The Diurnal Cycle of Land-Sea Breeze Circulations in the Presence of a Synoptic Pressure Forcing

The diurnal cycle of land-sea breezes (LSBs) resulting from time-varying surface thermal contrasts Δθ(t) are explored in the presence of a constant synoptic pressure forcing, M_g. Two orientations of the synoptic wind are compared: from sea to land (α=0°) versus from land to sea (α=180°). Large eddy simulations are conducted with a constant sea surface temperature, but a diurnally varying land temperature. The results reveal the development of four distinctive regimes depending on the joint interaction between (M_g, α) and Δθ(t) in modulating the fine-scale dynamics. The direction of the synoptic forcing plays a key role in the dynamics. For example, time lags — computed as the shifts that maximize correlation coefficients of the dynamics between transient and the corresponding steady state scenarios at Δθ=Δθ_max — are found to be 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. The mean flow is found to be out of equilibrium with the thermal forcing, and the turbulence fields are also shown to be out of equilibrium with the mean fields. 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. For the highest simulated M_g blowing from sea-to-land (α=0°), the simulations show one case of equilibrium unlike α=180° where the hysteretic response is shown to strengthen. Finally, vertical velocity profile at the shore and shore-normal velocity transects at the first grid level are used, respectively, to examine the multiscale regimes of the LSBs evolution using k-means clustering.