Page 18 - Volume 15 Number 1

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words, with a model-based solution, can you really be that sure of an icing event that’s nearly 12 hours in the making with any certainty? The old potential field didn’t take this forecast uncertainty into account, so it was common to still see some icing events with a very high likelihood even at 12 hours.
There’s no doubt that this new presentation will cause a lot of confusion with some pilots. It will
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be interpreted as, “Hey, later in the afternoon, there’s not going to be as much icing so I’ll delay my flight by 9 hours when things look better.”
Ah, no. Icing will still likely be there, it’s just that FIP can’t be as sure exactly where nearly 9 hours earlier. Consequently, the probabilities are calibrated lower and are often spread out over larger areas due to this uncertainty.
For example, if you look at the new calibrated FIP probability forecast starting at 2 hours you will notice that the icing probabilities will generally decrease through the available forecasts at 3, 6, 9 and 12 hours. It may appear to the unwary eye as if the icing is actually forecast to dissipate over time. That very well may be what is forecast to happen; however, the most likely reason for the decrease in probability is the uncertainty in the forecast and not the dissipation of icing potential. Consequently, don’t expect to see the new calibrated FIP probability product exceed 45 percent probability for a 12-hour forecast or 55 percent probability for a 9 hour forecast – you won’t see many yellow, orange or red colors on the longer range forecasts.
This will also affect the masked FIP severity charts as well. With a 12 hour forecast, for example, the 50 percent masked severity field will rarely show anything more than grey, which reflects the increase in uncertainty with forecast time. Even at 6 and 9 hours, the lack of forecast probabilities above 50 percent will be evident.
According to Dr. Marcia Politovich who is the in-flight icing research team lead for the FAA Aviation Weather Research Program (AWRP), “Other ‘under the hood’ improvements in the new FIP include a better scheme for icing in convection, more accurate cloud top estimation, and enhanced supercooled large drop (SLD) forecasts. These won’t impact the look-and-feel of the product, but should provide an added margin of safety for the flying public.”
Future FIP Enhancements
Icing analyses and forecasts are always being improved. Nextgen will certainly play an integral role in this. According to Dr. Politovich, “The Nextgen functional requirements document is asking for icing forecasts out to 60 hours.” She further adds, “My take on this is hey, give us a
JANUARY 2011