GCIP LSA-NW EAOP 1999 ASOS Surface "Specials" Data in QC format 1.0 General Description This dataset contains the Automated Surface Observing System (ASOS) "Special" observation surface data in Quality Control (QC) format. Data for the GEWEX Continental-Scale International Project (GCIP) Large Scale Area-North West (LSA-NW) Enhanced Annual Observing Period (EAOP) 1999 domain (36N to 51N latitude and 90W to 115W longitude) and time period (01 April 1999 through 31 Mar 2000) are contained within this dataset. The ASOS "Nominal" hourly surface data in QC format for the GCIP LSA-NW EAOP 1999 domain and time period are provided as a separate dataset. These data were not quality controlled by the University Corporation for Atmospheric Research/Joint Office for Science Support (UCAR/JOSS). 2.0 Detailed Data Description Automated Surface Observing System (ASOS) Algorithms The following are descriptions of the algorithms used by the Automated Surface Observing System (ASOS) to produce hourly surface data. These algorithms were also used to generate the ASOS specials. Complete details may be found in the ASOS User's Guide (1992). The ASOS hourly values were produced from ASOS 5-minute data by extracting all parameters (except precipitation) from the 55-minute observation of the previous hour and assigning those values to the current hourly observation. The precipitation is the sum of the precipitation values from the 05-minute of the previous hour through the 00-minute of the current hour. The following are descriptions of the algorithms used by ASOS to produce five minute surface data. Complete details may be found in the ASOS User's Guide (1992). 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 degree 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, then 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 are 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. 2.1 Detailed Format Description The GCIP LSA-NW EAOP-99 Surface "Special" observation 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 GCIP LSA-NW EAOP 1999 "Specials", the UTC Nominal and UTC actual times are equal to the time of observation and are always identical. Days begin at UTC hour 0100 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 are assigned by the Joint Office for Science Support (JOSS). For a list of possible QC Flag values see the Quality Control Section 3.0. No additional QC was performed by UCAR/JOSS. 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 the GCIP LSA-NW EAOP 1999 Surface "Special" observation data. The GCIP LSA-NW EAOP 1999 ASOS Hourly Surface data contains only the "Nominal" hourly observations for the GCIP LSA-NW EAOP 1999 domain and time period. 3.0 Quality Control Processing These data were not quality controlled by UCAR/JOSS. See Table 3-1 for a list of the possible quality control flags and their meanings. Table 3-1 - Quality Control Flags 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 value exceeds output format field size or was negative precipitation. T Trace precipitation amount recorded I Derived parameter can not be computed due to insufficient data. 4.0 References ASOS User's Guide, ASOS Project Office, NOAA, National Weather Service, Washington D.C., June 1992. World Meteorological Organization (WMO), 1988: Manual on Codes Volume I, Part B - Binary Codes. WMO, Geneva, Switzerland.