Tag Archives: turbulence

Low-Level Turbulence Climatology

Note:  I originally wrote this article for the General Aviation Council of Hawaii Winter 2013 newsletter. The results weren’t quite what I expected, but still proved interesting. Instead of highlighting where turbulence occurs, the pilot reports instead highlighted areas where aircraft tend to fly (i.e., Oahu, mainly with reference to HNL or CKH). –JB

In the Winter 2012 newsletter, I talked about mechanical turbulence and mountain waves. Moderate turbulence is the most common reason for an AIRMET around the Main Hawaiian Islands. In the 12 year period from 2001 to 2012, an AIRMET for turbulence was in effect for at least a portion of the day for over half of the time. By contrast, an AIRMET for mountain obscuration/IFR conditions was in effect for less than a quarter of the time, and an AIRMET for icing was in effect for less than five percent of the time.

Even though turbulence is common, there is little specific information available as to where it occurs (other than “over and downwind of the mountains”). The National Weather Service in Honolulu will begin a project this summer to quantify where turbulence is most likely to be encountered. By taking pilot reports of turbulence and sorting them based on atmospheric stability and low-level wind fields, we will be able to map where turbulence occurs during different weather patterns.

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Mountain Turbulence

Note:  I originally wrote this article for the General Aviation Council of Hawaii Winter 2012 newsletter.  Hopefully you will find it interesting and educational as well. –JB

Two common types of turbulence associated with mountains are mechanical turbulence and mountain waves. Mechanical turbulence is a result of an obstruction to the wind flow. Obstructions can range in size from trees and buildings to rough terrain and mountains. The degree of the turbulence depends on the strength of the wind speed and the size and shape of the obstruction. The stronger the wind or the rougher the terrain, the stronger the turbulence will be.

Mechanical turbulence usually occurs within 20 miles of the mountain, and is located at an altitude near or below the height of the terrain. As a rough estimate, low-level winds of 20 knots may lead to light turbulence, winds of 25 knots may lead to moderate turbulence, and winds greater than 30 knots may lead to severe turbulence.

Trapped lee waves downwind of a mountain range. Image courtesy of COMET/UCAR

Trapped lee waves downwind of a mountain range. Image courtesy of COMET/UCAR

For mountain waves, there are two main types: trapped lee waves and vertically propagating mountain waves. Mountain waves are a type of gravity wave, meaning that they are forced to oscillate because of gravity. (Waves on the ocean are another type of gravity wave.) When strong winds blow across a mountain, the air is forced upward by the terrain. If the atmosphere over the mountain is stable (that is, if the temperature of the air increases with height, which is typically the case in Hawaii due to the trade wind inversion), then the air that is forced upward by the mountain will be more dense than the air around it. The air will sink back toward the ground, where it will begin an up and down oscillation. These oscillations can continue for over 50 miles downstream of the mountain, and are known as trapped lee waves.

Visible satellite image of wave clouds over and downstream of Oahu under strong southwest winds.

Visible satellite image of wave clouds over and downstream of Oahu under strong southwest winds.

If the atmosphere has enough moisture, the trapped lee waves may be visible as wave clouds. Wave clouds form near the crests of the trapped lee waves, and appear as distinct lines that are oriented parallel to the terrain and perpendicular to the wind. While the clouds may appear to remain stationary, the wind blowing through them is actually quite strong.

Trapped lee waves tend to form when the low-level flow is perpendicular to the mountain range, there is an inversion located near the top of the ridge, and ridge-top winds are 25 knots or greater. Wave clouds typically form at an altitude within a few thousand feet of the ridge top. Turbulence is most often encountered below the crests of the mountain waves, or, in other words, below the wave clouds if they are present.

Trapped lee waves are the most common type of mountain wave in Hawaii, because of the persistence of the trade wind inversion. However, if the atmosphere is unstable, vertically propagating mountain waves may form. In this situation, there is no inversion for the mountain waves to reflect off of to begin the oscillation. Instead, the mountain waves will spread upwards through the atmosphere, and also tilt upstream in the direction from which the wind is blowing. Vertically propagating mountain waves may extend through the troposphere and even into the stratosphere.

The presence of vertically propagating mountain waves does not necessarily mean that there will be turbulence. If the amplitude of the waves becomes large enough, they become unstable and break–just like ocean waves–which leads to turbulence. If the waves don’t break, an aircraft may encounter significant wave action, but not the severe to extreme turbulence that may be encountered within breaking mountain waves.

The conditions necessary for mechanical turbulence and mountain waves are similar: strong winds blowing across a mountain. Whether the cause is mechanical turbulence or mountain waves, the results can be the same: turbulence. As a basic rule of thumb, be alert for turbulence if low-level winds are 25 knots or greater. The National Weather Service will issue an AIRMET if moderate turbulence is expected, and will issue a SIGMET if severe turbulence is expected. When an AIRMET or SIGMET is in effect, the forecaster will also provide the reasoning behind those products in the Area Forecast Discussion. These products (and others) are available through the aviation page of the WFO Honolulu website.

John Bravender
Aviation Program Manager
National Weather Service Honolulu

A Collaborative Turbulence Observation Project

Note:  I originally wrote this article for the General Aviation Council of Hawaii January 2015 newsletter, hence the aviation focus.  Hopefully you will find it enjoyable and educational as well. –JB

As the forecaster who oversees the aviation weather program at WFO Honolulu, I’m always looking for ways to work with the aviation community to improve the services we provide. Both meteorologists and pilots can benefit from more detailed observations of turbulence around the islands. In this article I’ll outline an idea for a “citizen science” project that could do just that.

Visible satellite image showing wave clouds over and to the southwest of Oahu. 7am HST, August 18th, 2012.

Visible satellite image showing wave clouds over and to the southwest of Oahu. 7am HST, August 18th, 2012.

One thing that forecasters struggle with is how far away from the terrain that turbulence occurs. As mentioned in the January 2012 article on mountain waves, we use general proxies to determine when turbulence may occur—for example, low-level winds of 25 knots or greater—and correlate these with pilot reports. PIREPs are very important in this process, because they are the only way we have to know what is actually occurring. However, PIREPs are usually provided at just one point and the pilot is looking to get out of the turbulence as fast as possible, not trying to see how far it extends. Satellite imagery can help in some instances, but the cloud features that indicate turbulence—such as wave clouds extending downstream of the mountains, as seen over and to the southwest of Oahu in this image—aren’t always present. We need some way to get more information about where turbulence occurs.

Aircraft reporting eddy dissipation rate (EDR) from December 24th, 2014.

Aircraft reporting eddy dissipation rate (EDR) from December 24th, 2014.

Increasingly, large aircraft are being equipped with instrument packages that include accelerometers. An algorithm was developed that uses the vertical acceleration of the aircraft to estimate a parameter called eddy dissipation rate (EDR). It provides an objective measure of how much turbulent motion the aircraft encounters, and is continuous along the flight track. The adjacent image shows a number of flights reporting EDR from December 24th, 2014. The blue and orange dots indicate no turbulence, magenta indicate light to moderate turbulence, and red (like the one circled) indicate moderate to severe turbulence. However, there is no need to resort to expensive new equipment to get this type of measurement. Many of us have smart phones that can do the same thing.

Map of bicycle "turbulence" measured using a smartphone accelerometer.

Map of bicycle “turbulence” measured using a smartphone accelerometer.

The adjacent map shows a short bicycle ride that I took using the accelerometer in my phone to measure the bumpiness of the route. The larger orange and red circles are bumpier conditions: potholes in the lower right, speed bump in the upper left, and embedded reflectors in the lower left. (The data were plotted using GPS Visualizer). Something similar could be used in aircraft as well. By using an app that logs GPS, altitude, and acceleration, we can get a measure of how much turbulence occurred and where it occurred during the course of a flight. We also need to normalize the data. (What does “20 [units]” mean after all?) For the commercial aircraft mentioned above, they use a mathematical model to determine how the aircraft responds to turbulence. However, this can also be done in a subjective manner. The pilot can take the flight track and highlight a few points—it was smooth in these areas, I had light turbulence here and here, and at this point I encountered moderate. With a few reference points, we can apply those values to the rest of the route and get turbulence observations for the entire flight track.

This would be an immense help to forecasters, since it will show us in much greater detail where turbulence occurs and how it changes across the area. The information can also be made available to other pilots (as an aggregate product, stripped of identifying information) in near real time as a flight planning tool, as well as part of a climatology that can help new pilots learn of dangerous areas to avoid.

However, this is just an idea right now—which is why this article includes a bicycle track instead of a flight track. Eventually we’ll be looking for pilots who want to participate by measuring data as they fly. In the meantime I would love to hear comments or suggestions. Is this something that is feasible? Is it something that you’d be willing to do on a regular basis? As I mentioned, this is just the starting point of the process, and now is one of the best times to share your insight and expertise to help make it successful.

Thanks for your help!

John Bravender
Aviation Program Manager
National Weather Service Honolulu