Comparison of Precipitation Rate Intensities as Determined by Visibility Versus Liquid Water Equivalent Measurements

Automated Surface Observing System (ASOS) stations use an algorithm based on visibility to determine precipitation intensity (light, moderate or heavy) for METAR reports. This method of determining intensity can misrepresent the actual precipitation rate because it doesn't measure the actual amount of water in the precipitation. This can pose a serious threat to aircraft de-icing procedures, which base de-icing holdover times on intensity. Since the amount of liquid water in falling precipitation relates directly to the failure of aircraft de/anti-icing fluids, the determination of intensity should be based on liquid equivalent precipitation rate.

De/anti-icing fluids are tested using a liquid water equivalent (LWE) rate calculated from manual snow pan measurements. These manual pan measurements have been compared against various precipitation gauges to develop transfer functions that correct the precipitation rates to match the manual pan rates. LWE rates are also used in the holdover timetables, issued by SAE International, which are used in ground de-icing to determine the allotted time a plane has before it must take off after being de-iced to avoid having to be de-iced again. This study compares the ASOS visibility based intensity algorithm to LWE precipitation gauge intensities.

Manual snow pan measurements were taken at the Marshall field site in Boulder, Co, for all snow events during the 2007/2008 winter season. The LWE rates calculated from this data set are used along with precipitation rates from a co-located GEONOR in a Double Fenced Inter-comparison Reference (DFIR) windshield to compare to output from a co-located visibility sensor. Another comparison was conducted using visibility measurements from different sensors verses precipitation rates from various LWE precipitation gauges, i.e. a Hotplate and a GEONOR in a DFIR shield, at multiple instrumentation sites. Results of these comparisons will be presented at the conference.