| Exporting TAB Files | |
Windographer can export WAsP .tab files that indicate occurrences or frequency versus direction sector and wind speed bin. If the dataset contains multiple wind speed and direction sensors, you can choose the speed and direction sensors upon which to base the bins. You can also choose the number of wind direction sectors, the wind speed bin size, and whether to use the entire dataset or just a subset in the calculations.
If your workbook has multiple datasets, you can select the dataset from the top of the Export Data window.
Tip: To make the bins center on integer wind speeds, set the bin size to 1 m/s and choose the option to make the first bin half size. Then the first bin will cover 0-0.5 m/s, the next will cover 0.5-1.5 m/s, the next will cover 1.5-2.5 m/s, and so on.
If you have chosen to export frequencies and you check the Remove seasonal bias checkbox, Windographer will calculate the mean of monthly means for each cell in the frequency table. This avoids seasonal bias in datasets that cover a non-integer number of years. An 18-month dataset, for example, that covers two winters and one summer, is biased towards the winter season because it contains twice as much winter data as summer data. The remove seasonal bias option corrects that bias, which should make the TAB file more representative of the true long-term average conditions.
The table that appears in the top right corner of this window reports the mean wind speed and mean wind power density of the original time series data and of the frequency distribution contained in the TAB file. The article on mean of distribution explains the calculation of the mean of a frequency distribution.
To calculate the mean speeds and power densities that appear in the Time Series column of this table, Windographer uses the same time series it uses to create to TAB file itself, drawn from the selected speed and direction sensors in the time steps that meet your filter criteria. The number of time steps in this time series appears below the table. If the mean speed in this table differs from one calculated elsewhere in Windographer, the reason is probably that this calculation encompasses only those time steps that contain valid speed and direction values, whereas most other mean wind speed calculations in Windographer require only valid speed values.
To calculate the mean wind speed of the TAB file, Windographer calculates a weighted average of the midpoints of the speed bins, weighted by the number of occurrences in each bin. This TAB file mean will often differ slightly from the time series mean. Reducing the bin size tends to reduce this discrepancy.
If you have chosen to scale the TAB file, the mean wind speed of the TAB file should closely match the mean value to which you chose to scale. Otherwise, the TAB file mean wind speed should closely match either the mean of monthly means of the original time series, if you have chosen to remove seasonal bias, or the simple mean of the original time series, though again the limited resolution of the TAB file will likely result in a small discrepancy.
You may wish to scale a TAB file to a particular mean wind speed, for example to account for a long-term climatic correction, or to do a sensitivity analysis. To do that, check the checkbox labeled Scale to mean wind speed of and entering the desired mean. Windographer will temporarily scale the wind speed time series as required to make the TAB file conform to that mean wind speed as closely as possible. It will try multiple different constant scaling factors and use the one that leads to a TAB file mean speed nearest to the target. Some discrepancy is likely, due to the finite resolution of the TAB file; the smallest possible effect of scaling would be to push a single 10-minute wind speed from one bin to the next, causing a small step change to the weighted average TAB file mean. Again, reducing the bin size tends to reduce this discrepancy.
If you have chosen to remove seasonal bias, then the ‘Scale to’ mean will default to the mean of monthly means of the unscaled wind speed time series data. Otherwise, the ‘Scale to’ mean will default to the simple mean of the unscaled wind speed time series data.
Line 1 contains a text description. Windographer writes the name of the dataset, the names of the speed and direction sensors that you have chosen to specify the bins, and whether you have chosen to remove seasonal bias or scale to a particular mean value.
Line 2 contains the latitude, longitude, and the height above ground (in metres) of the wind speed sensor.
Line 3 contains the number of direction sectors, the scale factor (which Windographer always reports as 1.00, and the direction offset value, which Windographer always reports as 0.00.
Line 4 contains the overall frequency (in percent) for each direction sector. These numbers will sum to 100.
Lines 5 and up contain the main frequency table of the TAB file. The rows of this table correspond to the wind speed bins, and the columns correspond to the direction sectors. The first column, however, contains the upper limit of the wind speed bin, in m/s. In the preview that appears in the screenshot above, the '0.5' value that begins the fifth line indicates that the first wind speed bin ends at 0.5 m/s. The '1.5' indicates that the second wind speed bin ends at 1.5 m/s. Each wind speed bin begins at the endpoint of the previous bin, with the first bin always starting at zero m/s.
The values in the other columns indicate number of occurrences or frequency, depending on whether you have chosen to export occurrences or frequencies. Frequency values appear in per mille, and refer to the frequency within each direction sector. So each column of frequency values will sum to 1000. The screenshot above, for example, shows that the wind direction fell within the northerly direction sector 6.00% of the time, within which the wind speed fell within the range of 5.5-6.5 m/s for 94.699‰ of the time. You would need to multiply those two numbers together to find that the wind blew from the north in the range of 5.5 to 6.5 m/s with a frequency of 0.0600 * 0.094699 = 0.00568194, meaning 0.57%.
Windographer can export TAB files that indicate occurrences or frequency versus direction sector and wind speed bin. If the dataset contains multiple wind speed and direction sensors, you can choose the speed and direction sensors upon which to base the bins. You can also choose the number of wind direction sectors, the wind speed bin size, and whether to use the entire dataset or just a subset in the calculations.
See also