GIST 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) Integrated Systems Test (GIST) domain and time period (01 April 1994 through 31 August 1994) are contained is this dataset. The NCDC SAO "Nominal" hourly data for the GIST domain and time period are included in the GIST Hourly Surface Composite. The GIST domain is approximately 91W to 107W longitude and 31N to 40N latitude. This NCDC SAO "Specials" dataset contains data from approximately a hundred stations and is identical in quality control processing, and similar in format, etc. to the data in the GIST Hourly Surface Composite. 2.0 Detailed Data Description 2.0.1 NCDC SAO Algorithms The NCDC SAO dataset consist of principle reporting stations which are usually fully instrumented and therefore record a complete range of meteorological parameters. Most of these stations reside at airports and provide aviation support, but with the increasing installation of automated observing instrumentation, more SAO observations are obtained from remote locations. This dataset contains only the NCDC SAO "special" observations. The NCDC SAO "nominal" observations are included in the GIST Hourly Surface Composite. For details on the observer procedures see the Federal Meteorological Handbook No. 1 (1988). Requirements for taking "Special" observations are summarized below: if the ceiling forms or dissipates below, decreases to less than, or if below, increases to equal or exceed either 3000 ft, 1500 ft, 1000 ft, 500 ft, or all nationally published landing minimums that are applicable to a specific airport. if a layer of clouds or obscuring phenomena aloft is present below 1000 ft and no layer aloft was reported below 1000 ft in the preceding observation. if a layer of clouds or obscuring phenomena aloft is present below the highest published landing minimum, including circling minimums, applicable to the airport and no sky cover aloft was reported below this in the previous observation. if the prevailing visibility decreases to less than, or if below, increases to equal or exceeds 3 mi, 2 mi, 1.5 mi, 1 mi, or any of the nationally published landing minimums applicable to the airport. if the highest value from the Designated Runway Visual Range (RVR) Runway during the preceding 10 minutes decreases to less than, or if below, increases to equal or exceed 2400 ft. if the RVR applicable to touchdown for the in use runway decreases to less than, or if below, increases to equal or exceeds 6000 ft, 4000 ft, and 2400 ft. if a tornado, funnel cloud, or waterspout is observed, disappears from sight, or occurred within the past hour according to outside sources, but was not observed or recorded at the station. if a thunderstorm begins, increases in intensity or ends. if hail begins or ends. if freezing precipitation begins, ends, or changes intensity. if ice pellets begin, end, or changes intensity. if any other type of precipitation begins or ends. if the average 1-minute wind speed suddenly increases to twice or more than twice the currently reported 1-minute wind speed and exceeds 25 knots. if the wind shifts. if a pressure jump is recorded at stations equipped with 12-hour microbarographs. if a volcanic eruption begins or erupts. if any other meteorological situation which in the opinion of the observer is critical to the safety of inbound aircraft occurs. A special observation is taken, recorded, and disseminated after an observer returns to duty following a break in observer coverage at the station unless a Record observation is filed during that 15-minute period. A special observation is taken, recorded, and disseminated after the observer returns to duty following a break in Hourly coverage if a Record observation was not filed as scheduled during that 15-minute period. This requirement is applicable only to breaks in coverage which occur during regularly scheduled hours of operation. 2.1 Detailed Format Description The NCDC SAO "Special" observation surface data contains ten metadata parameters and 38 data parameters and flags. The metadata parameters describe the station location and time at which the data were collected. The time of observation is reported both in Universal Time Coordinated (UTC) Nominal and UTC actual time. For the NCDC "Specials", the UTC Nominal and UTC actual times are equal to the time of observation and are always identical. Days begin at UTC 0100 and end at UTC 0000 the following day. 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 Nominal Time of Observation UTC Nominal 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 ---- ---------- blank No Squall or Gust S Squall G Gust The code values for the ceiling flag Indicator are: 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. 2.2 Data Remarks This dataset contains only the NCDC SAO "Special" observation surface data. The GIST Hourly Surface Composite contains only the "Nominal" hourly observations for the NCDC SAO data. A value of 999.00 in the ceiling height field of the NCDC SAO data means the sky condition was either clear or a height was not reported. 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 GIST 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 not 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 <2.00mb >=2.00mb >=5.00 mb Sea Level Pressure <2.00mb >=2.00 mb >=5.00 mb Calculated Sea Pressure <4.00mb >=4.00mb >=10.00 mb Dry Bulb Temp <2.50 C >=2.50 C >=5.00 C Dew Point Temp <2.50 C >=2.50 C >=5.00 C Wind Speed <5.00 m/s >=5.00 m/s >=10.00 m/s Wind Dir (Ws<10m/s) <=90.00 deg >90.00 deg >=180.00 deg Wind Dir (Ws>=10m/s) <=50.00 deg >50.00 deg >=90.00 deg Precipitation <25.00 mm >=25.00 mm >=75.00 mm The squall/gust wind speed data were not quality controlled. 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 were 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. I Derived parameter can not be computed due to insufficient data. 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". Wind directions of less than zero or greater than 360 degrees were coded "unlikely". 4.0 References 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. 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. United States Department of Commerce, 1988. Federal Meteorological Handbook No.1 - Surface Observations, FCM-H1-1988, National Oceanic and Atmospheric Administration. World Meteorological Organization (WMO), 1988: Manual on Codes Volume I, Part B - Binary Codes. WMO, Geneva, Switzerland.