A Status Report on ASOS Ceilometer Algorithm Development

The Vaisala CT 12K cloud height indicator (CHI) is the standard ceilometer installed on the Automated Surface Observing System (ASOS). The ASOS sky condition algorithm processes Vaisala CT 12K ceilometer data over a 30 minute interval to formulate cloud amounts and heights. One goal of ASOS Planned Product Improvement (PPI) effort is to improve the ASOS Sky Condition Algorithm below 1000' in certain conditions such as low clouds, fog, and precipitation. The current ASOS sky condition algorithm has historically reported cloud heights at 100' when they are actually 200'-300'. These heights can cause airports to remain in limited operation for long periods, which is very costly to both the airport and the airline industry.

The ASOS program initiated a sky condition algorithm development effort in 1998 to investigate ways to improve the ASOS sky condition algorithm below 1000'. Two proposed modifications to the ASOS sky condition algorithm are presented in this paper. These proposed modifications were tested on a case study basis using data from Vaisala CT 12K ceilometers at Sterling, VA from January 1998 to March 1999. Statistical results, that compare algorithm output from the current ASOS sky condition algorithm with the two proposed algorithm improvements, will be presented.