Lake-ICE 20-Minute Surface Composite 1.0 General Description The Lake-Induced Convection Experiment (Lake-ICE) 20-Minute Surface Composite is composed of data from three sources (i.e., Artais Automated Weather Observation System (AWOS), Handar AWOS, and Qualimetrics AWOS) for the Lake-ICE domain. Data from these sources (165 stations) were merged and quality controlled with the Lake-ICE 5- Minute Composite and the Lake-ICE Hourly Surface Composite. This Surface Composite contains 20-Minute data for the Lake-ICE time period (28 November 1997 through 25 January 1998) and for the Lake-ICE domain. The Lake-ICE domain is approximately 37N to 55N latitude and 73W to 104W longitude. 2.0 Detailed Data Description The Lake-ICE 20-Minute Surface Composite is composed of data from three different AWOS sources which record data at 20-minute frequencies. The AWOS twenty minute data are from different instrumentation vendors which record data on-site at a 20-minute frequency. * Data were collected from 15 AWOS stations manufactured by Handar Inc., Sunnyvale, CA. * Data were collected from 10 AWOS stations manufactured by Qualimetrics Inc., Sacramento, CA. * Data were collected from 140 AWOS stations manufactured by Artais of Columbus, OH. 2.0.1 AWOS 20-Minute Algorithms The following are descriptions of the various types of AWOS station data. Further details can be found in the AWOS Operations Manual (1988). Details regarding the data collection from each type of AWOS can be found at the end of this section. 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 degree 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 dew point 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 dew point exceeds the 5-min average temperature by more than 2 degrees, the dew point 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 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 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 (magnetic north) and wind speed is rounded to the nearest knot. Note that AWOS makes no correction to true north in the archived data. 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). For Artais AWOS only, the raw data are reported with respect to magnetic north. No corrections were applied to these Artais AWOS winds. Precipitation AWOS takes 1-min accumulated measurements and computes total precipitation over the period specified in the AWOS selected archival interval (usually 5-min or 20-min). The total accumulation counter is automatically reset each hour. Present Weather Present weather is not reported in the AWOS data. Handar 20-Minute AWOS data Handar AWOS data were based upon original 20-minute data. There are no present weather or sea level pressures reported for Handar data. Precipitation is the accumulated precipitation for the 20-minute period. All other parameters are the values reported for the 20-minute observation. Qualimetrics 20-Minute AWOS data Qualimetrics AWOS data are based upon 20-minute data. Precipitation is the accumulated precipitation for the 20-minute period. All other parameters are the values reported for the 20-minute observation. Station pressure is calculated from altimeter setting. Sea level pressure is not reported for the Qualimetrics AWOS data. Present weather is not reported for Qualimetrics AWOS data. Artais 20-Minute AWOS data Artais AWOS data are based upon 20-minute data. Precipitation is the accumulated precipitation for the 20 minute period. All other parameters are the values reported for the 20-minute observation. Present weather and Sea level pressure are not reported for the Artais AWOS data. 2.1 Detailed Format Description The Lake-ICE 20-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 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 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 Joint Office for Science Support (JOSS) 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 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 dew point is computed using the formula from Bolton (1980). Calculated Sea Level pressure is computed from station pressure, temperature, dew point, and station elevation using the formula of Wallace and Hobbs (1977). 3.0 Quality Control Processing The Lake-ICE 20-Minute Surface Composite was formed from three datasets (i.e., 20-minute Artais AWOS, 20-minute Handar AWOS, and 20-minute Qualimetrics AWOS) for the Lake-ICE domain. These datasets were collected over the Lake-ICE domain (i.e., 37N to 55N and 73W to 104W) and time period (28 November 1997 through 25 January 1998). The data from the Lake-ICE 20-Minute Surface Composite, Lake- ICE 5-Minute Surface Composite, and the Lake-ICE Hourly Surface Composite were quality controlled together. The 5-minute, 20-minute, and Hourly data were then divided into their respective composites. During the JOSS Horizontal Quality Control (JOSS HQC) processing, station observations of pressure, temperature, dew point, wind speed and wind direction were compared to "expected values" computed using an objective analysis method adapted from that developed by Cressman (1959) and Barnes (1964). The JOSS HQC method allowed for short term (>/= 30 day) variations by using 30 day standard deviations computed for each parameter when determining the acceptable limits for "good", "questionable", or "unlikely" flags. "Expected values" were computed from inverse distance weighted station observations within a 300 300 km Radius Of Influence (ROI) centered about the station being quality controlled (the station being quality controlled was excluded); i.e.; theta_e = / Where theta_e is the "expected value" of the parameter at the site in question, theta(i) is the observed value of the parameter at site i, w(i) is the weighting factor for site i (here the inverse of the distance between site i and the station being quality controlled), and <...> is the sum over all stations "i" in the current ROI that have valid observations of the parameter at the time in question. Data were always compared at like solar times. To determine an observation's HQC flag setting, the difference between the actual observation and its "expected value" was compared to that parameter's normalized standard deviation. Normalizing factors (also called the sensitivity coefficients) were chosen to control the "good", "questionable", and "unlikely" flag limits for each parameter. See Table 3-1 for Lake-ICE normalizing factors. Table 3-2 contains the HQC flag limit ranges derived from the normalizing factors given in Table 3-1 and estimated standard deviations for each parameter so that 95% of the QC limits applied to the Lake-ICE data fell within these ranges. For example, 95% of the observed station pressure values that were flagged as "good" were within 1.6 mb of the expected value. The significant overlap of the ranges seen in Table 3-2 was partially due to seasonal and station differences in standard deviations. The actual HQC limits applied at any particular time depended upon the dynamic nature of the particular station's parameter values over time. Data were never changed, only flagged. HQC was only applied to station pressure, sea level pressure, calculated sea level pressure, temperature, dew point, wind speed and wind direction. If the calculated sea level pressure quality control information was available, its flag was applied to the station and sea level pressures. If the calculated sea level pressure could not be quality controlled, the sea level pressure quality control flag was applied to the station pressure. If the sea level pressure could not be quality controlled, the station pressure quality control flag was not overridden. Table 3-1 Normalizing factors used for Lake-ICE 5-Minute Surface Composite Parameter Good Questionable Unlikely --------- ---- ------------ -------- Station Pressure 0.2 0.2 0.5 Sea Level Pressure (SLP) 0.2 0.2 0.5 Calculated SLP 0.4 0.4 1.0 Dry Bulb Temperature 0.5 0.5 1.0 Dew Point Temperature 0.5 0.5 1.0 Wind Speed 2.25 2.25 4.0 Wind Direction 1.22 1.22 2.2 Table 3-2 Ranges of HQC flag limit values for Lake-ICE 5-Minute Surface Composite Parameter Good Questionable Unlikely --------- ---- ------------ -------- Station Pressure (mb) < 1.7 [0.8-4.1] > 2.1 Sea Level Pressure (mb) < 1.9 [0.9-4.9] > 2.2 Calculated SLP (mb) < 3.6 [1.8-8.9] > 4.6 Dry Bulb Temperature (deg.C) < 3.8 [1.0-7.6] > 2.0 Dew Point Temperature (deg.C) < 4.2 [1.1-8.4] > 2.2 Wind Speed (m/s) < 7.0 [2.8-12.5] > 4.9 Wind Direction(degrees) < 173.6 [94.5-180.] > 170.4 The squall/gust wind speed data were not quality controlled. General consistency checks were also 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 0 degrees or greater than 360 degrees were coded "unlikely". If these consistency checks would have upgraded the quality control flags previously set by HQC or gross limit checks, then they were not applied. However, if these consistency checks would have degraded the previously set QC flags, they were applied. The JOSS HQC scheme relied on spatial and temporal continuity to flag the data. It has been shown that this method works very well for temperature, dew point, pressure, and wind speed, but is not a very good scheme for the wind direction. The flags appear to be overly lax and perhaps could be tightened. Gross limit checks were also used to determine the quality of the precipitation values. The gross limits are shown in Table 3-3. Certain "questionable" and "unlikely" data values were also manually inspected. After inspection, the quality control flag may have been manually modified to better reflect the physical reasonableness of the data. Data were never modified, only flagged. Negative precipitation was also coded "unlikely". The meanings of the possible quality control flags are listed in table 3-4. Table 3-3 - Precipitation Gross Limit Values Parameter Good Questionable Unlikely --------- ---- ------------ -------- 20-minute Precipitation < 3 mm 3-6 mm >= 6 mm Table 3-4 - 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 Barnes, S. L., 1964: A technique for maximizing details in numerical weather map analysis. J. Appl. Meteor., 3, 396-409. Bolton, D., 1980: The computation of equivalent potential temperature., Mon. Wea. Rev., 108, pp 1046-1053. Cressman, G. P., 1959: An operational objective analysis system. Mon. Wea. Rev., 87, 367-374. 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.