Variations in the Polar Mesospheric Summer Echoes during the Appearance of Irregularities of Noctilucent Clouds

Variations in the amplitude of the ordinary wave from a received signal on a partial reflection radar at a short-wave range on the Kola Peninsula during the appearance of noctilucent clouds on August 12, 2016, are examined. Noctilucent clouds are registered by the all-sky camera located 100 km southward of the partial reflection radar. They extended over the entire celestial hemisphere observed by the all-sky camera; all of them moved in the southern direction, and the clouds had a tenuous structure and showed gravity waves with spatial periods of 15–100 km. During the presence of noctilucent clouds over the partial reflection radar, polar mesospheric summer echoes (PMSEs) were recorded at heights of 83–86 km. It was found that the presence of only noctilucent clouds in diagram of the antenna pattern of partial frequency radar is not sufficient for the appearance of PMSEs; noctilucent clouds must also have irregularities of several kilometers. The PMSE heights decreased with a velocity of 0.5 and 1.3 m/s. The issue of aerosols that cause the appearance of PMSEs and noctilucent clouds is discussed.     

A case study of the evolution of a Kelvin–Helmholtz wave and turbulence in noctilucent clouds

Bright and extensive noctilucent clouds (NLC) were observed in Århus (Denmark) on 3/4 July of 2008 with an automatic digital camera taking images every minute. This event was unique in the sense that bright NLC were seen at high elevation angles (more than 30°) that allowed observing the evolution of a Kelvin–Helmholtz (KH) wave, resulted in well-developed turbulence. In particular, coherent vortex structures of a horseshoe-shaped form were observed for the first time in noctilucent clouds. The turbulent diffusion coefficient and turbulent energy dissipation rate around the mesopause are estimated in the range 162–667 m²/s and 300–1235 mW/kg, respectively, representing a case of strong neutral air turbulence in noctilucent clouds. Turbulent structures were observed to be in the vicinity of breaking small-scale gravity waves that seems to be responsible for a high level of turbulence.At the same time, it has been demonstrated that it is of importance to take into account non-turbulent process such as the gravity wave motion that is always present in NLC layers. Unless non-turbulent process is taken into account, this certainly leads to overestimating of the value of the turbulent diffusion coefficient. More accurate characteristics of turbulence in NLC can be obtained by analyzing a sequence of high-resolution images with a high frame-rate high-resolution digital camera.     

Periods of planetary waves in geomagnetic variations

Periods of planetary waves, especially the 10- and 16-day waves, were found in Fourier analyses of 10-year geomagnetic time series from two mid-latitude stations in the northern hemisphere. This suggests that planetary waves influence geomagnetic variations. Cross-spectral analysis of magnetic time series from seven stations located at around 50°N at the beginning of 1979, when a 16-day wave occurred in the stratosphere, also shows a 16-day oscillation. However, study of the phases does not reveal the horizontal direction of wave propagation. Furthermore, the temporal variations of the 16-day oscillation in magnetic time series are presented as dynamic spectra and the results are compared with global investigations of geopotential height data at 1 hPa (around 48 km) with respect to the 16-day wave for the same time interval. In some cases this comparison suggests a clear correlation between geomagnetic variations and planetary waves as well as a propagation of the 16-day wave up to the dynamo region (100-170 km).     

On the form of noctilucent clouds

Summary the possibility of the fine structure wave form of noctilucent clouds being due solely to variations in the dust density arising from the wave motions of the dust particles is discussed, with the conclusion that this state of affairs is unlikely.     

Simultaneous observations of planetary waves from 30 to 220km

Planetary wave activity at quasi 16-, 10- and 5-day periods has been compared at various altitudes through the middle and upper atmosphere over Halley (76°S, 27°W), Antarctica, during the austral winters of 1997–1999. Observational data from the mesosphere, E-region ionosphere and F-region ionosphere have been combined with stratospheric data from the ECMWF assimilative operational analysis. Fourier and wavelet techniques have shown that the relationship between planetary wave activity at different altitudes is complex and during the winter eastward wind regime does not conform to a simple combination of vertical planetary wave propagation and critical filtering. Strong planetary wave activity in the stratosphere can coincide with a complete lack of wave activity at higher altitudes; conversely, there are also times when planetary wave activity in the mesosphere, E-region or F-region has no apparent link to activity in the stratosphere. The latitudinal activity pattern of stratospheric data tentatively suggests that when the stratospheric signatures are intense over a wide range of latitudes, propagation of planetary waves into the mesosphere is less likely than when the stratospheric activity is more latitudinally restricted. It is possible that, on at least one occasion, 16-day planetary wave activity in the mesosphere may have been ducted to high latitudes from the lower latitude stratosphere. The most consistent feature is that planetary wave activity in the mesosphere is almost always anti-correlated to planetary wave activity in the E-region even though the two are in close physical proximity. The oscillatory critical filtering of vertical gravity wave propagation by planetary waves and the re-generation of the planetary wave component at higher altitudes through subsequent critical filtering or breaking of the gravity waves may provide an explanation for some of these characteristics. Alternatively the nonlinear interaction between planetary waves and tides, indicated in the E-region data, may play a role.     

Observations of noctilucent clouds from Lithuania

We present an analysis of systematic visual and photographic observations of noctilucent clouds seen from Lithuania in the years 1973–2009. The main trends in the noctilucent cloud occurrence frequency and the mean brightness are derived from statistical and correlation analysis. A clear signature of the solar activity cycle is imprinted on the noctilucent cloud occurrence frequency and mean brightness, both showing distinct anti-correlation with the sunspot numbers; however, no statistically significant increase of either noctilucent cloud occurrence frequency or brightness has been detected at least over past 19 yr (1991–2009). The only statistically significant positive trend is established for the numbers of very bright noctilucent cloud displays in the years 1973–2009. The most recent noctilucent cloud observations are linked to variations of local mesospheric temperatures, measured by the Aura satellite.     

A comparative study of winds and tidal variability in the mesosphere/lower-thermosphere region over Bulgaria and the UK

Meteor radars located in Bulgaria and the UK have been used to simultaneously measure winds in the mesosphere/lower-thermosphere region near 42.5°N, 26.6°E and 54.5°N, 3.9°W, respectively, over the period January 1991 to June 1992. The data have been used to investigate planetary waves and diurnal and semidiurnal tidal variability over the two sites. The tidal amplitudes at each site exhibit fluctuations as large as 300% on time scales from a few days to the intra-seasonal, with most of the variability being at intra-seasonal scales. Spectral and cross-wavelet analysis reveals closely related tidal variability over the two sites, indicating that the variability occurs on spatial scales large compared to the spacing between the two radars. In some, but not all, cases, periodic variability of tidal amplitudes is associated with simultaneously present planetary waves of similar period, suggesting the variability is a consequence of non-linear interaction. Calculation of the zonal wave number of a number of large amplitude planetary waves suggests that during summer 1991 the 2-day wave had a zonal wave number of 3, but that during January/February 1991 it had a zonal wave number of 4.     

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.     

Mesospheric planetary wave effects on global PMC variability inferred from AIM–CIPS and TIMED–SABER for the northern summer 2007 PMC season

Polar Mesospheric Cloud (PMC) observations from the Cloud Imaging and Particle Size (CIPS) instrument on the Aeronomy of Ice in the Mesosphere (AIM) spacecraft are used to investigate the role of planetary wave activity on global PMC variability in the summer polar mesosphere during the 2007 Northern hemisphere season. This is coupled with an analysis of contemporaneous measurements of atmospheric temperature by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere–Ionosphere–Mesosphere–Energetics and Dynamics (TIMED) spacecraft to characterize the importance of temperature as a dominant forcing mechanism of the dynamical state of the summer polar mesosphere. The study confirms results from a recent study using PMC data from the Student Nitric Oxide Explorer (SNOE) and temperature data from SABER, such that planetary wave activity is present in both PMCs and mesospheric temperature and that are strongly coherent and anti-correlated. The dominant wave present in the polar summer mesosphere in both PMCs and temperature is the 5-day wavenumber 1 Rossby normal mode. The maximum amplitude of the variation of the 5-day wave in temperature is small at 3 K but has a significant effect on PMC albedo. The phase relationship between PMC and temperature is variable between 150° and 180° out of phase, with PMC albedo reaching a maximum ～10 h before the minimum in temperature. We have identified two additional waves, the westward propagating 2-day wavenumber 2 (2DW2) and the eastward propagating 2-day wavenumber 1 (2DE1) are both present in PMC and temperature variability in the 2007 NH season. The 2DW2 wave is consistent with a Rossby normal mode excited by the instability in the zonal mean zonal wind. However, the source of the 2DE1 wave could be a nonlinear interaction of the 2DW2 with the migrating diurnal tide. This is the first time these two wave features have been detected in coincident PMC and temperature measurements. Analysis of the zonal variation of PMC occurrence and temperature shows they are also anti-correlated and supporting the conclusion that temperature is an important forcing mechanism in zonal variability.     

Atmospheric anomalies in the summer of 1908: Skyglow

After the Tunguska catastrophe, the region of “white nights” extended southwards as far down as ∼42°N at some locations. Impressive sunsets, colored in all shades of the solar spectrum were observed in the territory of Europe and some parts of Asia from 47 to 60°N. These phenomena can be explained by the reflection of sunlight from the internal (Earth-facing) surface of the noctilucent cloud field ∼10 mln km² in area. The reflective surface of the noctilucent cloud field after the Tunguska catastrophe was estimated to be by a factor of 104 larger than the reflective surface of usual noctilucent clouds.     

Gravity wave observations in the summertime polar mesosphere from the Cloud Imaging and Particle Size (CIPS) experiment on the AIM spacecraft

We present the first results of gravity wave signatures on polar mesospheric clouds (PMCs) during the summer of 2007, in the northern hemisphere polar region. The Cloud Imaging and Particle Size (CIPS) experiment has one of the three instruments on board the NASA Aeronomy of Ice in the Mesosphere (AIM) spacecraft, which was launched into a sun-synchronous orbit on April 25, 2007. CIPS is a four-camera, wide-field (120°×80°) imager designed to measure PMC morphology and particle properties. One of the objectives of AIM is to investigate gravity wave effects on PMC formation and evolution. CIPS images show distinct wave patterns and structures in PMCs that are similar to ground-based photographs of noctilucent clouds (NLCs). The observed horizontal wavelengths of the waves were found to vary between 15 and 320 km, with smaller-wavelength structures of less than 50 km being the most common. In this paper we present examples of individual quasi-monochromatic wave events observed by CIPS and statistics on the wave patterns observed in the northern hemisphere during the summer months of 2007, together with a map showing the geographic locations of gravity wave events observed from CIPS.     

Variations in the area of the global field of noctilucent clouds of the northern hemisphere in 2007–2012 seasons

Using daily distributions of noctilucent cloud fields obtained for 2007–2012 by the AIM satellite, we analyzed temporal changes in the area of the global field of mesospheric noctilucent clouds. These clouds have been shown to be characterized by some common features that can be approximated mathematically by simple functions reflecting the seasonal course of the temperature and humidity regime of the high-latitude mesosphere, allowing a clear physical interpretation. We discuss the specific features of changes in the cloud field area for individual seasons.     

On the transformation of planetary waves of tropospheric origin into waves in radio wave absorption in the lower ionosphere

Summary Calculations are carried out of upward propagation of a tropospherically forced 10-day planetary wave into the upper middle atmosphere with the use of the COMMA-R model of the University of Cologne, of its transformation into a wave in electron density by means of the model of the Comenius University, and of its final transformation into a wave in radio wave absorption in the lower ionosphere applying the computer code of the Geophysical Institute. The calculations show that the absorption may be used for investigating the planetary wave activity, particularly of its long-term trends. The possibility of propagation of planetary waves from the winter hemisphere to the summer hemisphere is illustrated, which could contribute to explanation of the occurrence of travelling planetary waves in the mesosphere in summer.Dedicated to the Memory of Professor Karel P     

Traveling planetary waves in the stratosphere

This paper reviews the theory and observations of some traveling planetary waves in the stratosphere. Two categories of waves which appear prominently in the literature are discussed: westward propagating waves of periods in the range 3–7 days (the 5-day wave) and in the range 10–20 days (the 16-day wave). Although the observations seem to indicate that these waves are waves of the Rossby type (planetary waves), the evidence is less clear regarding (1) the question of whether these waves are forced internal waves or free (resonant) external waves, and (2) the identification of the observed waves with specific theoretical waves of the Rossby type. When recent observations are compared with theory, the evidence seems to favor the notion that the 5-day and 16-day waves of longitudinal wave number 1 may be identified, respectively, with the gravest and next gravest symmetric free Rossby modes. However, the observational evidence seems to be less clear regarding the nature of the 16-day wave than the 5-day wave.     

Influence of wave activity on the composition of the polar stratosphere

The planetary wave impact on the polar vortex stability, polar stratosphere temperature, and content of ozone and other gases was simulated with the global chemical–climatic model of the lower and middle atmosphere. It was found that the planetary waves propagating from the troposphere into the stratosphere differently affect the gas content of the Arctic and Antarctic stratosphere. In the Arctic region, the degree of wave activity critically affects the polar vortex formation, the appearance of polar stratospheric clouds, the halogen activation on their surface, and ozone anomaly formation. Ozone anomalies in the Arctic region as a rule are not formed at high wave activity and can be registered at low activity. In the Antarctic Regions, wave activity affects the stability of polar vortex and the depth of ozone holes, which are formed at almost any wave activity, and the minimal ozone values depend on the strong or weak wave activity that is registered in specific years.     

Simultaneous observations of the 2-day wave at London (43°N, 81°W) and Saskatoon (52°N, 107°W) near 91 km altitude during the two years of 1993 and 1994

Simultaneous observations are valuable in providing further insights into the character of the quasi 2-day wave. In this study we investigate the period and amplitude for the quasi 2-day wave near 91 km using MF radars at London and Saskatoon, Canada, and in addition look at possible values of the zonal wave number. The results of the present study bring out certain new aspects of the quasi 2-day wave at mid-latitudes in the Northern Hemisphere. In particular we show that the period of the quasi 2-day wave determined from our study (specially at large amplitudes) is smaller (46-47 h) than the 51-52 h period often suggested by other Northern Hemisphere results, and that the periods also showed variability as a function of time. We also draw attention to the annual variability, and especially highlight the occurrence of the wave in non-summer months. Our observations show significant wave correlation between the London and Saskatoon sites during time intervals of strong 2-day wave activity. These results suggest that the 2-day waves of 1993/4 are westward propagating waves of zonal wave number 3, although sometimes the zonal wave number 5 is also indicated (specially at large amplitudes). Our study also contributes additional mid-latitude geographical data which should aid in developing a better picture of the quasi 2-day wave.     

Variability of planetary waves as a signature of possible climatic changes

The long-term variability of stationary and traveling planetary waves in the lower stratosphere has been investigated using the data of NCEP/NCAR reanalysis. The results obtained show that during the last decades winter-mean amplitude of the stationary planetary wave with zonal wave number 1 (SPW1) increases at the higher middle latitudes of the Northern Hemisphere. It has been suggested that the observed increase in the SPW1 amplitude should be accompanied by the growth in the magnitude of the stratospheric vacillations. The analysis of the SPW1 behavior in the NCEP/NCAR data set supports this suggestion and shows a noticeable increase with time in the SPW1 intra-seasonal variability. The amplitudes of the long-period normal atmospheric modes, the so-called 5-, 10- and 16-day waves, diminish. It is supposed that one of the possible reasons for this decrease can be a growth of radiative damping rate caused, for instance, by the increase of CO₂. To investigate a possible climatic change of the middle atmosphere dynamics caused by observed changes in the tropospheric temperature, two sets of runs (using zonally averaged temperature distributions in the troposphere typical for January 1960 and 2000) with the middle and upper atmosphere model (MUAM) have been performed. The results obtained show that on average the calculated amplitude of the SPW1 in the stratosphere increased in 2000 and there is also an increase of its intra-seasonal variability conditioned by nonlinear interaction with the mean flow. This increase in the amplitudes of stratospheric vacillations during the last four decades allows us to suggest that stratospheric dynamics becomes more stochastic.     

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.     

Global distribution and climatological features of the 5–6-day planetary waves seen in the SABER/TIMED temperatures (2002–2007)

The paper is focused on the global spatial structure, seasonal and interannual variability of the ～5-day Rossby (W1) and ～6-day Kelvin (E1) waves derived from the SABER/TIMED temperature measurements for 6 full years (January 2002–December 2007). The latitude structure of the ～5-day W1 wave is related to the gravest symmetric wave number 1 Rossby wave. The vertical structure of the ～5-day Rossby wave amplitude consists of double-peaked maxima centred at ～80–90 km and ～105–110 km. This wave has a vertically propagating phase structure from the stratosphere up to 120 km altitude with a mean vertical wavelength of ～50–60 km. The ～6-day E1 wave is an equatorially trapped wave symmetric about the equator and located between 20°N and 20°S. Its seasonal behaviour indicates some equinoctial and June solstice amplifications, while the vertical phase structure indicates that this is a vertically propagating wave between 20–100 km altitudes with a mean vertical wavelength of ～25 km.     

Observation of 6.5-day waves in the MLT region over Wuhan

In this paper, the 6.5-day planetary waves over Wuhan (30.5°N, 114.3°E) were investigated on the basis of the meteor radar measurements within 78–98 km height region during February 2002–December 2005. The observations show that 6.5-day waves have a prominent seasonal variability and have larger amplitudes at equinoxes than at solstices. We also found that intensive waves occur mostly between 84 and 98 km, and the zonal components of 6.5-day waves are a little larger than its meridional components on average. The main periods of 6.5-day waves are near 6–7 days in spring/winter season and 5–7 days (even extend to 8 days) in autumn months. However, these waves exhibit a downward progression when the amplitude is large. Robust wave events occur basically in eastward background winds. During the 4-year interval, the strongest waves were found in Apri–-May of 2003 and 2004.