Tuesday, 13 January 2009: 9:15 AM
Evaluation of the Use of Visibility to Estimate Snowfall Rate Using ASOS data
Room 121BC (Phoenix Convention Center)
Operational estimates of light, moderate, and heavy snowfall intensity are currently based on visibility using a table given in the Federal Meteorological Handbook (FMH-1). This table indicates that light snow is greater than or equal to 3/4 miles visibility, while heavy snow less than or equal to 1/4 mile, with moderate snow in between these values. Previous research by Rasmussen et al. (1999) has shown that the use of this table leads to significant uncertainty as to the actual liquid equivalent rate. In an attempt to improve this situation, a revised table was developed by the FAA using the Rasmussen et al. (1999) research, hereafter called the FAA/Rasmussen table. This table takes into account both the time of day and ambient temperature in an attempt to improve the snowfall rate estimate. This paper provides an evaluation of the use of both the FMH-1 table and the FAA/Rasmussen table using data collected by NCAR/FAA from five field sites located at Denver, Chicago, Pittsburgh, and Minneapolis/St. Paul airports and one minute ASOS data. The NCAR field sites includes visibility estimates from the Vaisala PWD-22, and snowfall rate estimates from a GEONOR weighing gauge and a Yankee Hotplate liquid equivalent gauge. ASOS sites include visibility from a Belfort sensor and liquid equivalent from the All Weather Precipitation Accumulation Gauge (AWPAG).