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The impact of vertical resolution on the identification of the tropopause altitude

Poor vertical resolution of temperature profiles hampers the identification of the tropopause altitude. The figure shows the differences between tropopause heights from high-resolved radiosonde data and tropopause heights from the same data but represented at degraded vertical resolution.

In their paper (https://doi.org/10.5194/amt-12-4113-2019), Nils König and colleagues have investigated the dependence of the tropopause height offset on the vertical resolution of the underlying temperature profile for both the lapse rate tropopause and the cold point tropopause. Both tropical and mid-latitudinal radiosonde stations were assessed. The mean (median) displacements of the lapse rate tropopause altitude when inferred from a temperature profile of 3 km vertical resolution and a Gaussian kernel are -130 m, -400 m, -730 m, and -590 m (-70 m, -230 m, -390 m, and -280 m) for Nairobi, Hilo, Munich, and Greifswald, respectively. In case of a Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) averaging kernel the displacement of the lapse rate tropopause altitude is -640 m. The mean (median) displacement of the cold point tropopause inferred from a temperature profile of 3 km vertical resolution (Gaussian kernels) was found to be -510 m, -610 m, -530m , and -390 m (-460 m, -510 m, -370 m, and -280 m) for the stations mentioned above. Unsurprisingly, the tropopause altitude displacement is larger for coarser resolutions. The effect of the tropopause displacement on the water vapour saturation mixing ratio is roughly proportional to the vertical resolution. In tropical latitudes the resulting error is about one to two parts per million by volume per vertical resolution in km. The spread of the tropopause displacements within each sample of profiles seems too large as to recommend a correction scheme for tropical temperature profiles, while for midlatitudinal temperature profiles of vertical resolutions of 1 to 5 km a lapse rate of -1.3 K/km reproduces tropopause altitudes determined from high-resolution temperature profiles with the nominal lapse rate criterion of -2 K/km fairly well.

For more information see: https://doi.org/10.5194/amt-12-4113-2019