A new method to detect and monitor Sudden Stratospheric Warming events based on radio occultation: demonstration using the Jan-Feb 2009 event
Presenter:
Ying Li
State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and Geophysics (IGG), Chinese Academy of Sciences, Wuhan, China.
(1)State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and Geophysics (IGG), Chinese Academy of Sciences, Wuhan, China. (2)Wegener Center for Climate and Global Change (WEGC) and Institute for Geophysics, Astrophysics, and Meteorology/Institute of Physics, University of Graz, Graz, Austria.
Poster
Global Navigation Satellite System (GNSS) Radio Occultation (RO) provides globally distributed and accurate long-term atmospheric profiling of the thermodynamic structure of the stratosphere with 1-km-level vertical resolution in temperature, density and related variables. Since continuous multi-satellite RO started in 2006, with the COSMIC and MetOp missions, the geographic data coverage is also sufficiently dense for monitoring and analyzing regional-scale thermodynamic phenomena. In this respect, so-called Sudden Stratospheric Warming (SSW) events are one process of interest at high northern latitudes during winter months.
Building on this capability, we developed a new method to detect and monitor SSW events based on the RO temperature, density, and bending angle profiles available north of 50°N and demonstrated it for the well-known Jan-Feb 2009 event. We first construct RO temperature, density, and bending angle anomaly profiles (against long-term gridded RO climatology fields interpolated to RO locations) and extract from these anomaly magnitudes in selected stratosphere and stratopause region altitude layers. The anomalies are then averaged into a suitable space-time-binned grid over 50–90°N (5° latitude x 20° longitude grid, updated daily). Subsequently we compute, based on the gridded anomaly estimates, three basic daily metrics for detecting an SSW and for tracking its extent and intensity while it evolves. We based these metrics on the concept of Threshold Exceedance Areas (TEAs), the geographic areas wherein temperature, density and bending angle anomalies exceed predefined thresholds such as 40 K or 40 %. Together with obtaining primary-, secondary-, and trailing-phase TEA metrics of potential SSWs this way at daily sampling, geographic center location and anomaly maximum values of TEAs are tracked, adding dynamical characterization. If the main-phase TEAs (primary- plus secondary-phase) are found above 3 Mio. km2 (an area of ~1000 km effective radius around center location) over a full week (7 days) or longer, classification as an SSW event is assigned and main-phase duration and event strength are recorded.
We present the method based on the prominent SSW event of Jan-Feb 2009, where we also cross-validated it with results using collocated profiles from ECMWF analysis fields, or directly the gridded analysis fields themselves, rather than RO profiles. We found the method robust and the TEA metrics of the 2009 event to show a strong SSW emerging Jan 17, reaching a maximum on Jan 23, and the strong primary-phase temperature anomaly fading by Jan 27. On Jan 22-23 a MSTA-TEA40 value (threshold exceedance area of middle stratosphere temperature anomaly > 40 K) of about 9 Mio. km2 was reached. It covered a geographic region with a diameter of almost 3500 km that was centered over East Greenland, covering Greenland entirely and extending from Western Norway to Eastern Canada. The secondary and trailing-phase metrics (based on lower and upper stratosphere density and temperature anomalies) neatly track the further SSW development, where the thermodynamic anomaly propagates downward and is fading with a transient upper stratospheric cooling, spanning until end February and beyond. We expect the method robust and long-term stable for monitoring how SSW characteristics evolve under climate change and variability. Hence we started to apply it also to the longer RO and ERA5 data records.
Poster in PDF:
