A NEW APPROACH TO THE CLIMATOLOGY OF CONVECTIVE ACTIVITY

Abstract

This paper describes a new approach to the climatology of thermal soaring conditions that allows a systematic comparison of different sites. The key idea is to apply a numerical convection model to a series of operationally measured radiosoundings and to calculate the "potential flight distance" in a thermal cross- country flight for each day. A climatology of thermal soaring cohditions can be based on these PFDs. It could be useful for the preparation of soaring championships and also for opening up new exciting soaring sites. A climatological map based on the PFD would show atmospheric and topographical influences on thermal soaring. As a case study we selected the Sahara site Tamanrasset (DZ, Algeria, 22o49'N, 5o24'E) located near the Tropic of Cancer and used all operationally measured radiosoundings for the year 1996 to compute the daily potential flight distances with the numerical convection model <<ALPTHERM>>. The height of convection reveals a bi-seasonal characteristic. In summer (from the middle of May through the end of October) it reaches 4000+/-500 m AGL. In winter (from November through the end of February) heights of 2000+/-500 m AGL are typical. March and April are transition months with the height of convection varying between 1500 and 3800 m AGL. Potential flights distances mainly reflect the annual variation of the length of the day. The lowest PFDs in winter are around 300 km. From March through October 500 km are possible. From the middle of April through the end of August 600 km is a typical figure. Cumulus formation is rare. On the rare days with cumulus formation (20% of the days from April through September over the plateau of Tamanrasset) the potential flight distance exceeds 750 km. The largest PFD obtained was 921 km. Cumulus formation should be more frequent over the desert mountains (Hoggar, Tibesti, Atlas).