Enroute Icing

### Produced by The COMET® Program

### Step 1: Diagnosing Locations of Icing » NWP Analyses » Vertical X-C and Plan View Maps - Description

The most commonly used tools for NWP analyses of icing risk are vertical cross-sections and plan view maps.

Vertical cross sections (X-Cs) show a detailed “slice” of the model atmosphere, plotting temperature T, relative humidity RH, and negative pressure vertical velocity –VVEL. Negative of pressure vertical velocity is used so that upward vertical motion has a positive value. The cross sections help forecasters locate layers of potential icing in or near to that model slice.

P lan view maps are used to find horizontal areas where layer mean temperature T, relative humidity RH, and –VVEL meet icing criteria. The layers often used include:

- 925-850 hPa (about FL20 through FL45)
- 850-700 hPa (about FL45 through FL110)
- 700-500 hPa (about FL110 through FL180)

Be aware that icing can occur above and below these levels. You can use the vertical cross-section to determine the best mean layer map to use.

Icing is likely where the NWP forecast variables all meet the following criteria:

- T (see -VVEL for exceptions to the -22°C lower bound value)
  - 0°C to -22°C in supercooled saturated/near-saturated layer
  - T < ‒22°C within any moist layer(s) usually means trace to mainly light icing; ice crystals remove or grow at the expense of supercooled water
- RH:
  - RH > 80% for icing; > 90% icing near-certainty
- Minus (−) VVEL (positive means upward motion)
  - −VVEL > 0: rising motion, enhanced risk of icing
  - −VVEL < 0: sinking motion, reduced risk of icing
  - Adjust -22°C to a warmer threshold as ‒VVEL increases for non-thunderstorm situations
- Supercooled large droplets (SLDs, droplet diameter > 50μm) associated with severe icing is diagnosed in some NWP models

Note that the Rapid Refresh (RAP) model predicts SLD explicitly, while the Rapid Update Cycle (RUC2) model used for aviation in 2011 (the time of this case) did not.

Use T/RH/−VVEL to identify multiple layers with icing potential, separated by one or more layers with little icing risk. The model data should be validated against icing reports and other observations.

The main advantage of plan view maps and cross sections of model analyses is that they can provide information to diagnose supercooled cloud droplets, including SLDs. The Rapid Refresh (RAP) and High-Resolution Rapid Refresh (HRRR, 4-km grid spacing) have this capability, with hourly (to quarter-hourly for some data) output to 18 hrs. These models also have explicitly predicted cloud liquid/ice, supercooled water, and precipitation type and mass for each model layer.

The limitations primarily relate to the accuracy of the explicit cloud water and precipitation analyses and forecasts. In past verification studies, the RAP and HRRR have captured only about 40% of icing PIREPs. A new data assimilation system was implemented in the RAP and HRRR in 2014, which may improve explicit analyses and predictions of cloud condensate and precipitation, especially in convective situations over the first few hours. This will, in turn, improve model icing diagnosis in the short range (1 to 6 hours).