FATE OF MERCURY IN THE ARCTIC Michael Evan ... - COGCI

Fate of Mercury in the Arctic 49
The temperature reading was however, used to control that the sonic anemometer system was
running properly, since the temperature calculation is related to the flight time determination, as
seen in (4), and if the path length determination in (4) manifested in unusual temperature readings,
then necessarily the vector determination was affected, as seen from (3).
Throughout the experiments, the sonic anemometer functioned well, though as will be
discussed later, in Barrow, the sonic was not set on the tower properly, causing the down channel to
be preferentially sampled. This was corrected analytically after the measurements as discussed later.
It can be seen from (3) that the wind speed is not affected by the speed of sound in air and is
therefore independent of temperature and thus pressure, as well as humidity, though temperature
determination is humidity dependent.
The METEK sonic anemometer, like all 3 dimensional sonic anemometers, arranges three pairs
of transducers such that the three dimensional wind vector can be unambiguously derived for the
local vector going through the centrum of the sonic anemometers plane. It does this by a non-
orthogonal arrangement around the vertical axis of the instrument which has a rotational symmetry
= 120 o , with each path having an angle of 45 o with reference to the horizontal.
The instrument is set up completely level with a reference point on the axis aligned to magnetic
North, so that information about the arrival direction of the wind parcel is properly recorded and the
software provided transforms the information into the Cartesian coordinate system, which is sent to
the REA system and translated to a switching decision.
The system utilized had software that allowed it to run for 10 minutes, correcting for the tilt of
the terrain. The tilt of the terrain creates a flow distortion that will bias results if not corrected for. If
the instrument is not set up completely level, as turned out to be the case in Barrow, an artificial
flow bias will be introduced, since the terrain correction program assumes the system is level.