Marine Boundary Layer Decoupling and the Stable Isotopic Composition of Water Vapor

Abstract

Marine boundary layer (MBL) stratocumulus clouds play a crucial role in the climate system, radiatively cooling the Earth through strong reflectance of incoming shortwave radiation compared to the underlying ocean, with comparatively little impact in the longwave portion of the spectrum due to cloud top temperatures that are not significantly different than the surface. These important cloud types are maintained by turbulence driven by cloud top radiative cooling that generates mixing between the oceanic source of water vapor and the cloudy layer (Hartmann et al., 1992;Klein & Hartmann, 1993;Wood, 2012). When the MBL is shallow (below around 1 km), the boundary layer is generally well-mixed, capped by a strong temperature inversion, and the MBL cloud fractions are greatest (Albrecht et al., 1995). Cloud top evaporative cooling and entrainment of free-tropospheric air into the MBL, among other mechanisms, leads to a deepening of the MBL beyond 1 km, and the longwave cooling at the cloud tops cannot mix the negatively buoyant entrained air through the full depth of the MBL. The cloudy layer can thus become decoupled from the oceanic moisture source, reducing or eliminating mixing through the full depth of the MBL, and reducing cloud fractions (Bretherton & Blossey, 2014;Bretherton & Wyant, 1997). The MBL can also become decoupled through radiative heating of the cloud layer and evaporative cooling in the sub-cloud layer, both processes further stabilizing the MBL and suppressing vertical mixing (Ghate et al., 2013;Nicholls, 1984;Xiao et al., 2011). The details of this decoupling process and how it impacts water vapor transport within the MBL and the exchanges of water vapor between the subcloud layer (SCL), the cloudy layer, and the

Distinct Dynamical and Structural Properties of Marine Stratocumulus and Shallow Cumulus Clouds in the Eastern North Atlantic

“…The large-scale environment for coupled Sc exhibited a similar pattern, with the Azores surface high located over Graciosa Island (Figure S4a in Supporting Information S1 and Galewsky et al, 2022, their Figure 5). Contrary to our findings, Galewsky et al (2022) showed that the Azores surface high for decoupled cases was shifted to the northeast. The center of the Azores surface high for this study moved toward the southwest even though we consider a longer time period (October 2015 to September 2017; Figures S4a and S4b in Supporting Information S1).…”

“…Contrary to our findings, Galewsky et al. (2022) showed that the Azores surface high for decoupled cases was shifted to the northeast. The center of the Azores surface high for this study moved toward the southwest even though we consider a longer time period (October 2015 to September 2017; Figures S4a and S4b in Supporting Information ).…”

“…Galewsky et al. (2022) considered only very strongly decoupled states with high $${\increment}{q}_{t}$$ for 1 yr (March 2018 to February 2019).…”

“…For instance, Galewsky et al. (2022) used the threshold $${\increment}{q}_{t} > 5.7$$ g kg −1 for decoupled conditions while we used 0.5 g kg −1 for this study following the threshold of Jones et al. (2011).…”

Non‐Equilibrium Fractionation Factors for D/H and 18O/16O During Oceanic Evaporation in the North‐West Atlantic Region

“…However, several recent studies have questioned the assumed role of SST and wind speed on the controls of nonequilibrium fractionation based on water vapor d ‐excess observations (Bonne et al., 2019 ; Pfahl & Sodemann, 2014 ; Pfahl & Wernli, 2008 ; Steen‐Larsen et al., 2014 , 2015 ; Uemura et al., 2008 ). Other studies argued that water vapor d ‐excess above the ocean surface may not be influenced solely by ocean surface evaporative conditions, namely RH and SST, but also by the coupling between the marine boundary layer (MBL) and the free troposphere (Benetti et al., 2018 ; Galewsky et al., 2022 ) as well as by air–sea interactions during cold and warm advection (Thurnherr et al., 2021 ). Consequently, it can be concluded that a substantial uncertainty exists on the magnitude of nonequilibrium fractionation during evaporation in real‐environmental conditions and, still, no agreement exists on the controls of nonequilibrium fractionation by SST and wind speed (e.g., Gonfiantini et al., 2020 ).…”