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Large-Eddy Simulations of Entrainment of Cloud Condensation Nuclei into the Arctic Boundary Layer: 18 May 1998 FIRE/SHEBA Case Study |
Colorado State University, CIRA/CSU/NOAA/ETL, Colorado State University, Institute for Marine and Atmospheric Research Utrecht (IMAU),The Netherlands
jiang@atmos.colostate.eduThree-dimensional simulations of a spring time Arctic boundary layer cloud observed during the FIRE/SHEBA 1998 spring IOP were conducted to study the influence of entrainment of cloud condensation nuclei (CCN) at cloud top on cloud microphysical and dynamical structure, radiative properties, and cloud evolution. The model is a large-eddy version of the Regional Atmospheric Modeling System (RAMS) with explicit representation of the CCN spectrum and cloud droplet spectrum.
Results show that droplet concentrations increase about two-fold, effective radii decrease by 25 - 30%, liquid water content increases about 21%, and no drizzle reaches the ground when the air containing higher CCN concentration at the inversion is entrained into the cloudy boundary layer in comparison with the simulation using a CCN profile of smaller constant value. More vigorous eddies are attributed to the significant dynamical response of the simulated cloud to the increased CCN concentration due to entrainment. The response of the cloud optical properties to entrainment occurs from the begining of the simulations. Cloud albedo shows a 12% increase, while cloud optical depth has a 33% increase due to the entrainment of higher CCN concentrations. These results are consistent with both observations and modeling studies. Thus entrainment alters a drizzling case to a nondrizzling case and helps sustain higher liquid water path, more persistent, and more reflective clouds, provided the entrainment is moderate.
