Assessing the mechanisms governing the daytime evolution of marine stratocumulus using large‐eddy simulation

Abstract

Large-eddy simulation (LES) output for a case of thin stratocumulus off the coast of California is examined in a mixed-layer analysis framework to identify the specific mechanisms responsible for governing the evolution of the cloud system. An equation for cloud-base height tendency isolates the individual cloud-modulating mechanisms that control the evolution of boundary-layer liquid-water static energy (S l ) and total water mixing ratio (q T ). With a suitable spin-up procedure, the control simulation performs admirably compared with observed estimates of liquid water content, vertical velocity variance, and radiative fluxes sampled during an aircraft field campaign. Investigation of the cloud response to various environmental forcing scenarios was addressed through a suite of sensitivity simulations, including variations in subsidence velocity, surface fluxes, wind shear near the inversion, and radiative forcing. In the control simulation, rising cloud-base tendencies are associated with entrainment warming/drying and short-wave absorption, whereas lowering cloud-base tendencies are driven by long-wave cooling. Even in the presence of substantial afternoon solar heating, entrainment fluxes remained active. The thin cloud demonstrated unexpected resiliency, with mixed-layer analysis indicating that, as the short-wave flux decreases later in the afternoon, the relative contribution of long-wave cooling often becomes large enough to offset entrainment warming/drying and result in a reversal of cloud-base tendency. The evolution of cloud-base tendency is found to be insensitive to the net radiative flux divergence for most of the simulations (liquid water path ranging from~10-50 g/m 2 ). Error analysis in comparison with LES S l and q T budgets suggests that our method of entrainment flux calculation could be improved by a more complete understanding of entrainment-layer physics. KEYWORDS afternoon, large-eddy simulation, marine, mixed-layer model, stratocumulus 1 Q J R Meteorol Soc. 2019;145:845-866.wileyonlinelibrary.com/journal/qj

The diurnal stratocumulus-to-cumulus transition over land

“…9a). These results agree with the findings by Wang et al (2008) and McMichael et al (2019), who studied cloud-top shear effects on marine Sc clouds. Such agreement reinforces the analogy between the night-time Sc cloud studied here before sunrise and the typical marine Sc, given the low values of the 5 surface fluxes.…”

Coastal Stratocumulus Dissipation Dependence on Initial Conditions and Boundary Forcings in a Mixed-Layer Model

“…5-n), agreeing with the single variable changes ( Fig. 6-d), as well as the response in LWP in previous sensitivity studies (McMichael et al 2019;Ma et al 2018;van der Dussen et al 2016;Noda et al 2014;Blossey et al 2013) and the response in cloudiness for independent changes of subsidence (Myers and Norris 2013).…”

Liquid Water Path Steady States in Stratocumulus: Insights from Process-Level Emulation and Mixed-Layer Theory

“…These insights into the entrainment velocity can be linked to a cloud-radiation-entrainment feedback (Zhu et al 2005;McMichael et al 2019): While the LWP determines the entrainment velocity by controlling radiative and evaporative cooling at the cloud top, as well as the general release of latent heat during condensation, the warming and drying by entrainment tend to reduce the LWP. Zhu et al (2005) argued therefore that minuscule differences in the entrainment velocity lead to significant changes in the LWP, as indicated by the aforementioned first condition for a stable steady state.…”