Estimating the Decoupling Degree of Subtropical Marine Stratocumulus Decks From Satellite

Abstract

Abstracts The decoupling degree of stratocumulus (Sc) decks is an important quantity dictating evolutions of Sc. In subtropical oceans, the Sc decoupling is a key intermediate process of the Sc‐to‐cumulus transitions, a persistent phenomenon that is not fully understood. This study introduces a new approach for estimating the degree of decoupling of subtropical Sc decks using passive satellite sensors. This method is limited to regions where Sc decks are advected over progressively warmer water. This is most common in the subtropics. The estimation concept is that decoupled Sc clouds under cold‐advection conditions are fed by spreading of the tops of cumulus clouds that are coupled. The cumulus clouds constitute a much larger liquid water path over small areas, which is identified by a positive skewness of the liquid water path, a quantity measurable from high‐resolution satellite data. The decoupling degree here is defined as the difference between the Sc cloud‐base height and lifting condensation level that is the cumulus cloud‐base height under cold‐advection conditions. This concept and the satellite‐based estimations are supported by ship measurements over the Northeast Pacific. One‐year climatology of the satellite‐inferred decoupling degree was generated over the same region, revealing a coherent pattern of offshore decoupling, consistent with previous theory and field‐campaign observations.

Episodes of Warm‐Air Advection Causing Cloud‐Surface Decoupling During the MARCUS

“…It is instructive to compare this maintenance mechanism of decoupled clouds under LLWAA with that of LLCAA. The LLCAA allows for a conditionally unstable environment that is favorable for the vertical growth of cumulus clouds underneath the decoupled the Sc decks (Bretherton, 1997; Miller & Albrecht, 1995; Zheng et al, 2018a). Although these cumulus clouds can help sustain the stratocumulus decks by transporting moisture to the upper level (Jensen et al, 2000; Miller & Albrecht, 1995; Zheng et al, 2018b), once the cumulus clouds become vigorous enough, they penetrate through the overlying Sc decks and cause strong ventilation that is effective at drying out the Sc decks (Bretherton, 1997).…”

“…A key assumption underlying Lily's model is that a cloud‐topped MBL is always well mixed with vertically uniform moist conserved variables (e.g., liquid water potential temperature and total water vapor mixing ratio). This well‐mixed state of the MBL is conventionally called “coupled” and, on the contrary, a state deviated from the well‐mixed state(e.g., vertical stratification of moist conserved variables) is called “decoupled” (Bretherton & Wyant, 1997; Dong et al, 2015; Jones et al, 2011; McGibbon & Bretherton, 2017; Nicholls, 1984; Stevens, 2000; Zheng et al, 2018a).…”

“…This allows the underlying cumulus clouds to penetrate through the decoupled layer and feed the decoupled Sc decks, forming a cumulus‐coupled MBL. This is the reason why decoupled MBLs are often more precipitating than the coupled MBLs due to the deep cumuliform clouds (Martin et al, 1995; O et al, 2018; Zheng et al, 2018a). This mechanism, however, is unlikely to be present under low‐level warm air advections (LLWAA; Figure 1b).…”

The Relationships Between Cloud Top Radiative Cooling Rates, Surface Latent Heat Fluxes, and Cloud‐Base Heights in Marine Stratocumulus

“…This is because of the prevailing equatorward flow that advects Sc decks over progressively warmer water, a large‐scale condition favorable for the initiation, and vertical penetration of Cu clouds that feed moisture to the previously decoupled Sc decks. This leaves the decoupled single‐layer Sc, a marginal category of cloud over the subtropical northeast Pacific, which serves as the basis for a recently developed satellite remote sensing technique that infers the decoupling degree of Sc decks from the spatial distribution of LWP (Zheng et al, ). In absence of the decoupled Sc decks, two STBL regimes dominate: well‐mixed and Cu‐coupled STBLs.…”

Coupled and decoupled stratocumulus-topped boundary layers: turbulence properties

“…Decoupling can be caused either by reducing the intensity of radiatively driven circulation in relation to STBL depth or by stabilizing the subcloud layer (Zheng et al, 2018b). The first possibility might be realized with daytime shortwave radiative heating which offsets longwave cooling (Nicholls, 1984;Turton and Nicholls, 1987) or by extensive entrainment of warm and dry free-troposheric air which deepens the STBL to such an extent that the turbulence is no longer sufficient to sustain the mixing (Bretherton and Wyant, 1997).…”