ASOS Blowing Snow (BLSN) Algorithm

The ASOS precipitation identification sensor (Light Emitting Diode Weather Identifier (LEDWI)) may identify the occurrence of rain during blowing snow and ice pellet events. In the past, this resulted in erroneous reports of rain at low temperatures, i.e., at 32F and below. If a freezing rain sensor was installed and was not detecting freezing rain, or if a freezing rain sensor was not installed at the site, then ASOS would have reported rain at low temperatures if rain was identified by the LEDWI. In turn, this would be encoded in the METAR/SPECI reports.

With the implementation of software version 2.60, ASOS will address the erroneous reporting of rain at low temperatures. This will be accomplished by using the newly developed blowing snow (BLSN) algorithm. One feature of the algorithm is to infer the occurrence of BLSN, when conditions warrant. ASOS currently infers the presence of mist (BR), fog (FG), freezing fog (FZFG), and haze (HZ) based on ambient and dew point temperatures and surface sensor visibility. The BLSN algorithm also has the capability to report Unknown Precipitation (UP) if BLSN conditions are not met. Or, in cases where the ambient temperature is less than 0F, and in some cases where the visibility is 7 statute miles or greater, the reports of rain will not be used to report the present weather occurring. (See the BLSN algorithm logic flow diagram.) Of course, if snow (SN) or freezing rain (FZRA) is detected, the BLSN algorithm will not be used and SN or FZRA will be reported.

The BLSN algorithm was developed by using 30 years of climatological data and tested and improved over a winter season. The algorithm is used if the temperature (T =15-minute average of the 5-minute averages) is less than or equal to 32F and the LEDWI (Pres Wx) is identifying rain (RA). When both of these conditions exist, then the wind speed (Ws = 15-minute average of the 2-minute averages), visibility (VIS), sky condition (SKY) layer amount and height, are examined to determine if BLSN or With the implementation of software version 2.60, ASOS will address the erroneous reporting of rain at low temperatures. This will be accomplished by using the newly developed blowing snow (BLSN) algorithm. One feature of the algorithm is to infer the occurrence of BLSN, when conditions warrant. ASOS currently infers the presence of mist (BR), fog (FG), freezing fog (FZFG), and haze (HZ) based on ambient and dew point temperatures and surface sensor visibility. The BLSN algorithm also has the capability to report Unknown Precipitation (UP) if BLSN conditions are not met. Or, in cases where the ambient temperature is less than 0 F, and in some cases where the visibility is 7 statute miles or greater, the reports of rain will not be used to report the present weather occurring. (See the BLSN algorithm logic flow diagram.) Of course, if snow (SN) or freezing rain (FZRA) is detected, the BLSN algorithm will not be used and SN or FZRA will be reported.

The BLSN algorithm was developed by using 30 years of climatological data and tested and improved over a winter season. The algorithm is used if the temperature (T =15-minute average of the 5-minute averages) is less than or equal to 32 F and the LEDWI (Pres Wx) is identifying rain (RA). When both of these conditions exist, then the wind speed (Ws = 15-minute average of the 2-minute averages), visibility (VIS), sky condition (SKY) layer amount and height, are examined to determine if BLSN or UP is occurring. Generally, if the temperature is 32F or less, but 26F or greater, then UP would be reported. Climatological analysis has shown that it is likely that ice pellets (PL) would be occurring in this temperature range. At temperatures less than 26F but greater than or equal to 0F, BLSN could be reported depending on the other parameters. If the wind is less than 23 knots, the sky is either overcast or obstructed, or if the temperature is greater than 14F, but less than 26F, it is more likely that BLSN is not occurring. Therefore, UP would be encoded.