Scaling Away

by Chip Kasper

Scaling Away

The term “scale” in fishing conversations usually refers to weighing or cleaning a recent catch.  However, the term “scale” also means the “relative size or extent of something”.  Economists refer to the “macro” and “micro” scales because various aspects of an economy have been shown to be more or less influential at either a very large (macro) or very small (micro) scale.  Physicists have long studied the very large (astrophysics), and the very small (atomic physics) substances in nature.  And, meteorologists and oceanographers too, must consider the various scales in nature on which the fluid motions of the atmosphere and ocean operate, in order to both understand and predict those motions.  Atmospheric motions, for example, may range between global fluctuations in the jet stream over the course of an entire year, to a three-second wind gust at a single, remote corner of a subtropical island.  Ocean circulations also operate on the very large and very small scales, ranging from the North Atlantic Gyre, which helps power the ever-present Gulf Stream, to the small whitecaps which form on the windward side of an island during a freshening breeze.

In fact, scale may help explain why the wind, waves, and weather differ greatly between the Islamorada Humps and the Cay Sal Bank, or even between Western Dry Rocks and Eastern Dry Rocks in any given day. Why is scale so important then for interpreting weather forecasts?  First, weather systems important to everyday life vary significantly across space and time.  

In meteorology, four fundamental scales are considered, encompassing diverse weather phenomena:  (1) planetary scale; (2) synoptic scale; (3) mesoscale; and (4) microscale.  Planetary-scale phenomena cover thousands of miles over time periods of a week or more.  Examples include the polar jet stream and the trade winds.  Synoptic-scale phenomena cover distances of a few hundred to a few thousand miles over time periods of about 1–7 days.  Examples include upper-level troughs, tropical waves, and hurricanes.  Mesoscale phenomena cover distances of a few miles to a few hundred miles, usually over a period of about 1–24 hours.  Examples include thunderstorms and sea breezes.  Finally, microscale phenomena operate over distances less than a mile, and time periods under an hour.  Examples include waterspouts and turbulence.  If a region is primarily influenced by mesoscale weather systems (as the Florida Keys often are during the summer), then this would help explain why it may be cloudy and raining in Key Largo, but sunny and dry in Key West, or why winds may suddenly surge to 25 knots at Cosgrove Shoal, but remain nearly calm at Carysfort Reef.  If, on the other hand, a region is primarily influenced by synoptic-scale weather systems (as the Florida Keys often are during the winter), then this would help explain why northeast winds may blow 15 to 20 knots for days on end, uniformly, from Fowey Rocks all the way past the Dry Tortugas.  Second, weather systems of different scales interact in complex, often nonlinear ways, posing a limit to predictability.  Dr. Edward Lorenz, a meteorologist and mathematician, was an early pioneer of the mathematical field of study called “chaos theory”.  Lorenz defined chaos as “when the present determines the future, but the approximate present does not approximately determine the future”.  This is one of the reasons why weather forecasts fail – we can only approximately define the present (initial conditions), and our models also are only approximate.  Lorenz famously posed the question in a 1972 paper, “Does the flap of a butterfly’s wings in Brazil set off a tornado in Texas?”  This concept became known as “the butterfly effect”, highlighting the importance of both scale interactions and initial conditions in modern numerical weather prediction.  Many of the routine information products from your NOAA/ Florida Keys National Weather Service provide numbers, graphics, and automated phrases derived from a digital forecast database created locally by the duty forecaster, based on his or her diagnosis of the weather situation, analysis of weather prediction model output, and experience.  If you wish to gain further insight into the forecast process, predictability, potential scenarios, and confidence that a duty forecaster possesses in a given forecast situation, then you may consult a little-known product called the “Area Forecast Discussion” (available online at http://forecast.weather.gov/product.php?site=NWS&issuedby =KEY&product=AFD). Remember to always check the weather before heading out on the water, and as always, be weather-ready, and stay safe!




Chip Kasper
Chip Kasper

Author

Chip is a senior forecaster and marine program meteorologist at the NOAA/National Weather Service Forecast Office in Key West. The National Weather Service provides weather, water, and climate information for the protection of life and property on land and at sea. Email Chip at kennard.kasper@noaa.gov.



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