Sensitivity of GPS occultation to the stratopause height

We scrutinize temperature profiles collected with radio occultation measurement for an imprint of the stratopause. In the retrieval step that integrates bending angle data to atmospheric refractivity, the falloff toward infinite altitude is constrained in a boundary condition with statistical optimization or extrapolation. We point to the coherence between temperature and density scale height as advocator for a fitting procedure that exploits data down to the stratopause. Data below have higher signal-to-noise ratio, but fitting them in costs additional parameters for the shifted sign in scale height gradient. On the basis of noise free simulation using a climatology covering all latitudes, seasons, and hours and on the basis of validation against data collected with weather balloons, laser imaging, and limb sounding, we find that adaptation to the fluctuating stratopause is crucial for the accuracy of the retrieved temperature. To facilitate investigation, the stratopause altitude was preset in the boundary condition according to climatological temperature. Biases between 10 and 50 hPa cancel out when the fitting interval is extended to 7.2 km above the stratopause, almost the average displacement of the maximum density scale height caused by nonlinearity in the temperature profile. This is a reminder that the stratopause should be read from bending angle, not temperature. Moreover, bias would be approaching zero at all levels if the a priori information for extrapolation had been lapse rate, not isothermal conditions. Keeping the model seed for temperature conversion to subsequent retrieval steps eliminates external information from the deconvolved refractivity. It will help argue for radio occultation as independent vehicle for climate monitoring.