Comment on acp-2021-981

Winds and tides of the Antarctic mesosphere and lower thermosphere: One year of meteor-radar observations over Rothera (68°S, 68°W) and comparisons with WACCM and eCMAM

Preliminary observations and simulation of nocturnal variations of airglow temperature and emission rates at Pune (18.5°N), India

<div> <p>Atmospheric gravity waves generated in the troposphere by a number of sources (convection, frontogenesis, orography etc.) can travel great vertical distances, propagating upwards to 80 - 120km where they influence the chemical and dynamical structure of the Mesosphere and Lower Thermosphere (MLT).</p> </div><div> <p>Current chemistry-climate models represent gravity waves, and their impact on the temperature and the chemical composition of the atmosphere, by means of parameterizations that take into account the turbulence and the mixing caused by breaking waves but largely neglect the dynamical and chemical constituent transport by vertically propagating non-breaking waves.</p> </div><div> <p>We present initial results from the WAVECHASM (Wave-Induced Transport of Chemically Active Species in the Mesosphere and Lower Thermosphere) project. By making use of a recent novel theoretical approach, where the effective wave diffusivity is expressed as a function of the eddy diffusivity and of the variances of the temperature perturbation and lapse rate fluctuations, the WAVECHASM project aims to incorporate the missing transport processes into global atmospheric chemistry models. We will show here that it is possible to modify the current gravity wave drag parameterization of NCAR’s Whole Atmosphere Community Climate Model (WACCM) to explicitly account for the wave-driven vertical mixing associated with non-breaking gravity waves. This additional source of vertical mixing is expected to induce significant constituent transport in the upper atmosphere.</p> </div>

The mesosphere and lower thermosphere (MLT), at heights of 80–100 km, is critical in the coupling of the middle and upper atmosphere and controls the momentum and energy transfer between these two regions. However, despite its importance, many General Circulation Models (GCMs) do not extend upwards into the MLT and those that do remain poorly constrained. In this study, we use a long-term meteor radar wind dataset from Rothera (67° S, 68° W) on the Antarctic Peninsula to test the Whole Atmosphere Community Climate Model with thermosphere-ionosphere eXtension (WACCM-X). This radar has an interferometer to determine meteor heights and has been running since 2005. This unique combination yields a dataset ideally suited to investigate interannual variability. We find that although some characteristic features in monthly median winds are represented well in WACCM-X, the model exhibits significant biases. In particular, the observations reveal a ∼10 ms-1 eastward wind at heights of 85–100 km in Antarctic winter, whereas the model predicts winds of the same magnitude but of opposite direction. We propose that this bias exists because WACCM-X is missing eastward momentum forcing in the MLT from the breaking of secondary gravity waves. Both the model and observations reveal significant interannual variability in monthly median winds. We investigate the role of particular key external phenomena in driving the winds in this region. These phenomena are; i) variations in Solar activity, ii) the El Nino Southern Oscillation (ENSO), iii) the Quasi-Biennial Oscillation (QBO) and iv) the Southern Annular Mode (SAM). We use a linear regression method to investigate how the observed and modelled winds, and modelled gravity wave tendencies in the Antarctic MLT vary in relation to the indices that quantify these phenomena. We find that there are some times of year and some height ranges at which there are significant correlations between the indices and the observed/modelled winds. In particular, in summer, there is a strong positive correlation in the modelled and observed zonal winds with the 11-year Solar cycle of magnitude up to 9 ms-1 per 70 Solar flux units. However, there appears to be little significant influence of the ENSO on the winds observed by the radar although WACCM-X zonal winds display a negative correlation throughout January–February and a positive correlation during March–May. Results from the QBO indices are varied and we find differing correlations in the model and observations. Finally, we find a positive correlation between observed summertime zonal winds and the SAM which has a magnitude of 9 ms-1 per 2.5 hPa change in the SAM index. However, in WACCM-X zonal winds the summertime response is negative and around 10 ms-1 per 2.5 hPa. The significance of this work lies in our quantifying the biases in a leading GCM and demonstrating there is significant interannual variability in both modelled and observed winds, some of which are consistent with the proposal of external forcing.

We derive the correlation patterns over the global latitudes and from the stratosphere to lower thermosphere (broadly referred to as teleconnection) using temperature data measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) from 2002 to 2010, and using 54 years of simulations of temperatures and winds by the Whole Atmosphere Community Climate Model (WACCM). We also analyze the possible mechanisms of teleconnection by investigating the correlations between the temperature and residual circulation. The correlation patterns show that teleconnection exists globally over the equatorial, mid‐ and high‐latitudes, and temperature anomalies correspond well to the anomalies of the residual circulations through adiabatic heating/cooling. A main new finding of this study is that the teleconnection extends well into the lower thermosphere, the thermospheric anomalies are consistent with the corresponding changes of the winter‐to‐summer lower‐thermospheric branch of the residual circulation, and the winter stratosphere perturbations influence the thermosphere globally. Using a reference point chosen in the northern winter stratosphere, we find that the teleconnection structures for time periods with and without Sudden Stratospheric Warmings (SSWs) display similar patterns in SABER, and teleconnection patterns in WACCM are nearly identical for days with major SSWs, minor SSWs and without SSWs. WACCM results show strong inter‐annual and intra‐annual altitude variations of the teleconnection patterns in the southern polar region but stable altitudes of correlation regions in the equatorial and northern latitudes. The altitude variations are likely responsible for the weak correlations poleward of 60°S when multiyear or multimonth data are used.

The Mesosphere and Lower Thermosphere (MLT) act as a critical region for the propagation and dissipation of atmospheric waves, such as gravity waves, tides, and planetary waves, playing a significant role in the global atmospheric circulation system. These waves, particularly gravity waves, dissipate and break in the MLT, converting their energy into turbulence and generating localized turbulent structures. The turbulence produced in turn can modulate wave propagation, with part of the dissipated energy potentially re-exciting new waves. Atmospheric turbulence in the MLT significantly influences the transport of energy, momentum, and matter, making it a key mechanism for understanding the coupling across the entire atmospheric system. The studies of MLT atmospheric turbulence can also promote the fine modeling of the middle and upper atmosphere.By integrating ground-based MF radar observations over (39.4°N, 116.7° E) with TIMED/SABER satellite data, we investigated the variations of atmospheric turbulence energy dissipation rate (ε) and the turbopause, as well as their relationship with atmospheric wave dynamics in the MLT region. Results show that the atmospheric ε is modulated by different periods at different altitudes. The ε is subject to 12 h and 24 h periodic variations. The 12 h periodic variation is more obvious at higher altitudes than the 24 h periodic variation at lower altitudes with the dividing layer at about 90 km. Advanced analysis of turbopause are based on the total wave variations based on SABER/TIMED. We first propose a new method for identifying the wave-turbopause by employing the conservation of energy principle, and introducing an energy index to delineate the turbopause layer’s boundaries. This method defines a set of parameters including the lower boundary height, upper boundary height, turbopause height, and turbopause layer thickness. Applying this method to long-term SABER data over Beijing, we find that the turbopause layer exhibits distinct seasonal and interannual variations. The average heigh of lower boundary is 69 km, and the average heigh of upper boundary is 94 km. Global characteristics of the turbopause layer are provided, which are quite valuable to enhancing our further atmospheric modeling and empirical studies.

The roles of planetary and gravity waves during a major stratospheric sudden warming as characterized in WACCM

Abstract. The mesosphere and lower thermosphere (MLT) wind structure over Wuhan (30° N, 114° E) in 2000/2001 winter and over Langfang (39.4° N, 116.6° E) in 2009/2010 winter are examined to reveal the effects of stratospheric sudden warming (SSW) in mid-low-latitude MLT region. The result shows that the MLT daily zonal wind over these two sites reversed from eastward wind to westward wind for several days during the SSW events. The reversals were almost coincident with the polar stratospheric temperature reaching its maximum at 10 hPa, 90° N and were about ten days prior to the reversal of high latitude stratospheric zonal wind at 10 hPa, 60° N. The temporal variations of tides, gravity waves and 2-day planetary waves in the mid-latitude MLT showed different behavior during the two SSW events. During the 2001 SSW event, MLT diurnal tide reached its maximum when the MLT zonal wind decreased rapidly and SSW event began in polar stratosphere; the activity of 2-day waves decreased after the onset of the 2001 SSW, while the gravity wave increased when the 2001 SSW developed into a major warming. However, in the 2009/2010 winter, the semidiurnal tide and 2-day wave in MLT over Langfang reached a peak about two days earlier than zonal wind reversal at 10 hPa, 60° N; no significant features were found in diurnal tides, terdiurnal tides and gravity waves related to the 2010 SSW event.

Previous observations and simulations are controversial as to whether El Niño will increase or decrease the diurnal tide (DW1) in the upper mesosphere and lower thermosphere (MLT) region. This study revisited the linear response of the MLT DW1 to El Niño during the winter (December-January-February) based on 19-year satellite observations of Sounding of the Atmosphere using Broadband Emission Radiometry (SABER). The MLT DW1 temperature amplitudes decreased by ~10 % during four El Niño winters from 2002 to 2020, consistent with the results from the simulation of the Specified-Dynamics version of the Whole Atmosphere Community Climate Model (SD-WACCM). According to the multiple linear regression analysis, the linear effects of El Niño-Southern Oscillation (ENSO) on tropical MLT DW1 are negative in both SABER observations and SD-WACCM simulations. In the SD-WACCM simulation, Hough mode (1, 1) dominates the DW1 tidal variation in the tropical MLT region. The consistency between the (1, 1) mode in the tropopause region and in the MLT region, as well as the downward phase progression from 15 to 100 km, indicates the direct upward propagation of DW1 from the excitation source in the troposphere. During 7 of 8 El Niño winters from 1979 to 2014, the anomalous amplitudes of the (1, 1) mode are negative in both the tropopause region and MLT region. The suppressed DW1 heating rates in the tropical troposphere (average over ~0–16 km and 35° S–35° N) during the El Niño events contribute to the decreased DW1 tide. The mesospheric latitudinal zonal wind shear anomalies during El Niño winters would lead to a narrower waveguide and prevent the vertical propagation of the DW1 tide. The gravity wave drag excited by convection also plays a role in modulating the MLT DW1 amplitude.

The transition between the middle atmosphere and the thermosphere is known as the MLT region (for mesosphere and lower thermosphere). This area has some characteristics that set it apart from other regions of the atmosphere. Most notably, it is the altitude region with the lowest overall temperature and has the unique characteristic that the temperature is much lower in summer than in winter. The summer-to-winter-temperature gradient is the result of adiabatic cooling and warming associated with a vigorous circulation driven primarily by gravity waves. Tides and planetary waves also contribute to the circulation and to the large dynamical variability in the MLT. The past decade has seen much progress in describing and understanding the dynamics of the MLT and the interactions of dynamics with chemistry and radiation. This review describes recent observations and numerical modeling as they relate to understanding the dynamical processes that control the MLT and its variability. Results from the Whole Atmosphere Community Climate Model (WACCM), which is a comprehensive high-top general circulation model with interactive chemistry, are used to illustrate the dynamical processes. Selected observations from the Sounding the Atmosphere with Broadband Emission Radiometry (SABER) instrument are shown for comparison. WACCM simulations of MLT dynamics have some differences with observations. These differences and other questions and discrepancies described in recent papers point to a number of ongoing uncertainties about the MLT dynamical system.

Gravity waves (GW) are important for the coupling between the different regions of the middle atmosphere. They are normally generated in the troposphere, are filtered by the wind field in the stratosphere and lower mesosphere and dissipate at least partly in upper mesosphere and lower thermosphere (MLT). The activity of gravity waves, their filtering by the mean circulation, and the variation of GW activity with solar activity have been studied using long-term wind measurements with Medium Frequency (MF) radars and meteor radars at high and middle northern latitudes. The GW activity is characterized by a semi-annual variation with a stronger maximum in winter and a weaker in summer consistent with the selective filtering of westward and eastward propagating GWs by the mean zonal wind. The latitudinal variation of GW activity shows the largest values in summer at mid-latitudes between 65 km and 85 km accompanied with an upward shift of the height of wind reversal towards the pole. Long-term observations of the MLT winds at mid latitudes indicate a stable increase of westward directed winds below about 85 km and an increase of eastward directed winds above 85 km especially during summer. The observed long-term trend of zonal wind at about 75 km goes along with an enhanced activity of GWs with periods of 3 to 6 hours at altitudes between 80 km and 88 km. In addition, the mesosphere responds to severe solar proton events (SPE) with increased eastward directed winds above about 85 km. The vertical coupling from the troposphere up to the lower thermosphere due to gravity waves and planetary waves is discussed for major sudden stratospheric warmings (SSW) for the winters 2006 and 2009.

<p>Energetic particle precipitation (EPP) impact on the middle atmospheric ozone chemistry plays potentially an important role in the connection between space weather and Earth's climate system. A variant of the Whole Atmosphere Community Climate Model (WACCM-D) implements a detailed set of ionospheric D-region chemistry instead of a simple parameterization used in the earlier WACCM versions, allowing to capture the impact of EPP in more detail, thus improving the model for long-term climate studies. Here, we verify experimentally the ion chemistry of the WACCM-D by analysing the middle atmospheric ozone response to the EPP forcing during well-known solar proton events<span> </span>(SPEs). We use a multi-satellite approach to derive the middle atmospheric sensitivity for the SPE forcing as a statistical relation between the solar proton flux and the consequent ozone change. An identical sensitivity analysis is carried out for the WACCM-D model results, enabling one-to-one comparison with the results derived from the satellite observations. Our results show a good agreement in the sensitivity between satellites and the WACCM-D for nighttime conditions. For daytime conditions, we find a good agreement for the satellite data sets that include the largest SPEs (max proton flux >10^<span>4 </span> pfu). However, for those satellite data-sets with only minor and moderate SPEs, WACCM-D tends to underestimate the sensitivity in daytime conditions. In summary, the comparisons WACCM-D ion chemistry, combined with the transportation, demonstrates a realistic representation of the SPE sensitivity of ozone, and thus provides a conservative platform for long-term EPP impact studies.</p>

We present the long‐term trends of the Arctic stratospheric polar vortex (SPV) using the Modern Era Retrospective Research analysis for Research and Application‐2 (MERRA‐2) reanalysis data, and assess its impact on the mesosphere and lower thermosphere (MLT) using Specified Dynamics Whole Atmosphere Community Climate Model with thermosphere‐ionosphere eXtension (SD‐WACCM‐X) simulations. Our findings reveal that the SPV showed a weakening trend from 1980 until the early 2000s, but a strengthening trend after the 2000s. This change is attributed to the weakening of the planetary wave (PW) activities, particularly the wavenumber‐1, which can be driven by long‐term Arctic surface changes. A strong link has been found between the observed PWs activity and the increasing sea surface temperature (SST) over/nearby the Barents‐Kara Sea before early 2000s and over the Central North Pacific region after the 2000s. Furthermore, we observe that the SPV trend influences the zonal mean zonal winds and migrating solar semidiurnal tide (SW2) in the MLT. The SW2 exhibits a ∼13%/decade positive trend from 1980 to the early 2000s and a ∼14%/decade negative trend after 2000s. These results provide evidence of the impact of Arctic surface variabilities, including sea‐ice, on the MLT dynamics, especially SW2 tide.

In August 1994, the Mesospheric and Lower Thermospheric Equatorial Dynamics (MALTED) Program was conducted from the Alcântara rocket site in northeastern Brazil as part of the International Guará Rocket Campaign to study equatorial dynamics, irregularities, and instabilities in the ionosphere. This site was selected because of its proximity to the geographic (2.3°S) and magnetic (∼0.5°S) equators. MALTED was concerned with planetary wave modulation of the diurnal tidal amplitude, which exhibits considerable amplitude variability at equatorial and subtropical latitudes. Our goals were to study this global modulation of the tidal motions where tidal influences on the thermal structure are maximum, to study the interaction of these tidal structures with gravity waves and turbulence at mesopause altitudes, and to gain a better understanding of dynamic influences and variability on the equatorial middle atmosphere. Four (two daytime and two nighttime) identical Nike‐Orion payloads designed to investigate small‐scale turbulence and irregularities were coordinated with 20 meteorological falling‐sphere rockets designed to measure temperature and wind fields during a 10‐day period. These in situ measurements were coordinated with observations of global‐scale mesospheric motions that were provided by various ground based radars and the Upper Atmosphere Research Satellite (UARS) through the Coupling and Dynamics of Regions Equatorial (CADRE) campaign. The ground‐based observatories included the Jicamarca radar observatory near Lima, Peru, and medium frequency (MF) radars in Hawaii, Christmas Island, and Adelaide. Since all four Nike‐Orion flights penetrated and overflew the electrojet with apogees near 125 km, these flights provided additional information about the electrodynamics and irregularities in the equatorial ionospheric E region and may provide information on wave coupling between the mesosphere and the electrojet. Simultaneous with these flights, the CUPRI 50‐MHz radar (Cornell University) provided local sounding of the electrojet region. A description of the campaign logistics and the measurements performed with the Nike‐Orion instrumentation and their implications for turbulence due to gravity waves and tidal instability in the mesosphere and lower thermosphere (MLT) are presented here. From a study of electron density fluctuations measured by rocket probes, we have found evidence for equatorial mesospheric neutral‐atmospheric turbulence between 85 and 90 km. Furthermore, falling‐sphere data imply that gravity wave breaking was a source for this turbulence. Mean motions and the various planetary, tidal, and gravity wave structures and their coherence and variability are the subjects of a companion paper.

The variability of the upper atmosphere is largely influenced by dynamical forcing from the lower and middle atmosphere. The Mesosphere and Lower Thermosphere (MLT) is the transition region between the middle atmosphere and the upper atmosphere, and it determines dynamical forcing to the upper atmosphere from below. It is thus of high importance to know, describe, and understand the dynamical processes within the MLT to quantify dynamics. Therein, General Circulation Models (GCMs) have been a significant tool to explain MLT processes.However, developing the right parameterizations that allow to accurately model near-to-realistic states of the MLT by GCMs is challenging, which is reflected in a large diversity of results from different models in comparison to observations, e.g., of winds and temperatures at the MLT.In recent years, the community model ICON (Icosahedral Nonhydrostatic Weather and Climate Model) has been expanded into altitudes up to 150 km, named the UA (Upper Atmosphere) branch. UA-ICON is increasingly being applied to model and to understand MLT processes and how they are controlled by the lower and middle atmosphere.We present newly developed capabilities of UA-ICON. Examples are mesospheric cooling during stratospheric warming events, low summer mesopause temperatures through appropriate specification of gravity wave parameterizations and runs of high spatial resolution. Results are discussed in comparison with observations and with predictions by other GCMs.