Summertime post-cold-frontal marine stratocumulus transition processes over the eastern North Atlantic

• Main Author: Kazemi Rad, Melissa
• Format: Text
• Language: unknown
• Published: No Publisher Supplied 2020
• Subjects: North Atlantic
• Online Access: https://dx.doi.org/10.7282/t3-h91j-mb73; https://rucore.libraries.rutgers.edu/rutgers-lib/65005/

The Marine stratocumulus cloud system is a major component of the Earth’s energy budget. Mid-latitude stratocumulus are known to transition from a single, continuous cloud layer to a hybrid configuration that includes both stratocumulus and cumulus, and eventually to trade cumulus toward the tropics. Stratocumulus transitions are often observed in the wake of cold air outbreaks in the mid-latitude summertime marine boundary layer (MBL). Cloud morphology associated with two summertime cold fronts over the Eastern North Atlantic (ENA) is investigated using high resolution simulations from the Weather Research and Forecasting (WRF) model and observations from the Atmospheric Radiation Measurement (ARM) ENA Climate Research Facility. Lagrangian trajectories are used to study the evolution of post-cold-frontal MBL clouds from solid stratocumulus to broken cumulus. 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. 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. Cloud-top entrainment instability (CTEI) is shown to operate only in the decoupled MBL. 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 alternative to CTEI. Overall results suggest that the deepening-warming hypothesis suggested by Bretherton and Wyant explains many of the characteristics of the summertime post-frontal MBL evolution of cloud structure over the ENA, thereby widening the phase space over which the hypothesis may be applied. A subset of the deepening-warming hypothesis involving warming initially dominating over moistening is proposed. It is postulated that changes in 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.