Doppler radar signatures of in-flight icing conditions

In-flight icing occurs when aircraft fly in regions where supercooled cloud, freezing rain, and especially freezing drizzle are present. Work on the detection of such conditions by remote sensing is being sponsored in many countries and resulted in dedicated field experiments like the Alliance Icing Research Study which occurred in the Montreal area last winter (1999-2000). Detection of supercooled cloud and drizzle is particularly difficult, especially in the presence of ice crystals of much stronger reflectivity. Although direct detection is difficult, the occurrence of icing conditions can often be determined by Doppler radars as supercooled water leave clues of its presence via its interaction with the snow crystals. When snowflakes fall through a supercooled cloud, they first grow quickly, resulting in a large dZ/dh, and then get rimed, resulting in denser crystals that fall faster. When rimed crystals melt, the resulting bright band will be considerably weaker. Some of these conditions can be detected by conventional scanning radars, while others require observations by a vertically pointing radar. Comparisons of prediction of icing conditions with radiometers and aircraft data show that these techniques have considerable potential. Finally, supercooled drizzle in the presence of snow can be detected by vertically pointing radars as it forms a distinct Doppler mode considerably slower than that of the rimed snow.