Validation of wind measurements for use in a scanning DOAS system at White Island, New Zealand

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Miller, C.A.; Werner, C. 2009 Validation of wind measurements for use in a scanning DOAS system at White Island, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2009/12 16 p.

Abstract: Accurate wind speed measurement is one of the most important components of determining SO2 emission rates from degassing volcanoes. It is also one of the most overlooked aspects of the emission rate calculation and often large errors (up to 50 %) are thought to be introduced through the use of an inappropriate wind speed. Here we present a comparison of wind speed and direction data recorded at White Island volcano, New Zealand, from a weather station on the summit of the island, from GPS-based airborne wind speed measurements and from the nearest weather station on the mainland, 50 km to the south. We show that the average White Island weather station measurements provide an accurate proxy to the true wind direction and speed (wind speed underestimated by ~13 %) for winds useful in DOAS measurements and that this variance also accounts for most of the difference in emission rates measured using a scanning DOAS system on the island and airborne COSPEC measurements. The mainland weather station is shown to under estimate the wind speed by 29 % on average compared to the flight data recorded at the island. Attempts to correct the White Island weather station wind speed data to the flight-derived wind speed data using a power law wind shear model were not successful and showed that most of the wind speed variation is due to complex turbulence affects caused by the steep topography on the island rather than simple laminar shear resulting from the different heights of the weather station and airborne measurements above ground. (auth)