Climatology of temperatures below -70°C near tropopause on Atlantic-tracks

On request of the Dutch Safety Board (Onderzoeksraad voor Veiligheid, OVV) the climatological occurrence of temperatures below -70C on flight tracks on the Northern Atlantic routes has been assessed, to develop more knowledge concerning the risk of fuel icing. Besides this climatology, the Dutch Safety Board had special interest in the upper-air temperatures that occured on December 28th, 2010 in an area NW of Schotland. This research is done as part of the program Knowledge for Climate, Hotspot Schiphol, aiming at climatology of Flight Information Region Amster- dam, full details available at the website http://www.knmi.nl/samenw/kbs. The required information is based on a reanalysis of the European Centre for Medium Range Weather Forecast (ECMWF), ERA-Interim. ERA-Interim is a reanalysis of the global atmosphere covering the data-rich period since 1989, and continuing in real time. This model improved existing reanalysis sets significantly, with the use of 4-dimensional variational analysis, a revised humidity analysis, the use of variational bias correction for satellite data, and other improvements in data handling. In the next chapters the average temperatures (and standard deviation) near tropopause level are presented per wintermonth (December, January and February) and winterseason (three months). The levels are 250, 225 and 200 hPa, corresponding to the ightlevels FL340, FL360 and FL390 in ICAO-standard atmosphere. As special interest is mentioned for temperatures near -70C the 1%- percentile of the temperature distribution per ightlevel is presented and the returntime of -70o as an indication of the climatological frequency of low temperatures near tropopause.