Summertime Post-Cold-Frontal Marine Stratocumulus Transition Processes over the Eastern North Atlantic

Abstract:

Thin marine clouds that lie just above the ocean surface are the most prevalent cloud type on Earth. They typically resemble vast, continuous cloud sheets in the high and mid-latitudes that transition into broken cloud elements toward the tropics. This study employs high-resolution regional supercomputer simulations using the Weather Research and Forecast model and observations from the Atmospheric Radiation Measurement (ARM) user facility's Eastern North Atlantic (ENA) observatory to better understand the atmospheric processes that lead to this cloudiness transition. Cloudiness transitions associated with two summertime cold fronts over the eastern North Atlantic (ENA) are investigated. Lagrangian trajectories are used to study the evolution of post-cold-frontal marine boundary layer (MBL) clouds from solid stratocumulus to broken cumulus and the Lagrangian derivative of the surface latent heat flux is found to be strongly correlated with that of the cloud fraction at cloud base in the simulations. Clouds within specified domains in the vicinity of transitions are classified according to their degree of decoupling, and cloud-base and cloud-top breakup processes are evaluated. Cloud-top entrainment instability (CTEI) is shown to operate only in the decoupled MBL in the WRF simulations. A new indicator of inversion strength at cloud top that employs the vertical gradients of equivalent potential temperature and saturation equivalent potential temperature, which can be computed directly from soundings, is proposed as an alternative to CTEI. Comparisons between ENA cloud and sub-cloud structural measurements and the simulations suggest that the simulated transition occurs prematurely, perhaps due to insufficient mixing in the marine boundary layer. Overall, the simulations suggest that the deepening–warming hypothesis suggested by Bretherton and Wyant (1997) explains many of the characteristics of the summertime postfrontal MBL evolution of cloud structure over the ENA, thereby widening the phase space over which the hypothesis may be applied. It is postulated that climate change–induced modifications in cold-frontal structure over the ENA may be accompanied by coincident changes in the location and timing of MBL cloud transitions in the post-cold-frontal environment.