Summertime Marine Stratocumulus Transition Processes over the Eastern North Atlantic

Marine boundary layer (MBL) clouds exhibit a seemingly endless array of complex configurations and cover a large fraction of the Earth's oceans. They are seminal components of the planetary radiation budget and potentially susceptible to change as a result of human activities. An extensive and variable MBL cloud system is present over the Eastern North Atlantic (ENA) that undergoes a morphological transition with latitude. Clouds over the northern reaches of the ENA tend to be layered while those to the south tend to be more broken. Conceptual models of this cloud transition depict reductions in inversion strength and increasing sea-surface temperatures toward the Intertropical Convergence Zone accompanying the evolution to broken cloud structure. Stratocumulus, shallow cumulus, and complex combinations of these cloud types, often exhibiting mesoscale organization, are present in the transition cloudiness over the ENA. Despite steady progress, significant uncertainty regarding the exact details of the stratocumulus cloud life cycle remain, and some basic questions have yet to be answered. For example, there is considerable uncertainty as to the various process combinations that lead to the transition of stratocumulus to more broken cloud structures. Given the known importance of the marine stratocumulus cloud system to the radiation budget, it is important to fully understand the life cycle of MBL clouds in the ENA cloud transition region and beyond. This is particularly true given the known challenges in simulating MBL cloud systems in models of all types.

We performed two high resolution (1250 m horizontal resolution) multiple-day simulations to investigate the break-up of a solid layer of stratocumulus with the Weather Research and Forecasting (WRF) model using the NCAR Cheyenne Supercomputer, and combined them with coincident measurements collected at the United States Department of Energy's Eastern ENA Climate Research Facility. The transition stratocumulus that we observed and simulated were found behind two summertime cold fronts over the Eastern North Atlantic (ENA). We analyzed the simulations in a Lagrangian framework to better understand the cloud life cycle. Overall, the simulations depict a stratocumulus breakup process that is consistent with the deepening-warming hypothesis and decoupling is identified as the key initial instigator of the break-up process. The simulations produced a range of cloud configurations associated with the transition region that are compared to similar cloud structures observed routinely in the summertime at the ENA site. Marine boundary layer depths simulated by WRF in the two runs were comparable to those that are measured in the region, as were the thermodynamic profiles that it produced. Hence, the simulations appear to bear a significant resemblance to reality.

Decoupling of the cloud layer from the surface supply of moisture and turbulent kinetic energy is known to be an important process in this region. It can be initiated by several mechanisms including solar radiation during the daytime, drizzle evaporating in the sub-cloud layer, or increases in the Lifting Condensation Level (LCL) height caused by warmer sea surface temperatures as the cloud advects southward toward the tropics. The simulations suggest a cloud layer erosion process that includes contributions from above and from below, and that the relative importance of these erosion processes depends critically upon the coupling state of the cloud layer. Erosion from below in the simulations was highly correlated with the latent heat flux at the surface and with the height of the LCL. Given the notable increase in the latent heat flux as the tracked columns advected south and an accompanying rise in the LCL, we concluded that warming from below was the likely perpetrator of the onset of decoupling. We also found that the onset of decoupling was characterized by variations in all variables in the column, especially the surface latent heat flux. Circumstantially, we concluded that the initiation of decoupling was stimulated by local surface variations in the latent heat flux and LCL.

We investigated several parametric relationships to determine the dynamic range that they produced across the transition from a solid deck of stratocumulus to broken cloud structure. Non-dimensional diagnostic parameters that used the LCL as the scaling depth produced a considerably greater dynamic range across the transition than any other parametric combination that we tested indicating that it is the most sensitive measure of the degree of decoupling, and coincidentally the likelihood of a transition. Viewed in the context of all results from our study, the key process that is implicated as the initial instigator of stratocumulus breakup into individual cloud elements in these simulations is the energy required to lift a parcel from the surface to the LCL.