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

Abstract

AbstractEffects of free‐tropospheric thermodynamic properties and large‐scale subsidence on the nocturnal growth of marine stratocumulus clouds are investigated through large‐eddy simulation based on Flight 5 of the Physics of Stratocumulus Top research campaign. It was characterized by a weak inversion and moist troposphere. Sensitivity simulations are performed using variations in the subsidence and free‐tropospheric temperature and humidity. The results confirm that the cloud‐top entrainment instability parameter κ alone cannot unambiguously control the behaviors of marine stratocumulus due to the opposite response of liquid water path (LWP) to changes in the humidity and temperature jumps with the same variation in κ, thereby both jumps should be considered separately. However, sometimes it could be considered as a joint factor of the humidity and temperature jumps controlling LWP even though cloud‐top entrainment instability does not occur or κ < 0. An obvious role of subsidence is to push the inversion layer down nearly identically and at the same rate under different ambient conditions. Enhanced subsidence, as expected, diminishes the entrainment rate, cloud‐top height, cloud thickness, and LWP. However, it has a small influence on the inversion thickness and the dynamic instability across the inversion. An LWP budget analysis shows that the direct effect of subsidence is a small reduction in the LWP, but the indirect effect of subsidence is large due to contributions from other physical processes. Therefore, subsidence is an important physical process controlling the LWP budget, even though its direct contribution is significantly smaller than the largest contributions from both radiation and entrainment.