Center for Ocean Atmosphere Prediction Studies
FSU WOCE Surface Meteorology DAC/SAC
Report WOCEmet 95-1
January 4, 1995
This report will consist of a discussion of the differences found when comparing the meteorological parameters reported in the transmitted digital COADS LMRF.6 data (Comprehensive Ocean-Atmosphere Data Set, Long Marine Reports Fixed) and those reported in the written METEOR cruise 18 station model record (Appendix A). Such a comparison serves as a preliminary step in quality checking two data sets which should be identical. Moreover, this report will serve to emphasize and to specifically examine the significant discrepancies found in each occurrence of the reported wind speeds.
The differences found between the digital data and the written standard meteorological station model record for the METEOR cruise 18, were in part consistent due to the conversion from one form of coding to the other, and in part sporadic. The consistent differences describe those discrepancies which contain a distinct pattern, while sporadic differences express irregular differences.
Consistent differences occurred in four different weather parameters. First, all the sea level pressure values were recorded to one decimal place in the station model record, but were reported to two decimal places in the COADS data, with the second decimal value being a nine. Further, digital air temperature, dew point temperature, and sea surface temperature were all given to two decimal places in the COADS data, but were rounded to the nearest whole number in the station model record.
Prior to looking closely at the specific sporadic differences in the two data sets, it is important to mention two points regarding the COADS data. First, one observation set that appeared in the station model record was missing in the digital data completely. The missing data occurred on September 23 at 18Z. Second, the data on September 13 at 18Z and September 16 at 12Z, were recorded twice in the COADS data, but the data contained some slightly different values for the same date and time. For example, all the weather parameters were identical for the duplicate reports given for 18Z on September 13, except the latitude and longitude values which were reported as zero in the second report. Further, on September 16 at 12Z there were two duplicate reports given; however, the two latitude values were reported as 52.90° N and 53.00° N.
The remaining weather parameters referred to in this report, differ in some form or other, when comparing the station model record to the COADS data. These differences; therefore, are described throughout the remainder of the report and are illustrated in the following tables.
In general, the cloud cover that was reported in the station model record was very difficult to decipher due to the poor quality of the drawing. Nonetheless, an attempt was made to compare the cloud cover data and to identify any differences that may exist (Table I). The fractional cloud covers in the station model record are reported in tenths, with 10/10 representing overcast skies, for example.
The differences in visibility are large, due partially to the extreme change in value that corresponds to a single increment step in the digital code. For example, a digital code of 96 refers to a value of 2-1/2 miles, while the next code of 97 corresponds to a value of 6-1/4 miles. For this reason, a discrepancy in the code number reported corresponds to a large difference in the visibility reported (Table II).
In comparing the latitude and longitude values, it was found that there were two notable discrepancies. One was the duplicate reports on September 13 and 16, as noted previously. The second occurred on September 12 at 18Z, when the COADS data reported a latitude value of 55.1° N, while the station model record gave a latitude value of 55.5° N.
There were no differences found in comparing the wind directions; however, large differences existed in the wind speeds of the two data sets. These differences were very sporadic, suggesting that such discrepancies could be related to differences in reporting systems. In all but two cases, the digital wind speeds, as reported by COADS, were slower than the station model wind speeds. The two exceptions occur on September 11 at 18Z and on September 13 at 12Z. In calculating the difference value of the station model wind speed minus COADS wind speed, a value of -3.99 knots was obtained for September 11 at 18Z, and a value of -0.02 knots was obtained for the difference on September 13 at 12Z. The overall mean difference; however, was +5.78 knots. A sample of the differences in wind speeds can be found in Table III.
Differences in sea-level pressure, dew point, and sea surface temperature, exist between the two data sets for September 23 at 18Z, as the COADS data is missing for this time, while the station model record gives 1011.3 mb, 16 degrees F, and 18 degrees F respectively.
The present weather parameter contains some discrepancies when comparing the model data with COADS data (Table IV).
The past weather, according to the two data sets, also differed (Table V).
Overall, the sporadic differences in reported meteorological parameters between the COADS and the station model records, have proven to be quite limited in number, with the exception of those inconsistencies found in reported wind speeds . For this reason, in an attempt to determine the possible cause of the large unexpected discrepancies in the wind speeds between the two reports, the digital wind speeds reported by other ships were compared with those of the METEOR (within a 100 kilometer and one hour data box (Fig. I) and within a 220 kilometer and one hour data box (Fig. II) from the latitude and longitude positions of the METEOR). The results were inconclusive. Since a reliable one-to-one correlation between the Meteor wind speeds and the other ships' wind speeds was not found, there is still uncertainty as to whether the winds reported in the station model report or those reported in the COADS report are the most accurate.
In conclusion, it appears that large and inexplicable differences exist between the digital and the written wind records. Other weather parameters differ, sometimes consistently, for specific conditions, as in the case of sea level pressure, air temperature, sea surface temperature, and dew point temperature, while other weather parameters differ sporadically, as in the case of wind speeds, between the two data sets. Some of the differences other than winds are nearly inconsequential and may be correctable. Missing data in the digital record may be inserted from the written reports. The large and unexpected discrepancies in the wind reports cannot be explained. We are pursuing contacts with the research vessel's (METEOR) scientific crew to investigate these and to make some attempt to determine the correct values.
(1). Herausgegeben von Jens Meincke, Ed. METEOR - BERICHTE Nr. 93 - 1, WOCE-NORD 1991 Cruise No. 18/Reise Nr. 18, 2 September - 26 September 1991. (Herein referred to as: Station Model Data)
(2). Slutz, R.J., S.J. Lubker, J.D. Hiscox, S.D. Woodruff, R.L. Jenne, D.H. Joseph, P.M. Steurer, and J.D. Elms, COADS (Comprehensive Ocean- Atmosphere Data Set) Release 1, pp. 300, CIRES University of Colorado, 1985. (Herein referred to as: Digital LMRF.6 data)
|DATE||TIME||COADS (code:miles)||STATION MODEL (code:miles)|
|September 11||06UTC||nothing reported||light fog|
|September 13||12UTC||sky unchanged in past hour||nothing reported|
|September 16||06UTC||sky unchanged in past hour||nothing reported|
|September 17||06UTC||sky unchanged in past hour||nothing reported|
|September 19||18UTC||sky unchanged in past hour||nothing reported|
|September 20||12UTC||sky unchanged in past hour||nothing reported|
|September 21||06UTC||sky unchanged in past hour||nothing reported|
|September 21||12UTC||sky unchanged in past hour||nothing reported|
|September 23||18 UTC||missing||light fog|
|September 3||18UTC||sky unchanged in past hour||nothing reported|
|September 11||12UTC||sky unchanged in past hour||nothing reported|
|September 17||18UTC||sky unchanged in past hour||nothing reported|
|September 23||18UTC||missing||nothing reported|
Figure I: The METEOR wind speeds as
compared to other ship's wind speeds within 111 km and one hour of the
METEOR, all data for COADS.
Figure II: A plot of the METEOR's wind speeds compared to other ship's wind speeds within 222 km and one hour of the METEOR, all data from COADS.