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E Automated Surface Observing System Impact on the Climate Record The ASOS hygrothermometer (McKee et al., and autumn (McKee et al., 1995). The HTB gauge also 1996b) is cooler than the conventional HO-83 hygro- reported too many days with 0.01 inch resulting from thermometer for both maximum and minimum tem- dew condensation, not precipitation. ASOS precipita- peratures and also has a smaller diurnal temperature tion undercatch ranged from two to 10 percent com- r ange (McKee and Doesken, 1997; McKee et al., pared to traditional manual observation. Further, the 1 996b; Schrumpf and McKee, 1996). The maxi - HTB evaporated or sublimated precipitation falling mum temperature differences are larger in magnitude below 15°F, recording almost no cold weather pre- (compared to minimums) and vary more with varying cipitation. The introduction into service, beginning in weather conditions. Individual test sites showed wide 1996, of a modified heated tipping bucket gauge for variation in ASOS-conventional differences, possibly ASOS resulted in an improved comparison between due to differences in instrument siting and surround- the ASOS and conventional measurements (McKee ings, as well as variable changes in the solar heating and Doesken, 1997). However, according to the CSD, effects; this local effect can vary from day to night, and the nearly 10 years of undercatch reported from the the effect of instrument location change can some- HTB gauge is still in the extant climate record. The times be as large as that resulting from the change in phased introduction of this new ASOS gauge will com- instrument. These local effects introduce a nonlinear plicate future precipitation comparison studies and any relationship between the ASOS and pre-ASOS data. adjustments that may be made to the data for normal Large ASOS-conventional differences in dew point computation. Further, ASOS is not equipped to mea- temperature can occur from station to station, but sure snowfall and snow depth amounts (NWS, 1992a). without systematic bias (McKee et al., 1996b). The cool Conventional NWS wind measurements use a three- temperature bias of ASOS means that relative humidity cup design with a continuous output to drive a dial indi- estimates are slightly higher than before, with seasonal cator or a strip chart recorder; ASOS uses a light chopper averages being one to three percent higher (McKee et rather than a voltage generator resulting in a lower start- al., 1996b). ing threshold and an accurate one-second average sample The ASOS Heated Tipping Bucket (HTB) gauge speed. The conventional wind vane reports in 10-degree consistently undermeasured precipitation compared to steps or a resolution of 6 five degrees, while the ASOS the standard universal weighing gauge, during heavy wind vane reports to the nearest whole degree (Lockhart, rain events (McKee et al., 1996b) and snow events 1995). ASOS also introduces a significant change in the (McKee et al., 1995). This difference showed a non- way wind speed is measured. All applications of maxi- linear seasonal pattern in the central United States, mum wind speed which relate to warnings have been with ASOS measuring significantly less precipitation based in the past on “instantaneous” values equivalent during winter and summer when compared to spring to an averaging time of 2 seconds, whereas ASOS uses 99
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100 APPENDIX E a five-second (soon to be changed to three-second) during periods of active weather (Cornick and McKee, average peak gust. ASOS provides a two-minute aver- 1993). age, continuously updated each minute, for the hourly ASOS is not equipped to measure sunshine dura- observation (Lockhart, 1995, 1996a, 1996b). Possible tion (NWS, 1992a). Conventional pressure observa- sources for differences in wind direction (Lockhart, tions are based on an aneroid altimeter indicator or a 1996b) are that measurements may not be taken at precision aneroid barometer with observations made exactly the same time, the instruments are not co- at hourly and special observation times (NWS, 1992a, located which would affect the character of the wind 1994a). The ASOS barometers consist of redundant flow, and the wind direction is determined differently. digital pressure transducers utilizing capacitive sensors, ASOS provides unweighted (objective) averages (scalar which compute and update the pressure report once or unit vector) from one-second samples taken for two every minute from readings obtained every 10 seconds minutes, whereas the conventional observation is the (NWS, 1992a). (subjective) average direction and speed inferred by an Manual observation of weather phenomena, includ- observer watching a dial for one minute. Analysis of ing obstructions to vision, has been based on personal five-second wind averaging indicates ASOS peak winds interpretation of the human senses (NWS, 1994a) for are lower than the previous subjective measurements almost all of history (Cornick and McKee, 1993), with (Lockhart, 1996b; McKee et al., 1996a). Differences intensity being based on visibility criteria. These phe- in the hourly wind speed observation show a nonlin- nomena include (a) rain, snow, fog, haze, and freezing ear wind speed-dependent bias (Lockhart, 1996b). A precipitation; and (b) tornadoes, funnel clouds, water comparison of the wind direction distributions at two spouts, thunderstorms, hail, ice crystals, snow pellets, sites indicated that there was no significant difference snow grains, ice pellets, drizzle, blowing obstructions between the ASOS and conventional hourly observa- (snow, sand, dust, spray), and smoke. The automated tions (Lockhart, 1996b). observation of these elements required a fundamental The ASOS cloud height indicator (CHI) is a change in observational technique and perspective. The laser ceilometer that differs from the standard NWS ASOS Precipitation Identification (PI) sensor can dis- ceilometer in the way it processes returns for low cloud criminate between the occurrence of rain and snow (and base and total obscuration. Both ceiling height and identify intensity) from an algorithm based on sensor cloud coverage (up to 12,000 feet only) are determined response (Cornick and McKee, 1993). Fog is reported if by time averaging over a 30-minute period the condi- visibility drops below seven statute miles and dew point tions directly overhead. In manual observations, the depression is 4°F or less. If the dew point depression observer subjectively evaluates the ceilometer trace at is greater than four degrees and no present weather is a single point in time to determine ceiling height, and indicated, then haze is reported. ASOS cannot report the cloud coverage is determined by visual examina- the weather phenomena from group (b) above (NWS, tion of the cloud conditions over the entire sky then 1992a). In a study of 13 sites, ASOS and human observ- subjectively forming a spatial average (Cornick and ers reported approximately the same number of total McKee, 1993). ASOS ceiling reports were highly minutes of freezing rain, however the coincidence rate correlated to conventional ceiling reports most of the (ASOS and human reporting freezing rain at the same time (92.7 percent), but the high level of equality drops time) was about 66 percent (Ramsay, 1997).