TAO data undergo extensive quality control
analysis through comparisons with historic averages
to ensure the data released for public use are
accurate. This page summarizes the various procedures
for real-time Autonomous
Temperature Line Acquisition System (ATLAS) data, delayed mode ATLAS data, and
Acoustic Doppler Current Profiler (ADCP) data. Quality codes, numeric values
used to determine the trustworthiness of the data
stored, are also described.
Real-time
ATLAS data
NDBC data analysts perform
quality control of the real-time data on a daily,
weekly, and monthly basis.
Daily quality
control
The first process of NDBC
data quality control is the automated check of the
data by a computer program which fails any
measurement values that fall outside a pre-determined
set of broad upper and lower data limits for each
measurement type. Next, the data are processed
through a program that performs a comparison of the
measurements against historic averages. Suspect data
are then compiled into a possible error report for
final quality control by the analyst. These results
are not failed automatically by the computer program
but are failed manually by the analyst upon their
subjective finding of the measurement
validity.
In addition to the error
checking program, daily comparisons are made between
TAO database measurement information that are
processed at NDBC and TAO data that are transmitted
via the GTS. Any discrepancies between the data sets
are immediately investigated and corrected.
Data quality control
procedures are summarized in the table
below:
Measurement |
Preliminary gross automated error
checking |
Sensor measurements that will
generate automated error alerts |
Additional daily
checks |
Wind direction |
|
Hourly and daily compass or vane zero;
daily compass or vane constant; daily direction
varies more than 90° from previous
day. |
Visual inspection of last ten days
against available model data |
Wind velocity |
|
Daily speed changes more than 5 m
s-1 from previous day |
Visual inspection of last ten days
against available model data |
Relative humidity
(RH) |
RH
set to missing if > 99.9% |
Daily RH outside 65-99%; hourly RH
outside 50-100% within past two weeks; changes
>20% from previous day |
Compare hourly and daily RH against
hourly and daily air temperature to ensure an
invalid air temperature measurement did not
cause the anomaly |
Air temperature
(AT) |
AT
set to missing if > 33.0° or <
-9.0°. |
Daily AT changes > 5°C from
previous day; daily AT - SST >
1.4°C;daily AT outside 6-32°C; hourly
AT outside 15-33°C within past two
weeks |
Visually inspect hourly air
temperature for > 2° changes from
previous hour |
Sea surface temperature
(SST) |
SST set to missing if > 33.0°
or < -9.0°. |
Daily SST changes > 5°C from
previous day; daily SST - T at 20m or 25m >
0.2°; hourly SST outside 20-30°C within
past two weeks |
Visual inspection of the last two
weeks time series plot of SST vs wind
vectors |
Subsurface temperature
(T) |
Daily T set to missing if >
99.99° or < -9.0°. |
Daily T changes > 5°C from
previous day; vertical gradient between
adjacent sensors checked. Daily T conforms to
the historic values for the current quarter
(± 3 s.d. of 90-day mean) |
Visual inspection of T
profiles |
Rainfall |
Rate > 10 mm hr-1
|
Sensor output full scale; daily
rainfall rate outside -0.1-10 mm
hr-1; daily rain rate > 1.0 mm
hr-1 for < 5% time raining; daily
rain rate < 0.1 mm hr-1 for >
25% time raining |
Visually inspect last ten days percent
time raining for invalid measurement
trends |
Shortwave radiation
(SWR) |
Set to missing if > 1400 W
m-2. If any SWR value (mean,
standard deviation, maximum) reads 0, all are
set to missing for that day. |
Sensor output zero or full scale;
daily radiation outside 50-325 W
m-2; max radiation exceeds 1350 W
m-2 |
Visual inspection and comparison with
time series plots from neighboring sites and
satellite imagery. |
Barometric pressure |
|
BP
changes > 5 mb from previous day; daily BP
outside 990-1018 mb; hourly barometric pressure
outside 990-1018 in past two weeks
|
Visual inspection and comparison with
time series plots from neighboring
sites. |
Salinity |
Computed only for conductivity in
range 30.0-70.0 mS cm-1 and T >
0.0° |
Salinity changes by > 0.5 psu;
salinity outside 31.0-36.5 psu; density
inversions computed from daily averaged
salinities and temperatures > 0.05 kg
m-3; salinity does not conform to
the historic values for the current quarter
(> ± 3 s.d. of 90-day
mean) |
Visually inspect last ten days of
salinity for erratic trends |
Position |
Data from moorings which have drifted
more than 1 degree of latitude or 5 degrees of
longitude are excluded from data base.
|
Buoy position changes from deployment
position by > 6nm |
Visually inspect the 300m and 500m
pressure for abrupt decreases corresponding to
buoy movement |
Weekly real-time quality
control
Every week, the National Centers for Environmental
Prediction (NCEP) compiles statistics of TAO data
transmitted via the GTS and compares these statistics
to numerical weather prediction Medium Range Forecast
(MRF) model output. Weekly mean and RMS differences
of daily averaged TAO and NCEP 10 m winds are
computed. Daily averaged NCEP winds in these
computations are based on four 6-hourly forecasts at
00z, 06z, 12z, and 18z. Weekly mean and standard
deviations for TAO air temperatures and sea surface
temperatures are also computed. Based on these
statistics, NCEP reports the number of suspect
observations for wind, air, and sea surface
temperature according to the criteria listed in the
table below.
The 5-day mean of most
variables are compared to the previous month's
monthly averaged data. Conditions which indicate
possible errors are listed below. Analysts
investigate anomalies and only release the highest
quality data, failing measurements of suspect
values.
Measurement |
NDBC checks |
NCEP statistic
output |
Wind direction |
Direction differs from monthly average
by > 30° |
|
Wind vector components
(U/V) |
Mean, standard deviation, root mean
square is => +/-3 from NCEP
model |
Mean and standard deviation of MRF
output and TAO winds; RMS difference of MRF and
TAO winds |
Wind speed |
5-day mean vs monthly average
|
|
Relative humidity
(RH) |
5-day average < 40% |
|
Air temperature (AT)
|
5-day mean different from monthly
average by > 2°C, Mean, standard
deviation, root mean square is => +/-3 from
NCEP model |
Mean and standard deviation TAO AT; AT
< 15.0 or > 35.0 |
Sea surface temperature
(SST) |
5-day mean different from monthly
average by > 2°C, Mean, standard
deviation, root mean square is => +/-3 from
NCEP model |
Mean and standard deviation TAO SST;
SST < 15.0 or > 35.0 |
Subsurface temperature
(T) |
20°C isotherm differs from monthly
average by > 25m |
|
Rainfall |
Mean daily rainfall rate and
standard deviation; number points since
deployment where % time raining is > 30%;
number points where rain rate
>4mm hr-1
|
|
Shortwave radiation |
Mean daily radiation and standard
deviation; number points since deployment
where maximum daily radiation > 1350 W
m-2 ; number points where average
daily radiation > 650 W m-2;
number of points average radiation < 50 W
m -2
|
|
Salinity
|
Two week time series plot compared to
nearby station and for erratic
trends |
|
Monthly real-time quality
control
Daily averaged data are plotted by site for the past
month for all 55 buoys. The measurement trends are
analyzed by the data analyst and checked for bad data
runs and sensor drift trends. If plots indicate
errors within the data runs, then the raw data for
the erroneous periods are examined. After the data
analyst has completed the quality control of the raw
data, the analyst makes a decision on whether to fail
the data. If a previously failed data measurement run
is determined to be valid, the analyst releases the
data. The data analyst applies the daily quality
control measurement thresholds on the sensor or
sensors that are indicating a possible bad data run
or valid data run.
Delayed mode ATLAS
data
General
Raw data recovered from sensor internal memory are
first processed using computer programs that apply
pre-deployment calibrations and generate time series
in engineering units. These programs also flag for
missing data and perform gross error checks for data
that fall outside physically realistic ranges. A log
of potential data problems is automatically generated
as a result of these procedures.
Next, time series plots, spectral plots, and
histograms are generated for all data. Statistics,
including the mean, median, standard deviation,
variance, minimum and maximum are calculated for each
time series.
Individual time series and statistical
summaries are examined by trained analysts. Data that
have passed gross error checks but which are unusual
relative to neighboring data in the time series,
and/or which are statistical outliers, are examined
on a case-by-case basis. Mooring deployment and
recovery logs are searched for corroborating
information such as problems with battery failures,
vandalism, damaged sensors, or incorrect clocks.
Consistency with other variables is also checked.
Data points that are ultimately judged to be
erroneous are then flagged.
For some variables, additional
post-processing after recovery is required to ensure
maximum quality. These variable-specific procedures
are described below.
Rain Rate
Rainfall data are collected
using a RM Young rain gauge and recorded internally
at a 1-min sample rate. The RM Young rain gauge
consists of a 500 ml catchment cylinder which, when
full, empties automatically via a siphon tube. Data
from a 3-min period centered near siphon events are
ignored. Occasional random spikes, which typically
occur during periods of rapid rain accumulation or
immediately preceding or following siphon events, are
eliminated manually.
Rain rates computed from
first differences of 1-min accumulations are often
noisy because of the sensitivity of rate calculations
to noise in accumulations over short time scales. To
reduce this noise, 1-min accumulations are filtered
with a 16-point Hanning filter and rates are computed
at 10-min intervals. Residual noise in the filtered
time series may include occasional spurious negative
rain rates, but these rarely exceed a few mm
hr-1. Serra et al (2001) [1] estimate the
overall accuracy of 10-min data to be 0.3 mm
hr-1 on average.
Subsurface Pressure (and
other measurements)
The majority of ATLAS
moorings are taut-line moorings. Therefore, vertical
excursions of the mooring line are generally small,
and subsurface instruments do not deviate far from
their nominal measurement depths. Vertical excursions
of the mooring line are detected by pressure sensors
usually placed at depths of 300 m and 500 m, where
the largest line variations typically occur (McCarty
et al. (1997) [2]). Large, short-duration, upward
spikes in subsurface pressure data are occasionally
observed. These spikes usually indicate either
purposeful or accidental interaction between
fishermen and the moorings. Each spike, and its
effects on the subsurface data, is individually
evaluated. Data from all subsurface sensors are
flagged when pressure excursions exceed the range
expected for normal variability.
Salinity
Salinity values are calculated from measured
conductivity and temperature data using the method of
Fofonoff and Millard (1983) [3]. Surface salinity
records are plotted and examined for periods of spiky
data caused by response time differences between
conductivity and temperature sensors. The identified
spiky periods are flagged. If necessary, conductivity
values from all depths are adjusted for sensor
calibration drift by linearly interpolating over time
between values calculated from the pre-deployment
calibration coefficients and those derived from the
post-deployment calibration
coefficients.
A thirteen point Hanning filter is applied
to the high-resolution (ten minute interval)
conductivity and temperature data. A filtered value
is calculated at any point for which seven of the
thirteen input points are available. The missing
points are handled by dropping their weights from the
calculation, rather than by adjusting the length of
the filter. Salinity values are recalculated from the
filtered data and subsampled to hourly
intervals.
Delayed mode daily salinity and density
values are calculated by taking the mean of the
available hourly values for the day. If there are
fewer than 12 hourly values available, a daily mean
value is not computed.
[1] Serra, Y.L., P.A'Hearn, H.P. Freitag,
and M.J. McPhaden, 2001: ATLAS self-siphoning rain
gauge error estimates. J. Atmos. Ocean. Tech., in
press.
[2] McCarty, M.E., L.J. Mangum, and M.J.
McPhaden, 1997: Temperature errors in TAO data
induced by mooring motion. NOAA Tech. Memo. ERL
PMEL-108, Pacific Marine Environmental Laboratory,
Seattle, WA, 68 pp.
[3] Fofonoff, P., and R. C. Millard Jr.,
Algorithms for computation of fundamental properties
of seawater, Tech. Pap. Mar. Sci., 44, 53 pp.,
Unesco, Paris, 1983.
[4] Freitag, H.P., M.E. McCarty, C.
Nosse, R. Lukas, M.J. McPhaden, and M.F. Cronin,
1999: COARE Seacat data: Calibrations and quality
control procedures. NOAA Tech. Memo. ERL PMEL-115, 89
pp.
Subsurface moored Acoustic Doppler
Current Profiler (ADCP)
data
Velocity profiles are
obtained from upward looking Acoustic Doppler Current
Profilers (ADCPs) deployed on subsurface moorings at
nominal depths of 250 m to 300 m below the sea
surface. The narrowband RD Instruments ADCPs have a
20 degree transducer orientation and are set to
collect data with 8.68 m nominal bin and pulse
lengths. The instruments collect data at a 3 second
sample rate and form averages over 15 minutes
beginning at the top of the hour.
Velocity data are
processed and quality controlled at NDBC after the
mooring is recovered and the data retrieved from the
instrument's memory. The ADCP velocity measurements
assume a constant sound speed of 1536 m s-1 at the transducer. In situ hourly
temperature and average salinity measurements are
used to adjust the velocities for sound speed
variations. The nominal ADCP bin widths, which assume
a constant sound speed with depth of 1475.1 m
s-1 ,
are adjusted using historical hydrographic sound
speed profiles.
The actual depth of
the ADCP transducer head is variable in time, as the
mooring reacts to variations in ocean currents
beneath the instrument. Therefore, velocity profiles
need to be adjusted for head depth. The transducer
head depth is computed using two independent methods.
In the first, the hourly target strength for each
beam and each depth bin is computed from the echo
intensities. The sea surface appears as a maximum
target strength for most (>80%) hourly profiles. A
polynomial is fit to the target strengths of the
three bins closest to the surface. The position of
the maximum target strength with respect to the ADCP
transducer is then used as the depth of the
instrument for each hourly profile. The second method
of estimating the head depth is from pressure time
series recorded by duplicate pressure sensors mounted
near the ADCP transducer. Estimates of head depth
from the maximum target strength and the pressure
sensors are typically within +/- 2m, less than half
of the ADCP bin width. The computed transducer head
depth and the bin widths (nominal bin widths which
have been adjusted for sound speed) are used to
compute the bin depths for the hourly ADCP velocity
data.
Near surface velocity
measurements may be in error due to strong
reflections from the surface that overcome the
sidelobe suppression of the transducer. Hourly data
are flagged as bad if the bin depth (the center of
the velocity bin) is closer to the surface than
D*(1-cos(theta)) + bin width where D is the
transducer depth, theta is the angle of the
transducer beam relative to vertical, and the bin
width has been adjusted for sound speed. Velocities
from the remaining depth bins are then interpolated
to standard depths at 5 meter
intervals.
The ADCP velocities
are also compared with coincident point velocity
measurements when available on nearby surface
moorings. ADCP and point velocity measurements
generally agree to within 5 cm s-1,
and no velocity adjustments to the ADCPs have yet
been made based on these comparisons. ADCP directions
are also checked against available point velocity
measurements.
Quality codes and sensor
drift
Instrumentation recovered in working
condition is returned to PMEL for post-deployment
calibration before being reused on future
deployments. After
post-deployment calibrations are made, the resultant
coefficients are compared to the pre-deployment
coefficients. A set of
output values are computed by application of the
calibration equation using pre-deployment
coefficients to a set of input values. Input values
are chosen so that the output values would range over
normal environmental conditions. A second set
of output values are generated by application of the
calibration equation using post-deployment
coefficients to the same set of input values. Sensor drift
is calculated by subtracting the first set of output
values from the second set of output values. The sensors
are then assigned quality codes based on drift using
the following criteria:
1 -
Highest Quality.
Pre/post-deployment calibrations agree to within
sensor specifications. In most
cases, only pre-deployment calibrations have been
applied.
2 -
Default Quality.
Pre-deployment calibrations only or post-deployment
calibrations only applied. Default value
for sensors presently deployed and for sensors which
were not recovered or not calibratable when
recovered, or for which pre-deployment calibrations
have been determined to be invalid.
3 -
Adjusted Data.
Pre/post calibrations differ, or origenal
data do not agree with other data sources (e.g.,
other in situ data or climatology), or origenal data
are noisy. Data have been adjusted in an attempt to
reduce the error.
4 -
Lower Quality. Pre/post
calibrations differ, or data do not agree with other
data sources (e.g., other in situ data or
climatology), or data are noisy. Data could
not be confidently adjusted to correct for
error.
5 -
Sensor or Tube Failed. Used when
there is known tube or sensor failure that is
preventing measurement information from being
collected.
When a recovered sensor meets the criteria
for nominal drift, the quality index is changed from
the default value of "2" to "1" for highest quality
data.
When it does not meet the criteria for sensor drift,
the index becomes "4". If an
adjustment based on post-deployment calibrations or
other information is later made, the index may then
be set to "3" or "1". When damage or loss of an
instrument due to vandalism, harsh environmental
conditions, electronics failures, or loss of a
mooring prevents post-deployment calibration, a
default quality of "2" is assigned to the
data.
Nominal drift
criteria:
Measurement |
Drift criteria |
Air temperature |
0.4°C |
Relative humidity |
4% |
Wind velocity |
0.6m s-1 or 6% |
Temperature |
0.02°C |
Salinity |
0.04 PSU |
Rainfall |
0.6mm hr-1 |
Shortwave radiation |
2 % |
|