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:
- The observed (8.4 m s-1; Foghin-Pillin, 2016) and
modeled speed of propagation (8.3 m s‑1) are
consistent with that predicted from gravity current theory
(\(C_{\text{gc}}\)= 8.3 m s-1),
- there is a continuous cooling of the surface after the arrival of the
SBF,
- a permanent rise in surface pressure, and
- the wind changes direction with an increase in speed.
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)], whereN is 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.