GIDS-1 NCDC SAO "Specials" Dataset 1.0 General Description This dataset contains only the National Climatic Data Center (NCDC) Surface Aviation Observation (SAO) "Special" observation surface data. Data for the GEWEX Continental-scale International Project (GCIP) Initial Dataset One (GIDS-1) area of interest and for a portion of the GIDS-1 time period (16 March 1992 through 30 April 1992) are contained is this dataset. The NCDC SAO "Nominal" hourly data for the GIDS-1 area and time period are included in the Hourly Surface Composite. The GIDS-1 area is approximately 85W to 106W longitude and 30N to 45N latitude. This NCDC SAO "specials" dataset contains data from hundreds of stations and is identical in quality control processing, and similar in format, etc. to the data in the Hourly Surface Composite. (Note: the "specials" for the STORM-FEST period, 01 February 1992 through 15 March 1992 are available as a seperate dataset.) 2.0 Detailed Data Description The NCDC SAO "Special" observation surface data format contains eight metadata parameters followed by 38 data parameters and flags. The metadata parameters describe the station location and time at which the data was collected. The time of observation is the actual UTC observation time that the data parameters are valid. Days begin at UTC 0000 and end at UTC 2300. All records are identical in length. The table below details the data parameter in each record. Several data parameters have an associated Quality Control (QC) Flag Code which are assigned by the Office of Field Project Support (OFPS) during quality control processing. For a list of possible QC Flag values see the Quality Control section 3.0. Parameters Units Date of Observation UTC Actual Time of Observation UTC Actual Network Identifier Abbreviation of platform name Station Identifier Network Dependent Latitude Decimal degrees, South is negative Longitude Decimal degrees, West is negative Station Occurrence Unitless Station Elevation Meters Station Pressure, QC flag Hectopascals (mb) Reported Sea Level Pressure, QC flag Hectopascals (mb) Computed Sea Level Pressure, QC flag Hectopascals (mb) Dry Bulb Temperature, QC flag Celsius Dew Point, QC flag Celsius Wind Speed, QC flag m/s Wind Direction, QC flag Degrees Total Precipitation, QC flag mm Squall/Gust Indicator Code Value Squall/Gust Value, QC flag m/s Present Weather, QC flag Code Value Visibility, QC flag Meters Ceiling Height (first layer) Hundreds of feet Ceiling Flag (first layer), QC flag Code Value Cloud Amount (first layer), QC flag Code Value Ceiling Height (second layer) Hundreds of feet Ceiling Flag (second layer), QC flag Code Value Cloud Amount (second layer), QC flag Code Value Ceiling Height (third layer) Hundreds of feet Ceiling Flag (third layer), QC flag Code Value Cloud Amount (third layer), QC flag Code Value The list of code values for the Present Weather is too large to reproduce in this document. Refer to WMO, 1988 for a complete list of Present Weather codes. The code values for the Squall/Gust Indicator are: Code Definition S Squall G Gust The following codes define ceiling information that can not be indicated by the numeric value: Code Definition 0 None 1 Thin 2 Clear below 12,000 feet 3 Estimated 4 Measured 5 Indefinite 6 Balloon 7 Aircraft 8 Measured/Variable 9 Clear below 6,000 feet (AUTOB) 10 Estimated / Variable 11 Indefinite / Variable 12 12-14 reserved 15 Missing The code values for the Cloud Amount Indicator are: Code Definition 0 0 ( or clear) 1 1 okta or less, but not zero or 1/10 or less, but not zero 2 2 oktas or 2/10-3/10 3 3 oktas or 4/10 4 4 oktas or 5/10 5 5 oktas or 6/10 6 6 oktas or 7/10-8/10 7 7 oktas or more, but no 8 oktas or 9/10 or more, but not 10/10 8 8 oktas or 10/10 (or overcast) 9 Sky obscured by fog and/or other meteorological phenomena 10 Sky partially obscured by fog and/or other meteorological phenomena 11 Scattered 12 Broken 13 13-14 Reserved 15 Cloud cover is indiscernible for reasons other than fog or other meteorological phenomena, or observation is not made. 2.1 Data Remarks This dataset contains only the NCDC SAO "Special" observation surface data. The Hourly Surface Composite contains only the "Nominal" hourly observations for the NCDC SAO data. 3.0 Quality Control Processing This dataset was quality controlled by comparing the station observations with the MAPS (Mesoscale Analysis and Predictions System) hourly gridded surface analyses. MAPS was developed and the analyses were produced by the Forecast Systems Laboratory of the NOAA Environmental Research Laboratories (Miller and Benjamin, 1992). The same method which was used to quality control the STORM- FEST data (Scully and McGuirk, 1993) was used to quality control the NCDC SAO "Specials" dataset. Hourly MAPS gridded values were interpolated to a station's latitude, longitude, elevation and observation time and were then compared to the observed values. Observed values were then flagged "good", "questionable" or "unlikely" based upon the comparison with MAPS. Data were never changed, only flagged. Only the station pressure, sea level pressure, temperature, dew point, wind speed and wind direction were quality controlled this way. The following table shows the allowed variance from MAPS values for each parameter. Parameters Good Questionable Unlikely Station Pressure <3.00mb >=3.00mb >=10.00 mb Sea Level Pressure <3.00mb >=3.00 mb >=10.00 mb Dry Bulb Temp <5.00 C >=5.00 C >=8.00 C Dew Point Temp <5.00 C >=5.00 C >=8.00 C Wind Speed <5.00 m/s >=5.00 m/s >=20.00 m/s Wind Direction <=90.00 >90.00 deg ------------- Squall/gust speed <25.00 m/s >=25.00 m/s >=35.00m/s Precipitation <25.00 mm >=25.00 mm >=75.00 mm Gross limit checks were applied to the squall/gust wind speed data. The wind speeds was compared with the maximum allowable speeds shown in the table above. Gross limit checks were also used to determine the quality of the precipitation values. Several "questionable" and "unlikely" data values were also manually inspected. After inspection, the quality control flag may have been manually updated to better reflect the likelihood of the actual occurrence of the precipitation value. Data was never modified, only flagged. Negative precipitation was also coded "unlikely". A table of the possible quality control flags and their meanings is listed below. QC Code Description U Unchecked G Good M Normally recorded but missing D Questionable B Unlikely N Not available or Not observed. X Glitch E Estimated C Reported precipitation value exceeds 9999.99 millimeters or was negative. T Trace precipitation amount recorded. General consistency checks were applied to the dry bulb temperature, wind direction, and the relationship between precipitation and cloud amount/cloud cover. If the dew point temperature was greater than the dry bulb temperature both values were coded "questionable". Also, wind direction for observed "calm" winds was given the same QC code as the wind speed. If precipitation was reported, but the cloud amount was "none" or "clear", then both the cloud amount and precipitation values were coded "questionable". Several impossible values were also checked. Negative wind speeds were coded "unlikely". Negative squall/gust wind speeds were coded "unlikely". Wind directions of less than zero or greater than 360 degrees were coded "unlikely". 4.0 References For additional station information, refer to the station list provided as a seperate dataset in CODIAC. Scully, K.W. and McGuirk, D.E., 1993: The Use of MAPS Analyses for Quality Control of Surface Observations from STORM-FEST. Preprints, Eight Symposium on Meteorological Observations and Instrumentation, Anaheim, California, 17-22. Miller, P.L. and Benjamin S.G., 1992: A System for the Hourly Assimilation of Surface Observations in Mountainous and Flat Terrain. Mon. Wea. Rev., 120, 2342-2359. World Meteorological Organization (WMO), 1988: Manual on Codes Volume I, Part B - Binary Codes. WMO, Geneva, Switzerland.