We present analyses of tropospheric diurnal heating, and middle atmosphere diurnal tides. Our stu... more We present analyses of tropospheric diurnal heating, and middle atmosphere diurnal tides. Our study highlights interannual variations, which have received comparatively little attention in the literature compared with seasonal studies. Analyses of 12 years of data from the Kauai MF radar, 8 years of data from the Christmas Island MF radar, and 3 years of data from the Rarotonga MF radar reveal significant interannual variations in the amplitude of the mesospheric diurnal tide. Striking enhancements are noted at both the Kauai and Christmas Island sites during 1992 and 1997. We have explored the roles of convective heating and IR absorption by water vapor in modulating tropospheric diurnal forcing. IR tidal heating has been derived using the NASA Water Vapor Project (NVAP) climatology. Diurnal latent heat release was inferred from ISCCP deep cloud (> 180 hPa) fractions. The enhancement seen in MF tidal amplitudes in 1997 is correlated with above-average tropical tidal heating due to IR absorption by water vapor. Little correlation was seen between mesospheric tidal variability on interannual timescales as seen by radars, and the variability associated with latent heat release. When the NVAP-derived monthly water vapor heating rates are bandpass-filtered between 1 and 12 years, a strong signal is observed in an equatorial, longitudinally narrow band that stretches from the Indian Ocean to the central Pacific. This relatively localized variance represents over 40% of the total heating variance in the 12-year time series.
A parametric instability at the lower hybrid frequency which is known as the ion quasi-mode insta... more A parametric instability at the lower hybrid frequency which is known as the ion quasi-mode instability may be excited in the ionosphere. The instability considered is a three-wave interaction in which an incident whistler mode wave near the lower hybrid frequency decays into a lower hybrid wave and an ion acoustic type of oscillation. Threshold calculations are made at intervals along the L = 4 magnetic field line starting at the earth's surface (the geomagnetic position of Siple Station, Antarctica) and proceeding to the equatorial plane. A model is developed for the ionospheric plasma parameters needed to evaluate the expression for threshold field strength. The fields directly radiated by the Siple transmitter are not strong enough to meet the requirement for threshold. However, it is possible that the threshold can be met for Siple transmissions which are amplified by natural processes and also for natural emissions.
Data obtained with multiple instruments at the main site of the 1999 Cooperative Atmosphere-Surfa... more Data obtained with multiple instruments at the main site of the 1999 Cooperative Atmosphere-Surface Exchange Study (CASES-99) are employed to examine the character and variability of wave motions occurring in the stable nocturnal boundary layer during the night of 14 October 1999. The predominant motions are surprisingly similar in character throughout the night, exhibiting largely westward propagation, horizontal wavelengths of
Journal of Atmospheric and Solar Terrestrial Physics, Apr 1, 2007
A cumulative total of over 5 years of data from an MF radar situated at Rothera (67°S, 68°W) on t... more A cumulative total of over 5 years of data from an MF radar situated at Rothera (67°S, 68°W) on the Antarctic Peninsula have been used to derive climatologies of periodic motions in the wind field in the mesosphere and lower thermosphere with periods less than or equal to 1 day. Strong tidal motions are observed at 24, 12 and 8 h and monthly mean climatologies are presented between 74 and 94 km altitude for comparison with the HWM-93 horizontal wind model. The 24 h tide shows a strong seasonal dependence in both the zonal and meridional components with a summertime maximum and wintertime minimum over all altitudes. The monthly mean maximum amplitude is 12(±2) ms-1 at 94 km in January and the minimum is <1 ms-1 around 86 km in early winter. The 12 h wave shows large short-term amplitude variability with a peak in amplitude around late autumn. It reaches a minimum at high altitudes in winter and below ˜80 km during summer, characteristic of a mixture of migrating and non-migrating modes. The phase of the 12 h wave is relatively constant throughout winter with a minimum mean vertical wavelength of ˜75 km around equinox. The 8 h wave is predominantly a summertime high altitude phenomenon. It is seen most strongly in the winds above 85 km and reaches monthly mean amplitudes of 6(±2) ms-1 in the zonal winds at 94 km altitude. Finally, a seasonal climatology of gravity wave variances is generated by calculating the daily mean variance in the raw winds after subtracting the fitted tidal components. This index shows a strong seasonal and height dependence in both components with a wintertime peak of ˜2000 m2s-2 in the zonal component at the highest altitudes. This peak occurs when the stratospheric zonal jets are strongest and therefore the filtering of upward-propagating waves in the stratosphere should be greatest; implying that either a significant part of this wintertime wave activity is generated from a region above the peak stratospheric wind or that there is a strong annual variability in the source or propagation of the gravity wave activity at Rothera.
The TIDI (TIMED Doppler Interferometer) instrument has been collecting neutral wind data from the... more The TIDI (TIMED Doppler Interferometer) instrument has been collecting neutral wind data from the lower thermosphere and mesosphere regions since December 2001. Preliminary winds from the O2 (0-0) emission have been produced and analyzed. These winds are being compared with ground-based measurements in our validation effort. Comparisons with model prediction are also being made. Although, the TIDI data are still preliminary, they show clear tidal features. We will describe our on going validation effort and progresses on processing and analyzing the TIDI neutral wind data.
Journal of Geophysical Research: Space Physics, 2013
ABSTRACT [1] A mesospheric horizontal wind data set measured during 1991–2006 by the medium frequ... more ABSTRACT [1] A mesospheric horizontal wind data set measured during 1991–2006 by the medium frequency (MF) radar at Kauai, Hawaii (22°N, 160°W) is analyzed to examine the long-term variability of the quasi two-day wave (QTDW). The QTDW over Hawaii is amplified twice a year, with the January and July events most likely being the representation of zonal wave numbers 3 and 4 modes, respectively. The amplitudes of the January monthly mean QTDW in both meridional and zonal winds and the July monthly mean QTDW in meridional component are nearly in phase with the solar cycle but with the solar maxima leading the QTDW maxima by 1 or 2 years. However, the July monthly mean QTDW in zonal wind is more antiphase with the solar cycle. Enhanced QTDW oscillations are evident in both wind components in January 1998, which is likely related to the strong El Niño event during the winter of 1997/1998. The enhanced gravity wave activity and the increased barotropic/baroclinic/inertial instability related to the strengthened stratosphere summer easterly jet might provide additional forcing to amplify the QTDW. Moreover, the enhanced migrating diurnal tide during warm El Niño-Southern Oscillation events could also contribute to the abnormally strong QTDW by increasing the refractive index and thus the growth rate of the QTDW. Additional enhancement of the QTDW with a short period of ~43 h is observed during the major sudden stratospheric warming in January 2006.
We present analyses of tropospheric diurnal heating, and middle atmosphere diurnal tides. Our stu... more We present analyses of tropospheric diurnal heating, and middle atmosphere diurnal tides. Our study highlights interannual variations, which have received comparatively little attention in the literature compared with seasonal studies. Analyses of 12 years of data from the Kauai MF radar and 8 years of data from the Christmas Island MF radar reveal significant interannual amplitude enhancements in the diurnal tide, particularly during 1992 and 1997. The amplitude maximum in 1997 is correlated with above-average tropical tidal heating due to IR absorption by water vapor. The tidal heating was derived using the NASA Water Vapor Project (NVAP) climatology. Examination of 10 years of monthly averaged tropospheric diurnal water vapor heating reveals an interannual component that maximizes over the Indian and tropical central Pacific oceans. This component explains over 40% of the total variance in the 10-year diurnal climatology of water vapor heating. We also explore the role of convective heating in modulating tropospheric diurnal forcing.
Summer middle atmosphere (MA) jet has been generally considered as a symmetric circumpolar jet in... more Summer middle atmosphere (MA) jet has been generally considered as a symmetric circumpolar jet in principle, associated with UV heating centered in the polar stratopause, and under no planetary-wave condition in westward winds. MF radar data at Poker Flat, Alaska (65N, 147W) and Andenes, Norway (69N, 16E) are analyzed for two years of 1999-2000, together with UK Met Office stratospheric
The power spectrum of vertically propagating waves excited by tropical heating has a strong diurn... more The power spectrum of vertically propagating waves excited by tropical heating has a strong diurnal component at planetary and inertia-gravity wave (IGW) horizontal scales. Observation-driven numerical modeling studies indicate that IGW's provide a significant source of momentum for the semiannual oscillation (SAO) in the equatorial zonal mean winds. We seek observatinal confirmation of the importance of diurnal IGW's for the SAO. We have analyzed global patterns of ascending and descending node differences in MLT tropical winds and temperatures. The difference fields are separated by 8-12 hours (depending upon the dataset), and are therefore considered diurnal proxies. Juxtaposed upon the planetary-scale features are localized longitudinal variations with zonal wavenumbers larger than 5. These "intermediate-scale" variations are vertically coherent, and can persist for several weeks. The total temperature variance of wavenumbers 9-16 is examined in relation to the underlying stratospheric mean winds. Stronger variances generally coincide with periods where the underlying zonal mean winds are either relatively weak, or unidirectional. The inverse association between variance and zoal mean wind magnitude is suggestive of a wave filtering mechanism.
We present analyses of tropospheric diurnal heating, and middle atmosphere diurnal tides. Our stu... more We present analyses of tropospheric diurnal heating, and middle atmosphere diurnal tides. Our study highlights interannual variations, which have received comparatively little attention in the literature compared with seasonal studies. Analyses of 12 years of data from the Kauai MF radar, 8 years of data from the Christmas Island MF radar, and 3 years of data from the Rarotonga MF radar reveal significant interannual variations in the amplitude of the mesospheric diurnal tide. Striking enhancements are noted at both the Kauai and Christmas Island sites during 1992 and 1997. We have explored the roles of convective heating and IR absorption by water vapor in modulating tropospheric diurnal forcing. IR tidal heating has been derived using the NASA Water Vapor Project (NVAP) climatology. Diurnal latent heat release was inferred from ISCCP deep cloud (> 180 hPa) fractions. The enhancement seen in MF tidal amplitudes in 1997 is correlated with above-average tropical tidal heating due to IR absorption by water vapor. Little correlation was seen between mesospheric tidal variability on interannual timescales as seen by radars, and the variability associated with latent heat release. When the NVAP-derived monthly water vapor heating rates are bandpass-filtered between 1 and 12 years, a strong signal is observed in an equatorial, longitudinally narrow band that stretches from the Indian Ocean to the central Pacific. This relatively localized variance represents over 40% of the total heating variance in the 12-year time series.
A parametric instability at the lower hybrid frequency which is known as the ion quasi-mode insta... more A parametric instability at the lower hybrid frequency which is known as the ion quasi-mode instability may be excited in the ionosphere. The instability considered is a three-wave interaction in which an incident whistler mode wave near the lower hybrid frequency decays into a lower hybrid wave and an ion acoustic type of oscillation. Threshold calculations are made at intervals along the L = 4 magnetic field line starting at the earth's surface (the geomagnetic position of Siple Station, Antarctica) and proceeding to the equatorial plane. A model is developed for the ionospheric plasma parameters needed to evaluate the expression for threshold field strength. The fields directly radiated by the Siple transmitter are not strong enough to meet the requirement for threshold. However, it is possible that the threshold can be met for Siple transmissions which are amplified by natural processes and also for natural emissions.
Data obtained with multiple instruments at the main site of the 1999 Cooperative Atmosphere-Surfa... more Data obtained with multiple instruments at the main site of the 1999 Cooperative Atmosphere-Surface Exchange Study (CASES-99) are employed to examine the character and variability of wave motions occurring in the stable nocturnal boundary layer during the night of 14 October 1999. The predominant motions are surprisingly similar in character throughout the night, exhibiting largely westward propagation, horizontal wavelengths of
Journal of Atmospheric and Solar Terrestrial Physics, Apr 1, 2007
A cumulative total of over 5 years of data from an MF radar situated at Rothera (67°S, 68°W) on t... more A cumulative total of over 5 years of data from an MF radar situated at Rothera (67°S, 68°W) on the Antarctic Peninsula have been used to derive climatologies of periodic motions in the wind field in the mesosphere and lower thermosphere with periods less than or equal to 1 day. Strong tidal motions are observed at 24, 12 and 8 h and monthly mean climatologies are presented between 74 and 94 km altitude for comparison with the HWM-93 horizontal wind model. The 24 h tide shows a strong seasonal dependence in both the zonal and meridional components with a summertime maximum and wintertime minimum over all altitudes. The monthly mean maximum amplitude is 12(±2) ms-1 at 94 km in January and the minimum is <1 ms-1 around 86 km in early winter. The 12 h wave shows large short-term amplitude variability with a peak in amplitude around late autumn. It reaches a minimum at high altitudes in winter and below ˜80 km during summer, characteristic of a mixture of migrating and non-migrating modes. The phase of the 12 h wave is relatively constant throughout winter with a minimum mean vertical wavelength of ˜75 km around equinox. The 8 h wave is predominantly a summertime high altitude phenomenon. It is seen most strongly in the winds above 85 km and reaches monthly mean amplitudes of 6(±2) ms-1 in the zonal winds at 94 km altitude. Finally, a seasonal climatology of gravity wave variances is generated by calculating the daily mean variance in the raw winds after subtracting the fitted tidal components. This index shows a strong seasonal and height dependence in both components with a wintertime peak of ˜2000 m2s-2 in the zonal component at the highest altitudes. This peak occurs when the stratospheric zonal jets are strongest and therefore the filtering of upward-propagating waves in the stratosphere should be greatest; implying that either a significant part of this wintertime wave activity is generated from a region above the peak stratospheric wind or that there is a strong annual variability in the source or propagation of the gravity wave activity at Rothera.
The TIDI (TIMED Doppler Interferometer) instrument has been collecting neutral wind data from the... more The TIDI (TIMED Doppler Interferometer) instrument has been collecting neutral wind data from the lower thermosphere and mesosphere regions since December 2001. Preliminary winds from the O2 (0-0) emission have been produced and analyzed. These winds are being compared with ground-based measurements in our validation effort. Comparisons with model prediction are also being made. Although, the TIDI data are still preliminary, they show clear tidal features. We will describe our on going validation effort and progresses on processing and analyzing the TIDI neutral wind data.
Journal of Geophysical Research: Space Physics, 2013
ABSTRACT [1] A mesospheric horizontal wind data set measured during 1991–2006 by the medium frequ... more ABSTRACT [1] A mesospheric horizontal wind data set measured during 1991–2006 by the medium frequency (MF) radar at Kauai, Hawaii (22°N, 160°W) is analyzed to examine the long-term variability of the quasi two-day wave (QTDW). The QTDW over Hawaii is amplified twice a year, with the January and July events most likely being the representation of zonal wave numbers 3 and 4 modes, respectively. The amplitudes of the January monthly mean QTDW in both meridional and zonal winds and the July monthly mean QTDW in meridional component are nearly in phase with the solar cycle but with the solar maxima leading the QTDW maxima by 1 or 2 years. However, the July monthly mean QTDW in zonal wind is more antiphase with the solar cycle. Enhanced QTDW oscillations are evident in both wind components in January 1998, which is likely related to the strong El Niño event during the winter of 1997/1998. The enhanced gravity wave activity and the increased barotropic/baroclinic/inertial instability related to the strengthened stratosphere summer easterly jet might provide additional forcing to amplify the QTDW. Moreover, the enhanced migrating diurnal tide during warm El Niño-Southern Oscillation events could also contribute to the abnormally strong QTDW by increasing the refractive index and thus the growth rate of the QTDW. Additional enhancement of the QTDW with a short period of ~43 h is observed during the major sudden stratospheric warming in January 2006.
We present analyses of tropospheric diurnal heating, and middle atmosphere diurnal tides. Our stu... more We present analyses of tropospheric diurnal heating, and middle atmosphere diurnal tides. Our study highlights interannual variations, which have received comparatively little attention in the literature compared with seasonal studies. Analyses of 12 years of data from the Kauai MF radar and 8 years of data from the Christmas Island MF radar reveal significant interannual amplitude enhancements in the diurnal tide, particularly during 1992 and 1997. The amplitude maximum in 1997 is correlated with above-average tropical tidal heating due to IR absorption by water vapor. The tidal heating was derived using the NASA Water Vapor Project (NVAP) climatology. Examination of 10 years of monthly averaged tropospheric diurnal water vapor heating reveals an interannual component that maximizes over the Indian and tropical central Pacific oceans. This component explains over 40% of the total variance in the 10-year diurnal climatology of water vapor heating. We also explore the role of convective heating in modulating tropospheric diurnal forcing.
Summer middle atmosphere (MA) jet has been generally considered as a symmetric circumpolar jet in... more Summer middle atmosphere (MA) jet has been generally considered as a symmetric circumpolar jet in principle, associated with UV heating centered in the polar stratopause, and under no planetary-wave condition in westward winds. MF radar data at Poker Flat, Alaska (65N, 147W) and Andenes, Norway (69N, 16E) are analyzed for two years of 1999-2000, together with UK Met Office stratospheric
The power spectrum of vertically propagating waves excited by tropical heating has a strong diurn... more The power spectrum of vertically propagating waves excited by tropical heating has a strong diurnal component at planetary and inertia-gravity wave (IGW) horizontal scales. Observation-driven numerical modeling studies indicate that IGW's provide a significant source of momentum for the semiannual oscillation (SAO) in the equatorial zonal mean winds. We seek observatinal confirmation of the importance of diurnal IGW's for the SAO. We have analyzed global patterns of ascending and descending node differences in MLT tropical winds and temperatures. The difference fields are separated by 8-12 hours (depending upon the dataset), and are therefore considered diurnal proxies. Juxtaposed upon the planetary-scale features are localized longitudinal variations with zonal wavenumbers larger than 5. These "intermediate-scale" variations are vertically coherent, and can persist for several weeks. The total temperature variance of wavenumbers 9-16 is examined in relation to the underlying stratospheric mean winds. Stronger variances generally coincide with periods where the underlying zonal mean winds are either relatively weak, or unidirectional. The inverse association between variance and zoal mean wind magnitude is suggestive of a wave filtering mechanism.
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