The lower thermosphere during the northern hemisphere winter of 2009: A modeling study using high‐altitude data assimilation products in WACCM‐X

Westward traveling planetary wave events in the lower thermosphere during solar minimum conditions simulated by SD-WACCM-X

The effects of breaking of traveling, planetary scale Rossby waves (TPWs) in the lower thermosphere are investigated with respect to the mixing of neutral constituents. We use numerical simulations of the Whole Atmosphere Community Climate Model, eXtended version, whose meteorology below 92 km is constrained by atmospheric specifications obtained from operational weather forecast/data assimilation system. The Fourier spectra show that the amplitude of TPWs with periods between 3 and 10 days are statistically significant in some years; the amplitude and phase of the band‐pass filtered behavior is consistent with the behavior of the 5 day wave. A wavelet analysis using the S‐transform shows that large variations with periods between 3 and 10 days can occur in relatively narrow temporal windows (20–30 days) during boreal winter. The momentum flux entering the lower thermosphere during the times of TPW amplification is shown to be large, and the amplifications of the TPWs in the thermosphere are not always associated with stratospheric sudden warming. The subtropical zonal accelerations are consistent with Rossby wave encountering a surf zone at low latitudes, resulting in wave breaking. The zonal acceleration is shown to be associated with a meridional diffusion, which is largest in the lower thermosphere where the wave activity and the wave breaking are also large. The ultimate effect on neutral density and composition is a meridional, down‐gradient mixing; although this horizontal diffusion is largest below 110 km, the effects on the composition are amplified with increasing altitude, due to the diffusive separation of the thermosphere.

Sudden stratospheric warmings (SSW) are large‐scale disruptions of the wintertime state of the stratosphere that can affect the circulation at synoptic and global scales, including altitudes up to the mesopause in both winter and summer hemispheres. In this study, the response of the summer mesosphere is analyzed during the SSW in the winter stratosphere. In particular, we focus on major SSW events where the climatological stratopause disappears and subsequently reforms at higher altitude, which we refer to as “extreme SSW” in this article. The summer mesosphere response to such extreme SSW events is analyzed in three different phases: (a) stratosphere warming phase, (b) stratopause discontinuity phase, and (c) stratopause reformation phase. Composites of anomalies with respect to climatology derived from the Microwave Limb Sounder and the extended version of the Whole Atmosphere Community Climate Model with specified dynamics are analyzed. The polar summer mesosphere cools during the stratospheric warming phase and warms in subsequent phases. A detailed lag‐correlation analysis shows strong negative correlation of −0.6 to −0.8 between the summer mesosphere and the winter stratosphere during the stratosphere warming phase, and a positive correlation of 0.4–0.6 in the phases thereafter. An attempt is made to explain the apparent drivers and dynamics responsible for these couplings, supported with evidence from observations and model output.

Impact of non-migrating tides on the low latitude ionosphere during a sudden stratospheric warming event in January 2010

Modes of climate variability, such as the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and Atlantic Multi‐decadal Oscillation (AMO), have been shown to have large impacts on North American hydroclimate through their atmospheric teleconnections. However, short instrumental records limit our ability to examine the long‐term stability (or stationarity) of hydroclimate teleconnections. Here, we use two last millennium (LM) paleoclimate data assimilation (DA) products to assess the stationarity of hydroclimate teleconnections over two key regions: the southwestern U.S. and Lower Mississippi River basin. Moving correlations between climate indices and regional hydroclimate expose highly nonstationary regional hydroclimate teleconnections with significant periodic variations on multi‐decadal and multi‐centennial timescales. Although limitations of the DA products prohibit a robust analysis of climate mode intensity, consistent sea surface temperature (SST) patterns between DA products during strong and weak teleconnection periods suggest that changing teleconnection strength is driven by the changing relationships between climate modes over the LM. Although our results are sensitive to proxy availability, the DA techniques provide novel insight into the nonstationarity and long‐term variability of North American hydroclimate teleconnections. This work provides a baseline for teleconnection behavior from DA products, which is potentially valuable for decadal prediction.

Sudden stratospheric warmings (SSWs) significantly influence Eurasian wintertime climate. The El Niño phase of the El Niño–Southern Oscillation (ENSO) also affects climate in that region through tropospheric and stratospheric pathways, including increased SSW frequency. However, most SSWs are unrelated to El Niño, and their importance compared to other El Niño pathways remains to be quantified. We here contrast these two sources of variability using two 200‐member ensembles of 1‐year integrations of the Whole Atmosphere Community Climate Model, one ensemble with prescribed El Niño sea surface temperatures (SSTs) and one with neutral‐ENSO SSTs. We form composites of wintertime climate anomalies, with and without SSWs, in each ensemble and contrast them to a basic state represented by neutral‐ENSO winters without SSWs. We find that El Niño and SSWs both result in negative North Atlantic Oscillation anomalies and have comparable impacts on European precipitation, but SSWs cause larger Eurasian cooling. Our results have implications for predictability of wintertime Eurasian climate.

Mesospheric signatures observed during 2010 minor stratospheric warming at King Sejong Station (62°S, 59°W)

Investigations of the winter-time behavior features of the horizontal wind velocity in the lower thermosphere measured by different ground-based methods showed that in periods of winter-time stratospheric warmings there are significant deviations of the azimuth and magnitude of the velocity vector from values typical fo the winter season. It is the winter months: December, January, February and March when the zonal and meridional flows exhibit the largest year-to-year variability. In a series of our papers, by generalizing results of analysis of the stratospheric winter-time warming effects on dynamic characteristics of the lower thermosphere, we stated the fact that the wind reversal in the lower thermosphere in the winter time from westerly to easterly is a fundamental property of the zonal circulation during warming periods in the stratosphere. Sudden stratospheric warmings are one of the main causes of the stability disturbance of zonal westerly winds in the mid-latitude lower thermosphere. The response to these meteorological phenomena changes from year to year and from station to station and depends on the climatic properities of the regions analyzed. The effect of strong stratospheric warmings of the 'major' type that were most frequently observed in the middle or at the end of the winter, manifest themselves in the reversal of westerly prevailing winds in the lower stratosphere. The 'reversal depth' of zonal winds depends on the location and dynamics of development of warm regions and is different for different climatic regions.

Numerical studies have shown that there is a lower thermospheric winter‐to‐summer circulation that is driven by wave dissipation and that it plays a significant role in trace gas distributions in the mesosphere and lower thermosphere, and in the composition of the thermosphere. However, the characteristics of this circulation are poorly known. Direct observations of it are difficult, but it leaves clear signatures in tracer distributions. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) onboard the Thermosphere Ionosphere Mesosphere Energetics and Dynamics satellite has obtained CO2 concentration from 2002 to present. This data set, combined with simulations by the Whole Atmosphere Community Climate Model, provides an unprecedented opportunity to infer the morphology of this circulation in both the summer and winter hemispheres. Our study show that there exists a maximum vertical gradient of CO2 at summer high latitudes, driven by the convergence of the upwelling of the mesospheric circulation and the downwelling of the lower thermospheric circulation; in the winter hemisphere, the maximum vertical gradient of CO2 is located at a higher altitude, driven by the convergence of the upwelling of the lower thermospheric circulation and the downwelling of the solar‐driven thermospheric circulation; the bottom of the lower thermospheric circulation is located between ~ 95 km and 100 km, and it has a vertical extent of ~10 km. Analysis of the SABER CO2 and temperature at summer high latitudes showed that the bottom of this circulation is consistently higher than the mesopause height by ~10 km.

Stratosphere-mesosphere coupling during stratospheric sudden warming events

This study uses ECMWF fifth-generation reanalysis, ERA5, which extends to the mesopause, to construct the Initial Conditions (IC) for WACCM (Whole Atmosphere Community Climate Model) simulations. Because the biases between ERA5 and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature data are within ±5 K below the lower mesosphere, ERA5 reanalysis is used to construct IC in the lower atmosphere. Four experiments are performed to simulate a Stratospheric Sudden Warming (SSW) event from 5 to 15 February 2016. The simulation using the WACCM default climatic IC cannot represent the sharp meteorological variation during SSW. In contrast, the 0~4 d forecast results driven by ERA5-constructed IC is consistent with ERA5 reanalysis below the middle mesosphere. Comparing with WACCM climatology ICs scheme, the ICs constructing method based on ERA5 reanalysis can obtain 67%, 40%, 22%, 4% and 6% reduction of temperature forecast RMSE at 10 hPa, 1 hPa, 0.1 hPa, 0.01 hPa and 0.001 hPa respectively. However, such improvement is not shown in the lower thermosphere.

Based on the data at ~40°N at different longitudes during different stratospheric sudden warming (SSW) events, the responses of zonal winds in the stratosphere, mesosphere and lower thermosphere to SSWs are studied in this paper. The variations of zonal wind over Langfang, China (39.4°N, 116.7°E) by MF radar and the modern era retrospective-analysis for research and applications (MERRA) wind data during 2010 and 2013 SSW and over Fort Collins, USA (41°N, 105°W) by lidar and MERRA wind data during 2009 SSW are compared. Results show that the zonal wind at ~40°N indeed respond to the SSWs while different specifics are found in different SSW events or at different locations. The zonal wind has significant anomalies during the SSWs. Over Langfang, before the onset of 2010 and 2013 SSW, the zonal wind reverses from eastward to westward below about 60–70♣km and accelerates above this region, while westward wind prevails from 30 to 100♣km after the onset of 2010 SSW, and westward wind prevails in 30–60 and 85–100♣km and eastward wind prevails in 60–85♣km after the onset of 2013 SSW. Over Fort Collins during 2009 SSW, eastward wind reverses to westward in 20–30♣km before the onset while westward wind prevails in 20–30 and 60–97♣km and eastward wind prevails in 30–60 and in 97–100♣km after the onset. Moreover, simulations by the specified dynamics version of the whole atmosphere community climate model (SD-WACCM) are taken to explain different responding specifics of zonal wind to SSW events. It is found that the modulation of planetary wave (PW) plays the main role. Different phases of PWs would lead to the different zonal wind along with longitudes and the different amplitudes and phases in different SSW events can lead to the different zonal wind responses.

A global model of sodium in the mesosphere and lower thermosphere has been developed within the framework of the National Center for Atmospheric Research's Whole Atmosphere Community Climate Model (WACCM). The standard fully interactive WACCM chemistry module has been augmented with a chemistry scheme that includes nine neutral and ionized sodium species. Meteoric ablation provides the source of sodium in the model and is represented as a combination of a meteoroid input function (MIF) and a parameterized ablation model. The MIF provides the seasonally and latitudinally varying meteoric flux which is modeled taking into consideration the astronomical origins of sporadic meteors and considers variations in particle entry angle, velocity, mass, and the differential ablation of the chemical constituents. WACCM simulations show large variations in the sodium constituents over time scales from days to months. Seasonality of sodium constituents is strongly affected by variations in the MIF and transport via the mean meridional wind. In particular, the summer to winter hemisphere flow leads to the highest sodium species concentrations and loss rates occurring over the winter pole. In the Northern Hemisphere, this winter maximum can be dramatically affected by stratospheric sudden warmings. Simulations of the January 2009 major warming event show that it caused a short‐term decrease in the sodium column over the polar cap that was followed by a factor of 3 increase in the following weeks. Overall, the modeled distribution of atomic sodium in WACCM agrees well with both ground‐based and satellite observations. Given the strong sensitivity of the sodium layer to dynamical motions, reproducing its variability provides a stringent test of global models and should help to constrain key atmospheric variables in this poorly sampled region of the atmosphere.

A high-altitude version of the Navy Operational Global Atmospheric Prediction System (NOGAPS) spectral forecast model is used to simulate the unusual September 2002 Southern Hemisphere stratospheric major warming. Designated as NOGAPS-Advanced Level Physics and High Altitude (NOGAPS-ALPHA), this model extends from the surface to 0.005 hPa (∼85 km altitude) and includes modifications to multiple components of the operational NOGAPS system, including a new radiative heating scheme, middle-atmosphere gravity wave drag parameterizations, hybrid vertical coordinate, upper-level meteorological initialization, and radiatively active prognostic ozone with parameterized photochemistry. NOGAPS-ALPHA forecasts (hindcasts) out to 6 days capture the main features of the major warming, such as the zonal mean wind reversal, planetary-scale wave amplification, large upward Eliassen–Palm (EP) fluxes, and splitting of the polar vortex in the middle stratosphere. Forecasts beyond 6 days have reduced upward EP flux in the lower stratosphere, reduced amplitude of zonal wavenumbers 2 and 3, and a middle stratospheric vortex that does not split. Three-dimensional EP-flux diagnostics in the troposphere reveal that the longer forecasts underestimate upward-propagating planetary wave energy emanating from a significant blocking pattern over the South Atlantic that played a large role in forcing the major warming. Forecasts of less than 6 days are initialized with the blocking in place, and therefore are not required to predict the blocking onset. For a more thorough skill assessment, NOGAPS-ALPHA forecasts over 3 weeks during September–October 2002 are compared with operational NOGAPS 5-day forecasts made at the time. NOGAPS-ALPHA forecasts initialized with 2002 operational NOGAPS analyses show a modest improvement in skill over the NOGAPS operational forecasts. An additional, larger improvement is obtained when NOGAPS-ALPHA is initialized with reanalyzed 2002 fields produced with the currently operational (as of October 2003) Naval Research Laboratory (NRL) Atmospheric Variational Data Assimilation System (NAVDAS). Thus the combination of higher model top, better physical parameterizations, and better initial conditions all yield improved forecasting skill over the NOGAPS forecasts issued operationally at the time.

Abstract. The forecast model and three-dimensional variational data assimilation components of the Navy Operational Global Atmospheric Prediction System (NOGAPS) have each been extended into the upper stratosphere and mesosphere to form an Advanced Level Physics High Altitude (ALPHA) version of NOGAPS extending to ~100 km. This NOGAPS-ALPHA NWP prototype is used to assimilate stratospheric and mesospheric temperature data from the Microwave Limb Sounder (MLS) and the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instruments. A 60-day analysis period in January and February 2006, was chosen that includes a well documented stratospheric sudden warming. SABER and MLS temperatures indicate that the SSW caused the polar winter stratopause at ~40 km to disappear, then reform at ~80 km altitude and slowly descend during February. The NOGAPS-ALPHA analysis reproduces this observed stratospheric and mesospheric temperature structure, as well as realistic evolution of zonal winds, residual velocities, and Eliassen-Palm fluxes that aid interpretation of the vertically deep circulation and eddy flux anomalies that developed in response to this wave-breaking event. The observation minus forecast (O-F) standard deviations for MLS and SABER are ~2 K in the mid-stratosphere and increase monotonically to about 6 K in the upper mesosphere. Increasing O-F standard deviations in the mesosphere are expected due to increasing instrument error and increasing geophysical variance at small spatial scales in the forecast model. In the mid/high latitude winter regions, 10-day forecast skill is improved throughout the upper stratosphere and mesosphere when the model is initialized using the high-altitude analysis based on assimilation of both SABER and MLS data.