Catch a Wave!
As knowledge in the fields of oceanography and meteorology increase it becomes more and more evident the two are closely related. Many of the same principles apply to both the atmosphere and the sea. Gravity waves are perfect examples, as they are seen in both water and air.
Gravity waves are harmonic impulses or “ripples” that can move vertically and horizontally through air or water. Gravity waves have two simple elements. Gravity is one element of the impulse. The other is buoyancy. In fact, gravity waves are more properly known as buoyancy waves because it is buoyancy, or the tendency to rise to the top, or float, that gets things started.
Gravity serves as the downward element. Although gravity waves occur at all levels of the sea and atmosphere, letís stick to atmospheric gravity waves in the troposphere to limit confusion. Besides, tropospheric gravity waves affect the business of operational forecasting the most.
Gravity waves are created in a number of ways; any mechanism that displaces air vertically is a source. Differing temperatures and densities of adjacent air parcels create movement, and terrain features disrupt flow and create eddies and whirls. More complex interactions that create disruptions are convective, volcanic, and geomagnetic. Geomagnetic sources do not generally impact the lower atmosphere, so Iíll spare you the details on that one.
When temperatures and densities of parcels differ, they “jostle” around seeking equilibrium. Colder parcels sink, warmer parcels rise, pressure gradient between parcels of differing density cause flow from higher to lower pressure to begin. Frontal activity, mainly cold fronts, gust fronts, squall lines and occlusions, create gravity waves by forcing warmer air aloft. Convection lifts parcels and displaces them and both volcanic activity and nuclear detonations produce sonic and debris laden heat bombs that shake up everything around them. Bingo, you have waves.
Gravity waves can propagate for a very long way under the right circumstances. They travel best and longest in a stable layer bounded above and below by unstable layers. This is a process called ducting and much of the energy a wave contains is preserved as it propagates downstream. These waves do not break. Those that escape the bounded layer do break, release energy into the upper layers, and dissipate in short order.
Things that cause atmospheric gravity waves:
- Orographic uplift over mountains or other higher terrain (lee waves)
- Penetrative convection (thunderstorms with overshooting tops) during the growth phase of thunderstorm development
- Subtropical westerly jet stream (radiates waves both in the troposphere and the stratosphere)
- Synoptic disturbances (fronts, lows of all intensities from minor depression to full- blown hurricanes and mid latitude storms).
- Volcanic eruptions and nuclear detonations
So why study gravity waves? They are difficult and time-consuming to study and the tools to do so effectively are only just now being fielded. Some of the tools we already have out there are being used now in new ways, such as the WSR-88D Doppler radar. Even with these new tools, it is hard to tinker forecast models to take gravity waves into account, but the impact they can have makes it important to try.
Here are two examples that highlight this need. In one case (Bosart and Seimon, 1988), rapid pressure falls with a strong gravity wave fooled forecasters into thinking a secondary low was developing along a coastal front when it wasnít. They made their forecasts based on this misconception. You can guess what happened; blown forecasts.
Illinois, December 1987 (Schneider, 1990); forecasters saw that the minimum pressure associated with a gravity wave trough was lower than that the synoptic scale cyclone through which it passed. The region of pressure falls with the wave was incorrectly interpreted to signal a direction and intensity change in the synoptic low, because the forecasters did not know about the gravity wave. They tried to adjust the forecast models of the synoptic storm and: another blown forecast.
Another reason we need to look at gravity waves is that they are both caused by and cause severe weather events. For example, when gravity waves “break” (break down abruptly), they create strong, sometimes severe turbulence that can be hazardous to aircraft. Even those that donít break cause turbulence. Think about the waves created by strong wind flow perpendicular to mountains or hills. That flow creates gravity waves, which create strong, sometimes severe turbulence that buffets the airframe and irritates the pilots.
Gravity waves and convection have an interesting relationship. Convection creates gravity waves, especially thunderstorms with overshooting tops, but waves from other sources can also cause convection to either develop explosively or fizzle like a dud firecracker. How they interact depends on the phase of the gravity wave at the time of intersection. A gravity wave can fire a squall line ahead of a cold front or turn a garden-variety thunderstorm into a supercell. On the other hand, a gravity wave can make convection collapse even when it gives every indication of developing quickly because the downward-moving part of the wave overlies the activity.
Lee waves that develop in the lee of mountains are standing waves (they donít move) but their effects are seen well downstream of their point of origin. When there is enough moisture available, this is evidenced by rotor clouds that appear at regular intervals downwind of a mountain/range. Rotor clouds appear at intervals downstream if they are above the lifting condensation level (LCL) because, as the waves travel downwind, they also oscillate vertically (up and down) in a sinusoidal pattern.
In the upward phase, cloud develops convectively and in the downward phase, it is suppressed or dissipated. Lee waves can also reintensify passing lows and develop frontal systems with the newly refurbished storm that can then bring considerable precipitation. Lee waves that interact with dry lines in the American Midwest from Texas to the Canadian border cause explosive thunderstorm growth and severe weather can result.
Understanding this relationship can make a forecasterís mental alarm bells go off when he or she sees a dry line and recognizes synoptic conditions are right for a lee wave to interact with it. It can go a long way to explaining why severe convection sometimes develops when conditions do not apparently support it and put a new tool in a forecasterís predictive toolbox.
Here are a few interesting tidbits about gravity waves. For an organized storm system, the strongest upward motions associated with gravity waves occur just behind the surface pressure trough and lead to maximum precipitation rates just ahead of the ridge. Gravity waves typically form within or near the back edge of a precipitation shield. Recent studies indicate that gravity waves may occur as often as a third of the time in the central US during the winter months because the jet stream is at its farthest south and is at its strongest. This is part, but not all, of the explanation for the higher frequency of big storms in winter.
Okay, while the explanations you have just read are very simplified, they do hit the high spots. Hopefully, they give you an idea of what gravity waves are and why they matter to every day forecasts. We can apply knowledge of gravity waves to improve our forecasts and do a better job for our customers. Now that you know what to look for, take another look around. Maybe you can catch a wave!
Written by Melody L. Higdon, 14 WS/DOPA