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

Abstract

Marine stratocumulus (MSc) cloud amount can decrease with an increase in the cloud-top instability parameter k, based on the cloud-top entrainment instability (CTEI) theory. Notice that if boundary layer temperature and humidity remain the same, a given k can correspond to different combinations of freetropospheric 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 k 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 k 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 k, 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 k. The spread can be so large that sometimes systems with larger k may have larger LWPs than systems with smaller k. More simulations are also performed covering other free tropospheric conditions and aerosol concentrations.

Influences of Subsidence and Free‐Tropospheric Conditions on the Nocturnal Growth of Nonclassical Marine Stratocumulus

“…Consequently, we see a reduction in the cloud cover down to ∼97%. The observed reduction in the LWP with an increase in |Δ q t | is in agreement with the results of classical cases reported by Xiao et al () and Xu and Xue ().…”

“…First, we note that a decrease in Δ θ l (moving left along the horizontal axis) leads to an increased entrainment velocity. This is in agreement with the results of Xu and Xue (), who studied the classical MSc case (second Dynamics and Chemistry of Marine Stratocumulus RF01). The inhibitory effect of the capping inversion on vertical mixing is reduced when the inversion strength is weakened.…”

“…In our case, however, we see the opposite response: 〈LWP〉 increases with a decrease in Δ θ l and an increase in 〈 w e 〉. We argue that (1) the increased w e (due to weakened inversion) directly causes the rise in the cloud top, which is consistent with Xu and Xue (); (2) the small |Δ q t | values characterizing the nonclassical MSc case studied here severely limit drying of the boundary layer due to entrainment. Thus, the cloud base does not rise as much as the cloud top, which is opposite to what was observed by Xu and Xue ().…”

“…Based on the results presented here and on previously published results regarding classical MSc cases (e.g., Bretherton et al, ; Noda et al, ; Xiao et al, ; Xu & Xue, ), we find that nonclassical and classical MSc cases have the following common points: (1) increased initial |Δ q t | results in cloud thinning, LWP reduction, and cloud‐base height lifting; (2) increased initial |Δ q t | causes inversion‐/cloud‐top height, cloud cover, and entrainment rate to decrease when the free‐tropospheric air is not too moist; and (3) enhanced inversion strength (i.e., increased Δ θ l ) reduces inversion‐/cloud‐top height and cloud‐base height. The most obvious difference between nonclassical and classical MSc is that increased initial Δ θ l reduces cloud thickness, LWP, and entrainment rate in nonclassical MSc but has the opposite effect in classical MSc.…”

“…The inhibitory effect of the capping inversion on vertical mixing is reduced when the inversion strength is weakened. In the study of Xu and Xue (), this was accompanied by a decrease in 〈LWP〉. In our case, however, we see the opposite response: 〈LWP〉 increases with a decrease in Δ θ l and an increase in 〈 w e 〉.…”