Buoy wind inhomogeneities related to averaging method and anemometer type: application to long time series

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Abstract

Moored buoy observations began two to three decades ago. These datasets have value for calibration of remotely sensed data, validation of weather and ocean wave models, input to reanalysis models, and studies of climate trend and variability. Changes in buoy wind observation methods have the potential to introduce inhomogeneities into the time series. These include changes in anemometer height (typically between 5 and 10 m) with changes in platform type; the transition in the 1980s and 1990s from use of a vector-mean to a scalar-mean averaging method; and, in recent years, the introduction of ultrasonic anemometers in the buoy networks, as well as the continued use of mechanical propeller-vane type of anemometers. This study examines differences in buoy wind measurements from RM Young propeller-vane and Vaisala ultrasonic anemometers installed on the same buoy, and differences in vector-averaged and scalar-averaged wind speeds from the same RM Young anemometer. This study also considers the effect of waves on these differences. The comparisons are based on large multi-year datasets from 6-m buoys with boat-shaped hulls (6N) and 3-m buoys with round hulls (3D), deployed off the coasts of Canada in the northeast Pacific and northwest Atlantic Oceans. Results of the anemometer type comparison suggest that the Vaisala ultrasonic winds are 0.16 ms−1 + 1.6% of the RM Young winds. Results of the vector scalar comparison of Canadian buoy data show that scalar mean winds were 2.1% and 2.7% greater than RM Young vector mean winds from 6N and 3D buoys, respectively. Vector-scalar differences increased with increasing wave height, more quickly with 3D than with 6N buoys. Results are used to adjust the winds from a long-term buoy in the northeast Pacific. The height adjustment is shown to be more important than the adjustment for vector or scalar averaging of the sustained wind speed, in terms of the impact of monthly mean wind speeds. A homogeneity testing program finds other unexplained significant shifts in the time series of monthly mean wind speeds. Copyright © 2011 Royal Meteorological Society and Crown in the right of Canada.

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