The USTAR (friction velocity) Filtering module examines whether the CO2 flux (FC) is filtered by using USTAR thresholds. Flux data submitted to AmeriFlux should not be USTAR-filtered.
The module is used to identify the following issues:
- Filtered FC by USTAR threshold
- Filtered USTAR
For each pair of USTAR and FC (i.e., values for the same year), the module finds the lower bound of USTAR when the concurrent FC is not missing (1 and 2 in figure below) and from all USTAR data in a year (3), respectively. The module expects that these two lower bounds match and are close to the expected lower range of USTAR, ~0.01 m s-1 in this example.
Example figure illustrating the USTAR filtering module with detection of potential issues. This example shows the lower bound of USTAR when concurrent FC is not missing (1 and 2) is higher than the lower bound from all USTAR data (3). The difference indicates that the FC data are filtered by using an USTAR threshold of 0.1 m s-1.