EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
(b). As shown in that section, it is not advisable to use a constant ratio between winds at the top of the
boundary layer and winds at a 10-m level. If a complete wind field is required for a particular application
it is recommended to use a planetary-boundary-layer (PBL) model combined with either a moving vortex
formulation or a numerical version of a parametric model.
(13) To provide some guidance regarding maximum sustained wind speeds at a 10-m reference level,
Figure II-2-17 shows representative curves of maximum sustained wind speed versus central pressure for
selected values of forward storm movement. It should be noted that maximum winds at the top of the
boundary layer are relatively independent of latitude, since the wind balance equation is dominated by the
cyclostrophic term; however, there is a weak dependence on latitude through the boundary-layer scaling,
which is latitude-dependent. This dependence and dependence of the maximum wind speed on the radius to
maximum wind were both found to be rather small; consequently, only fixed values of latitude and Rmax have
been treated here. From the methods used in deriving these estimates, winds given here can be regarded as
typical values for about a 15- to 30-min averaging period. Thus, winds from this model are appropriate for
use in wave models and surge models, but must be transformed to shorter averaging times for most structural
applications.
(14) Values for wind speeds in Figure II-2-17 may appear low to people who recall reports of maximum
wind speeds for many hurricanes in the range of 130-160 mph (about 58-72 m/s). First, it should be
recognized that very few good measurements of hurricane wind speed exist today. Where such measurements
exist, they give support to the values presented in Figure II-2-17. Second, the values reported as sustained
wind speeds often come from airplane measurements, so they tend to be considerably higher than
corresponding values at 10 m. Third, winds at airports and other land stations often use only a 1-min
averaging time in their wind speed measurements. These winds are subsequently reported as sustained wind
speeds. An idea of the magnitude that some of these effects can have on wind estimates may be gained via
the following example. The central pressure of Hurricane Camille as it moved onshore at a speed of about
6 m/s in 1969 was about 912 mb. From Figure II-2-17, the 15- to 30-min average wind speed is estimated
to be 52.5 m/s. Converting this to a 1-min wind speed in miles per hour yields approximately 150 mph,
which is in very reasonable agreement with the measured and estimated winds in this storm. It is important
to recognize though that these higher wind speeds are not appropriate for applications in surge and wave
models.
(15) Figures II-2-18 and II-2-19 are examples of the output from the hurricane model presented here.
Figure II-2-18 shows the four radials. Figure II-2-19 shows wind speed along Radials 1 and 3, as a function
of dimensionless distance along the radial (r/Rmax) for a central pressure pc of 930 mb and forward speeds of
2.5 m/s, 5.0 m/s, and 7.5 m/s. The inflow angle along these radii (not shown) can be quite variable. The
behavior of this angle is a function of several factors and is still the subject of some debate.
Meteorology and Wave Climate
II-2-31
Previous Page
