STORM CELL CHARACTERISTICS
The function of airborne weather radar is to allow pilots to identify and avoid potential weather hazards. The radar performs signal processing to estimate the radar reflectivity of the weather ahead. Reflectivity can be correlated to precipitation rate, and is displayed as green (light), yellow (moderate), and red (heavy) precipitation.
Reflectivity is used to identify the presence of potentially hazardous weather. However, reflectivity alone should not be used to determine the degree of hazard. Pilots should use the displayed reflectivity to recognize weather features that can indicate potentially hazardous conditions.
For example, a tight gradient (rapid transition of color levels with distance) in the reflectivity field is usually associated with severe turbulence generated by vigorous convection and should be avoided.
Other shapes are strong indicators of the presence of hail, such as:
Weather hazards that can be identified by the use of radar are generally associated with convective storms. Convection results in towering storm structures that can contain high wind gradients that lead to turbulent motion. Very vigorous convection can generate severe turbulence in the vicinity of the high reflectivity core, downwind of the core and at the top of the storm. The strength of the convection can be judged by the vertical size of the convective cell and the extent of high reflectivity portions of the storm, as well as the presence of shapes described above. At ranges less than 40 miles, the display of magenta will indicate areas of particularly turbulent activity.
Convective weather is associated with hazards due to turbulence, hail, and lightning strike. Recognizing convective weather is instrumental in avoiding these hazards. In addition to reflectivity associated with convective weather, the radar will typically display reflectivity associated with stratus, or stratiform, weather. Whereas convection is characterized by localized towers of updraft and downdraft features, stratiform precipitation results from much more widespread and much less vigorous uplift. As a result, stratus precipitation is more layered in form with much lower gradients in radar reflectivity. However, reflectivity of stratiform weather can be sufficient to cause yellow and red on the radar display. These high reflectivities result from relatively high rain rates, as well as from enhancement of reflectivity due to melting of snow particles just below the freezing level. High reflectivity of stratus weather does not indicate any significant hazard (with the exception of any potential for icing, or takeoff and landing performance issues associated with high rainfall rates). Therefore, it is important that pilots be able to recognize hazards based on the form of the weather (convective versus stratiform) and the other considerations described above, not by observing the reflectivity level alone.
Updrafts in thunderstorms support abundant water; when carried above the freezing level, this water becomes supercooled. As the temperature in the upward current cools to about -15°C, much of the remaining water vapor sublimates as ice crystals. Above this level, the amount of supercooled water decreases.
Supercooled water freezes on impact with an aircraft. Clear icing can occur at any altitude above the freezing level; but at high levels, icing may be rime or mixed rime and clear. The abundance of supercooled water makes clear icing occur very rapidly between 0°C and -15°C, and encounters can be frequent in a cluster of cells.
PLANNING A PATH
Remember to plan a deviation path early. Simply skirting the red or magenta portion of a cell is not enough. Wherever possible, plan an avoidance path for all weather echoes which appear beyond 100 nm since this indicates they are quite dense.
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