Proposed Improvements to the ASOS Sky Condition Algorithm

Proposed Improvements to the ASOS Sky Condition Algorithm

Joseph V. Fiore Jr. Raytheon Information Technology and Scientific Services 44210 Weather Service Road Sterling, VA 20166 703 661-1206 E-mail: Joseph.Fiore@noaa.gov

The Vaisala CT 12K ceilometer 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 the ASOS Planned Product Improvement effort is to improve the ASOS sky condition algorithm in IFR ceilings (below 1000'). The current ASOS sky condition algorithm has historically reported cloud heights at 100' when they are actually 200'-300'. The original motivation for this algorithm was to take a conservative approach to reporting low ceilings in the interest of safety. However, operational experience has shown that perhaps the algorithm is too conservative in reporting low ceilings. These low ceilings 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 in IFR conditions below 1000'. Two proposed improvements to the ASOS sky condition algorithm are presented in this paper. These proposed improvements were tested on a case study basis using data from Vaisala CT 12K ceilometers at Sterling, VA from January 1998 to April 2000. Statistical results that compare algorithm output from the current ASOS sky condition algorithm with the two proposed algorithm improvements will be presented.