On the Performance, Impact, and Liabilities of Automated Precipitation
Gauge Screening Algorithms

**X. Automated gauge QC and multi-sensor precipitation analysis at
NCEP**

The NCEP Stage II (http://www.emc.ncep.noaa.gov/mmb/ylin/pcpanl/stage2/)
is a real-time, hourly, multi-sensor (radar+gauges) national
precipitation analysis produced at NCEP and archived at NCAR (Lin and
Mitchell, 2005).

Currently the Stage II program uses a "gauge reject list" to screen out
gauges that have previously been flagged as problematic. This gauge
reject list is updated manually, following the procedure below:

1.  Visual inspection of display of Stage II analysis for persistent
    "bull's-eye" features

2.  Check the 24h accumulation of the hourly Stage II analysis against
    the NCEP/CPC 1/8 deg daily gauge analysis
    (<http://www.emc.ncep.noaa.gov/mmb/ylin/pcpverif/scores/docs/QandA.html#RFC8>)

3.  If Step 2 reveals that the bull's-eye in the Stage II analysis
    appears to be unrealistic, check the FSL gauge monitoring page
    (<http://precip.fsl.noaa.gov/hourly_precip.html>) to search for a
    possible HADS gauge that is responsible; verify suspected bad gauge
    reports against raw hourly gauge report files

4.  If an apparently-faulty gauge is identified in Step 3, add this
    gauge to the rejection list

This approach has several limitations:

1.  Very labor intensive

2.  Not always possible to flag a faulty gauge within a day or two of
    its misbehavior

3.  Only possible to identify gauges reporting abnormally large or
    persistent values (e.g. not possible to identify false zero reports)

4.  There is no measure in place to monitor rejected gauges to move it
    out of the reject list

Updates to the gauge reject list is done infrequently because of the
amount of work involved, and because a concern of thinning the gauge
population in the long run. As of June 2005, 80 gauges have been placed
on the reject list.

Figures 1 and 2 and Table 1 summarize a one-day snapshot of hourly
gauges flagged by the gauge QC program and those listed in the current
HADS reject list for the operational Stage II processing. Obviously the
automated gauge QC program is much more effective in screening out
problem gauges since it can check for more than the simple problem of
excessive amounts.

![](media/image1.png){width="4.927083333333333in"
height="3.8958333333333335in"}

*Figure 1. Good (black) and bad (red) hourly gauges from the gauge QC
program, for 16 June 2005.*

![](media/image2.png){width="5.0625in" height="4.0625in"}

*Figure 2. Gauges in the HADS reject list used by Stage II processing.
Red and black dots denote that the gauge QC program for 16 June 2005
found the gauges to be bad/good, respectively. Gauges marked in blue
were not found in that day's HADS reports*.

+----------+----------------+----------------------------------+-----+
| A        | Total number   |                                  | 6,  |
| utomated | of reporting   |                                  | 636 |
|          | HADS gauges    |                                  |     |
| gauge QC |                |                                  |     |
+----------+----------------+----------------------------------+-----+
|          | Number of good |                                  | 5,  |
|          | gauges         |                                  | 697 |
+----------+----------------+----------------------------------+-----+
|          | Number of      |                                  | 939 |
|          | flagged gauges |                                  |     |
+----------+----------------+----------------------------------+-----+
|          | Reason for     | Daily sum \> 12"                 | 9   |
|          |                |                                  |     |
|          | flagging (not  |                                  |     |
|          |                |                                  |     |
|          | all listed;    |                                  |     |
|          |                |                                  |     |
|          | some overlap   |                                  |     |
|          |                |                                  |     |
|          | between        |                                  |     |
|          | reasons)       |                                  |     |
+----------+----------------+----------------------------------+-----+
|          |                | ≥ daily sums ≥ 5"                | 80  |
+----------+----------------+----------------------------------+-----+
|          |                | Monthly Sum ≥20"                 | 172 |
+----------+----------------+----------------------------------+-----+
|          |                | Failed daily station neighbor    | 228 |
|          |                | check                            |     |
+----------+----------------+----------------------------------+-----+
|          |                | Failed hourly station neighbor   | 725 |
|          |                | check                            |     |
+----------+----------------+----------------------------------+-----+
| HADS     | Number of      |                                  | 80  |
|          | gauges on list |                                  |     |
| reject   |                |                                  |     |
| list     |                |                                  |     |
|          |                |                                  |     |
| used by  |                |                                  |     |
|          |                |                                  |     |
| Stage II |                |                                  |     |
+----------+----------------+----------------------------------+-----+
|          | Listed gauges  |                                  | 19  |
|          | classified     |                                  |     |
|          | also flagged   |                                  |     |
|          | as bad by      |                                  |     |
|          |                |                                  |     |
|          | automated QC   |                                  |     |
|          | program        |                                  |     |
+----------+----------------+----------------------------------+-----+
|          | Listed gauges  |                                  | 34  |
|          | classified as  |                                  |     |
|          | good by        |                                  |     |
|          | automated QC   |                                  |     |
|          | program        |                                  |     |
+----------+----------------+----------------------------------+-----+
|          | Listed gauges  |                                  | 27  |
|          | that did not   |                                  |     |
|          | appear in obs  |                                  |     |
|          | this day       |                                  |     |
+----------+----------------+----------------------------------+-----+

*Table 1. Summary of attributes of automated gauge QC results for 16 Jun
2005, and of the HADS reject list used by NCEP Stage II processing.*

The numbers shown in Table 1 are fairly representative of test
runs/cross comparisons conducted during 16-30 Jun 2005. Of the 80 gauges
on the manually edited HADS reject list, 27 did not appear on received
HADS reports during 12Z 15 -- 12Z 16 June 2005. The number of rejected
gauges listed as no show varies slightly from day to day, indicating
that while some of these stations might simply not have reported on any
individual day, others could be due to station IDs no longer used by
HADS network. On this day (for example), the percentage of bad gauges
from the automated gauge QC program is 14%, while the percentage of
"actually bad" gauges (when validated against the automated gauge QC,
and when the inactive gauges are excluded) on the manually edited reject
list is 56%. It appears that while some gauges placed on the reject list
over the years have since been judged to be no longer unreliable, others
do prove to be prone to persistent problems.

Parallel test was conducted during 17-30 Jun 2005 to use the bad gauge
list from the previous day's automated gauge QC program instead of the
80-member HADS reject list to screen out problematic gauges. A snapshot
of the comparison is shown in Fig 3.

![](media/image3.png){width="2.9694444444444446in"
height="2.7756944444444445in"}![](media/image4.png){width="2.941666666666667in"
height="2.7604166666666665in"}

![](media/image5.png){width="2.93125in" height="2.7604166666666665in"}

*Figure 3. Top left: Stage II analysis for the hour ending 08Z 17 June
2005, using the automated gauge QC-generated bad-gauge list for 16 June.
Top right: operational Stage II analysis using the 80-member HADS reject
list. Bottom: difference between the first (left) and second (center)
analyses.*

Overall the differences between the test Stage II analysis (excluding
gauges flagged by automated QC program) and the operational Stage II are
quite small. In the example shown in Figure 3, in southeast Oklahoma,
the test analysis has slightly higher value than the operational
analysis, and it appears to be caused by the excluded gauges (mostly by
failing the hourly/daily neighbor checks) reporting lower values of
precipitation amounts. The difference in South Dakota in Figure 3 is an
anomaly -- the gauge PIES2 (Pierre, ND) is listed in the operational
HADS reject list, and has been reporting \~0.2" per hour of rainfall on
and off, for roughly half of the hours each day, day after day, but it
has eluded the detection of the automated gauge QC program.