V-waves Trailing from Channel Islands

V-shaped waves (or V-waves) have been frequently observed from space over the leeside of the Channel Islands. These waves often reveal themselves as a pair of diverging cloud-free rays emitting from an island when the area is covered with stratocumulus clouds. A couple of V-wave events have been simulated using the Navy’s Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) with the finest horizontal grid spacing down to 0.5 km. These simulations capture the observed V-waves accurately, with the simulated angle between the two wave beams comparable to that observed from space.

Further diagnosis shows that the weakly convective marine boundary layer (MBL) is relatively shallow and capped by a strong MBL top inversion. Accordingly, the island-induced perturbations are largely trapped in the MBL. The simulated wave structure is consistent with the linear hydraulic wave theory of Jiang and Smith (2000). The MBL flow is supercritical relative to the hydraulic wave speed, and the cloud-free wave beams are associated with the strong subsidence portion of the V-waves.

The wave angle (α) is given by the ratio between mean MBL winds and the shallow water wave speed, namely, sin(α/2)=U/C, where U is the mean wind speed in the MBL on the upwind side of the island, and C=(g'H)^1/2 is the shallow-water wave speed, g'=gΔθ/θ₀ is referred to as the reduced gravity, Δθ is the potential temperature increase across the MBL top inversion, and θ₀ is the mean MBL potential temperature.