Table 4-6.—Relative Wind Modification
RELATIVE WIND MODIFICATION
WIND MODIFICATION
W
36-40KT
+2.0
3.0
4.0
1.5
2.0
4.0
I
31-35KT
1.5
2.0
3.0
1.0
1.5
3.0
N
26-30KT
1.0
1.5
2.0
0.5
1.0
2.0
D
21-25KT
0.5
1.0
1.5
0.0
0.5
1.5
16-20KT
0.0
0.5
1.0
0.0
0.0
1.0
S
11-15KT
0.0
0.5
1.0
0.0
0.0
1.0
P
06-10KT
0.0
0.0
0.5
0.0
0.0
0.5
E
0-05KT
0.0
0.0
0.0
0.0
0.0
0.0
E
00-30°
30-60°
60-90°
00-30°
30-60°
60-90°
D
ONSHORE
OFFSHORE
WIND DIRECTION RELATIVE TO BEACH FACE
The operation go/no-go decision is usually made by
SUROB REPORTING
comparing the MSI (6.5 in the example) to the
maximum, safe operating limits for each type of craft
provided in the Joint Surf Manual. The SUROB
observer should not be expected to make any
recommendations, but may be expected to provide the
modified surf index calculation. Recommendations will
be made by the forecaster. The MSI can also be
computed from the SURF program in TESS and MOSS
by using input from the surf observation. In addition to
MSI calculations, the SURF program produces a surf
forecast based on forecasted sea/swell, wind, and tide
information. Beach profiles can be created by manually
entering distance versus depth information obtained
from beach survey charts. This is usually accomplished
by the observer.
REVIEW QUESTIONS
Q22.
What is the purpose of the Modified Surf index?
Q23.
When calculating the MSI what will be the value
for a surf zone with 60% surging breakers and a
significant breaker height of 3 feet?
Q24.
When calculating the MSI, what will be the value
for a surf zone with an offshore wind at 70°
relative at 23 knots?
Q25.
What program in TESS/MOSS can be used to
compute MSI?
LEARNING OBJECTIVES: Explain when surf
observations are reported. Explain the purpose
of the SUROB Brevity Code.
Depending on conditions of the seas, tides, and
winds, SUROBS may show significant changes every
hour. No standard reporting times or reporting intervals
have been established. The Joint Surf Manual
recommends at least one SUROB every 12 hours, 2 to 3
days before an operation, and then increasing to hourly
within 6 hours of the landing. Normally, operation
planners will establish minimum reporting intervals and
assign a time to commence observations and reports.
The observer should routinely monitor the surf
conditions and submit intermediate or supplemental
reports whenever conditions change significantly.
Nighttime surf observations are not nearly as reliable as
daytime observations. Trends noted in the Modified
Surf Index in combination with current meteorological
parameters, such as wind and sea state, may provide the
best estimate of actual surf conditions at night.
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