Terdiurnal Oscillations Research Articles

Periodicities of polar mesospheric clouds inferred from a meteorological analysis and forecast system

AbstractThere is currently an ambiguity in what controls polar mesospheric cloud (PMC) periodicities near 83 km altitude. This is primarily because satellite and ground‐based data sets cannot resolve global mesospheric temperature variability over the diurnal cycle. To address this limitation, we employ a global meteorological analysis and forecast system that assimilates mesospheric satellite data with two significant advances. The first is that we use output at a more rapid one hourly cadence, allowing for a quantitative description of diurnal (24 h), semidiurnal (12 h), and terdiurnal oscillations. The second is that the output drives a simple PMC parameterization which depends only on the local temperature, pressure, and water vapor concentrations. Our study focuses on results from July 2009 in the Northern Hemisphere and January 2008 in the Southern Hemisphere. We find that the 24 h migrating temperature tide as well as the 12 h and 24 h nonmigrating tides dominate northern PMC oscillations whereas the 12 h and 24 h nonmigrating tides dominate southern oscillations. Monthly averaged amplitudes for each of these components are generally 2–6 K with the larger amplitudes at lower PMC latitudes (50°). The 2 day and 5 day planetary waves also contribute in both hemispheres, with monthly averaged amplitudes from 1 to 3 K although these amplitudes can be as high as 4–6 K on some days. Over length scales of ~1000 km and timescales of ~1 week, we find that local temperature oscillations adequately describe midlatitude PMC observations.

Global distribution of the migrating terdiurnal tide seen in sporadic E occurrence frequencies obtained from GPS radio occultations

Global Positioning System radio occultation measurements by FORMOsa SATellite mission-3/Constellation Observing System for Meteorology, Ionosphere and Climate satellites were used to analyse the characteristics of the 8-h oscillation in sporadic E (ES) layers. Six-year averages based on the 3-monthly mean zonal means from December 2006 to November 2012 were constructed for the amplitude of the terdiurnal oscillation in the occurrence frequency of ES. A global distribution from 60° S to 60° N is given, revealing two peaks above 100 km during solstice with one maximum at low and midlatitudes (approximately 10° to 40°) in each hemisphere. During equinox, the global distribution is marked by two dominant peaks centred at midlatitudes, while an additional weak maximum is located at very low southern latitudes. The seasonal characteristics around 110 km reveal large values during equinox at low and midlatitudes ( 40° S and in July near 30° S. The pattern around 90 km is dominated by a broad peak between 20° and 30° S from March to September. Comparisons with the terdiurnal oscillation in the neutral atmosphere derived from zonal wind and vertical zonal wind shear simulated with a circulation model of the middle atmosphere, as well as with satellite observations of the terdiurnal tide in temperature, fit quite well for the results above 100 km, but do not show agreement for lower altitudes.

Terdiurnal signatures in sporadic <i>E</i> layers at midlatitudes

Abstract. Global Positioning System radio occultation measurements by the FORMOsa SATellite mission-3/Constellation Observing System for Meteorology, Ionosphere and Climate satellites were used to analyse the behaviour of the signature of the terdiurnal tide in sporadic E (ES) layers at midlatitudes (43–63° N). According to theory, the occurrence of ES is expected when the vertical zonal wind shear, which is mainly owing to solar tides, is negative. 4 yr means, based on 3-monthly running mean zonal means from December 2006–November 2010, were constructed for the terdiurnal oscillation in the occurrence frequency of ES. Comparison of the results with VHF meteor radar observations of the terdiurnal tide and the 8 h oscillation in the vertical zonal wind shear at Collm, Germany (51.3° N, 13° E) shows a clear correspondence between the 8 h in ES and in wind shear signature.

MF radar observations of terdiurnal tide in the mesosphere and lower thermosphere at Wakkanai [formula omitted], Japan

MF radar observations of winds in the mesosphere and lower thermosphere (MLT) at Wakkanai (45.4° N, 141.7° E) have been significantly contributing to the studies of MLT dynamics. The present paper is an attempt to describe the characteristics of the terdiurnal tide at Wakkanai. We have used the continuous data gathered during the 5-year period 1997–2001. Mainly, the analysis focuses upon the seasonal aspects of the terdiurnal tide characteristics. The observed results have been compared with the results reported at London, Ontario, another station located at a similar midlatitude (43° N) sector. It is observed that the terdiurnal tide is a regular feature in the MLT region. There exists strong day-to-day variability in the terdiurnal tide amplitudes. Short-time variability confirms that the terdiurnal oscillation can have an amplitude comparable to that of the diurnal and semidiurnal tides on occasions. Presence of dominant 8-h oscillation is observed in all seasons. However, a strong seasonal dependence is not observed in their amplitude variations. Amplitudes show slightly larger values in the winter season than the corresponding values in the summer season. The terdiurnal amplitudes at Wakkanai are more comparable to the diurnal tide amplitudes than the semidiurnal values. Comparison of the observed characteristics at Wakkanai and London reveals some similarities as well as differences.

Terdiurnal oscillations in OH Meinel rotational temperatures for fall conditions at northern mid‐latitude sites

High‐precision (∼0.5 K) measurements of OH Meinel (M) (6,2) rotational temperatures above the Bear Lake Observatory, UT (42°N, 112°W) during October 1996 have revealed an interesting and unexpected mean nocturnal pattern. Ten quality nights (>100 h) of data have been used to form a mean night for autumnal, near‐equinoctial conditions. The mean temperature and RMS variability associated with this mean night were 203 ± 5 K and 2.4 K, respectively, and compare very favorably with expectations based on Na‐lidar measurements of mean tidal temperature perturbations over Urbana, IL (40°N, 88°W) during the fall 1996. Furthermore, this comparison shows that the 8‐h tide was the dominant source of the mean nocturnal temperature variability in the OH M region during this period. Additional data, obtained at Fort Collins, CO (41°N, 105°W) in November 1997, illustrate the occurrence of an 8‐h component of OH temperature variability about two months after the equinox and show that daily amplitudes as high as ≅15 K are possible.

On Krassovsky's ratio for ter-diurnal hydroxyl oscillations in the winter polar mesopause

Past observations of ter-diurnal oscillations in hydroxyl brightness and rotational temperature of the winter polar mesopause have revealed two apparently conflicting facets. The 8 h wave is a commonly observed feature, possibly the most recurring tide of that region. On the other hand, temperature changes were observed to lead changes in brightness, implying a negative phase of Krassovsky's ratio. (Krassovsky's ratio is the ratio of normalized airglow brightness variation to normalized temperature variation.) This qualitatively supports a gravity wave explanation according to predictions of current dynamical-chemical models of fluctuations in airglow emissions. But the ter-diurnal oscillations are not likely due to gravity waves because of the persistence of the oscillations. Here a statistically-significant sample (23 individual days) of clear ter-diurnal waves of the winter polar mesopause over Eureka, NWT (80°N), is considered in order to establish the range of observed amplitude and phase of Krassovsky's ratio and compare with the predicted values. The observed amplitude of Krassovsky's ratio was 2.6–8.1 with the majority of cases (74%) at 4±1. The phase of Krassovsky's ratio was negative for all 23 cases, i.e. changes in temperature led changes in brightness. In the majority of cases the negative phase was significantly different from zero. Theoretical calculations for two different model atmospheres (both isothermal and non-isothermal) were performed using a current model of tidally driven hydroxyl airglow fluctuations. The predicted phase of Krassovsky's ratio was slightly positive (≤ +30°) for an 8 h gravitational zonally symmetric tide of meridional index 2–5 and a migrating tide of meridional index 3–7. It is concluded that the theoretical predictions of Krassovsky's ratio of oscillations in hydroxyl emission induced by zonally symmetric tides and migrating tides as applied to the two model atmospheres do not agree with the above observations of the winter polar mesopause. Neither do the theoretical predictions of Krassovsky's ratio for gravity wave-driven fluctuations in OH nightglow from an extended, dissipative emission region match the observed value for the vast majority (87%) of days with clear ter-diurnal oscillations. The consistently negative phases over Eureka probably reflect unique conditions in the very high latitude winter mesopause region characterized by strong lapse rates in the emission layer or background minor constituent concentrations that differ significantly from model predictions.

Tidal oscillations of the Arctic upper mesosphere and lower thermosphere in winter

The upper mesosphere and lower thermosphere of the northern polar cap was investigated using Fabry‐Perot interferometer measurements of the horizontal wind velocity in the lower thermosphere, meridian scanning photometer observations of atomic oxygen green line brightness in the lower thermosphere, and Michelson interferometer records of the infrared Meinel OH bands brightness and rotational temperature in the upper mesosphere. Tidal airglow oscillations in the absence of local solar heating were studied using a superposed epoch analysis of observations obtained around three new‐Moon intervals (November 9–22, 1993, December 7–20, 1993, and January 4–17, 1994) in Eureka (80°N). Tidal harmonics were uncovered with an iterated least chi‐squared fit, and their existence in the data tested using a goodness‐of‐fit probability against a null hypothesis of no oscillations. No single dominant tide was found at all times and altitudes of the winter Arctic upper mesosphere and lower thermosphere. The horizontal wind velocity of the lower thermosphere exhibited all the first three harmonics of the tide, with the largest‐amplitude oscillations shown by the diurnal component of the meridional wave speed. The atmospheric layer near the mesopause that contributes to the OH airglow emission experienced terdiurnal oscillations throughout the winter season. Theoretical polarization relations for evanescent tides, considerations of energy density propagation with altitude, and a day‐by‐day analysis of the zonal and meridional wind speed and temperature all indicate that the terdiurnal tide observed on November was an evanescent zonally symmetric tide. Other data indicate propagating tides, migrating tides, or a mixture of both.

A comparison of three methods of measuring tidal oscillations in the lower thermosphere using EISCAT common programmes

The EISCAT Common Programme can be used in three ways to monitor tidal oscillations in the lower thermosphere. In Common Programme One (CPI) tristatic observations provide measurements of the ion-velocity vector at several heights in the E-region and one height in the F-region. In Common Programme Two (CP2) monostatic measurements give profiles of ion velocity in the E-region while tristatic measurements give continuous measurements of ion velocity in the F-region. From the ion velocities and the ion-neutral collision frequency, the vector of the E-region neutral wind can be determined and both east-west and north-south components of the diurnal, semi-diurnal and ter-diurnal oscillations can be identified. CP1 and CP2 also provide profiles of the field-aligned ion velocity, and these can be used to calculate the north-south component of the neutral wind without knowing the ion-neutral collision frequency, but the result is affected by any vertical component of neutral velocity. The three methods are compared and the advantages of CP2 demonstrated.

On the interactions between gravity waves and the diurnal propagating tide

The mutual interaction between a spectrum of gravity waves with phase speeds of 0, ±15, ± 30 m s −1 , and the diurnal propagating tide is investigated. The tide is simulated by an equivalent gravity wave formalism on a rotating plane, and the interaction is accomplished through an iterative scheme wherein the momentum deposition and effective eddy diffusivity generated by the breaking gravity waves are treated through a simple parameterization. The gravity wave stress is shown to be an effective mechanism for damping the diurnal tide. In addition, the interaction is shown to produce local time variations in eddy diffusivity which may have important photochemical consequences, as well as momentum forcing of secondary semidiurnal and terdiurnal oscillations. A Rayleigh friction parameterization of the wave stress effect is derived with potential use in two- or three-dimensional numerical models of the diurnal atmospheric tide.

Lunar and solar barometric tides at Rarotonga, Cook Islands

Lunar and solar atmospheric tidal oscillations have been determined with reasonable accuracy from a ten-year record of hourly mercury-barometer readings, corrected to mean-sea-level, at Rarotonga (Cook Islands), 21.2°S. For the lunar semidiurnal tide, the annual determination shows an amplitude (56 μb) slightly lower and a phase (51°) much smaller than the values (58 μb, 72°) that would be derived, for the position of Rarotonga, from the spherical harmonic analysis given byHaurwitz andCowley (1969). The seasonal variation of this oscillation, as given by the monthly and J, E, D values, shows most of the characteristic features found in world-wide determinations. In particular, the near equality of the J, D amplitudes at Rarotonga tends to support theHaurwitz andCowley (1969) suggestion of negative J-D values in southern middle latitudes. For the solar tides, the semidiurnal and terdiurnal oscillations at Rarotonga are similar to those found at other stations in the south-west Pacific region. However, for the diurnal oscillation, the annual amplitude (232 μb) is only about half the value (∼465 μb) indicated for the position of Rarotonga by the world maps of theS1(p) annual harmonic coefficients given byHaurwitz (1965). It thus seems likely that the relatively small area of lowS1(p) annual amplitude in the eastern part of the south Pacific, as indicated by these maps, is much more extensive than formerly supposed.

Electric field induced drifts from the French Incoherent Scatter Facility

The Saint-Santin facility has been operating in its quadristatic configuration since October 1973. A set of 8 days and 7 nights (winter and equinox) of F region drift measurements perpendicular to the magnetic field is presented and provides information on the ionospheric electric fields. For quiet magnetic conditions the day-to-day variability is found to be weak, and the corresponding daily variations show the dominance of a 50 m/s diurnal oscillation in east-west drifts and of a 15–20 m/s terdiurnal oscillation in north-south drifts. None of the presently available models of quiet time electric fields have been found to reproduce this data over 24 hours. Comparison with other mid- and low-latitude electric field results shows a rather good agreement with incoherent scatter results south of Saint-Santin but a strong contrast with Millstone Hill results despite comparable geographic latitudes. For disturbed magnetic conditions, perpendicular drift velocities appear displaced from their quiet day values with a tendency to westward deviations. On some occasions those displacements reach large values of 100 m/s (4 mV/m electric field) and more over 1 or 2 hours. An interpretation in terms of penetration of the plasmasphere by electric fields associated with magnetospheric substorms is suggested.

Mean and periodic components of ionospheric drifts in the D-region at 35°S during 1972

Observations of horizontal ionospheric drifts have been made near Adelaide (35°S) using the spaced receiver method as applied to partial reflections from the D-region in the height range 80–100 km. The technique offers a cheap alternative to the radio meteor systems which have previously been used for long term studies of winds at these heights. It has the decided advantage of a time resolution of the order of a few minutes. The 1972 data presented here are among the first partial reflection drifts to be obtained continuously through both day and night. This has permitted 24-h tidal components to be determined. The results are analysed to separate out the mean and periodic components of the motion. On the average, major and roughly equal contributions come from the diurnal oscillation, semi-diurnal oscillation and mean zonal flow. The terdiurnal oscillation and meridional flow emerge as secondary contributors to the overall motion. Significant seasonal variations are apparent in all components. At 85–90 km, the mean zonal component is strongly eastward in winter but westward in early summer. The diurnal tide is dominant in autumn whereas the semidiurnal tide is relatively more important during the spring.