QCF: overall quality control flag

QCF: overall quality control flag#

Note

The QCF (Quality Control Flag) is calculated from multiple flags from single tests.

QCF is a flag that shows the overall quality of the respective data point.
QCF can be: 0=best data, 1=OK data, 2=bad data
QCF is calculated from single test flags summed together:
- For records where the sum > 2: QCF=2
- For records where the sum = 2 from a single flag with value 2: QCF=2
- For records where the sum = 2 and no single flag is 2 (i.e., the sum comes from two single flags with value 1): QCF=1
- For records where the sum = 1 (i.e., only on single flag with value 1): QCF=1
- For records where the sum = 0 (i.e., all single flags are zero): QCF=0 (best quality fluxes)
Flags for single tests are created in Level-2, Level-3.2 and Level-3.3.
Genereally, single tests can be:
- Hard flags: Some tests yield either 0 or 2, but have no flag=1. This is the case if the test is so crucial that if the test fails, data are considered bad. If one test flag=2, then the QCF is automatically 2, no matter the other tests. Flags from such tests are also called hard flags.
- Soft flags: If a test results is flag=1, it can still be OK data for some analyses. Even if there is a second test with flag=1, data might still be OK. If one or two flags are 1, and there is no flag=2, then the QCF is also 1. These flags are less crucial and are therefore soft flags.
QCF always uses the same logic, for both flux and meteo data, only the single tests are different.
This was done for all fluxes, however, for NEE the requirements were stricter during the nighttime than during the daytime. For NEE, daytime QCF flags of 0 and 1 were accepted (flag 2 = bad data), but during nighttime only QCF flags with 0 were retained (flags 1 and 2 = bad data). For all other fluxes, QCF flags of 0 and 1 were accepted during daytime and nighttime (flag 2 = bad data).
Example:
- FLAG_L3.3_CUT_50_NEE_L3.1_QCF is the flag after Level-3.3, for flux NEE_L3.1 (storage-corrected) for the USTAR scenario CUT_50. This flag is applied to flux NEE_L3.1, producing the quality-filtered flux NEE_L3.1_L3.3_CUT_50_QCF. In addition, another flux variable is produced, containing only highest-quality fluxes: NEE_L3.1_L3.3_CUT_50_QCF0 (all single flags are zero).