SA41B-2340
Lunar tidal effects during the 2013 stratospheric sudden warming as simulated by the TIME-GCM

Thursday, 17 December 2015
Poster Hall (Moscone South)
Astrid I Maute, National Center for Atmospheric Research, Boulder, CO, United States, Jeffrey M Forbes, University of Colorado at Boulder, Boulder, CO, United States, Xiaoli Zhang, UCB 429, Boulder, CO, United States, Bela G Fejer, Utah State University, Logan, UT, United States, Valery A Yudin, NCAR/NESL, Boulder, CO, United States and Nicholas M Pedatella, University Corporation for Atmospheric Research, COSMIC Program Office, Boulder, CO, United States
Abstract:
Stratospheric Sudden Warmings (SSW) are associated with strong planetary wave activity in the winter
polar stratosphere which result in a very disturbed middle atmosphere. The changes in the middle atmosphere
alter the propagation conditions and the nonlinear interactions of waves and tides,
and result in SSW signals in the upper atmosphere in e.g., neutral winds,
electric fields, ionospheric currents and plasma distribution. The upper atmosphere changes
can be significant at low-latitudes even during medium solar flux conditions. Observationsalso reveal a strong lunar signal during SSW periods in the low latitude
vertical drifts and in ionospheric quantities. Forbes and Zhang [2012] demonstrated that during
the 2009 SSW period the Pekeris resonance peak of the atmosphere was altered such that the M2 and N2 lunar tidal components
got amplified.

This study focuses on the effect of the lunar tidal forcing on the thermosphere-ionosphere system during the
January 2013 SSW period. We employ
the NCAR Thermosphere-Ionosphere-Mesosphere-Electrodynamics General Circulation Model (TIME-GCM)
with a nudging scheme using the Whole-Atmosphere-Community-Climate-Model-Extended
(WACCM-X)/Goddard Earth Observing System Model, Version 5 (GEOS5)
results to simulate the effects of meteorological forcing on the upper atmosphere. Additionally lunar tidal forcing
is included at the lower boundary of the model. To delineate the lunar tidal effects a base simulation without lunar forcing
is employed. Interestingly, Jicamarca observations of that period reveal a suppression of
the daytime vertical drift before and after the drift enhancement due the SSW.
The simulation suggests that the modulation of the vertical drift
may be caused by the interplay of the migrating solar and lunar semidiurnal tide, and
therefore can only be reproduced by the inclusion of both lunar and solar tidal forcings in the model. In this presentation
the changes due to the lunar tidal forcing will be quantified, and compared to observations.

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