1. Waves for Weather Weenies
ATMO 456

2. Wave (and Marine Wind) Forecasting
Purview of the Meteorologist
National Weather Service
Industry
Oil and Gas
Ship Routing
Navy
Inseparable from Marine Meteorology

3. Definitions
From Oceanography: An Introduction, Wadsworth ©1985

4. Wave Period
The time it takes for successive wave crests to pass a fixed point.

5. Orbital Motions
From Oceanography: An Introduction, Wadsworth ©1985

6. Sea and Swell
Waves experienced at a location are a combination of the locally-generated wind waves (Sea) and the Swell propagating in from other source regions. L L
3 days for swell in N. Pacific to reach W. Coast, 2 weeks to transit Antarctica to Aleutians
Local Wind Wave spectrum v

7. Measures of Wave Height
Hs - Significant Wave Height – Most widely used and what customer usually wants
Avg. of the Highest 1/3 of all waves (wind wave or swell). AKA H1/3 or Combined Seas
SQRT(Sum of Squares of Sea and Swell) approximates
H1/10
Hmax and Havg

8. How Wind Waves are Made Ingredients Two governing equations
Straight Winds (more or less)
Don’t forget – less friction, less cross-isobar flow and greater speeds
Over a distance (Fetch)
For some period of time
Two governing equations
Depend on depth (shallow or deep)
Shallow is depth of one half wavelength (“feel bottom”)
Speed of Shallow water waves depends only on depth
Speed of deep waves depends on wavelength
Swell waves are generally much longer wavelength/period
West Coast waves much longer and larger (why?)

9. Concept of fully arisen sea
For a 30 Knot wind
From Oceanography: An Introduction, Wadsworth ©1985

10. Forecasting Waves Inland Waters vs. Offshore
Wind wave fetch limited or not
Phasing of wind speed and direction changes (duration of straight winds)
Account for Swell

11. Thankfully, there is a model!
Spectral Ocean Wave Models
Today’s primary model:
Wavewatch III
Run by NCEP (OPC) and by NAVY (FNMOC)
Of course, only as good as the model atmospheric winds driving it

12. Forecast Products
Coastal Marine Forecast/Inland Waters Forecast (and marine zones) (20nm out and 20 to 60nm)
Offshore Waters Forecast
Small Craft Advisories/Warnings
Open Ocean: High Winds and Seas Warnings
Gale (34 to 47 knots) and Storm (48 knots and up, although 50 is sometimes used)
Generally associated with Lows (“Gale” Low)
High Seas
12 ft or higher, with 6 ft increment boxes (12,18,24)

15. When Waves reach the Shore…
The term becomes Surf
Surf forecasting is usually a specialized business for recreation and for military operations
When waves break, mass is transported
Leads to a forecasting frontier:
Rip Currents

16. Rip Current (Runout)