High quality stratospheric radio occultation climate data

For around twenty years, satellite-based radio occultation (RO) technology has provided high-quality atmospheric data in the upper troposphere and lower stratosphere (5 km to 35 km altitude). These are used for meteorological applications such as weather forecasting, for climate monitoring and climate research, and also for space weather research. The first goal of this project is to increase the data quality of RO data in the middle and upper stratosphere (35 km to 50 km altitude). This implements an important recommendation of the International Radio Occultation Working Group, which is an RO expert group. Two easy-to-use methods developed in an earlier project will be used for the entire range of RO data since 2001. These new techniques can greatly reduce the measurement noise and effectively correct the residual influences of the Earth`s ionosphere. This improves the data quality across the entire stratosphere and thus enables more precise global climate monitoring, which in turn contributes to a better understanding of our climate system. After a thorough analysis, the complete available multi-satellite RO time series since 2001 will be processed. Based on this processed high-quality climate data, the second goal is to derive stratospheric winds and to analyse their circulation patterns over the tropics and subtropics. The advantages of the RO data used here are their high vertical resolution and long-term consistency over the entire time period. To ensure a high quality of climate monitoring, the wind fields are carefully compared with long-term weather analysis data fields, so-called reanalysis data, and these data are also used to extend them backwards to 1980. This finally makes it possible to investigate dynamic changes in height of the wind fields and changes in their circulation patterns under the influence of anthropogenic climate change of the last four decades with a reliability that was previously not possible. This exciting analysis is carried out with a focus on the changes in the tropics and subtropics, which are particularly important in the climate system. This allows us to expect new insights into the interplay between global warming and changes in circulation.

For almost 20 years, the satellite-based radio-occultation (RO) technique has been providing high-quality atmospheric data from the upper troposphere to the lower stratosphere (5 km to 35 km). The RO data are used e.g., for meteorological applications, climate analysis and monitoring. The original measurement of the phase shift scans a vertical profile up to a height of approximately 80 km. However, the data quality decreases with increasing altitude, due to an increasing influence of ionospheric refraction. The first goal of the project Strato-Clim was to improve the quality of the data obtained in the middle and upper stratosphere (roughly 30 to 45 km), by handling the problem of ionospheric influences. This enables a more precise analysis of processes in the stratosphere, which are also crucial for the climate system in the underlying troposphere (the weather layer) and for the Earth's surface. In this context, two different ionospheric corrections - the Kappa correction and the Bi-local correction - were specifically compared. Both improve the climate data in the stratosphere. The Kappa correction is easy to apply and requires only the measured phase shift and the F10.7 index. The Bi-local correction is more complex, offers however advantages under conditions of high magnetic activity and captures better regional variations. Overall, both methods are useful for reducing residual errors in RO data and allow the data to be used up to about 40 km altitude. Based on these high-quality RO climate data, the second objective of the research project was to derive tropospheric and stratospheric winds and study their global circulation patterns. Due to their global coverage, high-vertical resolution, and long-term stability, the RO climate data provide new insights into the interaction between global warming and changes in the atmospheric circulation. Within the Strato-Clim project, it was possible - as a pioneering new application - to extract global wind fields as a time series from RO climate data for the period 2006 to 2020. These new climatic winds resolve planetary wave interactions and could also be derived across the equator, which was previously not possible with RO data. In addition, an application study successfully analyzed for the first time tropical tropospheric wind features using these new RO climatic wind fields. In summary, Strato-Clim was an extremely successful FWF stand-alone project that contributed to the improvement of RO climate data and established a completely new, internationally recognized field of research in the area of radio occultation.