(Computer-Aided Forecasting Exhibition)
Gravity Waves: What's the Attraction?
PART I: What are "Gravity Waves"?
Deep convection produces waves over a broad
spectrum of frequencies. Gravity waves are one of many methods the
atmosphere uses to transfer momentum (as well as temperature and
moisture), and in the process, attempt to establish an energy equilibrium.
They are essentially a damped harmonic oscillator, like a spring, but
acting in three dimensions instead of one. A better 2-D analogy is a rock
tossed into a still pond. The size of the ripples depends on the size of
the rock and the depth of the water.
These "ripples" can be caused by flow over a
mountain range, a downdraft hitting the ground, or an updraft penetrating
the tropopause (i.e., entering the stratosphere). They are forced by
buoyancy and damped by gravity (in fact, Gravity Waves are
more correctly called Buoyancy Waves). They propagate most readily in a
stably stratified layer... in other words, potential temperature must
increase with height in the layer in order for a gravity wave to
propagate within that layer. Since the atmosphere has no upper boundary,
the waves also travel upward. These waves will not be discussed at length
here, but it should be noted that these upward-propagating waves can be
reflected into the horizontal (ducting), allowing the amplitude of the
horizontal waves to remain greater than it would otherwise.
The basic process responsible for the propagation
of gravity waves the atmosphere constantly trying to achieve pressure
equilibrium. A parcel of air will travel from an area of high pressure to
an area of low pressure due to the "pressure gradient force". Let Point A
be a local high and Point B be a local low. Air will travel from Point A
to Point B, eventually causing Point A's pressure to drop and Point B's
pressure to rise (conservation of mass). In some amount of time, Point A
will become a local low and Point B will become a local high. With a
little imagination, suppose there are Points C, D, E, etc that also
undergo similar pressure fluctuations. So the crests (and troughs) of the
waves travel outward in all directions from the source like the ripples
of water in a pond.
Some of the fundamental physical characteristics of
gravity waves are not well-understood or well-documented, but in general,
here are some basic guidelines: they typically have a 1-15 millibar
amplitude (vertical displacement), a 50-500 kilometer wavelength, and a
period of 1-4 hours. One can then easily calculate the maximum range of
wave speeds from this... 12-500 km/h. The gravity waves created by
tropopause penetration usually generate the highest amplitude and highest
What sort of environment is most favorable for the
creation, development, and propagation of gravity waves? As mentioned
earlier, the layer mus be stably stratified. This is very important; if
the atmospheric layer of interest is unstable, gravity waves do not
travel through it as easily. Furthermore, strong wind shear aloft is
seemingly beneficial. Both of these ingredients are found near a dryline
or a frontal inversion. And since drylines and fronts are quite adept at
forcing convection, it makes it very easy for thunderstorms along these
boundaries to generate very powerful gravity waves. These atmospheric
ripples can sometimes travel across the country for 1500 kilometers or more.
One final point... how do gravity waves interact
with convection? Recall the amplitude of these waves can be 15mb
(sometimes more). Well, this can be enough to force parcels to rise
<fall> above <below> their LCL or LFC; i.e., they are simply
displaced upward <downward> from their previous position. So
basically, powerful high-amplitude gravity waves can force
<hinder> condensation and convection. They are also responsible for
dangerous clear-air turbulence. In fact, if the amplitude isn't large
enough or if the air is too dry, condensation will not be
induced, and the result will be completely oscillatory and invisible
turbulence. The clear-cloudy ripple effect has been observed on several
occasions using visible satellite
imagery. In the case of the May 27, 1997 Jarrell, TX tornado outbreak,
the supercells were largely enhanced by gravity waves released several
hours earlier by a storm system hundreds of kilometers away.
Not only can gravity waves force convection, but
convection can create and amplify gravity waves. As mentioned earlier,
updrafts and downdrafts colliding with some form of boundary create
gravity waves. But the processes in a thunderstorm help to amplify
existing gravity waves. Latent heat release provides the waves with
energy. The thunderstorm's environment amplifies the crests by means of
evaporative cooling (when precipitation encounters dry air, it evaporates
and cools the air), while the troughs are amplified by means of
compensating subsidence (sinking that warms the air to compensate for
cooling caused by evaporation).
Understanding gravity waves can help severe weather
forecasters better predict where an outbreak is likely to occur, and how
powerful it could potentially become.
Credit for various parts of this section go to:
Steven Koch, Howard Bluestein, James Holton, and Joseph Schaefer.
PART II: Using Knowledge of Gravity Waves in Operational Forecasting
Now that you know what gravity waves are, how does
one detect them and use them in forecasting? Two key methods are
microbarographs and satellites.
A microbarograph is a sensitive instrument that can
record pressure fluctuations with 0.001 millibar precision. Armed with
this tool, forecasters can detect minute pressure changes associated with
the waves this is especially useful if there are no visible signs of the
waves (i.e., condensation).
Visible satellite imagery has 1-kilometer
resolution (from operational geosynchronous satellites such as GOES). This
is only useful if the gravity waves are strong enough to force parcels
above their LCL or LFC. If there are any clouds induced by the waves,
there's a good chance visible satellite imagery will detect them.
The forecaster now knows the extent and amplitude
of a series of gravity waves racing across a region. How is this knowledge
useful for predicting where and when thunderstorms will break out (or of
they will)? If one knows the ambient LFC and the amplitude of the
oncoming waves, one can predict whether or not the waves will induce
convection and where this is likely to occur. The easiest example to
understand is gravity waves approaching a dryline. A dryline is an
unstable boundary that typically only needs a little "trigger" to
initiate convection (a sharp temperature/dewpoint gradient). That trigger
can come in the form of gravity waves. This is precisely what happened in
the Jarrell, TX tornado outbreak on May 27, 1997. Gravity wave-induced
condensation ripples were traveling SSE across Texas, and by the early
afternoon, they encountered a substantial dryline. In less than one hour,
towering thunderstorms achieved severe limits and spawned numerous
tornadoes that day, including several F5's.
Credit for various parts of this section go to:
Steven Koch and Tobias Kerzenmacher
Brian McNoldy for MESO