Mesopause Region Research Articles

Impacts of the January 2005 solar particle event on noctilucent clouds and water at the polar summer mesopause

Abstract. The response of noctilucent clouds to the solar particle event in January 2005 is investigated by means of icy particle and ion chemistry simulations. It is shown that the decreasing occurrence rate of noctilucent clouds derived from measurements of the SCIAMACHY/Envisat instrument can be reproduced by one-dimensional model simulations if temperature data from the MLS/Aura instrument are used. The model calculations indicate that the sublimation of noctilucent clouds leads to significant changes of the water distribution in the mesopause region. These model results are compared with H2O measurements from the MLS and the MIPAS/Envisat satellite instruments. The pronounced modelled water enhancement below the icy particle layer and its decrease during the SPE are not observed by the satellite instruments. At altitudes >85 km the satellite measurements show an increase of H2O during the SPE in qualitative agreement with the model predictions. The discrepancies between model H2O and observations at lower altitudes might be attributed to the one-dimensional model approach which in particular neglects inhomogeneities and horizontal transport processes. Additionally, it is revealed that the water depletion due to reactions of proton hydrates during the considered solar particle event has only a minor impact on the icy particles.

Seasonal variations of midlatitude mesospheric Na layer and their tidal period perturbations based on full diurnal cycle Na lidar observations of 2002–2008

The Na lidar facility at Colorado State University (41°N, 105°W) started the full diurnal cycle observations of mesopause region temperature and zonal and meridional winds as well as mesospheric Na density in May 2002. In this paper, monthly means and seasonal variations of the density of mesospheric Na based on lidar observations from May 2002 to December 2008 are reported along with the amplitude and phase of tidal period perturbations. The revealed seasonal behaviors of mesospheric Na layer are generally consistent with published nocturnal climatology, with thick layers and high abundance in winter but thin layers and low abundance near summer. Tidal amplitudes of Na density are large in February–April and August–November with a dominant peak between 85 and 90 km; they are weak in summer months (May–July). The Na density tidal phase profiles, while showing downward progression, show a significant and abrupt phase shift (ideally 180 degrees). The center altitude of this phase shifting (termed switching altitude) is found to coincide with the fractional tidal amplitude (tidal amplitude over diurnal mean) minimum about 2–4 km above the centroid altitude of the associated Na layer. Taking advantage of the established temperature tidal climatology deduced from the same data set, the tidal phase behaviors between temperature and Na density and associated fractional Na density tidal amplitudes are discussed in terms of the theoretical prediction by Gardner and Shelton (1985).

Anthropogenic effects on the distribution of minor chemical constituents in the mesosphere/lower thermosphere – A model study

We investigate the influence that rising concentrations of methane, nitrous oxide and carbon dioxide have had upon the chemistry of the mesosphere since 1961. Calculations were performed using our global 3D-model LIMA (Leibniz-Institute Middle Atmosphere), designed for the investigation of the MLT-region (Mesosphere-Lower Thermosphere) and particularly for the extended mesopause region. LIMA utilizes observed tropospheric and lower stratospheric temperature and horizontal wind data up to 35km altitude by assimilating ECMWF/ERA-40 and ECMWF operational data. Real Lyman-α flux values are employed to determine the variable water vapor dissociation rate. Three different calculations were carried out and analyzed: (1) use of the same annual variation of the model dynamics in the chemical transport model (CTM) for all years according to the dynamics of the solar minimum year 1964 and employment of a realistic growth of the anthropogenic gases; (2) use of constant concentrations of the anthropogenic constituents at the lower border, but employment of the varying model dynamics; (3) the so-called realistic case, which considers both the long-term increase in the anthropogenic minor constituents and the varying dynamics according to LIMA calculations. The analysis of these three cases shows that the dynamics are able to counteract the impact of anthropogenic growth of minor constituents in the upper mesosphere-mesopause-lower thermosphere region in middle to high latitudes in summer. The water vapor mixing ratio increases due to rising methane concentration. The reason for this lies in a positive feedback process of autocatalytic water vapor production. The change in concentration of the minor constituents impacts both the cooling rate and the chemical heating rate. We present the relative and absolute deviations between the solar activity minimum years 1964 and 2008 for the most important minor constituents. We discuss the long-term behavior, particularly of water vapor, with regard to the impact on the NLC-region.

Wind and temperature response of midlatitude mesopause region to the 2009 Sudden Stratospheric Warming

In this paper we report winds and temperature in the mesopause region (80–102 km) over full diurnal cycles during the 2009 major Sudden Stratospheric Warming (SSW) at Fort Collins (41°N, 105°W). The measurements were made with the Colorado State University (CSU) sodium Doppler wind‐temperature lidar. We deduce the diurnal mean wind and temperature profiles by removing the tidal components from the 24‐h continuous observations and present their anomalous behaviors in connection with this event. These mean wind and temperature measurements reveal significant anomalies in the mesopause region: the mean temperature at 80 km was approximately 30 K lower than the climatological mean; the mean zonal wind ranged between ∼ −10 to 0 m/s from 80 to 97 km and then turned eastward in lower thermosphere in a reversal of the climatological mean wind profile. We further use observations from the TIMED/SABER satellite observations and simulations from the WACCM model to investigate the global structure of this dynamical anomaly at Fort Collins. The satellite observations and model reveal that the anomaly is part of a disturbance that extended from the polar region to Fort Collins. These simultaneous wind‐ and temperature‐lidar observations document the direct impact of a major SSW on the dynamic and thermal circulation of the midlatitude mesopause region.

A remarkable correlation between short period gravity waves and semiannual oscillation of the zonal wind in the equatorial mesopause region

Abstract. The variability of zonal winds and the horizontal wind velocity variance of short period (20–120 min) gravity waves (GWs) in the equatorial mesopause region are studied using medium frequency (MF) radar observations from Pameungpeuk (7.4° S, 107.4° E) during 2004–2010. The zonal winds display a distinct semiannual oscillation (called mesospheric semiannual oscillation, MSAO), with westward winds during equinoxes and eastward winds during solstices. Furthermore, the westward winds during March equinox are larger during 2008 and 2009. The short period GW variance also shows a semiannual oscillation with enhanced activity during equinoxes. A good correlation is observed between the zonal winds and the short period GW variance from 2008–2010, with the winds being westward during the times of enhanced GW activity. Such a correlation, however, is less obvious during 2004–2006. The long period (10–20 h) GW variance, on the other hand, does not show such a correlation throughout the observation period.

Mesospheric concentric gravity waves generated by multiple convective storms over the North American Great Plain

We report on six continuous hours of OH airglow imager observations (at z ∼ 87 km) of convectively generated gravity waves (GWs) near Fort Collins, Colorado, on the evening of 08 September 2005. These GWs appeared as nearly concentric rings, and had epicenters near the locations of deep convection in three thunderstorms in Colorado, Nebraska and South Dakota. Using GOES satellite and weather radar observations, we show that the GWs closely follow the thunderstorms. Using the background wind from a nearby radar, the intrinsic wave parameters and vertical wavelengths are calculated. The temperature perturbations are estimated to be ∼ 1–3% for GWs with horizontal wavelengths λh∼ 20–40 km and horizontal phase speeds ∼40–60 m/s. The horizontal wavelengths of GWs from a convective cluster decreased in time from 30 to 15 km. We employ convective plume and ray‐trace models to simulate the GW‐induced OH intensity perturbations from convective plumes, clusters and complexes. We find that the results using the background model wind (radiosonde/TIME‐GCM) agree well with the late‐time observations, when the images are dominated by southwestward, short‐wavelength, high‐frequency GWs. These late‐time GWs propagate against the background wind, and haveλh ∼ 30–40 km and periods of τ ∼ 20–30 min. The OH intensity perturbations are enhanced because the vertical wavelengths λz increased, increased, and the vertical velocity perturbations w′ decreased (because the GWs were near their reflection levels). We also find that these short‐wavelength GWs were created ∼5 h earlier by an extremely energetic, deep convective plume in South Dakota, thereby showing that small‐scale, convective GWs directly link the troposphere and mesopause region.

Mesopause region temperature and wind measurements with pseudorandom modulation continuous-wave (PMCW) lidar at 589 nm: Erratum

Mesopause region temperature and wind measurements with pseudorandom modulation continuous-wave (PMCW) lidar at 589 nm: Erratum

Influences of non-isothermal atmospheric backgrounds on variations of gravity wave parameters

Because of the importance of gravity waves (GWs) in coupling different atmospheric regions, further studies are necessary to investigate the characteristics of GW propagation in a non-isothermal atmosphere. Using a nonlinear numerical model, we simulate the propagation of small amplitude GWs with various wavelengths in different non-isothermal atmospheres. Our results show that the GW vertical wavelength undergoes sharp changes above the stratopause and mesopause region. Specifically, for a GW with an initial vertical wavelength of 5 km, the seasonal background temperature structure difference at 50° latitude can cause the vertical wavelength to vary by ∼2 km in the mesosphere and by as large as ∼4.5 km in the lower thermosphere. In addition, the GW paths exhibit great divergence in the height range of ∼65–110 km. Our results also show that the variations of GW path, vertical wavelength and horizontal phase velocity are not synchronized in a non-isothermal atmosphere as in an isothermal atmosphere. Despite the fact that all GWs change their characteristics as they propagate upward in a non-isothermal atmosphere, the variations relative to the initial parameters at a reference height are similar for different initial vertical wavelengths. Our results indicate that the changing characteristics of a gravity wave in a non-isothermal atmosphere need to be considered when investigating the relationship of GWs at two different heights.

Meteor‐radar observed mesospheric semi‐annual oscillation (SAO) and quasi‐biennial oscillation (QBO) over Maui, Hawaii

We observed the variations of the semi‐annual oscillation (SAO) in the mesopause region of northern subtropics, and we describe its origin and forcing implications. Using the measurements of horizontal wind profiles made by the University of Illinois meteor radar in Maui, Hawaii (20.7°N, 156.3°W) from May 2002 to June 2007 and the European Centre for Medium‐Range Weather Forecasts (ECMWF) interim data set, we find that the mesospheric SAO, with the winter westerly near 80–90 km clearly stronger than the summer westerly, is out of phase with the stratospheric SAO near 1 hPa (∼50 km). The mesospheric SAO easterly is strong during the easterly phase and weak during the westerly phase of the stratospheric quasi‐biennial oscillation (QBO) near 10 hPa (∼30 km), suggesting the modulation of the mesospheric SAO by the stratospheric QBO. The mesospheric QBO has an amplitude of approximately 5 m/s near 80 km. It is in phase with the stratospheric QBO near 10 hPa and out of phase with the QBO‐like oscillation near 1 hPa. The correlation of the gravity wave (GW) and the quasi‐two‐day wave (QTDW) activities with the mesospheric SAO and QBO suggests that the GW drags and the QTDW Eliassen‐Palm flux divergences likely contribute to the QBO modulation of the mesospheric SAO. The winter easterly wind in the tropical upper stratosphere pushes further into the northern subtropics during the QBO westerly phase than during the easterly phase. This may have impacts on the upward propagation of westward‐propagating GWs originated from the middle latitudes, and thus the westward GW forcing in the upper mesosphere of northern subtropics.

Summer to winter variability in mesospheric calcium ion distribution and its dependence on Sporadic E at Arecibo

We present a new investigation of the variability in the metallic calcium ion concentration near the mesopause region, and its relation to the electron concentration during summer and winter seasons at the Arecibo Observatory. During the summer months the ion layer is broader, extending to 87–88 km compared with winter months where it occurs above this altitude around midnight. The concentration maximizes to ∼200 ions cm−3 around 90–95 km close to midnight during the summer. However, for the winter months, the peak occurs during the early morning hours in thin descending layers above 98 km. Summer to winter variation in the calcium ion to electron ratio displays an average value of ∼0.15 and 0.05 during these seasons, respectively. A good correlation between them suggests that Ca+ densities are directly related to the strength of the Sporadic E, which is stronger in the summer. The average abundance of ions is 5.7 × 107 cm−2 and 4.6 × 107 cm−2 during summer and winter months respectively, while that for electrons is 1.2 × 1010 ions cm−2 and 5.8 × 109 ion cm−2 for these seasons. Both Ca+ and Ne display strong descending layers at different altitudes during summer and winter. Calcium ion lifetimes against neutralization are a factor of two lower during the summer than in the winter months around 90 km but similar at altitudes exceeding 95 km.

Using TIMED/SABER nightglow observations to investigate hydroxyl emission mechanisms in the mesopause region

Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Sounding of the Atmosphere Using Broadband Emission Radiometry (SABER) observations of vertical profiles of the OH nightglow emission rates, temperature, and ozone are used along with a theoretical model of the OH nightglow to distinguish the dominant mechanism for the nightglow. From the comparison between the model fit and the observations we conclude that the chemical reaction O3 + H→OH(v ≤ 9) + O2 leads to population distributions of vibrationally excited states that are consistent with the measurements. The contribution of the reaction HO2 + O→OH(v ≤ 6) + O2 to the nightglow is not needed to reproduce the measurements above 80 km, at least for the emissions originating from vibrational transitions with v ≥ 4. The analysis also determines the best fits for quenching of OH(v) by O2 and O. The results show that the quenching rate of OH(v) by O2 is smaller and that the removal by O is larger than currently used for the analysis of SABER data. The rate constant for OH(v) quenching by O2 decreases with temperature in the mesopause region. The vertical profiles of atomic oxygen and hydrogen retrieved using both 2.0 and 1.6 μm channels of Meinel band emission of the OH nightglow and the new quenching rates are slightly smaller than the profiles retrieved using only the 2.0 μm channel and the quenching rate coefficients currently used for the analysis of SABER data. The fits of the model to the observations were also used to evaluate two other assumptions. The assumption of sudden death quenching of OH by O2 and N2 (i.e., quenching to the ground state rather than to intermediate vibrational levels) leads to poorer agreement with the SABER observations. The question of whether the reaction with or quenching by atomic oxygen depends on the OH vibrational level could not be resolved; assumptions of vibrational level dependence and independence both gave good fits to the observed emissions.

Neutral air density variations during strong planetary wave activity in the mesopause region derived from meteor radar observations

Meteor radar observations of upper mesospheric/lower thermospheric winds and temperatures are suitable to investigate planetary waves in the mesosphere. Here we present a case study of the winter season 2009/2010 for two mid-latitude stations at the edge of the polar vortex and one station within the polar vortex on the island of Andøya. The selected interval begins with the build up of a long period oscillation at the end of November 2009 and ends in March 2010 after the January 2010 stratospheric warming. Planetary wave signature is visible in the temperature as well as the wind field derived from standard meteor radar and EOS MLS observations. Further, we infer neutral air density variations caused by this long period oscillation from meteor peak altitude variations of the sporadic meteor component. The results are compared with density variation computed from MLS temperature and pressure data.

Influence of the Antarctic ozone hole on the polar mesopause region as simulated by the Canadian Middle Atmosphere Model

It is well established that variations in polar stratospheric winds can affect mesospheric temperatures through changes in the filtering of gravity wave fluxes, which drive a residual circulation in the mesosphere. The Canadian Middle Atmosphere Model (CMAM) is used to examine this vertical coupling mechanism in the context of the mesospheric response to the Antarctic ozone hole. It is found that the response differs significantly between late spring and early summer, because of a changing balance between the competing effects of parametrised gravity wave drag (GWD) and changes in resolved wave drag local to the mesosphere. In late spring, the strengthened stratospheric westerlies arising from the ozone hole lead to reduced eastward GWD in the mesosphere and a warming of the polar mesosphere, just as in the well known mesospheric response to sudden stratospheric warmings, but with an opposite sign. In early summer, with easterly flow prevailing over most of the polar stratosphere, the strengthened easterly wind shear within the mesosphere arising from the westward GWD anomaly induces a positive resolved wave drag anomaly through baroclinic instability. The polar cooling induced by this process completely dominates the upper mesospheric response to the ozone hole in early summer. Consequences for the past and future evolution of noctilucent clouds are discussed.

Sporadic metal atom and ion layers and their connection to chemistry and thermal structure in the mesopause region at Arecibo

We present an analysis of two separate and distinct sporadic layer events in the mesosphere and lower thermosphere region above the Arecibo Observatory. These layers were observed in both neutral K and ionic Ca + with lidars, and in electron density with incoherent scatter radar. Temperature profiles were determined from the K lidar. One sporadic event was a high altitude layer, in which both atomic and ion sporadic layers were positioned above 100 km altitude. This represents a relatively common sporadic layer visible in both ions and neutrals at Arecibo. The other observation was less typical, with a lower altitude and more diffuse sporadic E layer, extending from below 90 km to above 95 km, which dissipated coincident with growth of a sporadic neutral K layer. We analyze these separate events using a temperature-dependent chemical model, which employs commonly accepted chemical processes. We find that the model successfully reproduces the high altitude layer of June 12–13, 2002. The result shows a temperature dependence related to the chemical lifetimes of the metallic constituents, and that the neutral layer would not have formed had the temperature profile matched that of the MSIS-90 model. Second, the temperature dependent chemistry model also reproduced K + in close agreement with electron densities on June 14–15, 2002. However, the modeled neutrals do not agree well with the observation above 90 km, and it likely requires inclusion of dynamical forcing and advection. We speculate that model and observational deficiencies, primarily exclusion of dynamics such as advection and wave interactions, are the likely shortcomings in the failure to reproduce the observations.

On the seasonal onset of polar mesospheric clouds and the breakdown of the stratospheric polar vortex in the Southern Hemisphere

[1] Southern Hemisphere (SH) polar mesospheric clouds (PMCs), also known as noctilucent clouds, have been observed to be more variable and, in general, dimmer than their Northern Hemisphere (NH) counterparts. The precise cause of these hemispheric differences is not well understood. This paper focuses on one aspect of the hemispheric differences: the timing of the PMC season onset. Observations from the Aeronomy of Ice in the Mesosphere satellite indicate that in recent years the date on which the PMC season begins varies much more in the SH than in the NH. Using the Canadian Middle Atmosphere Model, we show that the generation of sufficiently low temperatures necessary for cloud formation in the SH summer polar mesosphere is perturbed by year-to-year variations in the timing of the late-spring breakdown of the SH stratospheric polar vortex. These stratospheric variations, which persist until the end of December, influence the propagation of gravity waves up to the mesosphere. This adds a stratospheric control to the temperatures in the polar mesopause region during early summer, which causes the onset of PMCs to vary from one year to another. This effect is much stronger in the SH than in the NH because the breakdown of the polar vortex occurs much later in the SH, closer in time to the PMC season.

Long-term trends in Antarctic winter hydroxyl temperatures

[1] Observations of the hydroxyl nightglow emission with a scanning spectrometer at Davis station, Antarctica (68°S, 78°E), have been maintained over each winter season since 1995. Rotational temperatures are derived from the P-branch lines of the OH(6–2) band near λ840 nm and are a layer-weighted proxy for kinetic temperatures near 87 km altitude. The current 16 year record allows tentative estimation of the atmospheric response in the mesopause region to solar cycle forcing and the underlying long-term linear temperature trend. Seven years of new data have been added since the last reported trend assessments using these data. A multivariate regression analysis on seasonally detrended winter mean hydroxyl temperatures yields a solar cycle coefficient of 4.8 ± 1.0 K/100 solar flux units (SFU) and a linear long-term cooling coefficient of −1.2 ± 0.9 K/decade. These coefficients are consistent within uncertainties for nightly, monthly, and annual mean trend evaluations. A distinct seasonal variation in trend coefficients is found in 30 day sliding window or monthly trend analyses. The largest solar activity response (∼7 K/100 SFU) is measured in March, May–June, and September, and there is little or no solar response in April and August. The long-term trend coefficient shows the largest cooling rate (4–5 K/decade) in August–September, through to warming (2–3 K/decade) for the March and May–June periods. Comparisons of trend results are made with other important hydroxyl measurement sites. Variability in the remaining residual temperatures is examined using lag correlation analyses for the influence of planetary waves, the quasi-biennial oscillation, the polar vortex intensity, and the southern annular mode.

Meteor radar measurements of mean winds and tides over Collm (51.3° N, 13° E) and comparison with LF drift measurements 2005–2007

Abstract. An all-sky VHF meteor radar (MR) has been continuously operated at Collm (51.3° N, 13° E) since summer 2004. The radar measures meteor parameters, diffusion coefficients, and horizontal winds in the mesopause region. There exists a temporal overlap of the MR wind measurements with co-located low-frequency (LF) ionospheric drift measurements until 2007. Comparison of MR and LF semidiurnal tidal phases allows to empirically determine the virtual height overestimation of LF reflection heights due to the group retardation of LF waves. LF reference heights have to be reduced by up to 20 km to match real heights. Correction of LF heights for group retardation allows to determine the wind underestimation by the LF method compared with meteor radar measurements and opens the possibility to continue long-term trend analysis using mesosphere/lower thermosphere winds.

Comparison of ground-based and satellite measurements of atmospheric temperature in the mesopause region in high-latitude eastern Siberia

The mesopause kinetic temperature at an altitude of 87 km measured with a SABER broadband radiometer installed on the TIMED satellite and the hydroxyl molecule rotational temperature measured with a ground-spectrograph installed in high-latitude eastern Siberia (Maimaga optical station; φ = 63°N, λ = 129.5°E) are compared. The data of the observations performed from 2002 to 2006 have been analyzed. The temperatures measured during the satellite passes at distances not larger than 300 km from the intersection of the spectrograph sighting line with the hydroxyl emitting layer (∼87 km) have been compared. An analysis of 130 cases of coincident measurements indicated that the average hydroxyl molecule rotational temperatures are systematically lower than the average kinetic temperature at an altitude of the hydroxyl layer measured with SABER by 4.4 K (with a standard deviation of 11.4 K). A seasonal dependence is observed regarding the difference between the ground-based and satellite measurements. The difference decreases from 10 K in January to zero towards March. However, the time variations in the temperature obtained with the ground-based device and on the satellite are similar. Based on the performed analysis, it has been concluded that a series of hydroxyl rotational temperatures can be used to study temperature variations on different time scales, including long-term trends at the temperature emission altitude (∼87 km).

Size and shape of ice grains in the mesopause region

The paper is devoted to optical remote-sensing techniques successfully applied to investigations of the mesospheric aerosol layer, starting with a short history of the development of optical sounding experiments. In the following part, a theoretical basis of scattering measurements is discussed. A brief description of the theoretical principles of optical analysis of aerosol properties, ending with a survey of current activity and, as yet, unresolved problems is presented. It can be concluded that there is a consensus regarding the size range of mesospheric aerosol particles. The contribution of the smallest particles cannot be assessed by current optical methods. The combination of the scattering measurement with an electromechanical impact detector is a promising technique. The true size distribution, more correctly a statistical average of many possible distributions, must rely on numerical modelling. Regarding the shape of the particles, the noctilucent-cloud particles are not perfect spheres. New insight into this problem can be expected from the ongoing observational program of the Atmospheric Chemistry Experiment (ACE) and Aeronomy of Ice in the Mesosphere (AIM).

Uncertainties in mesopause temperature trend over Fort Collins, Colorado (41°N, 105°W), using the bootstrap Monte Carlo method

[1] Two methods, based on different assumptions, have been investigated for computing the uncertainty in the trend of 960 mesopause region temperature profiles taken over 20 years. In addition to the trend, the fitting model includes annual and semiannual variations, a solar effect, and an episodic term that can be associated with the Mount Pinatubo volcanic eruption. In our standard procedure we assume that the unknown uncertainties of each measurement, and thus the weights of the observations, are constant with time and are determined by assuming that we have a good model. In addition we investigate including second derivatives along with first derivatives of the Hessian matrix. In the Monte Carlo bootstrap method one resamples the data with replacement to obtain new data sets which are fit to the model and yield a distribution of fitting parameters. The standard deviation of the distribution for a parameter is taken to be the uncertainty in the parameter. A possible assumption to justify this method is that the data are “independent and identically distributed.” Although we have not been able to justify the assumptions of either method, the two methods support each other since they give essentially the same results.