Techniques for Forecasting Freezing Spray at WFO Anchorage
Compiled by Joel Curtis, NWS Lead forecaster, NWA Marine Committee
Vessel Icing on Alaska waters is one of the most hazardous conditions encountered by mariners. Ice accumulation can sink smaller vessels in minutes, and actions by the crew to counteract icing are usually ineffective and always dangerous. The superstructure configuration of many types of commercial fishing boats that operate in the Anchorage forecast area makes them quite susceptible to rapid ice build-up from freezing spray. Vessel icing can occur as early as October in the Anchorage forecast area, but usually the frequency of icing encounters increasing dramatically in November. Vessel icing can occur as late as May, but the threat of significant icing drops dramatically during the month of April.
The threat of vessel icing is dependent on the physical ingredients necessary to allow for the formation of ice on superstructures. The necessary ingredients are:
1) cold ambient air temperature
2) liquid water colliding with the vessel superstructure
3) temperature of the superstructure below freezing
4) “near” freezing temperature of the liquid water available for the collision.
These necessary physical elements for vessel superstructure are the same as those required in aircraft icing. The significant difference is that there is a third and most predominant method for providing the liquid water for collisions with ocean vessel superstructures. Both freezing rain and accretion from fog can cause ocean vessel icing. The most frequent and intense ocean vessel icing is from freezing spray, which is when cold water droplets are brought into suspension by the surface wind and waves.
There is a long history of techniques employed by the Anchorage Forecast Office to forecast vessel icing, and these techniques have focused on a single element in the forecast: The potential for vessel icing is expressed marine forecasts as “FREEZING SPRAY” or “HEAVY FREEZING SPRAY”. This element has been included in marine forecasts since their implementation in Alaska. Note that the important concept of rate of the icing is implied in the two forecast categories. For all of the following techniques, the forecaster must forecast individual parameters that relate to the necessary ingredients listed above.
The first technique is a very simple one that is quoted here directly from the WFO Anchorage Station Duty Manual (SDM):
Freezing Spray - Wind > 15 kt and Air Temperature < 30F .
Heavy Freezing Spray - Wind > 15 kt with Air Temperature < 26F and SST < 5C.
The SDM also clearly states that: “ ...the rules are general and some discretion must be used”.
The rules listed above are employed cautiously because the forecasters are aware of the shortcomings of such general rules.
A more sophisticated approach is employed by Anchorage forecasters is an adaption of one presented by Overland, et al., (1986) which gives a nomogram of a simple wind versus ambient air temperature relationship and is attached to this paper. This technique is well described in the U.S. Navy manual: Forecaster’s Handbook for the Arctic by Sechrist, Fett, and Perryman, 1989. A copy of this nomogram is posted at the East, Central, and West forecast desks at WFO Anchorage . Forecasters modify this nomogram by splitting the “moderate” area between the “light” and “heavy” areas, because the “moderate” category has been shown to not be useful to the customers.
Two other aids to freezing spray forecasting relate to using guidance for obtaining the necessary forecast elements to arrive at the vessel icing decision. The first was developed by the Japanese Meteorological Agency (JMA) based on empirical data from their fishing fleet. This was related to Craig Bauer, a former Ice Forecaster at the WFO. JMA forecasters use the -6C 850 MB isotherm as a guide to correspond to +21F at the surface. This guidance, usually from models, combined with 20 knots of surface wind becomes a guidance forecast of “HEAVY FREEZING SPRAY”. Russ page, WFO Anchorage Ice forecaster, notes that heavy icing can occur with SSTs as high as 45F when the ambient air temperature is very cold, such as 0 F or colder.
Forecasting vessel icing in the WFO Anchorage Forecast area is a complex and subjective process. Experienced forecasters, however, can project the necessary weather elements into a forecast of freezing spray fairly rapidly. Value added to these forecasts comes from both experience an and understanding of the vessel icing process. The hazard of vessel icing due to freezing spray is extreme, and the service provided to the marine community by these forecasts is essential.