Stratocumulus-to-Cumulus transition: A case study from the MAGIC field campaign

 
Poster PDF

Authors

Virendra Prakash Ghate — Argonne National Laboratory
David B. Mechem — University of Kansas
Edwin W. Eloranta — University of Wisconsin
Maria Paola Cadeddu — Argonne National Laboratory
Michael Jensen — Brookhaven National Laboratory

Category

Warm low clouds, including aerosol interactions

Description

Longitude-height plot of the radar reflectivity from the vertically pointing ka-band radar (top) and the particulate backscatter from the High-Spectral-Resolution Lidar (bottom). The ceilometer reported first cloud base height (black) is shown in the top panel, while the surface lifting condensation level (red) is shown in both panels. The black line at 2200m in the top panel denotes daytime hours.
Marine boundary-layer clouds cover vast areas of the eastern subtropical oceans and have a significant impact on the Earth’s radiation budget. Marine stratocumulus (Sc) clouds form in regions with cold sea surface temperatures (SSTs) and strong boundary-layer inversion that is maintained by a large-scale subsidence. As these clouds are advected towards the trade-wind regions that have warmer SSTs and weak inversion, they decouple from the surface and transition to broken cumulus (Cu) clouds. This transition from stratocumulus to cumulus cloud (Sc-to-Cu) regime is thought to occur due to a complex interplay of processes modulated by surface fluxes, boundary-layer radiative cooling, inversion strength, and precipitation. Previous modeling studies have shown this transition to occur over a span of three days with the nighttime radiative cooling on the first two days able to recouple (well-mix) the boundary layer after daytime decoupling, with the nighttime radiative cooling on the third day being too weak to be able to recouple the boundary layer. Additionally, there is considerable debate over the relative influence of different mechanisms (precipitation versus entrainment) in causing the decoupling. In this study, we have used the data collected during the MAGIC field campaign from a five-day period to study the boundary-layer cloud transitions. The cloud and precipitation macro- and micro-physical properties were retrieved using the combination of data collected by vertically pointing Doppler cloud radar and High-Spectral-Resolution Lidar (HSRL). The retrieved microphysical properties and thermodynamic profiles were then used as an input to a one-dimensional radiative transfer model called the Rapid Radiative Transfer Model (RRTM), for retrieving profiles of radiative fluxes and heating rates. Satellite-reported cloud top height estimates were used to retrieve the boundary-layer entrainment rates through the mass budget equations. Our initial results suggest the drizzle evaporation flux to be similar in magnitude to the surface latent heat flux, with both of them about half the magnitude of the radiative flux divergence within the cloud layer. We present the day-to-day changes in the cloud and boundary-layer properties, along with the relative magnitude of the different factors responsible for the cloud transitions.