Ocean Waves and Surf Forecasting

Stuart H. SweeneyDepartment of GeographyUniversity of California, Santa Barbara

Winter 2006

Overview

• Ocean regions

• Global Circulation Patterns

• Characterizing and describing ocean waves

• Wave theory, propagation, and dispersion

• Refraction, shadowing, and bathymetry

• Wind and wave measurement

• Forecasting

Overview Overview

• What are waves?

OCEAN WAVES

Characterizing and describing waves

• “…at the present day, understanding of the mechanism of wave formation and the way that waves travel across the ocean is by no means complete.”

• models of idealized fluids

• complexity of real ocean waves

Characterizing and describing waves

Characterizing and describing waves

• Basic terminology

� T = period = time interval between two peaks passing a fixed point

� f = (1/T) = number of peaks passing a fixed point per second.

� L = wavelength

� H = wave height

Characterizing and describing waves

CDIP data

overlay

Characterizing and describing waves

CDIP data

overlay

Characterizing and describing waves

• What are waves?� Transfer a disturbance (energy) through material

� Material remains essentially stationary

� The wave form is relatively stable

� The disturbance moves at approximately a constant speed

• Wave types – surface waves� Ocean-atmosphere interaction

� Surface tension

Characterizing and describing waves Characterizing and describing waves

• Wind Waves� Wind Frictional Stress Energy

� Wave generation reaches maximum at wavelength (L) corresponding to a speed 1/3 that of wind speed. After that point transfer continues at decreasing rate.

� Fully developed sea: equilibrium state

- size and character of waves stop changing

- reality >> wave field

Characterizing and describing waves Characterizing and describing waves

• Wave field data

Source: http://cdip.ucsd.edu

0

50

100

150

200

250

22+ 22-18 18-16 16-14 14-12 12-10 10-8 8-6 6-

Period (T)

Ener

gy (i

n2 )

643684

Characterizing and describing waves

• Wave fields (by day)

Source: http://cdip.ucsd.edu

Characterizing and describing waves

• Directional spectrum and wave field

Source: http://cdip.ucsd.edu

Wave theory, propagation, and dispersion

Motion of water particles

� Surface: orbital diameter = wave height

� Diameter decreases exponentially with depth

� Zero displacement at depth = L/2

� With depth < L/2, orbital pattern is flattened

Wave theory, propagation, and dispersion

Wave speed, c

� c = L/T

� (variable depth)

� (deep water) [c=1.56T]

� g = 9.8 m s-2

Wave theory, propagation, and dispersion

Wave dispersion and sets

� “Those deep-water waves that have the greatest wavelengths and longest periods travel fastest, and thus are first to arrive in regions distant from the storm which generated them.”

� Radial Dispersion = waves separate based on differing speeds, c

� cg = c/2 if c1 c2.

� “Groupiness”: two wave trains in “phase” are a “set” or “group”

~~

Wave theory, propagation, and dispersion

Wave dispersion and sets

Wave theory, propagation, and dispersion

Swells (energy dispersion)

� Local waves and wind have little influence on the size and progress of swell waves.

� 90% of sea surface energy radiates out within 30-45 degree angle of wind direction.

� Circumferential Dispersion (Spreading loss): wave energy dispersed over increasing wave front.

� (recall: Radial Dispersion: Farther from the storm, more organized the wave energy)

Waves near storm

Waves far from storm

Refraction, shadowing, and bathymetry

Waves in shallow water

� As depth decreases,

- wave speed slows

- height and steepness increase

- period remains fixed

� Some wave energy lost to friction; energy is also focused (or defocused)

Refraction, shadowing, and bathymetry

Refraction

� Wave rays “turn” toward shallow water

-What is a wave ray?

� Concave refraction: If wave rays converge, energy focused, H increases

� Convex refraction:If wave rays diverge, energy diffused, H decreases

YZ

Snell’s law:

sin(Z) = sin(Y) x sqrt(d2/d1)

Refraction, shadowing, and bathymetry

Shadowing

� Wave energy blocked by islands

� Swell windows

Diffraction

� Change in direction and intensity of waves from fixed object.