Untersuchung von Feuchteprofilen mit Hilfe von der EOF-Analyse unter besonderer Berücksichtigung der zeitlichen Variationen

Abstract

For the three Northern Atlantic weatherships C, L and K, profiles of humidity are investigated with the help of empirical orthogonal functions (EOF's) for the period April 22 and October 11 1979. The EOF's are eigenfunctions of the coveriance mntrix of the humidity values at different pressure levels. With the aid of such functions and the connected time dependent coefficients the original humidity profilcs can be approximated. Each of these EOF's contains independent information about the various physical proccsses influencing the humidity profiles. Thus the first EOF represents dry or moist air in the whole troposphere. A positiv second EOF characterizes a stable troposphere with a boundary layer present (typical for a high pressure area), a negativ second EOF marks unstable conditions without a boundary layer (typical of low pressure area). The zero-crossing of the second EOF represents the average heigth of the boundary layer. Criteria are discussed, which can be used to determine the number of orthogonal functions which are relevant and significant to describe the humidity profiles. The temporal developments of the EOF's are analysed. The errors of the reproduced vertical profiles and of the nearsurface humidity are calculated and discussed. The nearsurface humidity can be connected with an error of 0.66 g/Kg if two EOF's are used. Whilst no trend is found for the EOF's of the two northern weatherships (C+L), a rise could be determined of the zero-crossing of the second EOF (mean boundary layer heigth) towards midsummer at weathership K. The EOF's are derived with SST taken into considaration, to investigate its effect on the vertical humidity profile. An experiment is carried out with data ensembles of different time periods to find out whether the EOF's are constant during the whole period of consideration. The errors of the calculated near surface humidity are compared using the EOF's derived for the different periods. An investigation did not reveal strong correlation between the time coefficients and synoptical observations as cloud cover, wind speed and direction