Spectral wave energy parameters offer captains valuable insight into approaching sea state conditions.
Ocean buoys can deliver a wealth of valuable marine information to seafarers. The National Data Buoy Center (NDBC), operated by the National Oceanic and Atmospheric Administration (NOAA), provides free public access to its network of real-time and archived buoy data. In addition to numeric data, graphics showing a time series of recent observations are also available at the click of a mouse.
While many are familiar with the traditional buoy parameters such as wave heights and wind speeds, there are other derived parameters containing nuggets of valuable insight that can alert captains of approaching sea state conditions. Spectral wave energy is one such example of an often-overlooked derived parameter that describes how uniform the wave field will be at a specific location.
Wave energy is a combination of wave height and wave intervals are waves that are separated between approximately 14-25 seconds within a set and appear well-organized and groomed, whereas short period intervals tend to be separated by less than 8 seconds and are generally choppy with poor or chaotic form.
Long-period waves are known as “swell” and form from distant storms, sometimes thousands of kilometers away, while short-period waves are known as “wind swell” and are observed within a local wind field. For example, the eastern Caribbean islands receive wind swell from the local trade winds, but also receive powerful winter swells that can be generated far away off the east coast of Canada. It is also important to know that long period waves travel faster than short period waves in deep water. Therefore, wave energy is highest when fast-moving large swells are present.
The spectral wave energy plots can be accessed at specific spectral wave buoys from the NDBC.
The spectral wave energy plot for a given location illustrates the distribution of wave energy separated into a range, or spectrum, of wave periods. On these plots, wave period (in seconds) is marked along the top of the plot (x-axis) and wave energy is indicated along the side of the plot (y-axis). Energy peaks near the shorter wave periods indicate choppy and somewhat unorganized waves being generated by local wind conditions, while peaks near the longer wave period indicate well-groomed, long-rolling waves generated from a remote storm far away.
A broad spectrum wave field is present when the wave energy is relatively high across many periods of the wave energy spectrum (Figure 1). This scenario can cause a high amount of wave interaction and give the appearance of a non-uniform surface. Often outlier sets are common in this type of environment. On the other hand, a narrow spectrum field is caused by the presence of only wind swell or only long period swell, with little mixture of the two (Figure 2). A narrow energy spectrum associated with high wave periods is characterized by a semi-clean sea state with a uniform and organized appearance consisting of long rolling waves out to the horizon. Therefore, the real-time spectral wave energy plot can provide valuable assistance when determining the sea state characteristics at a particular location: smooth and rolling, or choppy and disorganized.
The spectral wave energy plots can be accessed at specific spectral wave buoys from the NDBC (www.ndbc.noaa.gov). Just click “view details” from a yellow buoy marker and scroll all the way down the page to the link titled “Plot of wave energy versus frequency (and period).”
TEDDY ALLEN, PH.D., IS THE CO-FOUNDER AND CEO OF HENET AERIAL BUOY SYSTEMS (HENETWAVE.COM) AND A CLIMATOLOGY CONSULTANT FOR THE CARIBBEAN INSTITUTE OF METEOROLOGY AND HYDROLOGY IN BARBADOS.