Monday, September 28, 2015

Anemometers and wind

Antarctica is a notoriously windy place.  The windiest place on Earth is said to be Commonwealth Bay, Antarctica, where winds often exceed 240km/h (150mph) and the average wind speed is 80km/h (50mph). The cold dense air that forms high on the Antarctic plateau can rush down to the coast at tremendous speeds under the force of gravity; these are known as Katabatic Winds. On the night of 17th Sept. the peak gust at McMurdo was 67.5 knots (125km/h, 78mph).

Wind carries particles (aerosols) and blows snow around so it is an important quantity for us to measure. We are using two instrument for measurement; a traditional cup anemometer on our weather station and a sophisticated sonic anemometer. The "sonic" measures the travel time of pulsed sound waves, which will speed up if assisted by wind - it can measure wind speed and direction up to 20 time a second (20 Hz).

Our weather station, with cup anemometer, at Zodiac Camp. A faint solar halo can be seen around the sun; this is caused by light refracting off ice crystals high in the atmosphere.

Aligning cable to the sonic anemometer has to be done very carefully; we have to lay cables out quickly while still they are still warm as the very cold temperatures makes some cables brittle and they will shatter if moved too much afterwards.

Doug drilling anchor holes while Lars and Mike "supervise" and fine tune their shovel leaning technique. It was a very pleasant -20C/-2F during this installation so none of us are wearing our big jackets.

To anchor the mast of the sonic anemometer we drill a V-thread into the sea ice and loop the guy lines through it. The ice anchor is very strong and can easily take the body weight of a couple of people. (Photo: D. Goetz)

The head of the sonic anemometer senses wind speed in 3-dimensions (north-south, east-west, up-down)

Once installed and guyed, we mark the lines so we don't trip on them in low visibility. (Photo: D. Goetz)
The wind rose from Zodiac Camp for 17 days of the campaign. The dominant southerly winds of Antarctica are wrapped around Mt Erebus on Ross Island to hit our camp from the east and south-east. We have chosen the camp location carefully so that we are sampling clean air and are not downwind of McMurdo Station. The maximum wind speed at Zodiac Camp was 31.3 m/s (70mph, 61kts, 113km/h).