Gravity Wave Variation from the Troposphere to the Lower Thermosphere during a Stratospheric Sudden Warming Event: A Case Study

Abstract

High resolution Whole Atmosphere Community Climate Model (WACCM) simulations are used to study how gravity waves vary during a stratospheric sudden warming (SSW) event from the source region to the lower thermosphere. The variation of zonal mean momentum flux of resolved gravity waves (with zonal wavelengths less than 1600 km) during SSW are qualitatively consistent with those obtained from parameterized studies, mainly caused by the change of filtering by the mean zonal wind. At high latitude in the winter hemisphere, stratospheric and mesospheric momentum fluxes vary rapidly during SSW, and their magnitudes decrease significantly following SSW, agreeing with satellite observations. Gravity waves are also found to vary as wave sources change. At tropical regions (especially in the summer hemisphere), convectively generated gravity waves increase due to enhanced deep convection following SSW. At higher latitudes, orographic waves vary during SSW as the wind changes extend from the stratosphere down into the troposphere. Gravity waves generated from adjustment of the polar jet also undergo significant changes during SSW. These changes lead to strong longitudinal variation of gravity waves.(Citation: Liu, H.-L, 2017: Gravity wave variation from the troposphere to the lower thermosphere during a stratospheric sudden warming event: A case study. SOLA, 13A, 24−30,

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“…It is possible that the GW enhancement in the mesosphere is not only due to less filtering of tropospheric GW -the GW source could also be at higher altitudes. Liu (2017) argues that the rather rapid change in the winter jet system is expected to be a source of GW variability during SSW as GWs can be excited by imbalance of jet flow (O'Sullivan and Dunkerton, 1995;Zhang, 2004). Gerrard et al (2011) found evidence that upward-propagating gravity waves were generated in situ by a stratospheric temperature enhancement.…”

“…As SSW events are associated with dynamical changes in the stratosphere and mesosphere over several days, effects on gravity waves in the upper mesosphere and lower thermosphere can be expected and have also already been observed and/or modelled: Yigit and Medvedev (2012), for example, report that gravity wave (GW) activity increases by a factor of 3 in the course of the warming modelled by them. Liu (2017) point out that at high latitude in the winter hemisphere the momentum flux varies rapidly during the SSW. Afterwards, the magnitude of the mesospheric momentum flux decreases significantly.…”

“…His findings agree with the observations of GW momentum flux changes during a SSW published by Wright et al (2010), France et al (2012), Thurairajah et al (2014), and Ern et al (2016), for example. Liu (2017) argues that the rather rapid change in the winter jet system is expected to be a source of GW variability during SSW as GWs can be excited by imbalance of jet flow (O'Sullivan and Dunkerton, 1995;Zhang, 2004).…”

Reply to reviewer 2

“…Yigit and Medvedev (2012), for example, used a global circulation model in order to show that the activity of GW of lower atmospheric origin is enhanced by a factor of 3 in the course of the modelled warming. Based on WACCM, Liu (2017) point out that the magnitude of the mesospheric momentum flux decreases significantly after the SSW event, but it varies strongly during the event. His findings agree with the observations of GW momentum flux changes during a SSW published by Wright et al (2010), France et al (2012), Thurairajah et al (2014), andErn et al (2016), for example.…”

Reply to reviewer 1

“…Yigit and Medvedev (2012), for example, used a global circulation model in order to show that the activity of GW of lower atmospheric origin is enhanced by a factor of 3 in the course of the modelled warming. Based on WACCM, Liu (2017) point out that the magnitude of the mesospheric momentum flux decreases significantly after the SSW event, but it varies strongly during the event. His findings agree with the observations of GW momentum flux changes during a SSW published by Wright et al (2010), France et al (2012), Thurairajah et al (2014), and Ern et al (2016, for example.…”