rates. Our analysis identifies 'top-heavy' profiles characterized by maximum evaporative cooling near the cloud base, featuring smaller Dg and larger N0. Conversely, 'bottom-heavy' profiles exhibit larger Dg and smaller N0, with maximum evaporative cooling closer to the surface. Notably, our findings reveal that top-heavy profiles, especially when cloud bases are higher, tend to be more decoupled than bottom-heavy profiles. The higher decoupling of the top-heavy profiles is attributed to the stable configuration of the evaporatively-cooled moisture layer just below the warmer cloud layer, hindering moisture transport to the cloud. In contrast, for a bottom-heavy profile where the evaporatively-cooled moisture layer is accumulated closer to the surface over a warmer sea surface, surface-driven mixing promotes moisture transport to cloud bases, resulting in less decoupling. The decoupling index, independently estimated from the difference between ceilometer-based cloud base height and empirically determined lifting condensation level, enhances the robustness of our results. While emphasizing the significant influence of sub-cloud rain evaporation on the decoupling of cumulus-topped raining boundary layers, our study has not explored other factors like surface and radiative fluxes, which could also contribute to the boundary layer decoupling.