Sea Breeze/Land Breeze and Differential Heating Introduction

Sea Breeze/Land Breeze and Differential Heating Introduction


Today we’re going to be talking about
how horizontal temperature differences play a role in driving vertical
circulations that could potentially support thunderstorm development, provide
that ingredient of lift that supports thunderstorms and these temperature
differences can be driven by land-sea interfaces and differential heating
rates, sloped terrain for instance where we get elevated heat sources, provide the
impetus for these vertical circulations that support the development of
thunderstorms. We’re going to start by talking about the horizontal temperature
differences that arise from differential heating rates and say this is a poorly
drawn sketch of Florida, let’s kind of adjust this a little bit, not drawn to
scale over here, Florida Keys here, and say we’re dealing with the diurnal
heating cycle. Here’s the Sun as it comes out, Atlantic waters and the Gulf
waters over here. We know that the heat capacity of water is around four times
that of the land areas and so when the Sun comes out, the land areas are going
to heat up much more so, much faster rate than over the surrounding waters and
as a result, we’re seeing that the temperature change over the land areas
is going to be far more sensitive to the solar input, the solar radiation input,
than over the water areas because of the differential heat capacity and so we’re
going to have a much more abrupt increase in temperatures over the land
areas that we have over the over the waters. Temperatures will stay nearly
similar over the water locations than the air temperatures over land and as a
result, temperatures increasing, the molecules of air over the land areas are
going to be effectively energized substantially more, there’s much more
kinetic energy, they’re going to be moving faster, dispersing more and more and so density in the
lower atmosphere will be decreasing and as a result, pressure will be decreasing
as well over the land areas, density staying nearly similar values over
the waters and pressure similar values over the water. So as a result, we get
effectively something over the land areas that appears to be
a thermal low begin to evolve over the land compared to over the water
locations. Again, that’s because the temperature, the heating rates are
going to be much more sensitive over land as we’re dealing with a much lower
heat capacity, four times lower over land than over the water locations and so as
a result, we get a pressure gradient force that is directed inland compared
to relatively higher pressure over the Gulf of, over the Atlantic Ocean,
Gulf of Mexico for instance as you can see over here, and as a result, we have a
flux of mass that is directed inland from the water locations just like this
over here. And at the leading edge of that flux of cooler, denser, higher
pressure air emanating over the waters inland, we get a convergence band that
forms along coastal areas and extends inland, that’s called a sea breeze. It’s
associated with the vertical circulation, a zone of ascent where we potentially
get storms form if the ascent extends deep enough such that parcels can get to
the levels of free convection and potentially have the development of
thunderstorms if other conditions are met as we talked about throughout this
class. This sea breeze boundary is going to be critical with time, convective
outflows, rain cooled air will merge with the sea breeze boundary and potentially
accelerate its inland advance and we get storms effectively cluster increasingly
over interior sections of the Florida peninsula. The sea breeze boundary exists
across many other coastal interfaces as well. During the night time, I’ll draw a
moon of over here to indicate what’s going on at night, and you can see here
Florida peninsula and here’s the panhandle, here the Keys, and we’re going
to be having much more substantial cooling rates over the land areas than
over the water owing to the differential heat capacity,
so we’re going to be much more sensitive to temperature changes over here with
outgoing long-wave radiation over here across the inland areas driving much
lower, or a much stronger cooling rate. A higher density over land, a higher
pressure over land compared to over the water locations which drives a relatively
higher area, or I should say an area of higher pressure over the interior
section, interior sections of the state for instance driving an outward
directed pressure gradient force across the water locations. And you can see
similar to what we have during the day time, we’ll end up forming a convergence band at the
leading edge of that colder air that’s going to be extending outward from the
land areas, and that is called a land breeze boundary, and that can sometimes
be a focus for thunderstorm development at night. But the difference between the
sea breeze boundary and its propensity for thunderstorm development during the
daytime compared to the land breeze boundary at night is that during the day
when we have the land areas heat up more substantially, we’re having much
more buoyancy provided, much more erosion of inhibition and so the thermodynamic
character over the land areas is going to be more favorably phased with that
ascent associated with this sea breeze boundary in support of storm development,
whereas that buoyancy is not going to be as favorable over water locations at
night, for instance, owing to the smaller changes in temperature over the water
than over the land. And as a result, once that convergence boundary extends over
the water areas, we’re going to have less propensity for storm development owing
to smaller buoyancy aligning with that boundary compared to over land areas
during the day. So now let’s consider and what’s going on over the mountain areas.
We know that mountains can sometimes be a source for thunderstorm development, or
at least individual locations where thunderstorm development is favored, but
what exactly is going on in support of these differences in heating rates that
could potentially support the development of thunderstorms over
mountains? Well let’s consider a mountain slope over here. We’ll consider during
the daytime, sun comes out, here’s our mountain slope over here.
And in the atmosphere, we know that density decreases, background density
decreases as we go up through the atmosphere. Gravity is pulling all the
air molecules down, they bunch up in the lower part, the lowest part of the
atmosphere closest to the ground and density is much higher near the
ground and then decreases as we go higher in the atmosphere. So with the sun
coming out during the daytime, we know that we’re going to have relatively
similar input of solar radiation along the slope of the mountain. It’s not going
to vary tremendously as we go along the mountain slope. We’re going to have similar
input of solar radiation, insulation, incoming solar radiation along this
mountain slope up high as down low, but the difference lies in the density along
that slope. Lower density up high, higher density down low. So as a result, we’re
going to have fewer molecules of air up here that are going to be
experiencing the same amount of solar radiation. So with the same input of
solar radiation, we’re going to be able to heat up much faster higher along the
mountain then down low. There’s going to be effectively, we’re going to be much
more sensitive to the incoming solar radiation in terms of heating up the top
of the mountain than we are at the bottom of the mountain slope. And so as a
result, we’re going to be driving again a density difference now, not along, you
know, this flat surface, but along this sloped surface over here such that our
density will be falling much faster up high then down low. We’re going to have
relative lower pressure, pressures will be falling much more so up high along
the mountain slope than down low and instead of having our pressure gradient
force extend during the day for instance with a sea breeze circulatio just
along our horizontal surface, we’re going to go along our sloped surface over here
towards the stronger pressure falls towards the top of the mountain where we
have strong heating rates owing to relatively less density towards the top
of the mountain, much stronger temperature increases at that elevated
heat source over here driving a flux of air up the mountain
slope that is providing that circulation over here, orographic
circulations that are associated with that upslope flow associated with the
possibility of that lift extending high enough towards the level of free
convection for thunderstorm development if we have enough moisture present and
we’re able to potentially get the development of storms from these
elevated heat sources as you can see here owing again to this difference in
density, in this case, for the sea breeze circulation over here, lower density over
land, land breeze, lower density over the water locations. Over here, top
of the mountain, elevated heat source, that’s driving that upslope flow over
here, and then which is what we call an anabatic flow, and then at
night, we’ll have relatively higher density store up at the top of the
mountain, which will descend down the slope in a katabatic flow and we’ll
downslope flow favored at night. The main point here is that these differences in
density that can be favored by differential heating or differential
cooling rates across land surfaces or between land and water can drive the
ascent ingredient, the lift ingredient, necessary for thunderstorm development
in some cases. Sometimes we’ll have a storm develop over a mountain location and
then that storm will advance off the mountain potentially encountering higher
inhibition over the lower elevations. This is sometimes an issue in Montana
where you get storms form over the northern Rocky Mountain Front Range vicinity and
those storms then move off the terrain, encounter higher inhibition over the
lower elevations potentially where there’s less lift, and storms would
dissipate or sometimes they will intensify as they encounter relatively
higher buoyancy over the lower elevations, so we have to consider the
thermodynamic profiles particularly that these storms will be encountering as
they develop. Sometimes storms will have developed and then interact with these
orographic circulations that could potentially result in influencing their
character, their behavior, their intensity, and their structure. And so all of these
much smaller scale circulations can play a major role not only in
terms of supporting storm development, but affecting storms that have already
occurred. And now the next thing we’re going to be talking about is the
mathematical derivations behind these vertical circulations, what’s going on in
the theoretical side in terms of how we can use mathematics to illustrate these
vertical circulations that support the development of storms.

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