Tuesday, January 6, 2009

Is an Unexpected Wave a Rogue?

This YouTube clip is labeled "freak wave" but is this a freak (rogue) wave, or not? (The wave is at the very end of the clip)

The Ocean Prediction Center (OPC) defines a rogue:
They are generally considered to be unexpectedly high waves which in some instances come from a direction different from the predominant waves in the local are
The wave that came aboard was not much bigger then the ones previously encountered. An experienced mariner should not have been surprised to ship water in that sea. In fact the informed mariner should expect an occasional wave twice as high as stated in the forecast (for details, see below). So - no rogue in this case.

Here is another example of a boat being struck by an unexpected wave, in this case fishing vessel The Aleutian Ballard jogging in reported 40 kts seas and 60 kts winds in the Bering Sea - this clip is from Deadliest Catch:

In this case the wave that struck from the starboard does seem larger then the sea at the time and it apparently came from an unexpected direction.
Was it a rogue? A case could be made that it was but it is not uncommon during heavy weather to encounter two separate swell systems. The boat may have been struck by a breaking wave from a second, larger, swell system.

An unexpected wave is not necessarily a rogue (as seen in the case of the yacht). This raises the question, what size waves should a mariner expect to encounter at any given time? The answer is, as stated above, we should expect an occasional wave twice as high as stated in the forecast.

From the OPC again:
The wave height most commonly observed and forecast is the significant wave height. This is defined as the average of the one third highest waves. The random nature of waves implies that individual waves can be substantially higher than the significant wave height. In fact, observations and theory show that the highest individual waves in a typical storm with typical duration to be approximately two times the significant wave height.
For a specific example from the NWS (PDF file) :
a forecast of 10-foot seas in open waters means a mariner should expect to encounter a wave spectrum with many waves between 6 and 10 feet along with a small percentage of waves up to 16 feet and possibly even as large as 20 feet!
So in a typical storm we can expect a wave approximately twice as high as the height used in the forecast. So what is a rogue?

A rogue wave estimated at 18.3 meters (60 feet) in the Gulf Stream off of Charleston, S.C. At the time, surface winds were light at 15 knots. The wave was moving away from the ship after crashing into it moments before this photo was captured. Photo from NOS

A common definition of a rogue is a wave that is more then twice the significant wave height this is the definition found in Wikipedia.

Digging a little deeper and looking at some other definitions of a rogue wave, the photo above comes from this National Ocean Service site. They define a rogue as follows:
Rogues, called 'extreme storm waves' by scientists, are those waves which are greater than twice the size of surrounding waves, are very unpredictable, and often come unexpectedly from directions other than prevailing wind and waves.
This makes the case for calling the wave that struck the Aleutian Ballard a rogue stronger. We don't know if the forecast included a large swell from that direction but in any case it is not possible to predict when a wave in the open sea will break.

The weather services intended audience for forecasts is the mariner. Scientist and engineers are interested in the phenomena of rogue waves as well. They create and use their own definitions.
For example the book "Rogue Waves" By Michel Olagnon, Marc Prevosto differentiates between what they call classic extreme waves and rogue waves.

Classic extreme waves are rare members of a population of wave events defined by modeling the surface process as a piecewise stationary and homogeneous slightly non-Gaussian random field.

In other words classic extreme waves are rare waves that fit inside the forecast model.

Freak (rogue) waves are defined as:
A freak wave event is an event that represents an outlier (crest height, wave height, steepness or group of waves) when seen in view of the population of events generated by a piecewise stationary and homogeneous second order model of the surface model.
In other words waves that are not predicted by the model, an outlier.

This leaves us with the following:

1. The unexpected wave. - Waves that strike the uninformed and the unwary are often bigger then expected. These waves may be both common and easily explained.

2. Freak or rogue - These terms are defined in different ways. They are unexpected and sometime not understood.

3. Extreme wave - Sometimes used interchangeable with #2 but also may mean waves that are rare and while the may be unexpected, can be simply explained.

So, the unwary yacht was hit by an unexpected wave and the Aleutian Ballard - well, it got slammed by what mariners call "a big one".


Here is a good site with some interesting photos and graphics:Freak waves, rogue waves, extreme waves and ocean wave climate

Waves and the concept of a wave spectrum

In Search of Severe Weather has a great post: Rogue Waves

Here is my post: Significant Wave Height - A quiz

No comments: