Stratocumulus cloud-top inhomogeneous entrainment parcel model parameterization
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 in models due to the 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.
In this study 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. Variability in the entrainment is shown to vary the SWCF by up to 34 W m-2. In addition, simulations with varying updraft velocities and below cloud particle concentrations indicate the SWCF is 4 to 20 times 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.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A11J2897R
- Keywords:
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- 0320 Cloud physics and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0321 Cloud/radiation interaction;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3349 Polar meteorology;
- ATMOSPHERIC PROCESSES