How does NDBC adjust C-MAN and buoy wind speed observations to a standard height?

Anemometer heights on NDBC buoys vary according to buoy type. Anemometers on 3-meter discus and 6-meter NOMAD buoys are located approximately 5 meters above the waterline. Anemometers on 10 and 12-meter buoys are located 10 meters above the waterline. Anemometer heights at CMAN stations vary widely, depending on site structure and elevation above sea level. NDBC adjusts wind speeds to conform to the universally accepted reference standard of 10 meters. NDBC also adjusts wind speeds to 20 meters, a height closer to that typical of ship anemometers. This is done to provide marine forecasters a means to directly compare buoy observations with ship observations. Wind speeds adjusted to 10 and 20 meters are posted in the real-time data section of each station page on the NDBC website, as are the actual wind speeds measured by the anemometer. Only unadjusted wind speeds as actually measured by station anemometer are archived in the historical data pages.

The method used by NDBC to adjust wind speeds to a standard reference level is described by W. T. Liu et al. (1979). The method iteratively solves equations involving the air-sea exchanges of momentum, heat, and water vapor to arrive at the wind speed profile in the lower atmospheric boundary layer. Air and water temperature, wind speed at the height of observation, and relative humidity are required inputs to the algorithm. Since relative humidity is usually not measured, it is assumed to be 85 percent.

A simpler method is described by S. A. Hsu et al. (1994). Although never used operationally by NDBC, the method was tested and found to compare favorably with the more elaborate method under near-neutral stability. This is the condition most frequently encountered at sea and occurs when air and water temperatures are not too far apart. The method, referred to as the Power Law Method, is offered here for those who may want to explore the nature of the marine wind speed profile without having to deal with the complexity of the above method. The relationship is:

where u₂ is the wind speed at the desired reference height, z₂, and u₁ is the wind speed measured at height z₁. A value for the exponent, P, equal to 0.11 was empirically determined to be applicable most of the time over the ocean.

References:

Hsu, S. A., Eric A. Meindl, and David B. Gilhousen, 1994: Determining the Power-Law Wind-Profile Exponent under Near-Neutral Stability Conditions at Sea, Applied Meteorology, Vol. 33, No. 6, June 1994.

Liu, W. T., K. B. Katsaros, and J. A. Businger, 1979: Bulk Parameterizations of Air-Sea Exchanges of Heat and Water Vapor Including Molecular Constraints at the Interface, Journal of Atmospheric Science, Vol. 36, 1722-1735.