Marine Boundary Layer Clouds Associated with Coastally Trapped Disturbances: Observations and Model Simulations

Abstract

Modeling marine low clouds and fog in coastal environments remains an outstanding challenge due to the inherently complex ocean–land–atmosphere system. This is especially important in the context of global circulation models due to the profound radiative impact of these clouds. This study utilizes aircraft and satellite measurements, in addition to numerical simulations using the Weather Research and Forecasting (WRF) Model, to examine three well-observed coastally trapped disturbance (CTD) events from June 2006, July 2011, and July 2015. Cloud water-soluble ionic and elemental composition analyses conducted for two of the CTD cases indicate that anthropogenic aerosol sources may impact CTD cloud decks due to synoptic-scale patterns associated with CTD initiation. In general, the dynamics and thermodynamics of the CTD systems are well represented and are relatively insensitive to the choice of physics parameterizations; however, a set of WRF simulations suggests that the treatment of model physics strongly influences CTD cloud field evolution. Specifically, cloud liquid water path (LWP) is highly sensitive to the choice of the planetary boundary layer (PBL) scheme; in many instances, the PBL scheme affects cloud extent and LWP values as much as or more than the microphysics scheme. Results suggest that differences in the treatment of entrainment and vertical mixing in the Yonsei University (nonlocal) and Mellor–Yamada–Janjić (local) PBL schemes may play a significant role. The impact of using different driving models—namely, the North American Mesoscale Forecast System (NAM) 12-km analysis and the NCEP North American Regional Reanalysis (NARR) 32-km products—is also investigated.

Stratocumulus cloud clearings: statistics from satellites, reanalysis models, and airborne measurements

“…The combination of higher N d and lower LWP by the coastline results in smaller r e on clearing days. The more polluted clouds along the coastline during clearing days, especially south of major capes, are analogous to CTD clouds being more polluted during southerly wind regimes in the study region ( Juliano et al, 2019a , b ). An intriguing aspect of clearing days was that although a significant section of the study region was cloud-free, the mean cloud albedo ( A ) over the entire study domain was actually slightly higher than on non-clearing days ( Fig.…”

“…The 09:00 PST map most clearly shows that those two topographical features potentially serve as “trigger points” for the majority of clearings, and as a typical clearing day develops, the CF gets reduced around those points by moving farther south and to the west. The significance of these capes is discussed in many previous studies ( Beardsley et al, 1987 ; Haack et al, 2001 ; Juliano et al, 2019a , b ), pointing to their ability to alter local dynamics, cloud depth, and various microphysical processes such as entrainment. Cloud thinning in the vicinity of the capes due to an expansion fan effect is reported for both northerly and southerly flow ( Beardsley et al, 1987 ; Juliano et al, 2017 ).…”

“…Targeted experiments to examine these types of events will help advance understanding of their nature, which can then be contrasted with clearings along other coastal regions such as the southeastern Atlantic Ocean. Also, the nature of clearings has direct relevance to CTD events that evolve in similar regions as discussed by Juliano et al (2019a , b ).…”

“…One of their three events was associated with a so-called “southerly surge”, also referred to as a coastally trapped disturbance (CTD). CTD events were recently characterized off the US West Coast by Juliano et al (2019a , b ). Clearing events have been examined over the southeast Atlantic Ocean, with the catalyst for cloud erosion shown to be atmospheric gravity waves ( Yuter et al, 2018 ).…”

Sensitivity analysis of an aerosol-aware microphysics scheme in Weather Research and Forecasting (WRF) during case studies of fog in Namibia

“…Fog formation and lifetime is dependent on the atmospheric thermodynamic (radiation, turbulence, and mixing) and surface conditions (albedo, soil characteristics, roughness length, and moisture content). Therefore, the simulation of fog first requires that state atmospheric variables related to temperature and moisture are simulated adequately by models, which implies representing the coupling of land-atmosphere interactions and good parameterisation schemes of the planetary boundary layer (PBL; Bergot and Lestringant, 2019;Boutle et al, 2018;Juliano et al, 2019;Maronga and Bosveld, 2017;Steeneveld and De Bode, 2018).…”