Inferred Climatology of Supercooled Large Droplet Icing for Europe
Ben C. Bernstein
Little information is available about the frequency, spatial and temporal
distribution and extent of supercooled large drop (SLD) icing conditions due to a lack of
regular, direct measurements. Research aircraft provide in-situ observations, but the
sample set is small and can be biased. Other techniques must be used to infer the
presence of such conditions to create a more unbiased climatology. Its presence and
absence can be inferred using surface weather observations in conjunction with vertical
profiles of temperature and moisture. In this study, a climatology of SLD icing was
created using 15 years of coincident, 12-hourly surface weather reports and balloon-borne
soundings taken across Europe and surrounding areas.
A special version of the Current Icing Potential (CIP) was used to infer the
presence of SLD icing aloft. Surface observations were checked for the presence of
broken or overcast skies and their ceiling height, as well as for six precipitation types
(freezing drizzle, freezing rain, ice pellets, rain, snow and drizzle) and thunder. The
surface information was examined in the context of the thermodynamic profile from the
sounding. If clouds were present, the data were passed through a decision tree to
determine the icing “scenario”. SLD icing forms via two mechanisms: classical and nonclassical. Classical SLD forms when snow falls into a layer of air with T>0oC, melts to
form rain or drizzle, then falls into a layer with T<0oC, to become freezing rain or drizzle.
Non-classical SLD forms via the collision-coalescence process, where cloud droplets
grow to precipitation sizes.
If clouds and an appropriate combination of surface
precipitation and thermodynamic structure are found, the SLD scenario for each sounding
is identified and the likelihood (0.0 to 1.0 scale) of the presence of SLD at each altitude
in the sounding is determined. The frequency of the SLD “potential” exceeding a certain
threshold is assessed for each sounding location and for 3000 ft (~1km) altitude bins.
SLD conditions were found to be most common across northern Europe,
especially over the northern U.K., Iceland, the Faeroe and Shetland Islands, western
Norway and northern Germany (Fig. 1). Annual frequencies were on the order of 10% in
these areas, nicely matching the high frequency of surface drizzle and warm-topped,
cloudy skies in these areas. Low frequencies were found along the Mediterranean, where
clouds and precipitation were less common, and across most of Russia. The Russian
minimum may be attributable to issues with surface precipitation type reporting. Prime
locations changed latitude and height during the year, moving northward into the Arctic
during the summer and southward toward the Alps and even the Mediterranean during
the winter. The frequency for all European stations combined was on the order of 2%,
when the entire column is considered (SLD>0.4 at any altitude is a “yes”, while the lack
of SLD>0.4 at all altitudes is a “no”). Most events appeared to occur below 15,000 ft,
were less than 2000 ft deep and formed via the non-classical mechanism.
Fig. 1 - Annual average column SLD frequencies (%) over Europe.