AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
Stratocumulus Cloud-top Inhomogeneous Entrainment Parcel Model Parameterization
KEVIN SANCHEZ, Greg Roberts, Minghui Diao, Lynn Russell, Scripps Institution of Oceanography
Abstract Number: 531 Working Group: Aerosols, Clouds and Climate
Abstract Observations of stratocumulus clouds from the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES) were used to quantify the effect of cloud-top entrainment on the shortwave cloud radiative forcing (SWCF). Marine stratocumulus clouds are an important part of the global heat budget because of their large horizontal extent. However, their radiative forcing is highly uncertain due to sensitivity of their radiative properties to slight variations in cloud microphysical properties. The marine boundary layer also has few sources of cloud condensation nuclei, especially in the Southern Ocean, adding to the uncertainty. A cloud parcel model was utilized to simulate the observed cloud profiles. A mixing line entrainment parameterization, first shown by Sanchez et al. 2017, is applied to the simulations to account for cloud droplet evaporation due to cloud-top entrainment of warm, dry free tropospheric air, improving closure of cloud droplet number concentrations (CDNC) and SWCF. The mixing line parameterization uses thermodynamically conserved variables, derived from measured state parameters, to identify mixing of air into the cloud-top. Previously this entrainment parameterization has been applied to marine stratocumulus clouds on the west coast of Ireland and over Cyprus and compared to in-cloud vertical profiles of solar irradiance and cloud droplet extinction. In this study the parameterization is compared directly to measured CDNC profiles. Results indicate cloud-top entrainment decreases the SWCF between 37 and 85 W m-2 and CDNC between 35% and 60% for the five cases in this study. In addition, simulations with varying updraft velocities and below cloud particle concentrations indicate the CDNC and SWCF are significantly more sensitive to drying from cloud-top entrainment than changes in updraft velocity and particle concentrations, emphasizing the importance of accurately accounting for cloud-top entrainment.