Island Wake Atmospheric Boundary Layer Characteristics across the Lesser Antilles

Operational numerical weather prediction (NWP) models often struggle to accurately predict coastal weather near islands for several reasons, including inadequate numerical grid resolutions needed to resolve coastlines and topography and physical processes arising due to the complex surface heterogeneity between the ocean and coastal terrain. The small islands belonging to the Lesser Antilles are one such challenging environment. Although the region serves as an ideal natural laboratory for studying the impacts of isolated topography on oceanic tropical flow unperturbed by continental landmasses, these islands feature a range of topographical profiles which are typically under-represented by NWP models. Previous field campaigns (e.g., DOMEX, BOMEX) and sub-kilometer scale numerical investigations have elucidated much regarding steady-state flow conditions and orographically-forced precipitation around these islands. Gaps in understanding of the impacts and physics of island-perturbed flows on multi-day timescales remain, however. This is especially true regarding the processes influencing the diurnal evolution of thermal and dynamical structures of the marine atmospheric boundary layer (MABL) within the island wake, which satellite and radar products often reveal can extend 100s of kilometers downstream.

Here, we discuss work examining the physical processes driving the diurnal evolution of the MABL downstream of the various islands of the Lesser Antilles using high-resolution (i.e., 1-2 kilometer horizontal grid increments) mesoscale simulations from the United States Navy's Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®*). Simulations cover the entire month of August 2023, coinciding with the first Intensive Operation Period (IOP) of the Moisture and Aerosol Gradients / Physics of Inversion Evolution (MAGPIE) Office of Naval Research Departmental Research Initiative field campaign. The extended simulation time-frame allows for novel examinations of the diurnal characteristics of the island wake MABL. Both uncoupled, atmosphere-only and coupled ocean-atmosphere configurations are employed to elucidate the role of interactive air-sea fluxes on the MABL structure. Results highlight the complex nature of the interactions between island terrain, cross-coastal diurnal circulations, feedbacks from the cloud morphology, and the environmental flow in shaping the characteristics of the wake MABL. Overnight, offshore cold air advection accelerated by the island orography reduces surface buoyancy, stabilizing the wake MABL. Surface wind stresses over the wake remain weak, dampening heat fluxes into the atmosphere. After sunrise, the combination of offshore advection of the deepening island ABL in addition to reduced mixing of the upper-ocean over the wake enhances MABL buoyancy. As the day progresses, the height of the peak island orography and the strength of the background winds determine where wake convective updrafts concentrate, and whether surface buoyancy is rapidly extinguished through convective cold pools, or long, thin cloud trails emanating downstream are sustained. This work enhances our understanding of complex physical processes across myriad scales, and has implications for improving NWP model forecasts.

*COAMPS® and COAMPS-OS® are registered trademarks of the U.S. Naval Research Laboratory