GIST Twenty Minute AWOS Surface Composite 1.0 General Description The GEWEX Continental-Scale International Project (GCIP) Integrated Systems Test (GIST) Twenty Minute Aviation Weather Observing System (AWOS) Surface Composite is composed of data from several sources (i.e., 20 minute Handar AWOS, 20 minute Qualimetrics AWOS, and 20 minute Artais AWOS for the GIST area of interest). Data from these sources (thirty-two stations) were merged and quality controlled to form this Surface Composite. This Surface Composite contains data for the GIST time period (01 April 1994 through 31 August 1994) and for the GIST domain. The GIST domain is approximately 91W to 107W longitude and 31N to 40N latitude. 2.0 Detailed Data Description The GIST Twenty Minute AWOS Surface Composite is composed of data from three different instrumentation vendors which record data on-site at a twenty minute frequency. * Data were collected from 4 AWOS stations manufactured by Handar Inc., Sunnyvale, CA. * Data were collected from 24 AWOS stations manufactured by Qualimetrics Inc., Sacramento, CA. * Data were collected from 4 AWOS stations manufactured by Artais of Columbus, OH. 2.0.1 AWOS Algorithms The following are descriptions of the algorithms used by AWOS to produce twenty minute surface data (USDOT, 1988). All three AWOS manufacturers implemented the same algorithms. Temperature/Dewpoint AWOS takes at least 1-min measurements and computes a 5-min running average. A minimum of four 1-min averages are required to compute a valid 5-min average. 5-min averages are rounded to the nearest deg F. AWOS will report the latest valid 5-min average during the previous 15-min period. If one is not available, the data are reported as "missing". If the 5-min average dewpoint is 1 or 2 degrees higher than the 5-min average temperature, than the dew point is reported equal to temperature. If the 5-min average dewpoint exceeds the 5-min average temperature by more than 2 degrees, the dewpoint is reported as "missing". Station Pressure and Derived Pressure Elements AWOS takes 10-sec measurements from at least two independent pressure sensors and computes respective 1-min averages. A minimum of 5 measurements is required to compute a 1-min average. The 1-min averages from each sensor are compared to verify that differences do not exceed 0.04" Hg. If the sensors are in agreement, the lowest pressure reading from all sensors is reported. If the sensor differences exceed 0.04" Hg, the data are reported as "missing". The reported pressure is then used in the computation of derived parameters (e.g., altimeter reading). There is no sea level pressure parameter reported in this AWOS twenty minute dataset. Wind AWOS takes 1-sec measurements of wind speed and direction and computes a 2-min running average every 5-sec. Wind direction is rounded to the nearest 10 degrees and wind speed is rounded to the nearest knot. If the 2-min running average is 2 knots or less, the wind is reported as calm. The Gust is computed using the highest 5-sec average wind speed during the past 10-min period. A gust is computed only when the 2-min running average exceeds 9 knots and the highest 5-sec measurement exceeds the 2-min running average by 5 knots (during the past minute). Precipitation AWOS takes 1-min accumulated measurements and computes total precipitation over the period specified in the AWOS selected archival interval (usually 20-min). The total accumulation counter is automatically reset each hour. Present Weather There is no present weather parameter reported in this AWOS twenty minute dataset. 2.1 Detailed Format Description The GIST Twenty Minute AWOS Surface Composite 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 UTC Nominal and UTC actual time. Days begin at UTC hour 0020 and end at UTC hour 0000 the following day. The data parameters are valid for the reported times. Missing values are reported as 9's in the data parameter's field. The table below details the data parameters in each record. Several data parameters have an associated Quality Control (QC) Flag Code which is assigned during the OFPS quality control processing. For a list of possible QC Flag values see the Quality Control Section 3.0. Units given below are for OFPS QCF format only. 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 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 When not present in the raw data, the dewpoint is computed using the formula from Bolton (1980). Calculated Sea Level pressure is computed from station pressure, temperature, dewpoint, and station elevation using the formula of Wallace and Hobbs (1977). 3.0 Quality Control Processing The GIST Twenty Minute AWOS Surface Composite was formed from several sub-datasets (i.e., 20 minute Handar AWOS, 20 minute Qualimetrics AWOS, and 20 minute Artais AWOS for the GIST domain). Each 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 Twenty Minute Surface Composite. 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 <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 <20.00 mm >=20.00 mm >=50.00 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 Bolton, D., 1980: The computation of equivalent potential temperature., Mon. Wea. Rev., 108, pp 1046-1053. 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. Wallace, J.M., P.V. Hobbs, 1977: Atmospheric Science, Academic Press, 467 pp. World Meteorological Organization (WMO), 1988: Manual on Codes Volume I, Part B - Binary Codes. WMO, Geneva, Switzerland. United States Department of Transportation (USDOT), 1988. AWOS Operations Manual, Federal Aviation Administration.