[THUNDER AND RAIN]
ED BENSMAN: So the main difference between
nighttime thunderstorms and daytime thunderstorms is the
mechanism behind what creates the thunderstorm itself. So I
think we're all familiar with a hot, humid, summer day and the
sun comes up and heats that moisture, evaporates it and that
moisture rises then it cools and condenses and begins to form
cloud matter and grows vertically and creates thunder
storms. So during the day these thunderstorms can grow to great
heights, say thirty or forty thousand feet even above that.
When those thunderstorms then rain out sort of in the late
afternoon or early evening, it tends to send out ripples or
waves if you want to think of it that way. And as the sun sets
and the ground begins to cool that mechanism of the sun
heating the ground no longer exists. So the layer of air near
the surface of the earth becomes very stable and there isn't that
lift mechanism. But what happens is the thunderstorms that grew
during the day tend to spread out and they become the forcing
mechanism that allows the evening thunderstorms then to
grow. There's still available moisture in the atmosphere and
these waves or ripples in the atmosphere interact with these
pools of moisture and then allow the thunderstorms to grow, but
rather than growing sort of from the surface or the lowest couple
thousand feet and above, they may grow sort of from the middle
layers of the atmosphere and grow vertically. The
thunderstorms at night can last longer, they can last for hours
at a time and cross, actually great distances. So this summer
we've initiated a field campaign called the Plains Elevated
Convection at Night, or PECAN experiment, and it's
specifically to go out and look at these nocturnal thunderstorms
and what causes them. And there are four main initiatives that
they're looking at. One is called the Afternoon and Evening
Transition or, AET, and that is where we go from the sun heating
the surface of the earth and driving those afternoon
thunderstorms versus the nighttime thunderstorms and that
transition from day to night is very important. They're also
studying something called Convective Initiation and that's
just the growth of thunderstorms vertically in the atmosphere,
and so that convective initiation is what initiates or
what causes those storms to grow. They're also studying the
low level jet stream, that's this jet stream of wind in the
lower atmosphere that's elevated off the surface that can drive
copious amounts of moisture from the Gulf of Mexico say up into
the Central and Northern Plains of the United States. They're
also looking at what's called, what are called mesoscale
convective systems, and so what these are is there tends to be a
preferential pooling of these thunderstorms that are initiated
at night. These clusters sort of grow preferentially to the air
around them, and so how these clusters of thunderstorms gather
and how they persist through the night is very important to
understanding and helping us model things better in computer
models. And, finally, the last thing is what are called bores;
if you've ever been outside in a thunderstorm, it may be a hot,
humid day and all of a sudden you get a burst of cool
refreshing wind and that cool refreshing wind is just the
rainfall falling out from that thunderstorm and initiates
downstream a wave of cooler air, rain cooled air if you will.
Over long distances that rain cooled air then not only hits
the surface and spreads out laterally, but it also interacts
with other areas that have been heated during the day. Long
distances downstream, that air can actually itself begin to get
lifted up above and so it might get lifted above the ground
level up to say, maybe one or two thousand or a few thousand
feet and persist there well into the night, and so it's these
bores of wind that become very important.
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