Unsteady Land-Sea Breeze Circulations in the Presence of a Synoptic
Pressure Forcing
Abstract
Unsteady land-sea breezes (LSBs) resulting from time-varying surface
thermal contrasts Δθ(t) are explored in the presence of a constant
synoptic pressure forcing, Mg, when the latter is oriented from sea to
land (α=0°), versus land to sea (α=180°). Large eddy simulations reveal
the development of four distinctive regimes depending on the joint
interaction between (Mg, α) and Δθ(t) in modulating the fine-scale
dynamics. Time lags, computed as the shifts that maximize correlation
coefficients of the dynamics between transient and the corresponding
steady state scenarios at Δθ=Δθmax, are found to be significant and to
extend 2 hours longer for α=0° compared to α=180°. These diurnal
dynamics result in non-equilibrium flows that behave differently over
the two patches for both α’s. Turbulence is found to be out of
equilibrium with the mean flow, and the mean itself is found to be out
of equilibrium with the thermal forcing. The sea surface heat flux is
consistently more sensitive than its land counterpart to the
time-varying external forcing Δθ(t), and more so for synoptic forcing
from land-to-sea (α=180°). Hence, although the land reaches equilibrium
faster, the sea patch is found to exert a stronger control on the final
turbulence-mean flow equilibrium response. Finally, vertical velocity
profile at the shore and shore-normal velocity transects at the first
grid level are shown to encode the multiscale regimes of the LSBs
evolution, and can thus be used to identify these regimes using k-means
clustering.