Jesse Enloe and Shawn R. Smith
World Ocean Circulation Experiment (WOCE)
Surface Meteorological Data Assembly Center
Center for Ocean Atmospheric Prediction studies
Florida State University
May 20, 1999
Report WOCEMET 99-06
Version 1.0
Introduction:
This report summarizes the quality of surface meteorological data collected by the Knorr (identifier: KCEJ) IMET system during one WOCE cruise made in 1995. The data were provided to the Florida State University Data Assembly Center (DAC) in electronic format by the Woods Hole Oceanographic Institute. They were converted to standard DAC netCDF format and then processed using an automated data screening program which adds quality control flags to the data, highlighting potential problems. Finally, the Data Quality Evaluator (DQE) reviews all the data and the processor flags. Flags are then added, modified, and deleted according to the judgement of the DQE and other DAC personnel. An in depth description of the WOCE quality control procedures can be found in Smith et al. (1996). The data quality control report summarizes all flags for the Knorr IMET data and explains the reasons why these flags were assigned.
Statistical Information:
The Knorr data were expected to include observations taken every
minute. Values for the following variables were collected on the cruise:
| Time | (TIME) |
| Latitude | (LAT) |
| Longitude | (LON) |
| Platform Heading | (PL_HD) |
| Platform Course Over Ground | (PL_CRS) |
| Platform Speed Over Ground | (PL_SPD) |
| Platform Speed Over Water | (PL_SPD2) |
| Platform Relative Wind Direction | (PL_WDIR) |
| Platform Relative Wind Speed | (PL_WSPD) |
| Earth Relative Wind Direction | (DIR) |
| Earth Relative Wind Speed | (SPD) |
| Atmospheric Pressure | (P) |
| Air Temperature | (T) |
| Sea Temperature | (TS) |
| Relative Humidity | (RH) |
| Precipitation | (PRECIP) |
| Atmospheric Radiation | (RAD) |
Details for the cruise are listed in Table 1 and include cruise dates,
number of records, number of values, number of flags, and total percentage
of data flagged. A total of 577,738 values were evaluated with 15,589 flags
added by the preprocessor and DQE for a total of 2.70% of the values being
flagged. Note that three parameters (DIR, SPD, and PRECIP) were found to
be of poor quality and are not included in the statistical results. These
values will not be released with the data for this cruise (see discussion
below).
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Summary:
The overall quality of the data collected from the Knorr proved to be good except for the variables DIR, SPD, and PRECIP. After evaluation of the three variables it was determined that they should not be included in the public release. Table 2 details the distribution of flags among the remaining 14 variables. Discussions of the flagged and removed variables follow.
Table 2: Number of Flags and Percentage Flagged for Each Variable
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Variable |
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LAT LON PL_HD PL_CRS PL-SPD PL_SPD2 PL_WDIR PL_WSPD P T TS RH RAD |
3,134
127 8,941 |
16
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2,655
211 355
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81 69
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0 0 0 2,655 0 3,134 0 0 16 292 424 127 8,941 |
0.00 0.00 0.00 6.43 0.00 7.59 0.00 0.00 0.04 0.71 1.03 0.31 21.67 |
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| *Percentage < 0.01 |
Deleted Data:
True Winds:
Using the platform wind and navigation data collected from the Knorr,
the true winds were calculated by the DAC employing a tested code. A detailed
description of the true wind calculation procedures can be found in Smith
et al. (1999). Viewing a time chart of the true winds using the Visual
Data Assessment Tool (VIDAT) determined that the calculated true winds
echoed the movement of the ship. Correct true winds show no signal of the
ship's movement. Not being able to pinpoint the source of the error (possibly
an error in the heading or the reference of the zero degree line on the
anemometer to the ship), both the earth relative wind direction and speed
were expunged.
Precipitation:
The precipitation instrument used by the Knorr was the R.M. Young
Self-Siphoning Rain Gauge; a gauge designed to collect water until it is
full (50mm) and then self-siphon in a relatively short amount of time.
There was an obvious leakage problem with this rain gauge, therefore preventing
any accurate measurement of precipitation. The precipitation data were
extracted as well.
Other Problems:
Platform course received 2,655 cautionary (K) flags over a period of
approximately two days. In this same 48-hour period the ship's platform
speed over ground was less than 1 m/s and the data from the speed over
water were missing altogether. The ship's course showed very little or
no variability in this time, which is highly uncharacteristic given the
low speed of the ship. Consequently, the DQE assessed these data as questionable.
The air temperature and sea temperature both experienced numerous spikes.
It appears that these data experienced more electronic noise as the cruise
progressed. Isolated spikes were appropriately flagged an S, while groups
of data that showed high variability (spikes too numerous to be flagged
individually) were all flagged as suspect with the K flag.
Relative humidity received 127 B flags from the preprocessor for values
over 100 percent. These values are probably due to instrument calibration
error rather than actual supersaturation.
Negative values of radiation were recorded at night and were assigned
the B flag by the preprocessor. Being that radiation is a measurement of
incoming solar radiation, these negative values would be physically meaningless
and are likely to result of a calibration problem at low values.
The G flags assessed to pressure by the preprocessor were left in place
to highlight values that are greater than four standard deviations from
the climatological means (da Silva et al. 1994).
At very low platform speeds it is possible to have negative values for
PL_SPD2, which are consequently assessed B flags by the preprocessor. The
user should note that the EDO speedlog uses Doppler technology to measure
the speed of the ship relative to the water. When the ship's speed is low,
wind waves, and currents can cause realistic negative values. Negative
values also occur when the vessel is in reverse.
Final Comments:
The quality of the Knorr IMET data ranges from very good to poor.
The use of the data is left up to the discretion of the user. The problem
experienced by PL_CRS was isolated to a few days in a month's worth of
data. The rest of its data is of excellent quality. Values of PL_SPD2 flagged
B due to negative recorded values could conceivably be good data. The user
may want to disregard the boundary flags.
References:
Smith, S.R., C. Harvey, and Legler, D.M., 1996: Handbook of Quality
Control Procedures
Smith, S.R., M.A. Bourassa, and Sharp, R.J., 1999: Establishing More
Truth in True Winds.
da Silva, A.M., C.C. Young and S. Levitus, 1994: Atlas of Surface
Marine Data 1994, Volume 1:
and Methods for Surface Meteorology Data. WOCE Report No. 141/96, Report WOCEMET
96-1, Center for Ocean-Atmospheric
Prediction Studies, Florida State University,
Tallahassee FL 32306-2840
J. Atmos. Oceanic Technol., 16, 939-952.
Algorithms and Procedures. NOAA Atlas Series.