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Impacts of Free-Tropospheric Temperature and Humidity on Nocturnal Nonprecipitating Marine Stratocumulus

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  • 1 Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Beijing, China
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Abstract

Marine stratocumulus (MSc) cloud amount can decrease with an increase in the cloud-top instability parameter κ, based on the cloud-top entrainment instability (CTEI) theory. Notice that if boundary layer temperature and humidity remain the same, a given κ can correspond to different combinations of free-tropospheric temperature and humidity. By employing large-eddy simulations coupled with bin microphysics, this study investigates the characteristics of three nocturnal nonprecipitating MSc systems with the same κ but different free-tropospheric conditions. It is found that the spread of liquid water path (LWP) among the three cases is large. The LWPs of these three cases are also compared with the base case where κ is smaller. One of the three cases even has larger LWP than the base case, which is not expected by the CTEI theory. Results indicate that the thermodynamic properties of the free-tropospheric air are important. For the three cases with the same κ, cooler and moister free-tropospheric air leads to a cooler and moister boundary layer through entrainment, hence a lower cloud base. A cooler and moister free troposphere also allows the turbulent boundary layer air parcels to overshoot to a higher height, leading to a higher cloud top. Therefore, there is a spread in LWPs among systems with the same κ. The spread can be so large that sometimes systems with larger κ may have larger LWPs than systems with smaller κ. More simulations are also performed covering other free tropospheric conditions and aerosol concentrations.

Corresponding author address: Huiwen Xue, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Room 518, North Physics Building, 209 Chengfu Road, Beijing 100871, China. E-mail: hxue@pku.edu.cn

Abstract

Marine stratocumulus (MSc) cloud amount can decrease with an increase in the cloud-top instability parameter κ, based on the cloud-top entrainment instability (CTEI) theory. Notice that if boundary layer temperature and humidity remain the same, a given κ can correspond to different combinations of free-tropospheric temperature and humidity. By employing large-eddy simulations coupled with bin microphysics, this study investigates the characteristics of three nocturnal nonprecipitating MSc systems with the same κ but different free-tropospheric conditions. It is found that the spread of liquid water path (LWP) among the three cases is large. The LWPs of these three cases are also compared with the base case where κ is smaller. One of the three cases even has larger LWP than the base case, which is not expected by the CTEI theory. Results indicate that the thermodynamic properties of the free-tropospheric air are important. For the three cases with the same κ, cooler and moister free-tropospheric air leads to a cooler and moister boundary layer through entrainment, hence a lower cloud base. A cooler and moister free troposphere also allows the turbulent boundary layer air parcels to overshoot to a higher height, leading to a higher cloud top. Therefore, there is a spread in LWPs among systems with the same κ. The spread can be so large that sometimes systems with larger κ may have larger LWPs than systems with smaller κ. More simulations are also performed covering other free tropospheric conditions and aerosol concentrations.

Corresponding author address: Huiwen Xue, Department of Atmospheric and Oceanic Sciences, School of Physics, Peking University, Room 518, North Physics Building, 209 Chengfu Road, Beijing 100871, China. E-mail: hxue@pku.edu.cn
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