Impacts of the penetration of the sea breeze on the behavior of surface variables at a selected location (8ºN– 66.5ºW) along its axis of propagation L, can be assessed from Fig. 9, where starting at 1900 LST with the arrival of the SBF, there is a sudden increase in wind speed and a change in its direction from ENE to a less zonal NE (Fig. 9a). The southward advance of the sea breeze causes the rapid increase in the negative meridional component of the wind (–\(\upsilon\)) and the decrease in magnitude of the easterly component (–\(u\)). At the same time, the mixing ratio content and surface pressure exhibit a similar increasing rate of change, while the potential temperature continues dropping with a slight variation on its rate of change (Fig. 9b).
Therefore, the Unare sea breeze exhibits all the characteristics of a gravity current as proposed by Simpson (1987), Koch & Clark (1999), and Koch et al. (2005), namely:
Some of the instantaneous or short-lived shifts (i.e., rises or drops) expected in temperature and surface pressure upon the arrival of the SBF, cannot be determined due to the current specifications of the model. An even finer spatial and temporal resolution (< 1-km grid spacing and one-minute outputs) is needed to analyze this phenomenon with enhanced detail.
Stratified fluid dynamics also predicts that the intrusion of a gravity current into a less dense, two-layer, stably stratified fluid system, as the one existing ahead of the SBF at 1900 LST (Fig. 8d), may generate wave-like perturbations (e.g., bores, solitons), as occurs in Australia’s Gulf of Carpentaria (e.g., Clarke et al., 1981; Goler & Reeder, 2004; Reeder et al., 2013; Smith et al., 1982) and other parts of the world (e.g., Coleman et al., 2010; Koch & Clark, 1999; Tsai et al., 2004; Watson & Lane, 2016). However, the Unare sea breeze does not spawn such perturbations because the depth of the gravity current is much larger (~ 5 times) than the thickness of the stable-stratified boundary layer being perturbed. In this case, the latter is mixed into the gravity current, causing it to behave as if the lower layer does not exist (Simpson, 1987).
Furthermore, according to Koch & Clark (1999)—based on the modeling work of Haase & Smith (1989)—one of two parameters that determine if a bore can be generated from the intrusion of a gravity current is the ratio (\(\mu\)) of the long gravity wave phase speed (\(C_{o}\)) to the gravity current speed (\(C_{\text{gc}}\)) [Eq. (3)], whereis the Brunt-Väisälä frequency, and \(h_{o}\) is the depth of stratified-boundary-layer inversion.
\begin{equation} \mu=\ \frac{C_{o}}{C_{\text{gc}}}=\ \frac{\frac{2Nh_{o}}{\pi}}{C_{\text{gc}}}\ >0.7\nonumber \\ \end{equation}
For the Unare sea breeze, the calculated nondimensional value of\(\mu=0.28\) indicates that the flow is in the “supercritical regime” where the gravity current propagates faster than any gravity waves so that no bores can be generated. The ratio \(\mu\) is calculated for 2000 LST when the stratified boundary layer ahead of the SBF is well established.