A new dimension in Victorian surf forecasting
Keen weather observers will have noticed new additions to the Cape du Couedic and Cape Sorell wave buoys. Each buoy was upgraded a fortnight ago.
Both buoys have had the dominant wave direction added to the graphs, however the Cape Sorell buoy is now showing data which isn't available on any other buoy around Australia: wave spectrum.
On the East Coast wave direction is crucial as swells can come from 180 degrees - north through to east and around to south. Keeping an eye on the different swell directions hitting the coast can make or break your search for waves.
Direction in the southern states isn't quite as important with most swells coming from a smaller window, that being west to south. However, knowing the direction has its benefits, as you'll soon see.
To begin with, the wave direction shown on the period graphs for Cape du Couedic and Cape Sorell (represented in graph below) is associated with the dominant swell in the water, ie if there are multiple swells in the water it isolates the swell with the most energy.
Knowing swell direction is useful when identifying strong westerly or southerly swells. For instance, a savvy Victorian surfer can calculate the travel time of a west swell hitting the Cape du Couedic wave buoy till when it hits their coast, or vice versa with a southerly swell hitting Cape Sorell and moving north towards South Australia.
In reality, this could have already be done by cross-checking the incoming swell direction with wave model forecasts, however the new data identifies shifts in the swell direction allowing a greater real time indication of the swells arrival.
The directional spread (the green bars on each graph) show the range of directions a swell is coming in from. For example, a swell may be generally coming in from a pure south-west (225 degrees) direction, but there'll also be swells coming from 235 and 215 degrees in the mix, hence the spread will be 225 +/- 10 degrees.
Pure directional groundswells will have less directional spread and a swell source that's arrived from a broader storm or conveyer belt of frontal systems will have a higher spread.
The exciting stuff is having access to the pure wave spectrum at Cape Sorell, and I'll work through it one dot point at a time:
The open ocean consists of multiple swells from a variety of directions - i.e capillary waves, wind waves, mid-range energy, and long-period groundswells
The wave spectrum is a breakdown of the sea state at the wave buoy for a period in time.
The following dot points refer to the graph below:
- Wave energy is recorded and sampled over a 27 minute period with the most recent sample labelled on the horizontal axis, bottom right.
- Wave frequency is shown on the top horizontal axis, with the BOM conveniently converting it to period on the bottom horizontal axis
- Lower frequency (longer period groundswell) falls to the left hand side of the graph and higher frequency (windswell) energy to the right
The graph below is identical to the graph above but has been reproduced for the sake of clarity.
- The Y-axis shows the energy (swell) aligned with each wave period and this is where the valuable data can be found.
- The peaks in energy indicate the amount of swell for the corresponding period providing a quick way to see how much pure groundswell is in the ocean, if any
- The direction for each period is shown (green), with the readings on this graph showing the groundswell is W of SW in direction
- The lower period energy is more southerly in nature, with some very weak westerly windswell (4 seconds)
- Arrows on the bottom axis point out the peak and average periods for the current sea state.
Making sense of this will take some time, but let's look at an example.
A singular spike of swell energy around 16 seconds with no other energy either side would indicate a pure groundswell in the 16s range, and the red Tp arrow would also hover around the 16s value.
However, if the energy spectrum is spread out and broad in nature this indicates a mix of different swells and the result will be mixed wave trains at the beach with varying size and power.
A few further points.
- The wave spectrum will be great for identifying the amount of energy/size in the fore-runners of a new long-period groundswell .
- If the red Tp arrow sits to the left of the peak in energy, ie around 19s, with the peak in energy sitting at 14s, this will show that the long-period forerunners have no size to them, and the bulk of swell is still mid-period energy.
- Once the red Tp arrow lines back up with the energy spike you can concur that most of the swell is of that period.
Lastly, there is a crude way convert the energy shown on the y-axis to an approximate height in metres. Just take the square root of the spectral density.
Over the coming weeks, as better examples show up on the wave spectrum, I'll post working examples showing how to interpret it.