GIST Five Minute Surface Composite 1.0 General Description The GEWEX Continental-Scale International Project (GCIP) Integrated Systems Test (GIST) Five Minute Surface Composite is composed of data from several sources (i.e., 5 minute Automated Surface Observation System (ASOS), 5 minute Oklahoma Mesonet (OKMESO), 5 minute Artais Aviation Weather Observation System (AWOS), and 5 minute ARM/CART Surface (ARMSFC) for the GIST domain). Data from these sources (183 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 Five Minute Surface Composite is composed of data from four different sources which record data on-site at either 1 minute or 5 minute frequency. The 1 minute frequency data were converted to 5 minute data using the same algorithms as used by the ASOS network. 2.0.1 ASOS Algorithms The following are descriptions of the algorithms used by ASOS to produce five minute surface data. Temperature/Dewpoint ASOS takes 30-sec measurements and computes a 1-min average. A 5-min running average of these 1-min averages is computed. 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. ASOS 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 ASOS 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 setting, sea level pressure, and pressure remarks such as tendency). Wind ASOS takes 5-sec measurements of wind speed and direction and computes a 2-min running average. Wind direction is rounded to the nearest degree 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 ASOS takes 1-min accumulated measurements and computes total precipitation over 5-min, 15-min, hourly, 3-hr, 6-hr, and daily increments. Monthly totals are summed from daily totals. Present Weather There are currently two automated ASOS present weather sensors. They are the Precipitation Identification (PI) sensor which discriminates between rain and snow and the Freezing Rain (ZR) sensor. Although there is no ASOS "Obstruction To Vision" (OTV) sensor, ASOS algorithms evaluate data from multiple sensors (i.e., visibility, temperature, dewpoint temperature, and PI) and infer the presence of obstructions to vision (fog or haze). Once each minute the PI sensor output is stored in memory (up to 12 hours). The latest 10 minutes of data are examined. If 3 or more samples are missing, ASOS reports "missing" for that minute. If 2 or more samples indicate precipitation, and at least 8 one minute samples are available, the algorithm determines the type and intensity to report. In general, to report anything other than light precipitation (P-), two of the samples are required to be the same type. If there is a tie between two types of precipitation, snow is reported. The highest intensity obtained from two or more samples determines the present weather type and intensity that is reported. Once each minute the ZR sensor output is stored in memory (up to 12 hours). Data from the latest 15 minutes are used to compute the current minute freezing rain report. If 3 or more sensor outputs in the past 15-minutes are missing, the report is set to "missing". If at least one positive freezing rain report occurs in the past 15-minutes, freezing rain is reported for the current minute. If freezing rain is reported, the PI sensor report is examined and a hierarchical scheme is used to compute the present weather report. This scheme follows the familiar reporting hierarchy of LIQUID-FREEZING-FROZEN in ascending order of priority. ASOS does not report mixed precipitation. The beginning and ending times of one minute freezing rain reports are used in the hourly SAO reports. Once freezing rain has been sensed and the ambient air temperature is 36 degrees F or below, it will be carried in subsequent SAO reports for 15-minutes after it is no longer sensed. The OTV algorithm continuously monitors the reported visibility once each minute. When visibility drops below 7 statute miles, the algorithm obtains the current Dew Point Depression (DD) to distinguish between fog and haze. If the DD is < or equal to 4 degrees F, then fog will be reported and appended to the present weather report. If DD is > 4 degrees F and no present weather is reported by the PI and ZR sensors, then haze is reported as present weather. When present weather is reported by the PI and ZR sensors, haze is not reported. In the event DD is missing, visibility is used to discriminate between haze and fog. If visibility is < 4 miles, fog will be reported. When present weather is also reported, fog will be appended to the report. If visibility is > or equal to 4 miles but < 7 miles and no present weather is reported, then haze is reported. 2.0.2 ARMSFC Algorithms The detailed descriptions of the algorithms used to produce ARMSFC one minute data are not currently available. The algorithms used by ASOS to produce 5 minute data were applied to the ARMSFC 1 minute data to produce ARMSFC 5 minute surface data. 2.0.3 OKMESO Algorithms Temperature/Dewpoint Campbell Scientific Inc. combined a thermistor, which measures air temperature, and a Vaisala HMP35 sorption probe, which senses relative humidity, into a dual probe known as the HMP35C. The HMP35C samples the atmosphere from inside an unaspirated multiplate radiation shield mounted at the end of a 1 m boom extending west of the tower, at a height of 1.5 m. The filter to keep dust off the sensors also reduces ventilation, slowing the parameter's response time to as much as 10 minutes in light winds. The temperature thermistor is quite rugged and accurate. Unfortunately, the unaspirated radiation shield can create temperature errors of several degrees Celsius when the wind is calm ( < 1 m/s) and radiation is strong ( > 800 W/m2). Radiation at high sun angles near 70 deg seem to induce the worst errors. The temperature sensor has a sampling frequency of 3 sec and averages 100 samples to produce a 5-minute average. The Relative Humidity (RH) sorption device results in a tendency for upward drift over time. This requires frequent monitoring to remain within calibration standards. The error for readings is +/- 2% for measurements 0 to 90%, and +/- 3% for measurements 90 to 100%. The RH sensor is protected from atmospheric contaminants by being enclosed in a cylindrical GORTEX membrane. The RH sensor has a sampling frequency of 3 sec and averages 100 samples to produce a 5-minute average. Station Pressure and Derived Pressure Elements The barometer, a Vaisala PTB 202, is a highly accurate and rugged instrument equipped with its own microprocessor, which controls the barometer and calibrates observations to correct for temperature and other factors. Because wind flow can exert a dynamic pressure and affect sensor readings, the barometer is protected with a static pressure port, a tube which extends down 0.5" from the barometer (2 feet above ground). Sample frequency is 12 seconds and an average is calculated using 25 samples over a 5-minute period. The average reading is accurate to 0.4 mb for temperatures between -30 and 50 deg C, and for pressures between 700 and 1100 mb. Barometric pressure is not corrected for the elevation of the station above sea level. Wind The sensor is a R.M. Young Model 5103 wind monitor, a propeller and wind vane unit oriented to true north. The propeller has a range of 1 to 60 m/s and can withstand gusts to 100 m/s; it has a distance constant of 2.7 m. Both the propeller and vane starting threshold is 1 m/s. A wind direction observation is taken every 3 seconds and wind speed is a three second average. One hundred samples from each sensor are averaged over a 5-minute period. The 5-min wind speed is an arithmetic average and is independent of wind direction. The 5-min wind direction is a vector average and is independent of wind speed. When wind speed equals zero (calm), the wind direction and all other computed parameters (i.e., gust, standard deviation, etc.) are set to zero. Precipitation The sensor is an unheated tipping bucket rain gauge with a 30 cm diameter opening located 0.6 m above ground. An Alter-style wind screen is used to minimize wind-induced errors. Each tip of the bucket is equivalent to 0.01" of rain. The number of tips are accumulated to determine total rainfall and the counter is reset daily at 0000 UTC. Rainfall rates are determined by computing the change of accumulation per unit time. Because of the design of the tipping bucket rain gauge, measurements made during heavy rainfall periods generally underestimate the total rainfall. Also, because of the unheated gauge, measurements may underestimate and/or delay liquid water content totals during periods of freezing precipitation. Present Weather Present weather is not reported in the OKMESO data. 2.0.4 AWOS The following are descriptions of the algorithms used by AWOS to produce five minute surface data. 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). Sea level pressure is not reported in the AWOS data. 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 Present weather is not reported in the AWOS five minute data. 2.1 Detailed Format Description The GIST Five Minute 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 Universal Time Coordinated (UTC) Nominal and UTC actual time. Days begin at UTC hour 0005 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 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. Note that the units give in Table 1 apply only to the OFPS QC Format (QCF). Table 1. 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 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 Five Minute Surface Composite was formed from several sub-datasets (i.e., 5 minute Automated Surface Observation System (ASOS), 5 minute Oklahoma Mesonet (OKMESO), 5 minute Artais Aviation Weather Observation System (AWOS), and 5 minute ARM/CART Surface (ARMSFC) 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 Five 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 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 <10.00 mm >=10.00 mm >=25.00 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 not 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 ASOS User's Guide, ASOS Project Office, NOAA, National Weather Service, Washington D.C., June 1992. 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. The Oklahoma Mesonet User's Guide, Oklahoma Climatological Survey, The University of Oklahoma, May, 1995 United States Department of Transportation (USDOT), 1988. AWOS Operations Manual, Federal Aviation Administration. 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.