Hurricane wind speeds (2)




Some Civil Engineers think they can tell the wind speed by the damage, and to a certain degree that is possible, but it is not precise.  Officially, the most credible measurement comes from NWS towers.   There are  also many deployable towers that collect all kinds of in situ data, including wind speed. In addition, Doppler Radar can measure wind speed, but only the component toward or away from the radar.  With towers it is possible to get an accurate reading. But for the maximum wind, the towers must be in the right place, and that is not always easy to anticipate.   Maximum wind speed is not an easy measurement to make over land, but it is even more difficult over water.


Over water there are instrumented buoys, but again they have to be in just the right spot to measure the maximum one-minute average wind.  Aircraft can measure flight level winds but these are high above the ground.  NOAA operates two P-3 Orion turbo prop airplanes,  and the Air Force a  WC – 130J.  Both workhorses. The NOAA planes have X band Doppler radars on them and they produce in flight real time plots of the winds. In addition they deploy dropsondes that are instruments that measure and telemetry all kinds of data, including the wind speed.  So as it descends near 10 m off the surface it gives the wind speed. But that is the winds where the sonde is, not necessarily the storm maximum.


Another way is that by analysis, it is found that the near surface winds are about 80% of the flight level winds. This give an approximate value..  Of course, one problem with an aircraft is that is must move.  Thus it is measuring wind along it’s track, and the track my not be over where the winds are the greatest.


For several years, the maximum one minute average winds are largely determined by a nadir pointing microwave radiometer that uses 6 channels in the microwave portion of the spectrum (where the wavelengths are of the order of centimeters in wavelength.  (Your microwave oven uses wavelengths in the x-band or about 3 cm in wavelength (a little over an inch from peak-to-peak of the EM wave).   I these instruments measure the strength of the upward radiation from the  sea.  Everything emits radiation, just as the sun does.  The colder the object, the less it radiates and the longer is the wavelength where the intensity is the greatest. The strength of the radiation that is measured is called the brightness and it is related to a temperature, the brightness temperature. Not all object radiate equally well.  A so-called black body both absorbs and radiates the maximum possible. 


The stronger the wind, the most disturbed the sea and the greater the amount of sea foam.  Sea foam radiates really well and is nearly a perfect black body, so more is radiated. Thus the stronger the intensity, the more disturbed the sea and the stronger the wind.  Now the temperature of the water is a factor and the intensity of the rain is a factor.  It turns out that as for the temperatures involve, the error is small (a fraction of a degree and a few m/s).  The rain rate can be deduced from two different frequencies and that can be accounted for.  Of course there are other problems such as RFI (radio frequency interference, etc. ) but they can also be minimized. 


What remains is a reasonable estimate of boundary layer winds along the airplane’s flight path.  It still is not guaranteed to be the maximum winds anywhere, but it is an estimate and not just a point measurement as is the

dropwindsondes.  It tends to over estimate the winds in the eye, because, while the winds are near zero, the sea has not adjusted to that low wind speed so it over estimates the winds. And it probably underestimates the maximum wind, but it does a pretty good job.


Keep in mind, the maximum sustained winds (msw) is the average over one minute and gusts can and will be 30% higher and lower than that average. It is an average and not the maximum wind. Given all the uncertainties, it is appropriate to view the numbers as qualitative important with an uncertainty of least 5 to 10 mph.