David L. Sheets1
National Weather Service Office
The Automated Surface Observing System (ASOS) was installed at Capital City Airport in Lansing, Michigan in September 1993. The system was accepted in October 1993 and since then has operated in a test mode, providing daily and monthly climatological data, along with weather observations.
Currently, the National Weather Service provides local climatological data for the city of Lansing. This service will end shortly when the office is slated to "spin-down," and eventually close, in accordance with the ongoing modernization and associated restructuring of the National Weather Service. At that time, ASOS will likely take over as the primary source of climatological data for the Lansing area. In this study, the differences between the climatological output from ASOS and the current official climatological out put were analyzed to determine the potential impact ASOS will have when it becomes the official source of climatological information.
SCOPE OF THE STUDY
The three primary climatological parameters of temperature, wind and precipitation were examined for the first six months of ASOS operation. This six-month period covered October 1993 through March 1994.
An example of ASOS climatological output from the ASOS software is shown in Figure 1. Daily temperature, wind, and precipitation data from this output was compared to official readings, measured by Weather Service Office Lansing (WSO LAN) personnel and entered the MAPSO (Micro-computer Aided Paper less Surface Observation) personal computer. The information entered into MAPSO is regularly archived and sent to the National Climatic Data Center, where it is certified, compiled, and becomes the official local climatological data for the Lansing station. This MAPSO data was compared to the ASOS output to document differences, which may ultimately affect local climatological data in the future.
Figure 1. Example of ASOS daily climatological summary as displayed and printed from the ASOS Operator Interface Device.
Figure 2. Layout of Lansing's Capital City Airport. The location of the National Weather Service Office is labeled WSO LAN in the lower left and the nearby instrument shelter and universal rain gage site is indicated to the right of the WSO. The ASOS field equipment is located in the upper right.
The standard equipment is located 841 feet above mean sea level on relatively flat ground, with a slight slope downward to the north toward the drainage ditch. Both the standard and ASOS temperature sensor and field equipment arrays are located in one of the lowest areas in the airfield. The drainage ditch is identified as the Reynolds drain in Figure 3. It originates from an area about 1 mile north of the airport and drains into the Grand River about 1 mile south of the airport. The entire area is relatively flat with small hills to the west, north, and east of the airport perimeter. The Grand River is located in a curving valley about 1 mile southwest to 1 mile south of the airport.
Figure 3. ASOS and standard equipment field locations. Area shown is enlarged from the rectangle outlined in upper right section of Figure 2.
The methods used by ASOS and observers to measure maximum and minimum temperatures are somewhat similar. ASOS software uses an algorithm that samples the ambient air temperature nominally every 30 seconds and computes a one minute average based on this reading. It then averages five consecutive one minute values to compute a five minute ambient air temperature. This temperature is updated every minute. The highest and lowest five minute average temperatures of the day are stored as the maximum and the minimum temperatures, respectively. All values are rounded up (NOAA et al. 1992).
The HO-83 hygrothermometer operates similarly, except that it samples ambient air temperature every 37.5 seconds. It then takes an average of eight of these samples to compute a running five minute temperature reading. The system compares each of these running five minute values to the maximum and minimum temperature values stored in memory. If this temperature is greater than the stored maximum temperature, or less than the stored minimum temperature, the maximum and minimum temperatures are adjusted accordingly (DOC et al. 1984). These maximum and minimum temperature values are read and reset by the observer, and recorded into the MAPSO computer and stored as the extreme values for the day. Both ASOS and MAPSO compute daily average temperatures by adding the maximum and minimum and dividing by two.
The actual sensor is a rotating cup anemometer, known as the "F420" series (DOC et al. 1992). The ASOS F420 anemometer operates by "counting" flashes of light that pass through the rotating wind cup shaft. The number of flashes is digitally converted to reportable wind speed values. The ASOS algorithm incorporates a smoothing technique that takes five second averages of one second wind speed and uses these five second averages over a two minute period to report the "instantaneous" wind speed. The daily peak wind value consists of the greatest five second average wind speed stored in ASOS for the 24-hour period (DOC et al. 1992). Daily average winds are computed from the stored two minute averages, which are updated every minute.
The standard F420 anemometer operates by using the wind-driven rotating wind cups to produce a DC current. An analog gage measures this current, which is converted into knots. This gage is read by observers to determine wind speeds for weather observations. A separate wind gust recorder chart also uses this DC input to drive pen movement across a moving paper roll, which records the instantaneous wind speed over time.
The determination of reportable one minute wind speed relies on observer judgment. For each hourly observation, the observer monitors the wind velocity dial, located within the observer's console in the Weather Service Office at Lansing, for a one minute period to determine the average wind speed. The observer then enters this into the MAPSO computer (DOC et al. 1988). These 24 hourly wind speeds are averaged by the MAPSO software to compute the daily average wind velocity. The peak wind gust is read from the 24-hour trace on the wind gust recorder chart and is also entered into the MAPSO program at the end of the day.
The basic components of the ASOS tipping bucket include a heated collector funnel, pivoted dual-chamber tipping bucket, and heated drain pan and drain tube. The tipping bucket tips with every .01 inches of liquid precipitation. Algorithms are used to correct the high rainfall rates and to operate thermostats that adjust for varied weather conditions (DOC et al. 1992).
The current standard method for measuring liquid precipitation incorporates a universal recording weighing rain gage. At WSO Lansing, this gage is located about 175 feet southeast of the General Aviation Building, the building that houses the WSO. This is about 3500 feet south-southwest of the ASOS combined sensor group, as seen in Figure 2. The universal rain gage operates by converting the weight of both liquid and frozen precipitation into pen movement on a drum-mounted, clock-driven, rotating chart. Precipitation values, in hundredths of an inch, are periodically read by NWS observers from this chart and entered into MAPSO for the daily liquid precipitation.