Long-term changes of (polar) mesosphere summer echoes

Strong VHF radar echoes have been observed not only during summer months at polar latitudes (polar mesosphere summer echoes, PMSE) but also at middle latitudes (mesosphere summer echoes, MSE). These echoes are closely connected with small ice particles, thus containing information about mesospheric temperature and water vapour content. But the (P)MSE also depend on the ionisation due to solar wave radiation and precipitating high energetic particles. Observations with VHF radars at Andenes (69.3°N; 16.0°E) since 1994 and at Kühlungsborn (54.6°N; 11.8°E) since 1998 are used for investigations of the solar and geomagnetic control of the (P)MSE as well as of possible long-term changes. The (P)MSE are positively correlated with the solar Lyman α radiation and the geomagnetic activity and have slightly positive trends. Due to the limited measuring period, the significance levels of the detected (P)MSE trends are small. Positive trends in noctilucent clouds (NLC) and polar mesospheric clouds (PMC) are in general agreement with (P)MSE trends.     

Inter-hemispheric asymmetry in polar mesosphere summer echoes and temperature at 69° latitude

An inter-hemispheric asymmetry is found in the characteristics of polar mesosphere summer echoes (PMSE) and upper mesosphere temperatures at conjugate latitudes (～69°) above Antarctica and the Arctic. The second complete mesosphere–stratosphere–troposphere (MST) radar summer observation season at Davis (68.6°S) revealed that PMSE occur less frequently, with lower strength and on average 1 km higher compared with their northern counterparts at Andenes (69.3°N). We consider the thermodynamic state of the mesosphere for conjoining hemispheric summers based on satellite and ground-based radar measurements, and show the mesopause region near ～80–87 km of the Southern Hemisphere (SH) to be up to 7.5 K warmer than its Northern Hemisphere (NH) counterpart. We show that this is consistent with our observation of asymmetries in the characteristics of PMSE and demonstrate how the mesosphere meridional wind field influences the existence and strength of the echoes in both hemispheres.     

Modeling the temperature of the polar mesopause region: Part I—Inter-annual and long-term variations generated by the stratospheric QBO

We present results from the Numerical Spectral Model (NSM), which focus on the temperature environment of the mesopause region where polar mesospheric clouds (PMC) form. The PMC occur in summer and are observed varying on time scales from months to years, and the NSM describes the dynamical processes that can generate the temperature variations involved. The NSM simulates the quasi-biennial oscillation (QBO), which dominates the zonal circulation of the lower stratosphere at equatorial latitudes. The modeled QBO extends into the upper mesosphere, due to gravity wave (GW) filtering, consistent with UARS zonal wind and TIMED temperature measurements. While the QBO zonal winds are confined to equatorial latitudes, the associated temperature variations extend to high latitudes. The meridional circulation redistributes the QBO energy—and the resulting temperature oscillations away from the equator produce inter-annual variations that can exceed 5 K in the polar mesopause region, with considerable differences between the two hemispheres. The NSM shows that the 30-month QBO produces a 5-year or semi-decadal (SD) oscillation, and stratospheric NCEP data provide observational evidence for that. This SD oscillation extends in the temperature to the upper mesosphere, where it could contribute to the long-term variations of the region.     

First HF radar measurements of summer mesopause echoes at SURA

HF sounding of the mesosphere was first carried out at SURA in summer 1994 at frequencies in the range 8–9 MHz using one of the sub-arrays of the SURA heating facility. The observations had a range resolution of 3 km. Almost all measurements indicated the presence of strong radar returns from altitudes between 83 and 90 km with features very similar to VHP measurements of mesopause summer echoes at mid-latitudes and polar mesopause summer echoes. In contrast to VHP observations, HF mesopause echoes are almost always present.     

High-altitude data assimilation system experiments for the northern summer mesosphere season of 2007

A global numerical weather prediction system is extended to the mesosphere and lower thermosphere (MLT) and used to assimilate high-altitude satellite measurements of temperature, water vapor and ozone from MLS and SABER during May–July 2007. Assimilated temperature and humidity from 100 to 0.001 hPa show minimal biases compared to satellite data and existing analysis fields. Saturation ratios derived diagnostically from these assimilated temperature and water vapor fields at PMC altitudes and latitudes compare well with seasonal variations in PMC frequency measured from the aeronomy of ice in the mesosphere (AIM) satellite. Synoptic maps of these diagnostic saturation ratios correlate geographically with three independent transient mesospheric cloud events observed at midlatitudes by SHIMMER on STPSat-1 and by ground observers during June 2007. Assimilated temperatures and winds reveal broadly realistic amplitudes of the quasi 5-day wave and migrating tides as a function of latitude and height. For example, analyzed winds capture the dominant semidiurnal MLT wind patterns at 55°N in June 2007 measured independently by a meteor radar. The 5-day wave and migrating diurnal tide also modulate water vapor mixing ratios in the polar summer MLT. Possible origins of this variability are discussed.     

Observations of the E-region horizontal winds in the auroral zone and at mid-latitudes by a ground-based interferometer

The MICADO instrument, consisting of a Michelson interferometer, has observed winds and temperatures during three winter campaigns in the auroral zone, and during 2 years at the Observatoire de Haute-Provence. The instrument observed the O(¹S) oxygen emission line. Emission from this line originates from both the E- and F-regions. A method to separate the contribution from these two regions is presented for cases when the thermospheric component is comparable to that for the mesosphere. For the auroral latitudes, a mean model of the meridional and zonal neutral wind components as a function of magnetic activity and time is presented and compared to predictions from recent empirical models. For the mid-latitudes, several properties of the semi-diurnal tides are shown and compared to radar observations and predictions from recent theoretical models.     

Effects of solar proton events in the mesosphere/lower thermosphere region according to the data of meteo radar wind measurements at high and middle latitudes

Data from meteo radar measurements of the wind in the mesosphere/lower thermosphere region at high latitudes of the Southern Hemisphere (Molodezhnaya station, 68° S, 45° E) and at middle latitudes of the Northern Hemisphere (Obninsk station, 55° N, 37° E) during solar proton events that took place in 1989, 1991, 2000, 2005, and 2012 are analyzed in the paper. In 1989 and 1991, we succeeded in observing the response to solar proton evens at both stations simultaneously. The results show that solar proton events lead to a change in the wind regime of the mesosphere and lower thermosphere. At high latitudes of the Southern Hemisphere, significant changes are observed in the values of the velocities of the meridional and zonal components of the prevailing wind. In the case of powerful solar proton events, the amplitude of the semidiurnal tide grows in the vicinity of the proton flux maximum. The response to these events depends on the season. The reaction of the prevailing wind at middle latitudes shows the same features as the reaction of the wind at high latitudes. However no unambiguous response of the tide amplitude is observed. In the summer season, even powerful events (for example, in July 2000) cause no changes in the wind regime parameters in the midlatitude region of the mesosphere/lower thermosphere.     

Dynamical influence of the Madden-Julian oscillation on the Northern Hemisphere mesosphere during the boreal winter

This study first investigates the effect of the Madden-Julian Oscillation(MJO) on the Northern Hemisphere(NH)mesosphere. Both observations and simulations suggest significant cooling in the NH polar mesosphere approximately 35 days after MJO phase 4(P4), which lags the MJO-induced perturbation in the upper stratosphere by 10 days. The enhanced planetary waves(PWs) propagate upward and result in wavenumber-1 pattern temperature anomalies in the mesosphere lagging MJO P4 by 25 days. The anomalous PWs also lead to the weaker eastward zonal wind in the upper stratosphere and lower mesosphere lagging MJO P4 by 30 days. Simultaneously, the weaker westerlies result in weaker climatological westward gravity waves(GWs) in the mesosphere due to critical-level filtering. The mesosphere meridional circulation is suppressed due to both anomalous PWs and GWs, and this suppression causes polar mesospheric cooling lagging MJO P4 by 35 days.     

Mesospheric mean winds and tides observed by the Imaging Doppler Interferometer (IDI) at Halley,Antarctica

This paper describes the first ever mesospheric wind observations from Halley, Antarctica, over a full year. The recent implementation of an Imaging Doppler Interferometer at Halley is providing a new, high quality and continuous dataset to investigate the dynamics of the Antarctic mesosphere. The mean winds show clear seasonal variations, with reversals in both zonal and meridional components near the equinoxes. The dominant tidal modes have periods of 12 h and 24 h but with significant variations in amplitude during the year. Waves with longer periods are also apparent at certain times of year. The seasonal variations and amplitudes of the winds and tides are compared with other high-latitude sites in the southern and northern hemispheres. It is found that the overall pattern of winds at Halley is broadly similar to that seen at similar geographic latitudes, but with noticeable differences which may be related to it being a southern hemisphere site.     

Long-term tendencies in the MLT prevailing winds and tides over Antarctica as observed by radars at Molodezhnaya,Mawson and Davis

Long-term tendencies in horizontal neutral wind parameters in the southern polar mesosphere/lower thermosphere are presented. The wind data analyzed were obtained from meteor and MF radars situated at Molodezhnaya (45.9°E, 67.7°S), Mawson (62.9°E, 67.6°S) and Davis (78.0°E, 68.6°S). The composite dataset covers years from 1970 to 2006. A Bayesian approach in the form proposed by Wang and Zivot [2000. A Bayesian time series model of multiple structural changes in level, trend and variance. Journal of Business and Economic Statistics 8, 374–386] is used for the trend assessment. This approach allows structural breaks to be identified in the trend parameters (slope, mean or variance of residuals) or demonstrates their absence. The results of our analysis have shown persistence through such structural breaks in trends of the winter and summer prevailing winds and in meridional tidal components. It is demonstrated that the wind parameters exhibit different stable states with transitions between the states. Correlations between the southern polar MLT wind and indices of atmospheric variability (Northern annular mode, Southern annular mode, Multivariate El-Niño/Southern Oscillation Index) were then considered. The results show that statistically significant correlations exist during some periods of observations that do not exist during others.     

A modelling study of the latitudinal variations in the nighttime plasma temperatures of the equatorial topside ionosphere during northern winter at solar maximum

Latitudinal variations in the nighttime plasma temperatures of the equatorial topside ionosphere during northern winter at solar maximum have been examined by using values modelled by SUPIM (Sheffield University Plasmasphere Ionosphere Model) and observations made by the DMSP F10 satellite at 21.00 LT near 800 km altitude. The modelled values confirm that the crests observed near 15° latitude in the winter hemisphere are due to adiabatic heating and the troughs observed near the magnetic equator are due to adiabatic cooling as plasma is transported along the magnetic field lines from the summer hemisphere to the winter hemisphere. The modelled values also confirm that the interhemispheric plasma transport needed to produce the required adiabatic heating/cooling can be induced by F-region neutral winds. It is shown that the longitudinal variations in the observed troughs and crests arise mainly from the longitudinal variations in the magnetic meridional wind. At longitudes where the magnetic declination angle is positive the eastward geographic zonal wind combines with the northward (summer hemisphere to winter hemisphere) geographic meridional wind to enhance the northward magnetic meridional wind. This leads to deeper troughs and enhanced crests. At longitudes where the magnetic declination angle is negative the eastward geographic zonal wind opposes the northward geographic meridional wind and the trough depth and crest values are reduced. The characteristic features of the troughs and crests depend, in a complicated manner, on the field-aligned flow of plasma, thermal conduction, and inter-gas heat transfer. At the latitudes of the troughs/crests, the low/high plasma temperatures lead to increased/decreased plasma concentrations.     

Comparison of geostrophic and nonlinear balanced winds from LIMS data and implications for derived dynamical quantities

Nimbus 7 LIMS geopotential height data are utilized to infer the rotational wind distribution in the Northern Hemisphere stratosphere and lower mesosphere during a period of substantial wave-mean flow interaction in January, 1979. Rotational winds are derived from the application of a successive relaxation numerical procedure which incorporates the spherical polar coordinate iterative algorithm ofPaegle andTomlinson (1975) for the nondivergent nonlinear balance equation. Optimum convergence of the numerical solutions is found to occur when under-relaxation is utilized. The LIMS height analyses were also latitudinally smoothed and constrained to obey the ellipticity criterion for spherical coordinates. The balanced winds are compared with geostrophically derived values and within situ radiosonde reports for 100 mb to 10 mb over Berlin.From a localized perspective, the Berlin-LIMS comparison indicates that radiosonde and balanced wind vectors exhibit somewhat closer agreement in direction than is associated with the geostrophic estimates. However, substantial quantitative differences between radiosonde, balanced, and geostrophic wind speeds are also evident, suggesting that caution should be exercised in the local application of derived winds, as for example in the quantitative interpretation of trajectories derived from satellite height analyses during periods of enhanced stratospheric wave activity.On a longitudinally averaged basis, balanced zonal-mean wind speeds are typically 20% weaker than geostrophic values in polar latitudes, and as much as 50% weaker in tropical and midlatitude regions. Meridional balanced wind velocities, at a given longitude, are generally within ±10% of geostrophic values. Although these alterations in horizontal wind components result in only modest differences between balanced and geostrophic meridional eddy heat fluxes, a more substantial change appears in the meridional eddy momentum flux analysis. The corresponding patterns of Eliassen-Palm flux divergence are found to be somewhat more (less) intense for the balanced wind case in the stratosphere (lower mesosphere) in polar latitudes.     

Responses of polar mesospheric cloud brightness to stratospheric gravity waves at the South Pole and Rothera,Antarctica

We present the first observational proof that polar mesospheric cloud (PMC) brightness responds to stratospheric gravity waves (GWs) differently at different latitudes by analyzing the Fe Boltzmann lidar data collected from the South Pole and Rothera (67.5°S, 68.0°W), Antarctica. Stratospheric GW strength is characterized by the root-mean-square (RMS) relative density perturbation in the 30–45 km region and PMC brightness is represented by the total backscatter coefficient (TBC) in austral summer from November to February. The linear correlation coefficient (LCC) between GW strength and PMC brightness is found to be +0.09 with a 42% confidence level at the South Pole and ?0.49 with a 98% confidence level at Rothera. If a PMC case potentially affected by a space shuttle exhaust plume is removed from the Rothera dataset, the negative correlation coefficient and confidence level increase to ?0.61 and 99%, respectively. The Rothera negative correlation increases when shorter-period waves are included while no change is observed in the South Pole correlation. Therefore, observations show statistically that Rothera PMC brightness is negatively correlated with the stratospheric GW strength but no significant correlation exists at the South Pole. A positive correlation of +0.74 with a confidence level of 99.98% is found within a distinct subset of the South Pole data but the rest of the dataset exhibits a random distribution, possibly indicating different populations of ice particles at the South Pole. Our data show that these two locations have similar GW strength and spectrum in the 30–45 km region during summer. The different responses of PMC brightness to GW perturbations are likely caused by the latitudinal differences in background temperatures in the ice crystal growth region between the PMC altitude and the mesopause. At Rothera, where temperatures in this region are relatively warm and supersaturations are not as large, GW-induced temperature perturbations can drive subsaturation in the warm phase. Thus, GWs can destroy growing ice crystals or limit their growth, leading to negative correlation at Rothera. Because the South Pole temperatures in the mesopause region are much colder, GW-perturbed temperature may never be above the frost point and have less of an impact on crystal growth and PMC brightness. The observed phenomena and proposed mechanisms above need to be understood and verified through future modeling of GW effects on PMC microphysics and ray modeling of GW propagation over the South Pole and Rothera.     

Interhemispheric comparison of mesospheric ice layers from the LIMA model

The new LIMA/ice model is used to study interhemispheric temperature differences at the summer upper mesosphere and their impact on the morphology of ice particle related phenomena such as noctilucent clouds (NLC), polar mesosphere clouds (PMC), and polar mesosphere summer echoes (PMSE). LIMA/ice nicely reproduces the mean characteristics of observed ice layers, for example their variation with season, altitude, and latitude. The southern hemisphere (SH) is slightly warmer compared to the NH but the difference is less than 3 K at NLC/PMC/PMSE altitudes and poleward of 70N/S. This is consistent with in situ temperature measurements by falling spheres performed at 69N and 68S. Earth's eccentricity leads to a SH mesosphere being warmer compared to the NH by up to approximately 85 km and fairly independent of latitude. In general, NH/SH temperature differences in LIMA increase with decreasing latitude and reach at 50. The latitudinal variation of NH/SH temperature differences is presumably caused by dynamical forcing and explains why PMSE are basically absent at midlatitudes in the SH whereas they are still rather common at similar colatitudes in the NH. The occurrence frequency and brightness of NLC and PMC are larger in the NH but the differences decrease with increasing latitude. Summer conditions in the SH terminate earlier compared to NH, leading to an earlier weakening and end of the ice layer season. The NLC altitude in the SH is slightly higher by 0.6–1 km, whereas the NLC altitudes itself depend on season in both hemispheres. Compared to other models LIMA/ice shows smaller interhemispheric temperature differences but still generates the observed NH/SH differences in ice layer characteristics. This emphasizes the importance of temperature controlling the existence and morphology of ice particles. Interhemispheric differences in NLC/PMC/PMSE characteristics deduced from LIMA/ice basically agree with observations from lidars, satellites, and radars.     

Variations in the phase of the semidiurnal tide over Davis,Antarctica

The amplitude and time of maximum of the semidiurnal tide in the mesosphere and lower thermosphere above Davis varies through the summer months. In particular, the time of maximum can oscillate around a fixed value or, at some heights, shift by a whole cycle of local time. Recent studies of the semi-diurnal tide at the South Pole have suggested that an s=1 mode is common at high-southern latitudes. At mid-latitudes, the s=2 mode is thought to dominate suggesting that a region where these modes overlap is a possibility. Although it is not possible to discern which of these modes are present using single station data, the concept that more than one 12-h wave is present allows a new interpretation of the Davis semi-diurnal amplitudes and times of maximum. In this study, wind data obtained at Davis, Antarctica, during the summer of 1996/97 are used to show that the complex variations in time of maximum and amplitude can be explained by a sum of two waves. The simplest of models combines an invariant and a varying 12-h wave and yields characteristics for each component that are less complex than the original observation. There is also potential for changes to the interpretation of time of maximum vs. height profiles.     

PMSE observations at three different frequencies in northern Europe during summer 1994

Simultaneous observations of polar mesosphere summer echoes (PMSE) have been carried out during summer 1994 in northern Norway using three radars on different frequencies: the ALOMAR SOUSY radar at Andenes on 53.5 MHz, the EISCAT VHP radar at Tromsø on 224 MHz and the MF radar at Tromsø on 2.78 MHz. During the common measuring period in July/August 1994, PMSE could be detected at 224 and 53.5 MHz, and there are strong hints that PMSE also occur at 2.78 MHz. Reliable correlations between hourly backscattered power values indicate that the PMSE structures have zonal extensions of more than 130 km and can be detected at very different scales (half wavelength) between 0.67 (EISCAT VHP radar) and 54 m (MF radar). Using the wind values derived by the MF radar it can be shown that the mesospheric wind field influences the structure of PMSE. The diurnal variation of PMSE is strongly connected with tidal-wind components, whereas spatial differences of PMSE can partly be explained by the mean wind field.     

Dynamical response of low-latitude middle atmosphere to major sudden stratospheric warming events

The zonally averaged UK Meteorological Office (UKMO) zonal mean temperature and zonal winds for the latitudes 8.75°N and 60°N are used to investigate the low-latitude dynamical response to the high latitude sudden stratospheric warming (SSW) events that occurred during winter of the years 1998–1999, 2003–2004 and 2005–2006. The UKMO zonal mean zonal winds at 60°N show a short-term reversal to westward winds in the entire upper stratosphere and lower mesosphere and the low-latitude winds (8.75°N) show enhanced eastward flow in the upper stratosphere and strong westward flow in the lower mesosphere during the major SSW events at high latitudes. The mesosphere and lower thermosphere (MLT) zonal winds acquired by medium frequency (MF) radar at Tirunelveli (8.7°N, 77.8°E) show a change of wind direction from eastward to westward several days before the onset of SSW events and these winds decelerate and weak positive (eastward) winds prevail during the SSW events. The time variation of zonal winds over Tirunelveli is nearly similar to the one reported from high latitudes, except that the latter shows intense eastward winds during the SSW events. Besides, the comparison of daily mean meridional winds over Tirunelveli with those over Collm (52°N, 15°E) show that large equatorial winds are observed over Tirunelveli during the 2005–2006 event and over Collm during the 1998–1999 events. The variable response of MLT dynamics to different SSW events may be explained by the variability of gravity waves.     

Polar and high latitude substorms and solar wind conditions

All substorm disturbances observed in polar latitudes can be divided into two types: polar, which are observable at geomagnetic latitudes higher than 70° in the absence of substorms below 70°, and high latitude substorms, which travel from auroral (<70°) to polar (>70°) geomagnetic latitudes. The aim of this study is to compare conditions in the IMF and solar wind, under which these two types of substorms are observable on the basis of data from meridional chain of magnetometers IMAGE and OMNI database for 1995, 2000, and 2006–2011. In total, 105 polar and 55 high latitude substorms were studied. It is shown that polar substorms are observable at a low velocity of solar wind after propagation of a high-speed recurrent stream during the late recovery phase of a magnetic storm. High latitude substorms, in contrast, are observable with a high velocity of solar wind, increased values of the Bz component of the IMF, the Ey component of the electric field, and solar wind temperature and pressure, when a high-speed recurrent stream passes by the Earth.     

Noctilucent clouds seen at 60°N: origin and development

A noctilucent cloud is seen at a particular time from a specified place. The journey of the cloud particles from nucleation to observation can be calculated by using a simple model of growth and taking account of the fall speed of the cloud particles. Cloud particles can be backtracked by bringing together growth and fall speed equations and a model of mesospheric winds to find where the particles of a cloud seen at a particular time and place have originated. The wind model that is used here suggests that there is a distinct outer edge to the summertime polar circulation pattern in which water vapour is being carried up from the lower mesosphere to the mesopause. The change in latitude of this outer edge during the summer season may well account for the observed seasonal change in occurrence of mesospheric clouds. Polar mesospheric clouds cause a drying of the upper mesosphere. It is suggested here that diffusion of water vapour dumped at the level of polar mesospheric clouds will take an appreciable time to carry water vapour back up to the mesopause. In consequence, there will be a significant separation between the observed location of a noctilucent cloud and its precursor polar mesospheric cloud.     

Vertical and off-diagonal eddy diffusivities in the Northern Hemispheric stratosphere and lower mesosphere

Meteorological rocket soundings, launched between 1968–74 at six locations representative of low, middle, and high latitudes in the northern hemisphere, are employed to determine the vertical, meridional and off-diagonal components of the eddy diffusivity in the northern hemispheric statosphere and lower mesosphere.It is shown that the distribution of the vertical and meridional components of the eddy diffusivity are similar in the northern hemisphere, although the magnitude of the former is 10⁷ smaller than that of the latter; the magnitude of the off-diagonal eddy diffusivity is about 10³ smaller than that of the meridional eddy diffusivity. In the troposphere, a maximum eddy diffusivity occurs in the mid-latitude at about 7 km above the mean sea level for both the summer and winter seasons. In the stratosphere, a maximum eddy diffusivity occurs in the mid-latitude at about 33 km in the winter, but no maximum in the summer.Paper presented at the World Meterological Organization Technical Conference on Global Observations of Atmospheric Pollution Relative to Climate, Boulder, Colorado, 20–24 August 1979.