Day 5: synthetic band difference utility for icing forecasts

Yesterday forecasters utilized the synthetic IR imagery with the ice color curve (tops from 0 to -25C) as a method to forecast potential icing conditions. Today the same has been used, but with an additional tool. The synthetic band difference (10.35 -3.9 um), though typically used to identify liquid water clouds associated with low clouds, has also been found to assist in icing forecasts.

The IR ice curve identifies all clouds with tops from 0 to -25C, including those that are glaciated. However, by using the synthetic band difference, which is designed to identify liquid water clouds, and comparing to the IR with the ice enhancement, forecasters can identify liquid clouds with tops below freezing. These areas would have a higher probability of icing conditions.

Check out the imagery below of the 1500 UTC forecasts of the NAM Nest (Fig. 1) and WRF (Fig. 2) simulated IR imagery with the icing enhancement, as well as the 1500 UTC WRF generated synthetic band difference (Fig. 3). Additionally, the 21Z icing forecast for this afternoon and the 1500 UTC PIREPs have been overlaid on all three. Note in particular our area of interest over southern FL.

While the synthetic imagery generated by each model varies slightly, both did fairly well in identifying the area of showers over the southern portion of FL, particularly those areas in which cloud tops were in the temperature threshold for icing conditions. The synthetic band difference shows some noticeable high clouds in the area (grays and blacks), obscuring the lower clouds below, but it is evident that there are liquid clouds in the area (blues). The presence of these liquid water clouds with tops below freezing indicates a higher probability of icing, and this was corroborated by a handful of icing PIREPs in the area around 1500 UTC.

Figure 1. 20130215 1500 UTC NAM Nest synthetic IR imagery, 1500 UTC PIREPs, and the 21Z icing forecast

Figure 2. 20130215 1500 UTC WRF synthetic IR imagery, 1500 UTC PIREPs, and the 21 Z icing forecast

Figure 3. 20130215 1500 UTC WRF synthetic band difference (10.35 - 3.9), 1500 UTC PIREPs, and the 21Z icing forecast