Climatology of icing areas derived from ERA40 analysis

Icing is a dangerous phenomenon for aeronautics, and very difficult to predict for forecasters. However it the spatial and temporal distribution of the available observations does not allow scientists to constitute a large data base, useful to the elaboration of a climatology of icing risk areas. A possibility to get information on the state of the atmosphere is to employ results from numerical weather prediction (NWP) models. The most recent available observations available both on the ground and in altitude (soundings, satellite..) are used to produce analyses, which constitute the best three-dimensional description of the atmosphere, at a fixed time. In this study, two data sets are used. The first set is constituted by 13 years (1989-2000), of the ERA40 reanalysis from the ECMWF (European Center for Medium-Range Weather Forecasts). The horizontal resolution is about 1° all over the globe. Meteorological fields are available every 6 hours and 5 pressure levels have been used ( 1000 hPa 925 hPa, 850 hPa, 775 hPa , 700 hPa ). The second data set is based on 10 years ( 1994-2003) of operational ARPEGE analysis. ARPEGE is Météo-France’s operational numerical. Its mesh is stretched, with an optimised resolution over western Europe. Fields are available twice a day (0 and 12 UTC) on 5 pressure levels ( 1000, 925, 850, 800 hPa and 700 hPa). A regular 0.5° grid have been derived for the Northern Europe area. The favourable icing conditions areas are defined using temperature (between 0°C and –10°C) and humidity(>80%) intervals. These very restrictive criteria are usually considered to be highly favourable to icing conditions. Thus frequencies are calculated and allowed to establish a climatology on different months of the year. The preliminary results shows a concordance between the two models over Northern Europe. Furthermore the areas with high icing risk are comparable with those obtained on the northern part of the american domain by the climatology issued from IIDA, calculated at the NCAR (National Center of Atmospheric Research).