NCEP Home > NCO Home > Production Management Branch > Quality Assessment Project > Surface Marine Monthly Statistics > Information on Surface Marine Monthly Statistics
Information on Surface Marine Monthly Statistics
Statistics on global surface marine meteorological and oceanographic
data are compiled monthly by personnel of NOAA's NWS/NCEP/Central Operations.
These data include those which are observed/measured by ships (including those vessels that are part of the U.S. Voluntary Observing Ship Project), moored and
drifting buoys,
Coastal Marine Automated Network (CMAN) stations, and tide gauge stations
(tide gauge data are provided by NOAA's National Ocean Service/Center for Operational Oceanographic Products and Services (NOS/COOPS)).
Separate statistics are compiled monthly for specified vessels in the Gulf of
Alaska and for specified member vessels of the
International SeaKeepers Society.
The data for which these statistics are
compiled include sealevel pressure, air temperature, and wind direction and
speed (for the surface meteorological data), and sea surface temperature (for
the oceanographic data).
Only 1 year's worth of statistics will be available at any given time
on this web site. Please note that the monthly statistics are updated
on the first business day of each month. I.e. if the 1st falls on a
weekend or on a Federal Government holiday, the statistics are updated on
the first business day which follows the weekend or holiday.
The global monthly statistics are sent to the
UK Met Office in Bracknell, England each month (the UK Met Office has
been designated by the
World Meteorological Organization (WMO) as the
lead center for monitoring surface marine observations), where
they are consolidated with additional
statistics and forwarded to platform managers. The platform managers may then
investigate platforms listed in the statistics to see if instrumentation needs
to be corrected. Please note that the statistical files available via this
web site are not identical to those passed to Bracknell, in that the
ship/platform names are included in the files on this web site (they are not
included in the Bracknell files).
The monthly statistics produced for the Gulf of Alaska vessels and for the
International SeaKeepers Society vessels are made available to those program
managers via this web site, so that they may evaluate the performance of
select vessels in their area of interest.
All of the monthly statistics are computed using differences between the
observed data and first guess fields from one of 2 NWS numerical models. This
file explains the method of comparison between the data and first
guess fields, and also provides the criteria used in determining which
platforms will be listed in the NCEP Central Operations (NCO) global marine
statistics each month.
Standard of Comparison
Data from each platform (ships, moored (fixed) buoys, drifting buoys,
CMAN platforms, and tide gauge stations) are compared to forecasts
from one of 2 NWS numerical models for each of 4 daily synoptic periods.
Each synoptic period is a 6 hour window centered on the times given in the
table below. The first guess valid times are also given below. All data
observed/measured during each synoptic period are compared to the first
guess valid for that synoptic period. The first guess is interpolated to
the platform's location and differences between observation and first guess
are computed. Differences between observed data and forecasts are archived
four times daily.
Synoptic Period 
Numerical Model Used 
First Guess Valid Time 
0000 UTC 
6hour forecast from the 1800 UTC run of NWS's Global Data Assimilation System (GDAS) 
0000 UTC 
0600 UTC 
6hour forecast from the 0000 UTC run of GDAS 
0600 UTC 
1200 UTC 
6hour forecast from the 0600 UTC run of GDAS 
1200 UTC 
1800 UTC 
6hour forecast from the 1200 UTC run of GDAS 
1800 UTC 
If the GDAS is unavailable for a particular synoptic period, a 6hour
forecast from the previous run of NWS's Global Forecast System (GFS) is used instead.
Each synoptic period includes data with observation times +/ 3 hours as
follows:
Synoptic Period 
Observation Times of Data 
0000 UTC 
2100 UTC (previous day) through 0259 UTC 
0600 UTC 
0300 UTC through 0859 UTC 
1200 UTC 
0900 UTC through 1459 UTC 
1800 UTC 
1500 UTC through 2059 UTC 
No time interpolation between observed data and first guess is done
(i.e. an observation from 1000 UTC or 1400 UTC will be compared to a first
guess valid at 1200 UTC).
Criteria for specific parameters
 Global monthly statistics:
For each of the 5 parameters (sealevel pressure, air temperature,
wind speed, wind direction, and sea surface temperature) included in the
monthly statistics, the following criteria must be met in order for a
particular platform to be listed in the NCO global monthly statistics:
Sealevel pressure (slp):
 20 or more slp observations for the month, and 1 or more of
the following:
 absolute value of bias (mean difference between
observation and forecast) is at least 4.0 mb
 standard deviation of difference is at least 6.0 mb
 percentage of reports that were gross errors is at
least 25% of the total number of reports received for this parameter (see Definition of gross errors, below).
Air temperature:
 20 or more air temperature observations for the month, and 1 or
more of the following:
 absolute value of bias is at least 4.0 deg C
 standard deviation of difference is at least 6.0 deg C
 percentage of reports that were gross errors is at
least 25% of the total number of reports received for this parameter.
Wind direction:
 20 or more wind direction observations for the month, and
1 or more of the following:
 absolute value of bias is at least 30.0 degrees
 standard deviation of difference is at least 80.0 degrees
 percentage of reports that were gross errors is at
least 25% of the total number of reports received for this parameter.
Wind speed:
 20 or more wind speed observations for the month, and 1 or
more of the following:
 absolute value of bias is at least 5.0 m/s
 percentage of reports that were gross errors is at
least 25% of the total number of reports received for this parameter.
Sea surface temperature (sst):
 20 or more sst observations for the month, and 1 or
more of the following:
 absolute value of bias is at least 4.0 deg C
 standard deviation of difference is at least 6.0 deg C
 percentage of reports that were gross errors is at
least 25% of the total number of reports received for this parameter.
 Gulf of Alaska monthly statistics and International SeaKeepers Society monthly statistics:
Specific callsigns were given to NCO/SIB to indicate those vessels which are
to be included in these monthly statistics. No other criteria must be met,
except that the vessels must have at least one observation during the
desired month.
If a vessel reports only once during a specific month, then the bias for
any reported parameter is not a mean bias but is merely the difference
between observed and first guess values for that parameter.
Standard deviation is only computed when
a vessel has 2 or more reports for any particular parameter
in the month. As with the global monthly statistics, gross errors are not
included in computations of bias or standard deviation (see Definition of
gross errors, below).
With the exception of wind direction, the bias for each parameter is
always computed as (observed data  first guess). A mean difference
(bias) for slp of 4.8 mb would mean that the observed slp for that
platform is, on average, 4.8 mb lower than the first guess slp. Also,
a low standard deviation (SD) of the difference would mean that the
platform's bias is consistent (the closer to 0 the SD is, the
more consistent that platform is for that parameter). For example,
if the slp bias was 4.8 mb and standard deviation of the difference
was 1.0, that would imply that the platform consistently was around
4.8 mb too low with its slp. If the SD is high, then the platform has
a lot of variability for the parameter in question, with regards to its
comparison with the first guess.
Calculation of bias and standard deviation for wind direction
Beginning with the February 2000 statistics, the computation of bias and
standard deviation for wind direction is being done using a new algorithm.
This new algorithm is the same one used by
MeteoFrance's
Centre for Marine Meteorology
(Centre de Meteorologie Marine (CMM)) in computation of wind
direction statistics for moored and drifting buoys. For platforms reporting
wind direction that is consistently close to 180 degrees off the first guess
wind direction, using the regular algorithm of (observed wind direction  first
guess wind direction) would result in a very large SD and bias, which would
make it difficult to detect the wind direction error. The ideal for this
situation would be to have a small SD with a bias on the order of 180
degrees. This is possible if CMM's algorithm is used. This algorithm
computes bias and SD for the four following series:
 (Obs  FG)
 (Obs + 90)  FG
 (Obs  90)  FG
 (Obs + 180)  FG
where Obs = observed wind direction and FG = first guess wind direction.
The bias and SD are computed for each of the above 4 series. The smallest
of the 4 SD values is selected and then (bias  i) (where i is 0, 90, 90, and 180, respectively) is selected as the actual bias if the series ((Obs + i)  FG) was the series associated with the smallest SD. See the following table for more details.
If smallest SD is computed for 
Bias used is 
(Obs  FG) 
bias = ((bias of (obs  FG))  0) 
((Obs + 90)  FG) 
bias = ((bias of ((obs + 90)  FG))  90) 
((Obs  90)  FG) 
bias = ((bias of ((obs  90)  FG))  (90)) 
((Obs + 180)  FG) 
bias = ((bias of ((obs + 180)  FG))  180) 
Using this algorithm for all wind direction bias and SD computations will
ensure that the most accurate representation of the difference between observed
wind direction and first guess wind direction will be used and displayed in
the monthly statistics.
Sample lines from global monthly statistics (and explanation)
NCEP CENTRAL OPERATIONS (NCO) PLATFORM STATS
NOVEMBER 2001
SEALEVEL PRESSURE FOR ALL HOURS
PLATFORM SHIP/PLAT. NAME TYPE NO. REPORTS MEAN DIFF SD DIFF # GROSS ERR
VVMA UNKNOWN SHP 20 11.2 2.1 1
The above lines were taken from the global monthly statistics produced for November 2001. The platform shown
has the callsign VVMA. Its name is unknown. The letters SHP in the TYPE
column indicate that the platform is a ship (see Platform
types seen in monthly statistics, below).
The number of reports used in calculation of the sealevel pressure statistics for ship VVMA is 20.
The mean difference is 11.2 mb and the standard deviation of the difference is 2.1 mb. The number of
gross errors for sealevel pressure from ship VVMA is 1
(see Definition of gross errors,
below). Please note that the total number of sealevel pressure reports received from ship VVMA for
the month of November is 21
(the sum of the numbers listed under NO. REPORTS and # GROSS ERR, as seen above).
Caveat
Please note that a platform's appearance in the monthly statistics
does not necessarily mean that the platform's data are bad. A high
bias can be indicative of a persistent problem with the first guess
at the platform's location. While the model's grid resolution and
topographical resolution are 1 degree x 1 degree, this is still not
fine enough to depict smallscale local meteorological and oceanographic
effects. In addition, the first guess in general performs more poorly
in the Southern Hemisphere than in the Northen Hemisphere, due to lower
data counts (much less land south of the Equator, so not nearly as many
landbased surface and upper air observations, plus major shipping lanes
are primarily in the Northern Hemisphere).
Definition of gross errors
An observation is considered to be a gross error if it differs
from the first guess by the following values:
slp 
abs(obs  fcst) GE 15.0 mb 
air temperature 
abs(obs  fcst) GE 15.0 deg C 
wind direction 
abs(obs  fcst) GE 100 degrees 
wind speed 
abs(obs  fcst) GE 15.0 m/s 
sst 
abs(obs  fcst) GE 15.0 deg C 
abs: 
absolute value 
obs: 
observation 
fcst: 
forecast (first guess) 
GE: 
greater than or equal to 
Please note that, prior to August 1998, the gross error limits for
wind direction and wind speed were the following:
wind direction 
abs(obs  fcst) GE 140 degrees 
wind speed 
abs(obs  fcst) GE 15.45 m/s (30kts) 
The wind direction and wind speed gross errors were changed in August
1998 to conform to gross error limits used by other international centers
producing similar statistics.
By convention, gross errors are not used in the computation of the mean
difference or standard deviation.
Platform types seen in monthly statistics
Abbreviation 
Type 
SHP 
ship 
DB 
drifting buoy 
FB 
fixed (moored) buoy 
CMN 
CMAN platform 
TG 
tide gauge station 
Statistics are not computed for platforms with the following
callsigns: 'SHIP', 'BUOY', 'BBXX', 'PLAT', 'RIGG', or 'METEO'.
These callsigns are not unique (i.e. more than one platform may
have the same callsign), so statistics for these callsigns would
be meaningless.
Ship and Platform Names
The ship and platform names are obtained from several sources.
Since callsigns and ship names change from time to time, a particular
callsign may be associated with 2 or more different names, or a ship
name may be associated with several callsigns. Although we will update the
names file several times a year, we may not know which callsign/name is
current.
A good web site for querying ship names and/or callsigns is that of the
ShipOfOpportunity Programme (SOOP), which is a joint program of the
WMO and the Intergovernmental Oceanographic Commission (IOC). On their
query page, one may enter a full or partial ship name or callsign,
and an attempt will be made to find a match on either. To access the query
page, go to the
SOOP home page, and click on "Network operations", then
click on "Ships", then "Search for ships". You'll be taken to the query
page, where you may then enter the callsign or ship name.
Statistics Updates
 Aug. 1998  gross error limits for wind direction and wind speed are modified (see Definition of gross errors, above).
 Feb. 2000  new algorithm for computation of bias and standard deviation for wind direction is implemented (see Calculation of bias and standard deviation for wind direction, above).
 Sep. 2000  tide gauge data are now being archived (effective Sep. 5, 0600 UTC synoptic period).
 Oct. 2000  tide gauge data are present in monthly statistics file for September 2000.
 Feb. 2001  separate statistics are now displayed for select vessels in the Gulf of Alaska.
 Jun. 2001  separate statistics are now displayed for International SeaKeepers Society vessels (data first received at NCEP/NCO on May 15, 2001).
 Apr. 2003  renaming Aviation model (AVN) to Global Forecast System (GFS) per changes made by NCEP's Environmental Modeling Center.
 May 2003  added table and links for Gulf of Alaska data counts.
 May 2008  added link for Gulf of Alaska  NOAA Oceanographic Research Ships data counts.
 Apr. 2013  removing Gulf of Alaska data counts (no longer used by AK PMOs).
Questions
Any questions regarding the NCO monthly marine statistics
may be directed to NCO's Statistics Specialist.
NOTE: if you are seeking climatological marine data such as average
temperature, wave heights, or prevailing winds for a particular oceanographic
region, please contact NOAA's National Climatic Data Center (NCDC).
For marine forecasts, please contact NOAA/NWS/NCEP's Ocean Prediction Center (OPC). Thank you.
Page last modified: Thursday, 11Apr2013 13:57:41 UTC

