Quality Assessment with QC_TND

QC_TND performs quality-control (QC) checks on the major broadband components:
  • Global or Total (T),
  • Direct Normal (N),
  • and Diffuse (D).
No QC will be performed if the solar zenith angle exceeds 75 degrees.

The QC_TND flags are explained in Table 1 below, while Table 2 outlines the process by which these flags are determined.



Table 1. QC_TND Flag Values
FlagFlag Value
0   untested parameter
1

  parameter took and passed tests for physical limits, model limits for tolerances less than 3%,
  and reasonable coupling with other parameters for tolerances less than 3%.
2   parameter inspected by hand and eye; passed inspection (superseded by 1, if 1 is true).
3   parameter inspected by hand and eye; did not pass inspection (supersedes 1).
4   parameter was received as missing; was interpolated from neighbors.
5

  parameter was received as missing; was able to be modeled by coupling with the other 2
  parameters. The modeled parameter passed all tests for tolerances less than 3%.
6

  parameter was received as missing; was patched by a method other than modeling
  or interpolation.
7   parameter lower than 0.
8   parameter higher than ETR.
10-97
parameter exceeded the 3% tolerance in one of 4 ways:
0 - too low when coupled with other parameters
1 - too high when coupled with other parameters
2 - too low when compared to a model
3 - too high when compared to a model

The particular error is determined by the remainder of MOD (Flag + 2, 4). The percentage error is determined by dividing (Flag + 2) by 4. The error is greater than or equal to the integer result, but less than the next highest, except for the 94-97 flags, which signify errors greater than or equal to 24%, no ceiling.

EXAMPLE:

Flag = 67 becomes 69 when 2 is added. MOD ( 69, 4 ) = 1, or a coupled high error was detected first. Percent = 69/4 = 17; so the mismatch was: the mismatch is no less than 17% and less than 18%.
Please note that the highest percentage problem is reported.

For radiation values less than 210 W/m2, absolute tolerances are used. In the example above, if the radiation value were 150 W/m2, the error would be between 17% and 18% of 210 W/m2, or about 36 W/m2.

99   parameter is missing.


Table 2. QC_TND Tests
GroupTest Performed
  I
Parameter within physical limits.
For each parameter:
  1. Is it less than 0?
  2. Is it greater than ETR?
  3. Is it greater than the model maximum?

If the parameter fails ("yes" to any question) any of these tests, it will be flagged according to the results of the Group II tests.

  II
A clear day was indicated.
Global above clear-day threshold (Bird):
  1. Is Direct lower than Global minus Bird Diffuse?
  2. Is Diffuse lower than Rayleigh Diffuse?
  3. Is Diffuse higher than Bird Diffuse?

Direct above clear-day threshold (Bird):

  1. Is Global lower than Direct plus Rayleigh Diffuse?
  2. Is Diffuse lower than Rayleigh Diffuse?
  3. Is Diffuse higher than Bird Diffuse?

Flags are assigned now. If a single parameter failed Group I testing, it is the only one flagged. Otherwise, all parameters that failed ("yes" to a Group II question) are flagged.

  III
  Check coupling among parameters (if a test fails ("yes" to a question) and there was no prior Group I or Group II failure flag all parameters).
  1. By what percent is Global less than the Direct?
  2. By what percent is Global less than the Diffuse?
  3. By what percent is Direct less than the Global minus the maximum Diffuse (KD = 0.55)?
  4. By what percent is Global less than the Direct plus the Diffuse?
  5. By what percent is Global greater than the Direct plus the Diffuse?

At this point, the largest percentage is applied to the questionable parameter(s).



NOTE:

Bird is the Bird clear-sky model, Tau500 = 0.85., water vapor = 6.0 cm.
These values have been selected to produce a very "dirty", dim sky.


  Rayleigh is the Rayleigh clear-sky model, the "cleanest" possible clear sky.



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