Predictability of cold drops based on ECMWF's forecasts over Europe
Gaál, Nikolett ; Ihász, István
Cold drops are closed cyclonic eddies in the middle and upper troposphere that are separated from the main western stream. Being elliptically shaped with a diameter of hundreds of kilometres, they resemble miniature cyclones on satellite images. Within a cold drop the air mass is isolated from the cooler air of higher latitudes and it carries air substantially colder than its surroundings to the warm regions of lower latitudes. Cold drops are sometimes called upper-level lows (ULLs).
It is important to study cold drops because they can sometimes be responsible for severe weather affecting a region for a couple of days. They can occur at any time of the year and often bring high amounts of heavy rainfall. The unstable nature of cold drops provides perfect conditions for the formation of massive thunderstorms in summer (these can lead to flooding, or in rare cases, even tornadoes), and intense snowfall during winter. Synoptic studies of the cold drops have received less attention during the past decades than their importance would suggest.<br>We have studied the formation, development conditions, and synoptic and dynamical backgrounds of cold drops using ECMWF's ERA-Interim reanalysis. The first step was to develop an algorithm to recognize the cold drops. Then a statistical investigation determined the typical horizontal and vertical structures of the cold drops. For the 280 selected cases, predictability of the intensity and position of the cold core was investigated using ECMWF's high-resolution forecast (HRES). Also, based on ECMWF's ensemble forecast (ENS), a new type of ensemble plume containing four meteorological variables was developed to help forecasters predict the onset of a cold drop.
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