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Wind

Wind parameters available through the POWER API and DAV include wind speeds and direction at various heights above the surface. The user community choice determines the exact height availability.

Mean Wind Speed and Wind Direction Parameters

The wind speed and wind direction parameters are to be used to determine the mean wind flow of a region rather than local area due to effects of buildings and local topography.

Corrected Wind Speed

In addition to the wind speed parameters available at set heights above surface, the POWER API allows the user to request a wind speed at a user defined height (between 10 and 300 m) for a particular wind-surface type.

Equation: Corrected Wind Speed (Adjusted For Elevation)

\begin{align}\ WSC_{hgt} = WS_{10m} \times \left(\frac{hgt}{WS_{50m} }\right) ^{\alpha} \end{align}
\begin{align} Where&: \\ &WSC_{hgt} : \text{ Corrected wind speed at user supplied height} \\ &WS_{10m} : \text{ Wind speed at 10 meters } \\ &hgt : \text{ User supplied height above surface in meters} \\ &WS_{50m} : \text{ Wind speed at 50 meters } \\ &\alpha: \text{ Surface roughness for user supplied wind-surface type } \\ \end{align}
wind-surface Definition wind-surface Definition
vegtype_1 35-m broadleaf-evergreen trees (70% coverage) vegtype_7 0.6-m perennial groundcover (100%)
vegtype_2 20-m broadleaf-deciduous trees (75% coverage) vegtype_8 0.5-m broadleaf shrubs (variable %) & groundcover
vegtype_3 20-m broadleaf and needleleaf trees (75% coverage) vegtype_9 0.5-m broadleaf shrubs (10%) with bare soil
vegtype_4 17-m needleleaf-evergreen trees (75% coverage) vegtype_10 Tundra: 0.6-m trees/shrubs (variable %) & groundcover
vegtype_5 14-m needleleaf-deciduous trees (50% coverage) vegtype_11 Rough bare soil
vegtype_6 Savanna:18-m broadleaf trees (30%) & groundcover vegtype_12 Crop: 20-m broadleaf-deciduous trees (10%) & wheat
vegtype_20 Rough glacial snow/ice seaice Smooth sea ice
openwater Open water airportice Airport: flat ice/snow
airportgrass Airport: flat rough grass

Wind Speed Validation

In this section, MERRA-2 10-meter wind speed is compared to observations reported to the National Center for Environmental Information (NCEI – formerly National Climatic Data Center). Selected surface sites from the NCEI Integrated Surface Database (ISD) files are used for the hourly MERRA-2 comparisons. Global "Summary of the Day" (GSOD) files are used for the comparison to the daily mean MERRA-2 temperature.

Hourly

Hourly Wind Speed

Scatter plot of the MERRA-2 hourly 10-meter wind speed vs. the hourly station observations values from the selected NCEI ISD files for the years 2001 - 2019.

Hourly Temperature

The hourly validation scatter plot exhibits a large concentration of pairs for wind speeds < 10 m/s and shows that the surface site values have a larger range in general than the MERRA-2 data. The total number of hourly pairs is 225,364,116.

Daily Average

Daily Average 10-meter Wind Speed

Scatter plot of the re-gridded MERRA-2 daily wind speed vs. the daily station observations values from the NCEI GSOD files for every 3rd year from 1981 – 2014.

Wind Speed

The color bar along the right side of the scatter plot provides a measure of the distribution of the NCEI, MERRA-2 wind speed pairs in bins of 1 m/s. For example, the left column along the vertical color bar shows that each data point in dark blue represents the number ground site/MERRA-2 data pairs within 10% (i.e. <=74,628.4) of the maximum number of data pairs within a 1 m/s bins in the plot (i.e. 746,284). Additionally, from the right side of the vertical color bar it can be seen that all the points shown in dark blue contain 17.25% of the total number of ground site, MERRA-2 data pairs (8,832,141). The remaining 82.75% of the data pairs are concentrated along the 1:1 grey line in the lower left corner for wind speeds < 10m/s.

Wind Direction

Wind Direction at a given height in degrees as the meteorological convention. The meteorological convention gives the direction from which the wind originates and wind direction is measured in degrees clockwise from due north.

The computation of wind direction from MERRA-2 u and v components is based upon the information from NCAR/UCAR/EOL's quick reference on wind direction. Verification of the equations is accomplished with the usage of MERRA-2 10 meter winds from July 11, 2015; shown in this table:

Latitude Longitude u (ms-1) v (ms-1) direction
38.0°N 80.0°W 1.14718 0.00922 270.46°
37.0°N 79.0°W 1.53487 -0.89496 300.25°
37.0°N 76.5°W -1.53666 0.21723 81.95°
35.0°N 75.5°W 1.85421 2.19976 220.13°
39.0°N 77.5°W -0.09501 1.50678 3.61°

These locations are shown as the red circles on the streamline map here.

Image of MERRA-2 streamline winds

Click to Expand - Python Code
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def WD(data, Level):
    '''
    Wind Direction

    - data: xarray dataframe with the variables units defined (required)
    - Level: the elevation to compute the wind direction (required)
    '''

    U_Values = Convert(data["U{}M".format(Level)]).To("m/s")
    V_Values = Convert(data["V{}M".format(Level)]).To("m/s")

    WD_Values = np.arctan2(-U_Values, -V_Values) * (180 / np.pi)
    WD_Values = np.where(WD_Values >= 0, WD_Values, WD_Values + 360)

    WD_Values = xr.DataArray(WD_Values, dims=('time', 'latitude', 'longitude',))

    WD_Values.name = "WD{}M".format(Level)
    WD_Values.attrs['units'] = "Degrees"

    return WD_Values