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I I I mounting arm pointed south of the HAS tower, about 5 m above ground level, with the sensing rod tilted toward the south. I 17.25 I 1.0 I I I I I I All dim,nsioflf in indl,s I Fig. II. Scale drawing of the Goodrich Model 872F I icing rote sensor. I The HAS data logger which collected measurements from the icing sensor and other instruments was programmed to calculate a running mean of three 1 min observations, centered on the middle minute. The running mean of three had to decrease by 34 Hz or more over exactly a 10 min period in order ofa pair of EUs to be declared. The 10 min period was specified to provide a reasonable time sample and to minimize the impact of any sudden frequency change, After the first of the EU pair was declared, the icing sensor continued to be monitored but was not used to declare another pair of EUs until two hours had passed. Another pair would then be declared as soon as the running mean for the past 10 min again showed a minimum decrease of34 Hz, equivalent to about 0.005 inches of ice buildup according to wind tunnel tests conducted by the manufacturer3. If this criterion was reached during the last ten minutes of the experimental pair, than a new pair would be initiated immediately following the original pair. The specific values from the same source were approximately 0.01 inches ice buildup, equivalent to about 40 mg mass per 67 Hz frequency decrease. A very similar Goodrich icing sensor is part of the ASOS used by the National Weather Service, Federal Aviation Administration and others to detect freezing drizzle. The ASOS algorithm uses an accretion of 0.005 inches as part of their freezing drizzle reporting threshold, the same value used to declare EUs for the Utah randomized experiment. I I I I I 3 Darren Jackson, Goodrich Aerodynamicist/Systems Engineer, personal communication, 18 Sep. 2003. 24 I