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Vector Instruments W200P wind vane, Vector Instruments A100R anemometer

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Campbell Scientific Surface Wind Sensors

The Capel Dewi Atmospheric Observatory (CDAO) operates a wind vane and anemometer on a 10 m tower at nearby (3 km away) Frongoch Farm. The wind speed and direction measured at this hill-top location can be considered to be representative of the background flow. This overcomes the fact that the surface wind direction at the CDAO is tightly constrained by the orientation of the valley in which it is located. Refer the Climatology section of the CDAO page for more details.

Instrument Details
  • Latitude: 52.423565°N
  • Longitude: -4.0539832°E
  • British National Grid Reference: SN6042882605
  • Altitude above mean sea level (base of instrument): 140 m
  • Height of sensors above ground: 10 m
  • Notes: this instrument is operated in support of NCAS’s long term measurement programme. It is not bookable.

Dataset Details

Refer to the Data section below for more details about the data files.

Sensor: Vector Instruments A100R anemometer

  • Measures: wind speed
    Threshold: 0.2 m s-1
  • Accuracy:
    • 0.1 m s-1 for speeds between 0.3 and 10 m s-1
    • 1% for speeds between 10 and 55 m s-1
    • 2% for speeds above 55 m s-1

Sensor: Vector Instruments W200P wind vane

  • Measures: wind direction
  • Accuracy: +/-3° in steady winds over 5 m s-1
  • Data product: Mean direction

A Campbell Scientific CR10 data logger was used to acquire the raw data until 4th June 2019, when it was replaced by a Campbell Scientific CR1000X. Both loggers sample(d) the sensors at 1 s intervals. However, the CR10 only made available one minute
representative values based on those measurements – i.e. minimum, mean, and maximum values. The CR1000X allows full access to the 1 s
samples, which will be stored in a new dataset in the near future.

The following operations were carried out by the CR10 in order to
derive the 1 minute representative values:

    • the mean wind speed is taken as the mean of the 1 s samples of wind speed
    • a 3 s running mean is applied to the 1 s interval samples of wind speed. The minimum and maximum gust wind speeds are taken as the minimum and maximum values of this 3 s running mean.
    • 1 s samples of wind direction are converted into unit vectors, which are averaged to give a mean wind direction.

Note that a more appropriate method to calculate the mean wind and direction would be to convert the wind speed and direction to (scaled)
vectors, to average these, and then to convert back to speed and
direction.

The data are currently only available in (ASCII) NASA Ames format files. Refer to Gaines and Hipskind (1998) for a full description of the file format. Here I will just describe the bare minimum that you need to know in order to read in the data. I’m using the convention that the first line/value number is 1 rather than 0. Where multiple values are given on a single line, they are separated by white space(s).

  • Line 1: two values. The first gives the number of (metadata-containing) header lines, nr_header_lines. The second gives is the File Format Index, whose value is always 1001.
  • Line 7: six values. These give the date (UTC) for which data are available followed by the date on which the file was created, both in YYYY MM DD format.
  • Line 12: four values specifying the missing datum values for the primary variables, i.e. for all of the variables except the first one, which records the the sample time. Note that there are occasional gaps in the data and so it is important to check for occurrences of missing datum values.
  • Lines (nr_header_lines + 1) to line (nr_header_lines+ 1441): 5 data values
    • Value 1: start time of 1 minute sampling period given as the number of seconds past 00:00:00 UTC for the day in question
    • Value 2: mean eastward wind component (m s-1), v_E
    • Value 3: mean northward wind component (m s-1), v_N
    • Value 4: ratio of minimum gust speed to mean speed, r_min
    • Value 5: ratio of maximum gust speed to mean speed, r_max

These values can be used to determine the mean wind speed (v_speed) and meteorological convention (i.e. from) direction (ϕ_from) and the minimum and maximum gust wind speeds (g_min and g_max, respectively)
as follows:

  • v_speed = sqrt(v_E^2 + v_N^2)
  • ϕ_from = arctan2(-v_E, –v_N)
  • g_min = r_min * v_speed
  • g_max = r_max * v_speed

where

  • sqrt implies the square root
  • ^2 implies squared
  • arctan2(y, x) implies the 2-argument arc tangent function. Note that in most programming languages/environments, the input arguments are expected in the order y then x. However, some – including Microsoft’s Excel – expect them in the order x then y. If you are using one of these, the order of the input arguments must be reversed.
  • * implies multiplication