Tropopause-Height

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The tropopause is the natural boundary between the troposphere and stratosphere. The location of the tropopause is an interesting feature of the whole troposphere-stratosphere system. Objective determination of the location of the tropopause is necessary in order to study stratosphere- troposphere exchange processes, to calculate radiative forcings, and to examine the behavior of the tropopause in climate change experiments. In most cases, gridded data sets in the form of analyses or model output are available. The low vertical resolution of those data in the tropopause region raises the question of how accurately the tropopause can be determined from such data.

Fig. 1. Climatological mean (1983-1998) tropopause pressure during Northern Hemisphere winter (DJF) calculated from daily temperature fields from NCEP/NCAR reanalysis.

We developed an accurate and robust method to determine the tropopause height from gridded data with low vertical resolution. The method is based on the thermal tropopause concept using the WMO stability criterion. The method is verified independently by comparing the calculated heights derived from ECMWF-analyses with observed heights from local radiosonde stations. Despite the coarse vertical resolution of the analyses, the error of the calculated tropopause heights is quite small. In the extratropics rms-errors are in the 30-40 hPa range, and in the tropics they are in the 10-20 hPa range (Fig. 2). Only in the subtropics, where the tropopause shows strong meridional gradients, significant deviations of up to 60 hPa can occur occasionally.

Fig. 2. Monthly mean differences between calculated and measured tropopause pressures during January as a function of latitude.

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