Mesonet Indiana Purdue Automated Agricultural Weather Station Network (PAAWS) [JOSS] CONTACTS: For general information on this BAMEX data set: Steve Williams P.O. Box 3000 Boulder CO 80307-3000 Email sfw@ucar.edu 303-497-8164 (voice) 970-491-8293 (fax) For detailed information on the PAAWS network: Ken Scheeringa kens@purdue.edu 1.0 DATA SET OVERVIEW This data set contains 30-minute resolution surface meteorological data from the Purdue Automated Agricultural Weather Station (PAAWS) Network. This network includes 7 stations around the state of Indiana. This data set covers the period from 20 May to 8 July 2003. These data are in columnar ASCII format. The PAAWS network is a system of remote automatic weather stations located at each of the eight regional Purdue Agricultural Research Centers (ARCs) throughout Indiana and the Purdue Agronomy Research Center. The purpose of the network is to continuously measure weather elements of special interest to Purdue agricultural researchers. 2.0 INSTRUMENT DESCRIPTION 2.1 Instrumentation Each automatic weather station datalogger measures: wind direction and speed at 10' air temperature at 4.5 ' soil temperature at 4" under a bare surface soil temperature at 4" under a sod surface precipitation solar radiation Most weather sensors and the datalogger are mounted on a 10 foot high tower mast. The wind vane and cup anemometer are mounted on a crossarm at the top of the mast. The solar radiaton sensor is also mounted on a crossarm. Dr. Grant modified the factory set height of the solar sensor to better expose the instrument to the sun. A cell phone antenna is also mounted on the mast. Mounted lower on the mast is the air temperature sensor, a thermistor shielded by an enclosure to avoid exposure to sunlight. The Zeno datalogger is mounted in an enclosure on the mast a few feet above the ground. All sensor leads run into the enclosure. The cell phone transceiver and modem are also inside the datalogger enclosure with a lead to the outside antenna. The voice synthesizer is located inside the enclosure as well. The leads of the two soil temperature sensors drop to the bottom of the mast, then run six feet parallel to and under the ground away from the mast. The sensors are buried exactly 4" below ground surface, closely intact with the surrounding soil. One sensor is placed below a sod-covered surface while the other sensor lies below a bare cover where the sod has been removed. The raingage is located about 20 feet away from the tower mast, with its leads buried underground. The gage has a very low profile, with its top funnel opening just one foot above ground level. 2.1.1 Wind Speed and Direction Sensor Information Wind direction is measured by a conventional balanced wind vane, sensitive to winds of at least 3 mph. The circular position of the wind vane is converted to an electrical signal by a conductive plastic potentiometer. A 15 volt signal is applied to the potentiometer. A percentage of this voltage is output by the potentiometer, directly related to the wind direction angle. A limitation of this sensor is that the potentiometer becomes worn over time, resulting in noisy or non-linear output. The only remedy is to replace the potentiometer. Wind speed is measured by a conventional rotating cup anemometer. As the cups rotate they produce an AC sine wave voltage signal with its frequency directly related to the wind speed. One complete sine wave corresponds to one rotation of the cup wheel. The AC sine wave is induced in a stationary coil by a two pole ring magnet mounted on the cup wheel shaft. A limitation of this sensor is that the precision ball bearings become worn rather quickly. As bearings wear they become noisy or the minimum detectable wind speed increases above an acceptable level. New bearings give the anemometer a starting speed of 2.5 mph, indicating it will start turning from a calm situation when wind speeds exceed 2.5 mph. Lower wind speeds are measureable if the winds were first above 2.5 mph. Bearings will be replaced at least yearly and probably twice a year. The measurement of wind gusts is highly sensitive to the number of samples included in a running average of wind speed. Our dataloggers at each Purdue ARC have been set to average the last 4 seconds of instantaneous samples when calculating wind gust. This is the factory recommended setting. A longer averaging period, such as the 5 seconds used by the National Weather Service in their automated ASOS system, will nearly always result in lower gust values. The National Weather Service will soon make a decision on whether to shorten the averaging time to 3 seconds, at the request of some of their field offices. 2.1.2 Air and Soil Temperature Sensor Information Air and soil temperatures use identical sensors. Air sensors are sheltered by a 6-plate passive ventilation shield to avoid exposure to sunlight while soil sensors are buried 4" under the ground surface. Thermistors are sensitive to small changes in temperature. The sensors used in this project are precisely manufactured so that they are directly interchangable should sensors require replacement. All thermistors are calibrated at the factory be measuring at multiple temperatures to insure that the sensor resistance and slope meet the device's interchangability specification. Thermistors cannot be repaired. Faulty devices must be replaced in their entirety. In winter months soil temperature sensors may become trapped in frozen or near frozen saturated soils. Soil temperature readings may report values right at 32F for extended periods of time. 2.1.3 Precipitation Sensor Information Precipitation is measured by a miniture version of the standard National Weather Service tipping bucket electronic raingage. The sensor has an aluminum collector funnel with a knife edge that directs incoming water into twin tiny buckets able to hold exactly .01 inch of water and counter-balanced on a central pivot. As one of the two chambers fills, it tips, spilling out to the bottom of the housing. A magnet is attached to a tipping bucket, which as the bucket tips, triggers a magnetic switch. A momentary switch closure occurs with each tip, which the datalogger senses and so increments an event counter. The alternate bucket is now exposed and begins to collect the next .01 inch of precipitation. After this bucket fills and tips the first bucket is returned to its original position and the whole cycle repeated. The total events counted at the end of a reporting period corresponds to precipitation accumulation in units of hundreths of inches. After the report the event counter is reset to zero. The tipping bucket has several limitations. First, the sensor must be kept clean. Any accumulation of bugs, dust, twigs, and other material can invalidate its measurements. Second, the tipping bucket has a maximum reliable flow rate of one inch of rain per hour. In more intense rain events the instrument will read low as a fixed amount of time is required for the bucket to tip and position the alternate bucket in place. Intermediate rainfall will be lost as water flows into the buckets rather than drips. The third deficiency of the tipping bucket is it does not work well in frozen precipitation events. Thus precipitation reports printed by the Indiana Climate Page when the air temperature is below freezing should be discarded. The data are suspect. This winter limitation could be partly resolved by adding heat tape around the gage funnel. This is not a foolproof solution, however, and creates its own set of problems. We have chosen to not install heat tape in the tipping bucket gages at any of the ARCs. Cold season precipitation data should be ignored as being unreliable. 2.1.4 Solar Radiation Sensor Information Solar radiation is measured by a LICOR pyranometer sensor. A pyranometer is an instrument for measuring solar radiation received from the whole sky hemisphere. It is suitable for measuring the global (direct) sun plus sky radiation (diffuse). A pyranometer is not the same as a quantum sensor. A quantum sensor measures photosynthetically active radiation (PAR), the portion of solar energy useful to plant growth. PAR is limited to the radiation waveband between 400 and 700 nanometers. In contrast the pyranometer measures solar radiation over the full solar range from 400 nm to 1100 nm. However the response of the LICOR pyranometer is not uniform over this range. The response begins around 400 nm and increases almost linearly to about 950 nm, then decreases nearly linearly to a cutoff near 1200 nm. The LICOR pyranometer has limitations. Buildings and trees can cause large variations in locations of small area. The instrument's response is not ideal, but the error is not serious provided it is used out in the open to measure natural unblocked sunshine. The sensor is not recommended for measurement in artificial lighting, inside plant canopies, or for reflected radiation. Testing has shown the LICOR pyranometer is remarkably constant for both clear and overcast days in the 400 nm to 700 nm spectral range. However a major change in the spectral distribution occurs in the near infrared wavelengths where water vapor absorption takes place on cloudy days. Data collected at low solar elevations can have significant error because of altered spectral distribution which changes in atmospheric transmission. Yet this is a small part of the daily total so the possible observed error usually has an insignificant effect on the daily integration. The area under the spectral irradiance curve of the sensor is directly proportional to the energy received by a horizontal surface. Under specific but typical conditions, energy received on a completely overcast day has been estimated to be 11.3 percent of that received on a clear day. The LICOR pyranometer is factory calibrated against an Epply Precision Spectral Pyranometer, an instrument known for its superior spectral response. 2.2 Station Locations Location Latitude(Deg) Longitude (Deg) Elevation (m) ---------------------------------------------------------- Butlerville 39.036 -85.531 242 Columbia City 41.107 -85.399 268 Farmland 40.250 -85.150 294 Lafayette 40.298 -86.903 224 Vincennes 38.739 -87.485 146 Wanatah 41.270 -86.560 224 West Lafayette 40.560 -86.930 215 3.0 DATA COLLECTION AND PROCESSING 3.1 Purdue Processing All weather sensor sampling, data storage, and data retrieval at the weather station is controlled by a datalogger, the Zeno 3200, manufactured by Coastal Environmental Systems of Seattle, Washington. Once each second the Zeno samples each weather sensor. The data are calibrated from electrical units (such as volts) into meteorological units (such as temperature degrees). Each weather sample may undergo further evaluation at this time depending on what type of sensor is involved and what extended processing the Zeno software manager has instructed the datalogger to do for that sensor. The results of all such Zeno sampling and processing are stored in its internal memory until the end of the sampling period is reached. The Zeno software manager can instruct the datalogger how long to continue sampling until a summary report is generated. The Zenos at the Purdue ARCs have been set to sampling periods of 30 minutes. At the end of each half hour, a summary table is generated which includes 30-minute averages of wind direction and speed, air temperature, soil temperatures, and solar radiation. The extreme wind gust for the 30-minute period, and the total precipitation are also calculated into the summary table. Whenever the datalogger senses that something is wrong with one of its weather sensors, it flags that value in its memory as being suspect. The limits on when a sensor is considered suspect may be set by the datalogger manager. In most cases we have accepted the factory defaults as to when a sensor is flagged. There are two circumstances in which a sensor is flagged. First, the sensor values have gone out of range, either too high or low to be regarded as reasonable under typical Indiana weather conditions. Second, the sensor value has been "stuck" or "nearly stuck" on one value much too long. This sensor is expected to vary in value quite frequently in Indiana but it has not. This may be a bad sensor or a sensor which has frozen over (in winter) and is no longer responsive to true environmental conditions. In any event any value which is flagged in the summary table should be used with caution. A flagged value in the hourly or daily option tables indicates that at least one value in the 30-minute source data from which the hourly or daily table was calculated was flagged. 3.2 UCAR/JOSS Processing UCAR/JOSS conducted no processing or quality control on these data. 4.0 DATA FORMAT AND FILE NAMING 4.1 Data Format These data are in columnar ASCII format. Each data file has a header line that describes the parameter and units contained in each column of data. 4.2 File Naming conventions butlerville_30min.txt where: butlerville is the station (see location table above). 30 min signifies the data are 30 minute values 5.0 DATA REMARKS None. 6.0 REFERNCES PAAWS Network Home Page: http://shadow.agry.purdue.edu/sc.zen-geog.html