Planetary wave 1 and 2 activity in MLT during stratospheric warmings
from a chain of SuperDARN radars and WACCM
N. H. Kleinknecht1, Y. J. Orsolini2,3, P. J. Espy1,2, V. Limpasuvan4, R. E. Hibbins 1,2
1Norwegian
University of Science and Technology (NTNU), Trondheim, Norway. 2Birkeland Centre for Space Science , Norway. 3Norwegian Institute for Air Research, Kjeller, Norway. 4 School of Coastal and Marine Systems Science, Coastal Carolina University, South Carolina, USA.
Contact: [email protected]
Abstract
This study investigates the effect of Sudden Stratospheric Warmings (SSWs) on Planetary Wave (PW) activity
in the mesosphere-lower thermosphere (MLT). PW activity is derived from meteor wind data using a chain of 8
SuperDARN radars around the North Pole that span longitudes from 25°E to 150°W and latitudes from 51°N to
66°N. Zonal wavenumber 1 and 2 components were extracted from the meridional wind for the years 20002008. Hovmöller diagrams of these components in the longitudinal direction represent the superposition of all
stationary as well as eastward and westward travelling waves. The observed behavior indicates the
propagation of planetary waves in the zonal background wind. The strength of the observed wintertime
activity is highly dependent upon SSWs and appears to be associated with the reduced zonal wind speeds in
the stratosphere they create. PW strength during SSW events are compared to results from the WACCM_SD
model.
Composites of SSWs with elevated stratopause events (ESE)
(day0= start of zonal wind reversal)
Planetary wave amplitudes; SuperDARN (~95 km)
SuperDARN meteor winds as a ground-based satellite
Used SuperDARN radars
between 51-66 N
Meridional Wind Anomaly at all used radar
stations (climatology removed)
Hovmöller diagram of retrieved zonal Rossby wave
number 1 and 2 (S1 and S2 ). The black line indicates
the SSW in winter 2005/2006
Planetary wave amplitudes; WACCM_SD
Measurements (SuperDARN) and
model
(WACCM_SD
=
Whole
Atmosphere
Community
Climate
Model with Specified Dynamics) show
enhanced planetary wave activity in
the MLT after a SSW (defined as a
zonal-mean wind reversal at 50km,
70-90°N for a minimum of 4 days).
Image from Boston University - Centre for Integrated Space Weather
Modelling
The zonal wind reversal occurs very
rapidly and is accompanied by strong
equatorward meridional winds. The
maximum in temperature at 50 km
marks the existence of a stratopause
jump.
Eight SuperDARN radars between 51°-66°N span latitudes from -150° to 25°E, and their meteor echoes provide hourly mean
winds at ~95 km that have been used to extract the longitudinal structure of the S1 and S2 planetary waves. After an initial quality
check, the tidal and 2-day wave components are removed by fitting 4-day segments of the hourly winds. The resulting 8-year
time series is then used to produce a daily climatology at each station which is then subtracted from the daily wind anomalies.
These anomalies as a function of longitude are then fitted to provide the amplitude and phase of the S1 and S2 planetary waves.
Planetary wave activity vs. zonal wind
Planetary wave activity in the MLT (blue) is highly variable. The
strongest planetary wave bursts occur approximately 3 days after strong
wind reversals in the stratosphere (westward wind peaks – magenta).
Such wind reversals are often associated with SSWs and strong vertical
coupling throughout the middle atmosphere. The SSWs shown (black)
are found to be followed by “Elevated Stratopause Events” (ESE). The
enhancement occurs in both wave components (PW1 and PW2).
However the timing and strength of the two components are variable.
This might be due to wave interaction and the mode of vortex
breakdown during the SSW.
A moderate but
significant correlation is
observed between
mesospheric planetary
wave activity and the
westward stratospheric
(50 km) wind bursts in
the polar cap (70-90°N)
at a 3 day lag (wind
leading pw activity).
References: Limpasuvan, V., et al. (2012), The roles of planetary and gravity waves during a major stratospheric sudden warming as characterized in WACCM, J. Atmos. Sol.-Terr. Phys. ,78-79, 84-98, doi:10.1016/j.jastp.2011.03.004
www.PosterPresentations.com
Tweedy,
O. V., et al. (2013), Nighttime secondary ozone layer during major stratospheric sudden warmings in specified-dynamics WACCM, J. Geophys. Res. Atmos., 118, doi:10.1002/jgrd.50651
RESEARCH POSTER PRESENTATION DESIGN © 2012
Wind and Temperature; UKMO
Conclusions
Moderate correlation has been found between planetary wave activity in the MLT and stratospheric westward
wind with a 3 day lag (wind leading). Composites of sudden stratospheric warmings (SSW) that are followed by
elevated stratopause events (ESE) were made and showed an increase of planetary wave activity after the wind
reversal. The observations (SuperDARN) show the strongest effect for the PW2 component. The model
(WACCM_SD) predicts slightly higher PW amplitudes that are equally strong for both wave components, with the
PW1 enhancement happening slightly earlier. The planetary wave activity observed in the MLT could be related
to Rossby waves from below (low zonal wind during SSW). However Rossby waves generated in-situ by
dynamical shear instability, or by breaking of gravity waves with zonally asymmetric upward flux are also
possible sources [Smith, 2003; Limpasuvan et al., 2012; Tweedy et al., 2013].
Smith, A. K. (2003), The Origin of Stationary Planetary Waves in the Upper Mesosphere, J. Atmos. Sci., 60, 3033–3041. doi: http://dx.doi.org/10.1175/1520-0469(2003)060<3033:TOOSPW>2.0.CO;2