Suggested procedure for nowcasting icing
Icing as such can observed only by flying through an icing cloud. A forecaster on duty can suspect icing in stratiform clouds indirectly by checking the following points:
Satellite images
- Are there icing clouds in automated interpretation products (ICLD etc.)?
- Are there stratiform cold water clouds in Meteosat 24hMicrophysics (yellow and/or greenish) and HRVFog (whitish pink) combinations?
- Are there stratiform cold water clouds in AVHRR images (light yellow in 124-VIS-combination, light red in 345-IR-combination)?
Soundings
- Are there 100 % humid layers with temperatures between 0…-15°C in nearby soundings?
If the answer is yes to any of the above questions, icing is possible.
Radar
- Are there strong echoes in CAPPI radar images among weaker ones in stratiform clouds?
- Are the strong echoes on the same height as the humid layer in soundings?
- Is the area of strong echoes classified as wet snow in hydrometeor classification etc. product?
If the answer is yes to all of the above questions, icing is likely.
The strength of the icing is not straightforward, because what big jets feel as light icing, may be severe icing for small aircrafts. Therefore, the nowcasted/forecasted icing class is only suggestive. Some hints, however, can be reduced from the observations:
- in 24hMicrophysical the yellow clouds are thicker and consist or bigger water drops, indicating stronger icing, whereas greenish clouds with smaller water drops indicate lighter icing
- with regard to soundings, the temperature range 0...-12 °C is favourable for strongest icing