Forecasting Tutorial: Reading Wave Models

16 May 2014 0 Share

Forecasting Tutorials is a series of in-depth articles where we teach you the techniques of surf and marine forecasting.

By Marine Forecaster Katie Jackson

Interpreting wave modelling isn’t rocket science, but it is isn’t necessarily as straightforward a process you might think. In this article Marine Forecaster Katie Jackson explains how to read the various wave model parameters to squeeze more information from the Wave Tracker and our virtual buoys. 

An accurate reading of wave models can bring you one step closer to a view like this. Photo: James Rheinberger.

An accurate reading of wave models can bring you one step closer to a view like this. Photo: James Rheinberger.

Part 1: Common errors reading swell data
Let’s start with the simplest of mistakes. Everyone can overlook the basic stuff and it is easy to skip ahead when we think we know everything. But it still gets us sometimes. Even if we think we know how to read a wave model correctly, that colourful blob on the chart gets us so excited that we miss something. So lets not skip the basics.

  • Know the time zone for where you are forecasting.
  • Check the units of measurement
  • Understanding your Virtual Buoys
  • Check all variables: height, period, direction, winds

Time zones
Weather and wave models run with UTC (also previously referred to as Zulu or Greenwich Mean Time/GMT) and given in 24hr format. Depending on the user interface you are using, a local time may or may not be adjusted for. Understand the time at your forecast location will likely be different from UTC and know the difference when you are looking at weather and swell charts. This could save you miscalculating your swell arrival time and hear the dreaded “shoulda been here yesterday”. Australia Eastern Standard Time (AEST) adjustment is +10hr, Australian Central Standard time is +9:30, and Australian Western Time is +8hr. If you are forecasting internationally be sure to check the time zone.

Coastalwatch Wave Tracker gives a local time in the top right corner for the model update. If you are reading a different wave forecasting interface, double-check the time stamps.

Measurements
As with the time zone, each forecast interface use a preferred measurement scale. Scientists and engineers develop weather and wave models so output is originally in international scientific units. This means wave heights are in decimal metres and wind speeds in knots (nautical mile per hour). However, surfers are picky with wave size so we like to break down measurements into feet and then complicate it by broadening it into a variable range (i.e. 2-3ft+, 4-6ft). Generally, human reports are not the actual measured size and skewed based on our egos, whereas wave models and buoys are based on math and hard science. Changing wave model units to be “surfer interpretable” should be simple since 1m is equal to 3.281ft but that’s not always the case. But if you want to adjust a wave report specific to your local surf spot, there are still adjustments only you can know. Figuring out local knowledge is key in translating your magic wave height number. For example, knowing the best tide at spot ‘x’ could make a model or buoy measured 1m/3.281ft swell change from breaking 2ft to 5ft.

Also look and note any changes between wave model legends. Different forecast interfaces vary in colour schemes and number intervals. Depending on the model you are looking at, the same colour could mean different wave heights. Notice if the model uses singular or multiple number intervals between their colour codes. Is the legend showing feet, metre, or half-metre intervals? Are the numbers uniform? CW Wavetracker lets you choose your units (metres or feet for waves and km/h or knots for wind) and you can adjust your reports to your preference. 

Paying close attention to wave model parametres can mean the difference between scoring and getting skunked.

Paying close attention to wave model parametres can mean the difference between scoring and getting skunked.

Reading Virtual Buoys
A virtual buoy takes wave data from a single location in a wave model and makes it easy for singular forecast analysis at that gridpoint. Each gridpoint in a model calculates a statistical wave distribution based on previously existing swells and seas generated by winds.  The highest energy swell calculated in the wave spectrum is displayed as a virtual buoy and gives the primary waves conditions you can expect for that precise latitude/longitude position over time. Surfers are generally most concerned with how wave height will vary but we also need to be aware of swell direction and swell period when looking at a virtual buoy. A common error that is largely overlooked when reading a virtual buoy is wave masking. This is when the primary swell forecast is hiding an underlying swell for the same location. More than one swell is usually present at any given time but a virtual buoy is only built to display one data at a time (the highest swell in the wave energy spectrum). This error in reading a forecast is often the case when large wind waves are generated and we fail to see an underlying secondary long period swell in the Virtual buoy information. 

We see swell variables displayed easily in our CW+ wavetracker graphs. Notice how changes occur across the forecast for one particular virtual buoy versus the next. How far apart are two virtual buoys with opposing swells? What is a surfer experiencing at the beach?

We see swell variables displayed easily in our CW+ wavetracker graphs. Notice how changes occur across the forecast for one particular virtual buoy versus the next. How far apart are two virtual buoys with opposing swells? What is a surfer experiencing at the beach?

Another common error is using a virtual buoy is not adjusting the swell data to how waves will behave in the surf zone. NOAA’s WavewatchIII (WWIII) is a wave model designed originally for predicting hazardous conditions. The intended use was for boating safety. Naval ships needed to know where seas were large and dangerous to forewarn and plan for conditions according. This means wave forecast will tend on the side of caution. Virtual buoys are located off the coast in deeper water than our surf zone in order to capture the most of open ocean conditions. The models do a fair job of forecasting surfing conditions but that is not what they were designed for. Swell on virtual buoys will be affected by elements of winds and waves more intensely in the open ocean than on the coast. Open ocean seas are hit hard with winds and multiple swells lumped together at once and swell needs time before waves disperse, approach the coast, and organise. Be sure to adjust your Virtual buoy readings for the changes in offshore swell conditions and understand how different swells undergo changes as waves propagate in towards the shore.  Waves refract to align into the coast in shallow water and changes direction. As well wave height steepens based on the wave period and water depth. Adjusting a virtual buoy forecast to what will actually occur at the beach will differ on location, distance from shore, angle to coast, etc.

Check where the Virtual Buoy is located compared to your forecast spot. Is your virtual buoy outside of an open beach or protected from a headland? On a straight, curved, or angled coastline? Is there a certain swell direction/period you are particularly looking for? It is commonplace that we want to click the closest Virtual buoy to us but look at the overall swell parameter first. Depending on swell characteristics, there are errors that may need to be accounted for on a virtual buoy. For example, are you better using a virtual buoy that is actually going to be further away because it is more in line with your coast? Offshore buoys may read larger wave heights but can capture a separate swell that is not evident nearshore.  Check the swell period and angle and mentally adjust surf heights for this. As mentioned, open water swell is going to undergo a lot of changes as it ventures into the surf zone and you may see swell not captured on a virtual buoy right near the coast.

Check all your virtual buoy variables and have a good look at what it is reading in all aspects.

Check all your virtual buoy variables and have a good look at what it is reading in all aspects.

Another less common error that widely overlooked on a virtual buoy is when an open ocean swell is moving out to sea.  Again, always keep in mind swell direction and period when you are looking at a virtual buoy reading (not just height). The beach won’t be able to capture offshore moving swell that is readable to a virtual buoy off the coast. If the virtual buoy is showing increasing waves but the beach is getting flatter then offshore winds are likely the cause. This happens when swell is angled slightly or directly offshore. Also, if a swell is small, look at the wave period and see if wave refraction may actually be able to bring in long period sets where surf is expected to be flat. Swell period can be an indication the flat conditions will still experience the occasional small sets even in sheltered areas (just expect lulls). Moral of story is don’t just focus on the wave height. Check swell period and swell direction along with height to get a better understanding of the whole picture.

The moral of story: Don’t just focus on the wave height. Check swell period and swell direction along with height to get a better understanding of the whole picture.

The moral of story: Don’t just focus on the wave height. Check swell period and swell direction along with height to get a better understanding of the whole picture.


Tags: wavetracker , virtual buoys (create Alert from these tags)

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