Based on a climate-chemistry model (constrained by reanalyses below~50 km), the zonal-mean composite response of the mesosphere and lower thermosphere (MLT) to major sudden stratospheric warming events with elevated stratopauses demonstrates the role of planetary waves (PWs) in driving the mean circulation in the presence of gravity waves (GWs), helping the polar vortex recover and communicating the sudden stratospheric warming (SSW) impact across the equator. With the SSW onset, strong westward PW drag appears above 80 km primarily from the dissipation of wave number 1 perturbations with westward period of 5-12 days, generated from below by the unstable westward polar stratospheric jet that develops as a result of the SSW. The filtering effect of this jet also allows eastward propagating GWs to saturate in the winter MLT, providing eastward drag that promotes winter polar mesospheric cooling. The dominant PW forcing translates to a net westward drag above the eastward mesospheric jet, which initiates downwelling over the winter pole. As the eastward polar stratospheric jet returns, this westward PW drag persists above 80 km and acts synergistically with the return of westward GW drag to drive a stronger polar downwelling that warms the pole adiabatically and helps reform the stratopause at an elevated altitude. With the polar wind reversal during the SSW onset, the westward drag by the quasi-stationary PW in the winter stratosphere drives an anomalous equatorial upwelling and cooling that enhance tropical stratospheric ozone. Along with equatorial wind anomalies, this ozone enhancement subsequently amplifies the migrating semidiurnal tide amplitude in the winter midlatitudes.

[1] The role of planetary waves in causing stratospheric sudden warmings (SSWs) is well understood and quantified. However, recent studies have indicated that secondary planetary waves are excited in the mesosphere and lower thermosphere following SSWs. We use a version of the Whole Atmosphere Community Climate Model constrained by reanalysis data below 50 km to simulate the SSW of January 2012, a minor warming followed by the formation of an elevated stratopause. We document the occurrence of enhanced Eliassen-Palm flux divergence in the mesosphere and lower thermosphere associated with faster, secondary westward-propagating planetary waves of wave number 1 and period <10 days. We confirm the presence of these secondary planetary waves using observations made by the Sounding of the Atmosphere using the Broadband Emission Radiometry instrument onboard NASA's Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite. Citation: Chandran, A., R.R. Garcia, R. L. Collins, and L. C. Chang (2013), Secondary planetary waves in the middle and upper atmosphere following the stratospheric sudden warming event

Elevated stratopause (ES) events occurring during Northern Hemisphere winter are identified in four climate simulations of the period 1953–2005 made with the Whole Atmosphere Community Climate Model (WACCM). We find 68 ES events in 212 winters. These events are found in winters when the middle atmosphere is disturbed and there are zonal wind reversals in the stratosphere at high latitudes. These disturbances can be associated with both major and minor stratospheric sudden warming events (SSWs). The ES events occur under conditions where the stratospheric jet, the gravity wave forcing, and the residual circulation remain reversed longer than in those winters where an SSW occurs without an ES. We compare ES events with the type of SSW (vortex splitting and vortex displacement) and find that 68% of ES events form after vortex splitting events. We also present a climatology of ES events based on NASA's Modern‐Era Retrospective Analysis for Research and Applications reanalysis data from 1979 to 2012 and compare it to the model results. WACCM composites of major SSW and ES also show enhanced Eliassen‐Palm flux divergences in the upper mesosphere after the stratospheric warming, immediately before the formation of an ES. However, the formation of an ES in WACCM is due primarily to adiabatic heating from gravity wave‐driven downwelling, which follows the reestablishment of the eastward jet in the upper stratosphere. We find nine winters where an ES forms in the absence of any significant planetary wave activity in the upper mesosphere and illustrate one such event.

[1] We report initial results of an effort to model the diurnal and seasonal variability of the meteor rate detected by high power and large aperture (HPLA) radars. The model uses Monte Carlo simulation techniques and at present assumes that most of the detected particles originate from three radiant distributions with the most dominant concentrated around the Earth's apex. The other two sources are centered 80°in ecliptic longitude to each side of the apex and are commonly known as helion and antihelion. To reproduce the measurements, the apex source flux was set to provide $70% of the total number of particles while the other $30% is provided by the combined contribution of the two remaining sources. The results of the model are in excellent agreement with observed diurnal curves obtained at different seasons and locations using the 430 MHz Arecibo radar in Puerto Rico, the 50 MHz Jicamarca radar in Perú, and the 1.29 GHz Sondrestrom radar in Greenland. To obtain agreement with the observed diurnal and seasonal variability of the meteor rate, an empirical atmospheric filtering effect was introduced in the simulation which prevents meteors with low-elevation radiants ( 20°) from being detected by the radars at mesospheric altitudes. The filtering effect is probably produced by a combination of factors related to the interaction of the meteor with the air molecules such as electron production and/or the ablation at higher altitudes. On the basis of these results we calculate the micrometeor global, diurnal, and seasonal input in the upper atmosphere.Citation: Janches, D., C. J. Heinselman, J. L. Chau, A. Chandran, and R. Woodman (2006), Modeling the global micrometeor input function in the upper atmosphere observed by high power and large aperture radars,

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The Whole Atmosphere Community Climate Model (WACCM) is used to study the influence of gravity waves on the generation and evolution of an elevated stratopause following a sudden stratospheric warming (SSW). By comparing WACCM simulations of two Arctic winters, where one is dynamically undisturbed and one is disturbed, we find that intense planetary wave activity during a SSW drives the reversal of the zonal mean wind in the stratosphere. This alters the penetration of eastward propagating, non‐orographic gravity waves into the mesosphere, which determine the extent of cooling in the lower mesosphere and upper stratosphere through the adiabatic effects of the gravity wave‐driven residual circulation, and play a crucial role in the reformation of the elevated stratopause in the lower mesosphere. Eventually, the forcing due to gravity waves returns to normal wintertime values as the stratospheric zonal wind recovers, and is then associated with the warming and lowering of the elevated stratopause, by wave induced diabatic descent. We find that SSW followed by an elevated stratopause is a climatologically robust phenomenon in free running WACCM with characteristics closely resembling recently observed events in the Arctic.

[1] The cloud imaging and particle size (CIPS) experiment is one of three instruments on board the Aeronomy of Ice in the Mesosphere (AIM) spacecraft that was launched into a 600 km Sun-synchronous orbit on 25 April 2007. CIPS images have shown distinct wave patterns and structures in polar mesospheric clouds (PMCs), around the summertime mesopause region, which are qualitatively similar to structures seen in noctilucent clouds (NLCs) from ground-based photographs. The structures in PMC are generally considered to be manifestations of upward propagating atmospheric gravity waves (AGWs). Variability of AGW effects on PMC reported at several lidar sites has led to the notion of longitudinal differences in this relationship. This study compares the longitudinal variability in the CIPSobserved wave occurrence frequency with CIPS-measured PMC occurrence frequency and albedo along with mesospheric temperatures measured by the sounding of the atmosphere using broadband emission radiometry instrument on board the Thermosphere Ionosphere Mesosphere Energetics and Dynamics spacecraft. Our results for the latitude ranges between 70°and 80°show a distinct anticorrelation of wave structures with cloud occurrence frequency and correlations with temperature perturbations for at least two of the four seasons analyzed, supporting the idea of gravity wave-induced cloud sublimation. The locations of the observed wave events show regions of high wave activity in both hemispheres. In the Northern Hemisphere, while the longitudinal variability in observed wave structures show changes from the 2007-2008 seasons, there exist regions of both low and high wave activities common to the two seasons. These persistent features may explain some of the observed differences in PMC activity reported by ground-based lidar instruments distributed at different longitudes. The statistical distribution of horizontal scales increases with wavelength up to at least 250 km. We also discuss the possibility of atmospheric tides, especially the nonmigrating semidiurnal tide, aliasing our observations and affecting the results presented in this analysis.

[1] Major stratospheric sudden warmings (SSW) occurring during Northern Hemisphere winter were identified in four runs of the Whole Atmosphere Community Climate Model (WACCM). Their characteristics are compared to those found by other authors using reanalysis data. The comparison shows that the frequency of occurrence of major SSW in the model is very similar to that found in reanalysis data, as is the occurrence of vortex splitting and displacement events. The main difference with respect to observations is that the modeled SSW are relatively longer lasting. WACCM simulates quite accurately some dynamical features associated with major SSW, despite the presence of outlier cases; however, the recently reported relationship between regional blocking and the type of SSW is only partially reproduced by WACCM. In general, the observed climatological and dynamical signatures of displacement SSW tend to be better reproduced by the model than those associated with splitting SSW. We also find that SSW in the model are often associated with an elevated polar cap stratopause, in agreement with recent observations. However, the simulations also show that there is not in general a close correspondence between major SSW and elevated polar cap stratopause events.