QC2d-1: Dip test

• fs=2 for A(t),
• fs=2 for A(t-1)
• fs=1 for A(t-2)

A dip is detected if and where is specified in the algorithm configuration. then receives a corrected value by interpolation.

The neighboring points at t-2 and t may not have missing original values and have to match the flags specified in linear_before_*flags and linear_after_*flags, respectively.

If there are 2 additional usable data points before and 1 usable additional data point after t-1, Akima algorithm are used for the interpolation and cfailed receives a message QC2d-1-A. To be usable, the original value may not be missing (-32767) and has to match the flag specification akima_*flags in the configuration if it is not an immediate neighbor of t-1, and .

If akima interpolation is not possible, linear interpolation is used and cfailed receives a message QC2d-1-L.

Beyond the standard configuration options detailed in the user guide, the following options are available for DipTest.

• candidate_cflags (default: fs=2&fhqc=0) and candidate_uflags (default: /no constraint/) specify the flags expected for A(t-1). Dips that do not match these flags will not be detected.
• akima_cflags (default: /no constraint/) and akima_uflags (default: U2=0) specify the requirements for additional points for akima interpolation
• linear_before_cflags (default: fs=1) and linear_before_uflags (default: /no constraint/) specify flag requirements for A(t-2)
• linear_after_cflags (default: fs=2&fhqc=0) and linear_after_uflags (default: /no constraint/) specify flag requirements for A(t)
• dip_flagchange (default: fs=9) specifies the flag modification for t-1 if an interpolation is performed
• afterdip_flagchange (default: fs=4) specifies the flag modification for t if an interpolation is performed
• ParValFilename specifies either (no default, must be specified, see below for examples)
  • a filename containing a list of parameter-delta pairs
  • or a list of parameter-value pairs when starting with list:

If using a file to specify parameter-delta pairs, the file must be formatted like this:

1   9.0
2   60.0
81  10.0
85  10.0
86  12.5

If using a list to specify parameter-delta pairs, the parameter must be given like this:

ParValFilename= list: 172 12.0, 174 12.0, 175 12.0, 176 12.0, 177 12.0, 211 7.5

Relevant parameters are all those subject to the step check (QC1-3a). The implementation is only tested for hourly measurements.

The report 24/93 KLIMA “Theoretical analysis of the dip-test in quality control of geophysical observations” by Petter Øgland describes a diptest run independent from a previous step test.

http://klima.dnmi.no/rapporter/Klimarapporter/1993/24_93.pdf

Comparison between Øgland's parameter δ and the current Kvalobs step criterium shows:

 δ(UU) = 30%  versus the Kvalobs step(UU) = 30%
 δ(FF) = 30 m/s versus the Kvalobs step(FF) = 10 or 12.5 m/s
 δ(TA) = 10.5 °C versus the Kvalobs step(TA) = 7.5 °C

For wind and temperature the Kvalobs dip test will flag more often than Øgland's formula. For humidity Øgland's formula will flag for errors without corresponding flag from the Kvalobs dip test. This occurs in a “skew” dip when the step test criterion is met in one of the two cases.