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  MADIS Snow Quality Control

MADIS Snow Quality Control


This page provides a detailed description of the quality control (QC) processing and data structures for the snow dataset. For a general overview of MADIS quality control, click here.

Automated Quality Control

Level 1 QC checks are considered the least sophisticated, level 2 the most sophisticated checks. The following table lists the snow variables* that are QC'ed, and the checks that are used:


  MADIS Snow Variables with QC
  ------------------------------------------------------------------------------------------
  Code       Name                      Max     Level | Level 1: |         Level 2:          
                                     Possible        |          |                           
                                     QC Level        | Validity |   Internal      Temporal  
                                                     |          | Consistency   Consistency 
  ------------------------------------------------------------------------------------------
  SD        snow depth                 2                  X            X             X
  SF6       snowfall -- last 6h        2                  X                          X
  SF24      snowfall -- last 24h       2                  X                          X
  SWED      snow water equiv - depth   2                  X            X             X
  SW6       snow water equiv - 6h      2                  X                          X
  SW24      snow water equiv - 24h     2                  X                          X


  Internal Consistency Checks
  ---------------------------

    1   snow depth vs. snow water equiv of depth

The level 1 validity checks restrict each observation to falling within a TSP-specified set of tolerance limits, while the level 2 temporal consistency checks restrict the temporal rate of change of each observation to a set of (other) TSP-specified tolerance limits. In both cases, observations not falling within the limits are flagged as failing the respective QC check. The following table lists the tolerance limits:


  -----------------------------------------
  Validity Checks
  -----------------------------------------
  snow depth                   0 -  300  in
  snowfall -- last 6h          0 -   50  in
  snowfall -- last 24h         0 -   50  in
  snow water equiv - depth     0 -  300  in
  snow water equiv - last 6h   0 -   50  in
  snow water equiv - last 24h  0 -   50  in

  ---------------------------------------------
  Temporal Consistency Checks
  ---------------------------------------------
  snow depth                       50 in/measurement
  snowfall -- last 6h               8 in/hour 
  snowfall -- last 24h              8 in/hour 
  snow water equiv - depth         50 in/measurement
  snow water equiv - last 6h        8 in/hour 
  snow water equiv - last 24h       8 in/hour 



The level 2 internal consistency checks enforce reasonable, meteorological relationships among observations measured at a single station. For example, the maximum water equivalent for the snow depth must not exceed the total snow water equivalent observation made at the same station. If it does, both snow depth and the total snow water equivalent are flagged as failing their internal consistency check.

Subjective Intervention

Two text files, a "reject" and an "accept" list provide the capability to subjectively override the results of the automated QC checks. The reject list is a list of stations and associated input observations that will be labeled as bad, regardless of the outcome of the QC checks; the accept list is the corresponding list of stations that will be labeled as good, regardless of the outcome of the QC checks.

Here are the current subjective intervention lists in use:

QC Data Structures

The MADIS QC information available for each variable includes the following QC structures: a single-character "data descriptor", intended to define an overall opinion of the quality of each observation by combining the information from the various QC checks, and for users desiring detailed information, a "QC applied" bitmap indicating which QC checks were applied to each observation, and a "QC results" bitmap indicating the results of the various QC checks.

The following table provides a complete list of the data descriptors and the bits used in the bitmaps:

  ------------------------------
  MADIS QC Information - Snow
  ------------------------------

  QC Data Descriptor Values
  -------------------------

  No QC available:

   Z - Preliminary, no QC

  Automated QC checks:

   C - Coarse pass, passed level 1
   S - Screened, passed levels 1 and 2
   V - Verified, passed levels 1, 2, and 3
   X - Rejected/erroneous, failed level 1
   Q - Questioned, passed level 1, failed 2 or 3

       where level 1 = validity
             level 2 = internal consistency, temporal consistency
             level 3 = N/A

  Subjective intervention:

   G - Subjective good
   B - Subjective bad

  Bitmask for QC Applied and QC Results
  -------------------------------------

   Bit       QC Check                      Decimal Value
   ---       --------                      -------------
    1        Master Check                        1
    2        Validity Check                      2
    3        Reserved                            4
    4        Internal Consistency Check          8
    5        Temporal Consistency Check         16

The QC bitmask is used in the QC applied and QC result "words" returned along with the QC data descriptor. By examining the individual bits, the user can determine which checks were actually applied, and the pass/fail status of each check that was applied.

In the QC applied word, a bit value of 1 means the corresponding check was applied, a bit value of 0 indicates the check wasn't applied.

In the QC results word, a bit value of 1 means the corresponding check was applied and failed, a bit value of 0 indicates the check passed (given that the check was applied).

The "Master Check" is used to summarize all of the checks in a single bit. If any check at all was applied, this bit will be set in the QC applied word. If the observation failed any QC check, it will be set in the QC results word.

When read as decimal numbers, the different bits that are set in the bitmask are summed together. For example, a QC applied value of 19 should be interpreted as 1 + 2 + 16, meaning the validity and temporal consistency checks were applied.


References

Technique Specification Package 88-21-R1 For AWIPS-90 RFP Appendix G Requirements Numbers: Quality Control Incoming Data, 1993. AWIPS Document Number TSP-032-1992R1, NOAA, National Weather Service, Office of Systems Development.


Last updated 16 June 2016