Gravity Waves Are Just Waves As Pressure He Was Observing

FlightOPS

Nearly impossible to predict and difficult to detect, this weather phenomenon presents Gravitya hidden risk, especially close to the ground.

regory Bean, an experienced pi- National Weather Service observer re- around and land back at Burlington. lot and flight instructor, says he ferred to this as a “gravity wave.” Neither The wind had now swung around to wasn’t expecting any problems the controller nor Bean knew what the 270 degrees at 25 kt. At this point, he while preparing for his flight weather observer was talking about. recalled, his rate of descent became fromG Burlington, Vermont, to Platts- Deciding he could deal with the “alarming.” The VSI now pegged at the burgh, New York, U.S. His Piper Seneca wind, Bean was cleared for takeoff. The bottom of the scale. Despite the control checked out fine. The weather that April takeoff and initial climb were normal difficulty, he landed the aircraft without evening seemed benign. While waiting but he soon encountered turbulence “as further incident. Bean may not have for clearance, he was taken aback by a rough as I have ever encountered.” He known what a gravity wave was, but report of winds gusting to 35 kt from wanted to climb to 2,600 ft. With the he now knew what it could do to an 140 degrees. The air traffic control- vertical speed indicator (VSI) “pegged,” airplane.1 ler commented on rapid changes in he shot up to 3,600 ft. Finally control- Gravity waves are just waves as pressure he was observing. He said the ling the airplane, Bean opted to turn most people normally think of them.

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BY ED BROTAK

Waves

Mountain waves can produce lenticular clouds.

Waves in the ocean are technically grav- waves. Some oceanic circulations Various disturbances produce gravity ity waves, for example. The “gravity” would also fall into this category. waves in the atmosphere. part of the term probably confuses the A gravity wave is the result of a How do gravity waves form? To start, meaning outside the field of meteorol- physical disturbance in an otherwise air must be physically forced up, form- ogy. Gravity is only one of the forces flat fluid flow. You can see waves in ing the crest of a wave. If the environ- that affect waves. Getting a little into water because the water itself is vis- ment is stable, this displaced air will the science here, gravity waves are ible. You can’t directly see waves in air tend to come back down to its starting physically induced waves, as opposed because air is invisible. A simple way of or equilibrium point. A stable atmo- to thermally induced waves. Thermally visualizing gravity waves, however, is to sphere is usually one in which a cooler induced waves are the result of tem- think of what happens when you throw layer of air sits under a warmer layer. perature and density contrasts. Most a rock in a pond. Waves move out in all The displaced air will start to sink and standard weather systems both at the directions from the disturbance. The gain downward momentum. Due to the

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starting elevation. This forms the trough associated with the jet stream, the for- are strongest just ahead of the trough. of the wave. But thermodynamic forces mation of a front or low, thunderstorms What does this mean in terms of flying — or buoyancy — will make the air rise and hurricanes. All of these events can conditions? Obviously, the updrafts and again toward its original level. If it over- produce gravity waves. These waves of- downdrafts would directly affect the ver- shoots again, this time on the upside, ten are damped out in the atmosphere tical motion of an aircraft. This is even another wave crest is formed. Usually and pose no problem. Other times they more of a concern if convection is also numerous waves in a set, an undulating can propagate for hundreds of miles. initiated, which would exacerbate the “wave train,” will be formed. Stable parts of the atmosphere are best lifting and sinking motions of the air. For pilots, the best-known atmo- for propagating waves. Moderate to extreme turbulence spheric gravity waves are the mountain Gravity waves are relatively small- could be expected. In addition, hori- and lee waves formed when winds blow scale features, with wavelengths vary- zontal air motions would increase, lead- across mountainous terrain. The hori- ing from roughly 5 to nearly 300 nm ing to strong and quickly shifting winds. zontal vortex, or rotor, which can form (approximately nine to 555 km). The The maximized lift area often could be immediately on the lee side is associ- period or time between waves in a set associated with heavier precipitation ated with extreme turbulence. These can vary from minutes to hours. They rates and subsequent decreases in vis- rotors are a threat to even large aircraft, can at times move very quickly, up to ibility. And, with gravity waves com- and numerous accidents have been as- about 80 mph (130 kph). The pres- monly occurring at low levels, these sociated with them. Often these waves sure amplitude — pressure change you effects would be maximized close to are indicated by specific cloud forma- would see on a barograph trace — can the surface, such as during takeoffs and tions such as lenticular or rotor clouds. vary between a few tenths of a millibar landings, with the greatest potential for If the air is drier, the danger zone may to more than 10 millibars (0.3 in Hg) the waves to affect aircraft flight path. not be clearly visible. However, these for the strongest waves. Forecasting gravity waves is ex- waves tend to be stationary or propa- A gravity wave’s effect on sensible tremely difficult. Computer models gate downstream with the wind. Thus, weather makes it important to meteo- handle large-scale features very well they normally can be avoided. rologists and aviators. The undulating but have trouble with factors as small What is usually referred to in me- waves and their interaction with the as gravity waves. The grids used by the teorology as a “gravity wave” is a mobile prevailing wind field instigate vertical models usually are too big to identify wave in the atmosphere, which pre motions in the atmosphere. Lifting mo- gravity waves. Another problem is that sents more of a threat. Gravity waves tions are maximized just ahead of the false indications of gravity waves can have a multitude of causes: wind shear wave crest (Figure 1). Sinking motions be generated by the models themselves. Very often, mathematical filters are used to eliminate gravity wave indica- Cross-Section of a Gravity Wave tions so the models can run efficiently.

Rising air Sinking air Meteorologists usually can determine situations where gravity waves Wave crest are likely. Even then, it is impossible to know if the waves will produce signifi- cant weather. On the other hand, once a Wave motion wave or set of waves gets started, meteorologists can track them and determine Wave trough speed and direction of movement. This information then can be used to make H L short-term forecasts to warn those pressure Convergent ow pressure Divergent ow Surface downstream of the coming weather. What gravity wave situations should Source: Ed Brotak pilots look for? The classic winter situ- Figure 1 ation involves gravity waves occurring

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north of a warm front and northeast Cold Season Gravity Waves of the surface low (Figure 2). Strong jet stream winds well aloft are to the southwest. The gravity waves propagate Jet stream northward within the sloping frontal Gravity wave zone layer above the ground. In these situations, the effects on sensible weather are L often profound. Surface low and fronts For example, a major winter storm Strongest winds was affecting northern Illinois and Chi- cago O’Hare International Airport. Light to moderate snow was falling with visibility reduced to ½ mi (0.8 km). Winds Source: Ed Brotak were blowing steadily from the northeast at 20 kt (37 kph). Then a gravity wave Figure 2 began to affect the region. Within minutes, the visibility dropped to 1/16 mi kt (93 kph). This variation was enough analyses are conducted, gravity waves (0.1 km) in heavy snow. Thunder was to convince the crew to conduct a missed may have not left any clue about their heard. Winds gusted to 56 kt (104 kph). approach. Researcher David W. Miller presence. Many such encounters go Gravity waves generated by thun- analyzed meteorological radar data rel- unreported in this situation. Clear air derstorms or thunderstorm complexes evant to the accident. At the time of the turbulence may be acknowledged but develop in a similar manner. In these crash, thunderstorms were at least 10 nm not cited as a causal factor. cases, cooler air from the thunderstorm (18 km) away from the crash site. Miller Until forecasting techniques improve, downdrafts hits the ground and spreads concluded that gravity waves generated the best defense that meteorologists can out, the leading edge of this outflow by the storms were the likely cause of the offer against the adverse effects of grav- forming the familiar gust front. Behind loss of control.2 ity waves is better detection. The first this feature, a sloping frontal surface Gravity waves can interact with exist- step in this is simply educating the avia- separates cooler air near the ground ing low-level boundaries and initiate new tion industry about the problem. The from warmer air above. Gravity waves convection. Some theories even sug- next step is for the aviation community can develop and propagate within the gest that gravity waves interacting with to be alert for the situations in which cooler, stable air. already severe thunderstorms can help gravity-wave generation is most likely. Often no clouds are associated with produce tornadoes. Offshore hurricanes The changes in pressure and wind due the gravity waves themselves, so they can can generate gravity waves that can move to gravity waves are measurable with approach unannounced to pilots. On ashore and produce thunderstorms. the meteorological instruments we now Dec. 6, 1998, a Beech Baron crashed dur- With such a variety of sources, grav- have if we know what to look for. ing approach to the airport at Norman, ity waves are common weather features Notes Oklahoma, U.S., killing the pilot. The around the world. From the cold cli- National Transportation Safety Board mates to the tropics, gravity waves may 1. Bean, Gregory. “Gravity Waves: A Pilot’s concluded that severe turbulence and be encountered. In fact, gravity waves Perspective.” AOPA Online: Never Again, wind shear were the probable cause of the in some form probably are omnipresent. April 8, 2004. loss of control. Pilot weather reports from Fortunately, most are fairly weak and 2. Miller, David W. “Exploring the Pos- nearby aircraft were part of the support don’t present a threat to flight crews. sibility of a Low Altitude Gravity Wave for this conclusion. The flight crew of a It is difficult to say how big a prob- Encounter as the Cause of a General Aviation Accident near Norman OK on Boeing 737 approaching Oklahoma City’s lem gravity waves are for the aviation Dec. 6, 1998.” A presentation to the 9th Will Rogers Field, about 30 mi from Nor- industry. As noted, they can be quite dif- Conference on Aviation, Range, and man, near the time of the crash reported ficult to detect. Thus, when incidents or Aerospace Meteorology, September 2000, encountering wind speed variation of 50 accidents occur and detailed post‑event Orlando, Florida, U.S. www.flightsafety.org | AeroSafetyWorld | February 2010 | 35