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Ocean Wave Spectra -- What the heck is that and what can it tell us?
Those pretty significant wave height and peak period maps we all love to look at are great, and I really mean that -- but these combined and dominant ocean wave characteristics (respectively) only tell half the story of the motion in the ocean. So what should we look at? The answer lies in ocean wave spectra.
In a nutshell, an ocean wave spectrum is a view of all of the different wave trains arriving at a certain point in the ocean -- from all different directions and all different periods. This information tells us which swells arriving at that point will make it to your beach of choice (“good swells”) and which swells are traveling away from the beach (“bad swells”). More importantly, a detailed analysis of a wave spectrum can tell you exactly when the good swells will show up on the beach and an idea of what kind of surf they will produce. The time it takes for the good swells to reach your beach is directly related to the period while the type and size of surf is a bit more complicated. Type and size of surf is related to many more things (energy of incoming train, local winds, tide, bottom, period, direction, decay distance from analyzed spectral point, etc.)
In reality (in an observational world), there may be an infinite number of directions and periods that ocean waves may be coming from at a single point. However, in model-model land this number is constrained. LOLA (and WW3) outputs wave energy values in 24 directional and 25 frequency (frequency is the inverse of period) bins -- which results in 600 total values. That’s 600 total wave energy values for each grid point. LOLA is a global model with a horizontal resolution of one degree, so that means she calculates a wave spectrum for 36000 ocean points -- a total of 21.6 million values, for you non math majors.
Lets put all that scientific mumbo jumbo to some use. Figure one shows a recent LOLA wave spectrum forecast for a point just east of Cape Hatteras, North Carolina. Each frame of the loop represents a particular forecast time as given by the top toolbar. The polar plots show every wave component that is expected to arrive at that point. The colors represent swell height as described by the color bar under the times. The directions labeled around the plot represent the directions the swells are coming from, and the period increases radially outward leaving short period seas closer to the center. In this figure note that short period S windswell increases at the beginning of the forecast period and then rotates NW by 72hrs out. At the end of the loop notice that a bit stronger and longer period NE/ENE ground swell is expected to pick up. More details on these polar spectral plots are provided here. Also, if you click on the “Swell Tracking Maps” option of LOLA you will see a text version of this plot, but it only includes the 3 most significant swell events occurring at a given time.
--Mark Willis
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