Monitoring sudden stratospheric warmings using radio occultation: a new approach demonstrated based on the 2009 event
Abstract. We introduce a new method to detect and monitor sudden
stratospheric warming (SSW) events using Global Navigation Satellite System
(GNSS) radio occultation (RO) data at high northern latitudes and
demonstrate it for the well-known January–February 2009 event. We first construct RO
temperature, density, and bending angle anomaly profiles and estimate
vertical-mean anomalies in selected altitude layers. These mean anomalies
are then averaged into a daily updated 5 ∘ latitude  ×  20 ∘ longitude grid over 50–90 ∘  N.
Based on the gridded mean anomalies, we employ the concept of threshold
exceedance areas (TEAs), the geographic areas wherein the anomalies exceed
predefined threshold values such as 40 K or 40 %. We estimate five basic
TEAs for selected altitude layers and thresholds and use them to derive
primary-, secondary-, and trailing-phase TEA metrics to detect SSWs and to
monitor in particular their main-phase (primary- plus secondary-phase)
evolution on a daily basis. As an initial setting, the main phase requires
daily TEAs to exceed 3×106  km 2 , based on which main-phase duration,
area, and overall event strength are recorded. Using the January–February 2009 SSW
event for demonstration, and employing RO data plus cross-evaluation data
from analysis fields of the European Centre for Medium-Range Weather
Forecasts (ECMWF), we find the new approach has strong potential for
detecting and monitoring SSW events. The primary-phase metric shows a strong
SSW emerging on 20 January, reaching a maximum on 23 January and fading by 30 January.
On 22–23 January, temperature anomalies over the middle stratosphere exceeding
40 K cover an area of more than 10×106  km 2 . The geographic tracking of
the SSW showed that it was centered over east Greenland, covering Greenland
entirely and extending from western Iceland to eastern Canada. The secondary-
and trailing-phase metrics track the further SSW development, where the
thermodynamic anomaly propagated downward and was fading with a transient
upper stratospheric cooling, spanning until the end of February and beyond. Given
the encouraging demonstration results, we expect the method to be very suitable
for long-term monitoring of how SSW characteristics evolve under climate
change and polar vortex variability, using both RO and reanalysis data.