N. J. Mitchell

A study of tidal and planetary wave periodicities present in midlatitude sporadic E layers

The diurnal and semidiurnal atmospheric tides are known to be of fundamental importance in the formation of midlatitude sporadic E layers, acting through their vertical windshear forcing of the long-living metallic ions in the lower thermosphere. Also, recent studies suggested that planetary waves play a role on sporadic E generation as well, a fact that went unnoticed in the long-going research of sporadic layers. In this paper a methodology is employed to investigate the tidal and planetary wave periodicities imprinted onto sporadic E critical frequencies foEs. In this approach, standard analysis techniques used in neutral atmospheric dynamics are applied on foEs time series obtained during summertime when sporadic E occurrence is nearly continuous. It is shown that besides the dominant and known 24-hour and 12-hour tidal periodicities in foEs, there is often a weaker terdiurnal (8-hour) oscillation present as well. In addition, there are planetary wave periodicites in foEs with periods near the normal Rossby modes, that is, 2, 5, 10, and 16 days. It is also found that the tidal oscillations in foEs undergo a strong amplitude modulation with periods comparable to the dominant planetary wave periodicities present in the data. Our results are in line with recent findings based on a single event study which suggested that sporadic E layers are affected indirectly by planetary waves through their nonlinear interaction and modulation of the atmospheric tides at lower altitudes. The close relationship between neutral wave dynamics and midlatitude sporadic E periodicities suggests that the ionosonde data can be used as an alternative means of studying tidal and planetary wave characteristics and their climatology in the lower thermosphere.

Planetary waves in coupling the stratosphere and mesosphere during the major stratospheric warming in 2003/2004

[1] The vertical coupling of the stratosphere-mesosphere system through quasi-stationary and traveling planetary waves during the major sudden stratospheric warming (SSW) in the Arctic winter of 2003/2004 has been studied using three types of data. The UK Met Office (UKMO) assimilated data set was used to examine the features of the global-scale planetary disturbances present in the winter stratosphere of the Northern Hemisphere. Sounding the Atmosphere using Broadband Emission Radiometry (SABER) satellite measurements were used as well for extracting the stationary planetary waves in the zonal and meridional winds of the stratosphere and mesosphere. Radar measurements at eight stations, four of them situated at high latitudes (63–69N) and the other four at midlatitudes (52–55N) were used to determine planetary waves in the mesosphere-lower thermosphere (MLT). The basic results show that prior to the SSW, the stratospheremesosphere system was dominated by an upward and westward propagating � 16-day wave detected simultaneously in the UKMO and MLT zonal and meridional wind data. After the onset of the SSW, longer-period (� 22–24 days) oscillations were observed in the zonal and meridional MLT winds. These likely include the upward propagation of stationary planetary waves from below and in situ generation of disturbances by the dissipation and breaking of gravity waves filtered by stratospheric winds. Citation: Pancheva, D., et al. (2008), Planetary waves in coupling the stratosphere and mesosphere during the major stratospheric warming in 2003/2004, J. Geophys. Res., 113, D12105, doi:10.1029/2007JD009011.

Planetary waves and variability of the semidiurnal tide in the mesosphere and lower thermosphere over Esrange (68°N, 21°E) during winter

[1] The main features of the planetary waves and the variability of the semidiurnal tide with planetary wave periods observed by meteor radar over Esrange (68°N, 21 °E) have been investigated. The interval of 39 months covering continuous measurements from October 1999 to December 2002 has been examined. The planetary waves most frequently observed by meteor radar measurements in the mesosphere and lower thermosphere (80-100 km) over Esrange are: 5-, 8- to 10-, 16-, and 23-day waves (the quasi-2-day wave is excluded in this study). They are strongly amplified in the winter. Some differences between high- and middle-latitude planetary waves notwithstanding, the 5-, 10-, and 16-day waves are most probably related to the well-known normal mode. There are some reasons to believe that the vertically upward propagating 23-day wave could be generated by solar forcing. The variability of the semidiurnal tide with periods of planetary waves has been thoroughly studied as well. It is found that in the winter when the planetary waves are significantly amplified, a very strong periodic variability of the semidiurnal tide is observed as well. This result indicates that the most probable mechanism responsible for the periodic tidal variability during winter is in situ nonlinear coupling between tides and planetary waves. Two winter periods have been examined (1999/2000 and 2001/2002) in order to find strong evidence supporting this suggestion. The validity of the frequency, phase, and vertical wavenumber (wavelength) relationship between the prime (the planetary wave and semidiurnal tide) and secondary waves has been established. The novel aspect of this work is that we show for the first time that the calculated vertical structures (vertical wavelengths) of the sum and difference secondary waves, which have very close periods, are actually very different.

The 8‐hour tide in the Arctic mesosphere and lower thermosphere

An all-sky VHF meteor radar at Esrange (68°N, 21°E) near Kiruna in Northern Sweden has been used to investigate the 8-hour tide in the Arctic mesosphere and lower thermosphere. We present a climatology of the 8-hour tide over the period October 1999 to April 2001. The tide appears to be a persistent feature of the Arctic atmosphere, although a large day-to-day variability of the tidal amplitude is observed. At times the 8-hour tide reaches amplitudes over 30 m s -1 . The amplitude of the tide increases with height across the observed height range of ∼80-100 km. Monthly mean tidal amplitudes range from <2 m s -1 to values as large as 10 m s -1 . A clear seasonal behavior is apparent with maximum amplitudes observed in the autumn. Vertical wavelengths are shortest in winter and spring (25-35 km) and longest in summer and autumn (50-90 km). At least on some occasions the vertical wave number relationships between the 8-, 12-, and 24-hour tides suggest that the 8-hour tide is being generated by nonlinear interaction between the 12- and 24-hour tides.

Nonlinear interactions between planetary waves in the mesosphere/lower-thermosphere region

A meteor radar has been used to measure horizontal winds during 1992 and 1993 in the mesosphere/lower-thermosphere region over the England at 54.5°N, 3.9°W. Evidence is found of quasi-periodic modulation of the amplitudes of planetary waves. Bispectral analysis applied to the data reveals a wealth of interactions occurring between different members of the population of planetary waves. The results of the bispectral analysis strongly support the idea that the mechanism of interaction is the quadratic nonlinear process proposed by Teitelbaum and Vial [1991] to account for tidal/planetary-wave interactions. The interactions are strongest in summer between the 2-day wave and waves with periods of about 10 and about 16 days. A number of secondary waves are produced in the interactions, and these could explain the apparent splitting of the 2-day wave into a number of closely spaced frequencies. Considerable interannual variability in the waves and interactions is evident.

Response of the mesopause region dynamics to the February 2001 stratospheric warming

Abstract The response of the mesosphere/lower-thermosphere (MLT) region to a major stratospheric warming in Europe during winter 2000/2001 has been investigated using mesopause-region winds measured by meteor radar or the LF-D1 method over three stations (Castle Eaton, 52°N; Collm, 52°N; and Esrange 68°N). The vertical wind structure measured over the three sites, and its time evolution, are found to be quite similar despite the different techniques used in the measurements. The effects of stratospheric warming are very clear, and are similar over both the mid-latitude and high-latitude sites. The warming resulted in a reversal of both the zonal and meridional wind. In the zonal component, this reversal was apparently associated with a planetary-wave oscillation with a period of ∼10 days. The effect was most conspicuous in the vertical prevailing wind gradients. The mesopause-region effects thus seem to be the results of a superposition of an intensifying planetary wave and a slow overall decrease in the strength of the zonal prevailing winds.

Quantifying gravity wave momentum fluxes with Mesosphere Temperature Mappers and correlative instrumentation

An Advanced Mesosphere Temperature Mapper and other instruments at the Arctic Lidar Observatory for Middle Atmosphere Research in Norway (69.3°N) and at Logan and Bear Lake Observatory in Utah (42°N) are used to demonstrate a new method for quantifying gravity wave (GW) pseudo-momentum fluxes accompanying spatially and temporally localized GW packets. The method improves on previous airglow techniques by employing direct characterization of the GW temperature perturbations averaged over the OH airglow layer and correlative wind and temperature measurements to define the intrinsic GW properties with high confidence. These methods are applied to two events, each of which involves superpositions of GWs having various scales and character. In each case, small-scale GWs were found to achieve transient, but very large, momentum fluxes with magnitudes varying from ~60 to 940 m2 s−2, which are ~1–2 decades larger than mean values. Quantification of the spatial and temporal variations of GW amplitudes and pseudo-momentum fluxes may also enable assessments of the total pseudo-momentum accompanying individual GW packets and of the potential for secondary GW generation that arises from GW localization. We expect that the use of this method will yield key insights into the statistical forcing of the mesosphere and lower thermosphere by GWs, the importance of infrequent large-amplitude events, and their effects on GW spectral evolution with altitude.

Combining AIRS and MLS observations for three‐dimensional gravity wave measurement

Gravity waves play a critical role in transporting energy and momentum between the troposphere, stratosphere, and mesosphere. Satellite measurements provide a powerful tool to investigate these waves across the globe. However, many present methods cannot yield reliable estimates of wave momentum fluxes or the directions of these fluxes. Here we present a new method which addresses this problem by combining observations from Atmospheric Infrared Sounder (AIRS) and Microwave Limb Sounder (MLS) in three dimensions. The method allows direct estimation of horizontal and vertical wavelengths as well as wave amplitude. This in turn allows estimation of both wave momentum flux and the full 3-D direction of propagation, crucially including the horizontal direction. The method thus allows separation of the data into, for example, eastward and westward momentum fluxes, allowing estimation of the net atmospheric forcing due to these waves. We illustrate this method with a proof-of-concept study over the Andes, arguably the largest source of gravity waves in the world. We further critically assess the advantages and disadvantages of our method. Our study highlights the importance of the difference between net and absolute measures of momentum flux.

Dynamical response to the QBO in the northern winter stratosphere:signatures in wave forcing and eddy fluxes of potential vorticity

AbstractWave–mean flow interactions associated with the Holton–Tan effect (HTE), whereby the tropical quasi-biennial oscillation (QBO) modulates the Northern Hemisphere wintertime stratospheric polar vortex, are studied using the ERA-Interim dataset. Significant evidence of the HTE in isentropic coordinates is found, with a weaker and warmer polar vortex present when the lower-stratospheric QBO is in its easterly phase (QBOe). For the first time, the authors quantify the QBO modulation of wave propagation, wave–mean flow interaction, and wave decay/growth via a calculation of potential vorticity (PV)-based measures, the zonal-mean momentum budget, and up-/downgradient eddy PV fluxes. The effect of the tropospheric subtropical jet on QBO modulation of the wave activity is also investigated. In the subtropical-to-midlatitude lower stratosphere, QBOe is associated with an enhanced upward flux of wave activity, and corresponding wave convergence and wave growth, which leads to a stronger poleward zonal-mean m...

Atmospheric Coupling by Gravity Waves: Climatology of Gravity Wave Activity, Mesospheric Turbulence and Their Relations to Solar Activity

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