Island Wakes As a Mechanism for Mixing of Free Tropospheric Aerosol into Marine Stratocumulus

Satellite remote sensing of boundary layer clouds over the Southern California Bight indicates that the clouds are often characterized by high cloud drop number concentrations N_d (N_d>300 cm^-3) in the near-coastal region, especially in the presence of shallow mesoscale eddy features (i.e., Catalina eddies) that preferentially occur during the late spring and early summer months. While maxima in N_d are occasionally observed immediately adjacent to the coast, suggesting offshore horizontal transport of continentally-sourced CCN within the boundary layer, it is more frequently the case that N_d maxima are observed 10s-100s of km offshore in the vicinity of the Channel Islands (specifically, Santa Catalina, San Clemente and San Nicolas Islands). The islands are unlikely to be major aerosol sources due to their small size and sparse populations, nor is it expected that local marine sources can explain high N_d as wind speeds are typically weaker over the Bight than over the open ocean to the west. In addition, direct transport of aerosol from the coast during eddy events is unlikely given that boundary layer flow over the islands typically has a westerly component. On the other hand, layers of enhanced aerosol concentration in the lower free troposphere have routinely been observed by aircraft for almost as long as airborne meteorological research has been conducted in the region. To evaluate whether free tropospheric aerosol is an important source of CCN for boundary layer clouds over the Bight, this work explores island wakes as a dynamical mechanism that enables efficient downward vertical mixing across the strongly stable boundary layer-capping inversion.

The Southern California Interactions of Low cloud and Land Aerosol (SCILLA) airborne experiment was undertaken in June 2023 to characterize aerosol and cloud properties over the Bight and evaluate aerosol transport hypotheses. SCILLA flights repeatedly sampled upwind (to the west) and downwind (to the east) of San Clemente Island (SCI) to characterize “clean marine” and “polluted” cloudy airmasses, respectively, with a comprehensive suite of instrumentation measuring aerosols, clouds, radiation, turbulence and trace gases. A sustained, multi-day Catalina eddy event occurred during the final week of the experiment and is the focus of the analysis presented here.

Enhanced concentrations of both fine and accumulation mode aerosols were observed above the boundary layer throughout the week, and there was frequently a large gradient in N_d of a few hundred drops per unit volume between upwind and downwind sampling modules. Analysis of sawtooth flight legs traversing cloud top and the overlying free troposphere during wake-impacted downwind modules show enhanced turbulence across the inversion and localized areas of N_d exceeding 600 cm^-3 (versus a typical “background” concentration of ~200 cm^-3). This suggests that turbulence induced by vertical shear in the island wake promotes mixing across a statically stable interface. In some cases, enhanced N_d is highly correlated with above-cloud accumulation mode aerosol concentration while in others it appears to vary more closely with above-cloud fine mode concentration. We speculate that day-to-day variability in aerosol-cloud interactions is due to differences in both aerosol source region and local meteorology/boundary layer properties, which are likely covarying factors. Overall, our results highlight an underappreciated vertical transport mechanism that may be important at global scale for understanding boundary layer aerosol-cloud interactions in the vicinity of islands with even moderate topography.