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- Author or Editor: Ruth S. Lieberman x
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Abstract
Data from the Nimbus-7 Limb Infrared Monitor of the Stratosphere (LIMS) are used to extend the analysis of diurnal tides in the middle atmosphere to include the zonally symmetric and eastward propagating components. These modes show vertical and meridional structures that are generally consistent with the predictions of linear tidal theory. The zonal and meridional wind fields are calculated from the tidal temperatures. Diurnal perturbations in LIMS temperature and derived winds agree well with equinoctial model calculations. Comparisons with day–night differences of radiosonde temperature and rocket winds show qualitative agreement in vertical structure and amplitude.
The zonally symmetric and eastward migrating tidal components show a higher degree of temporal variability than the sun-synchronous component. These modes at times contribute substantially to the diurnal signal in wind and temperature, with combined amplitudes equal to or in excess of the amplitude of the main migrating diurnal tide. The observed vertical phase variations suggest that their forcing may originate below the stratosphere.
Abstract
Data from the Nimbus-7 Limb Infrared Monitor of the Stratosphere (LIMS) are used to extend the analysis of diurnal tides in the middle atmosphere to include the zonally symmetric and eastward propagating components. These modes show vertical and meridional structures that are generally consistent with the predictions of linear tidal theory. The zonal and meridional wind fields are calculated from the tidal temperatures. Diurnal perturbations in LIMS temperature and derived winds agree well with equinoctial model calculations. Comparisons with day–night differences of radiosonde temperature and rocket winds show qualitative agreement in vertical structure and amplitude.
The zonally symmetric and eastward migrating tidal components show a higher degree of temporal variability than the sun-synchronous component. These modes at times contribute substantially to the diurnal signal in wind and temperature, with combined amplitudes equal to or in excess of the amplitude of the main migrating diurnal tide. The observed vertical phase variations suggest that their forcing may originate below the stratosphere.
Abstract
High-Resolution Doppler Imager (HRDI) winds and temperatures are used to diagnose a 2-day wave event detected in the Southern Hemisphere during January 1994. A novel aspect of this study is the focus upon the wave fluxes throughout the 65–100-km range, and their relationship to the background state. A wave “packet” composed of zonal wavenumbers 2, 3, and 4 propagates westward with a phase speed near 60 m s−1. The periods associated with zonal wavenumbers 2, 3, and 4 are 3.5, 2.1, and 1.7 days, respectively.
The morphology of the 2-day temperature and wind fields is consistent with that of a developing baroclinic wave. The divergence of the Eliassen–Palm flux of zonal wavenumbers 2–4 is dominated by the vertical convergence of meridional heat flux. The wave driving (or Eliassen–Palm flux divergence per unit mass) is predominantly westward, with magnitudes on the order of 5 m s−1 day−1. A steady-state quasigeostrophic model of the mean meridionaI circulation is used to estimate the mean wind response. The January 1994 event induces weak equatorward flow (less than 1 m s−1), together with westward winds on the order of 20 m s−1.
Abstract
High-Resolution Doppler Imager (HRDI) winds and temperatures are used to diagnose a 2-day wave event detected in the Southern Hemisphere during January 1994. A novel aspect of this study is the focus upon the wave fluxes throughout the 65–100-km range, and their relationship to the background state. A wave “packet” composed of zonal wavenumbers 2, 3, and 4 propagates westward with a phase speed near 60 m s−1. The periods associated with zonal wavenumbers 2, 3, and 4 are 3.5, 2.1, and 1.7 days, respectively.
The morphology of the 2-day temperature and wind fields is consistent with that of a developing baroclinic wave. The divergence of the Eliassen–Palm flux of zonal wavenumbers 2–4 is dominated by the vertical convergence of meridional heat flux. The wave driving (or Eliassen–Palm flux divergence per unit mass) is predominantly westward, with magnitudes on the order of 5 m s−1 day−1. A steady-state quasigeostrophic model of the mean meridionaI circulation is used to estimate the mean wind response. The January 1994 event induces weak equatorward flow (less than 1 m s−1), together with westward winds on the order of 20 m s−1.
Abstract
This paper reports a calculation of the acceleration of the zonal mean flow induced by dissipating tides in the equatorial lower thermosphere. Estimates of the gravest symmetric gravitational Hough mode (1,1) of the migrating diurnal tide are obtained from monthly composites of global winds observed by the Upper Atmosphere Research Satellite High Resolution Doppler Imager. Using the principles of classical tidal theory, the tidal momentum flux divergence is computed for a series of monthly mean (1,1) fields from January 1992 to May 1993. The contribution to the mean flow by the leading mode of the migrating tide ranges between −5 and −20 (easterly) m s−1 day−1 in the equatorial lower thermosphere. A semiannual variation is noted in the tidal amplitudes and the inferred tidal accelerations. These variations are consistent with observed trends in the zonal mean flow of the lower thermosphere.
Abstract
This paper reports a calculation of the acceleration of the zonal mean flow induced by dissipating tides in the equatorial lower thermosphere. Estimates of the gravest symmetric gravitational Hough mode (1,1) of the migrating diurnal tide are obtained from monthly composites of global winds observed by the Upper Atmosphere Research Satellite High Resolution Doppler Imager. Using the principles of classical tidal theory, the tidal momentum flux divergence is computed for a series of monthly mean (1,1) fields from January 1992 to May 1993. The contribution to the mean flow by the leading mode of the migrating tide ranges between −5 and −20 (easterly) m s−1 day−1 in the equatorial lower thermosphere. A semiannual variation is noted in the tidal amplitudes and the inferred tidal accelerations. These variations are consistent with observed trends in the zonal mean flow of the lower thermosphere.
Abstract
Observations of surface pressure and middle atmosphere temperatures and winds indicate that a substantial nonmigrating component is present in the diurnal tide. The nonmigrating tides, which propagate with a zonal phase speed that is different from the earth's rotation, are attributed to the diurnal heating of geographically fixed sources. In this study we utilize a classical tidal model to examine the propagation characteristics of diurnal tides. The global fields of tropospheric sensible, radiative, and latent heating used to drive the model are supplied from summer and winter diurnal climatologies of the NCAR Community Climate Model (CCM2). A novel aspect of this study is the focus on the relative importance of the nonmigrating components.
The classical model successfully reproduces many observed features of the low-latitude diurnal surface pressure tides. In the middle atmosphere, the simulated migrating (or sun-synchronous) tide shows qualitative agreement with November–March LIMS observations. Tropospheric solar heating is clearly the dominant driving force for the migrating tide, with secondary contributions from boundary-layer sensible heating and tropospheric latent heat release. The leading modes of the zonal mean tide are also driven chiefly by tropospheric solar heating. The higher-order modes of the zonal mean and eastward propagating tides may be attributed to the joint effects of tropospheric solar heating, sensible heating, and latent heat release. The LIMS and other data reveal features that cannot be explained or examined within the context of the classical model used in the present study. These include upward phase propagation, vertical attenuation, and temporal variations in the migrating diurnal tide.
Abstract
Observations of surface pressure and middle atmosphere temperatures and winds indicate that a substantial nonmigrating component is present in the diurnal tide. The nonmigrating tides, which propagate with a zonal phase speed that is different from the earth's rotation, are attributed to the diurnal heating of geographically fixed sources. In this study we utilize a classical tidal model to examine the propagation characteristics of diurnal tides. The global fields of tropospheric sensible, radiative, and latent heating used to drive the model are supplied from summer and winter diurnal climatologies of the NCAR Community Climate Model (CCM2). A novel aspect of this study is the focus on the relative importance of the nonmigrating components.
The classical model successfully reproduces many observed features of the low-latitude diurnal surface pressure tides. In the middle atmosphere, the simulated migrating (or sun-synchronous) tide shows qualitative agreement with November–March LIMS observations. Tropospheric solar heating is clearly the dominant driving force for the migrating tide, with secondary contributions from boundary-layer sensible heating and tropospheric latent heat release. The leading modes of the zonal mean tide are also driven chiefly by tropospheric solar heating. The higher-order modes of the zonal mean and eastward propagating tides may be attributed to the joint effects of tropospheric solar heating, sensible heating, and latent heat release. The LIMS and other data reveal features that cannot be explained or examined within the context of the classical model used in the present study. These include upward phase propagation, vertical attenuation, and temporal variations in the migrating diurnal tide.
Abstract
This paper presents analyses of nonmigrating diurnal tide signatures in High Resolution Doppler Imager mesospheric and lower-thermospheric winds and temperatures. A global comparison of both winds and temperature reveals equatorial features corresponding to nonmigrating tides. Structures interpreted as zonal mean and eastward nonmigrating diurnal tides display consistency between horizontal winds and temperatures. The second symmetric mode is prominent in the zonal mean and wavenumber 1. The gravest antisymmetric mode and the gravest symmetric or Kelvin mode are the main features in zonal wavenumbers 2 and 3. The amplitudes of the tides generally increase with altitude and maximize within 90–110 km.
Abstract
This paper presents analyses of nonmigrating diurnal tide signatures in High Resolution Doppler Imager mesospheric and lower-thermospheric winds and temperatures. A global comparison of both winds and temperature reveals equatorial features corresponding to nonmigrating tides. Structures interpreted as zonal mean and eastward nonmigrating diurnal tides display consistency between horizontal winds and temperatures. The second symmetric mode is prominent in the zonal mean and wavenumber 1. The gravest antisymmetric mode and the gravest symmetric or Kelvin mode are the main features in zonal wavenumbers 2 and 3. The amplitudes of the tides generally increase with altitude and maximize within 90–110 km.
Abstract
In this paper, the authors assess the suitability of the heating fields in the latest version of the NCAR Community Climate Model (CCM2) for modeling the thermal forcing of atmospheric tides. Accordingly, diurnal variations of the surface pressure, outgoing longwave radiation, cloudiness, and precipitation are examined in the CCM2. The fields of radiative, sensible, and latent beating are similarly analyzed. These results are subjectively compared with available data.
Equatorial diurnal surface pressure tides are fairly well simulated by CCM2. The model successfully reproduces the semidiurnal surface pressure tides; however, this may result in part from reflection of wave energy at the upper boundary. The CCM2 large-scale diurnal OLR is generally consistent with observations. The moist-convective scheme in the model is able to reproduce the diurnally varying cloudiness and precipitation patterns associated with land-sea contrasts; however, the amplitudes of CCM2 diurnal continental convective cloudiness are weaker than observations. The CCM2 boundary-layer sensible heating is consistent with a very limited set of observations, and with estimates obtained from simple models of diffusive heating. Although the CCM2 tropospheric solar radiative heating is similar in magnitude to previous estimates, there are substantial differences in the vertical structures. A definitive assessment of the validity of the CCM2 diurnal cycle is precluded by the lack of detailed observations and the limitations of our CCM2 sample. Nevertheless, the authors conclude that the global-scale components of the CCM2 diurnal heating are useful proxies for the true diurnal forcing of the tides.
Abstract
In this paper, the authors assess the suitability of the heating fields in the latest version of the NCAR Community Climate Model (CCM2) for modeling the thermal forcing of atmospheric tides. Accordingly, diurnal variations of the surface pressure, outgoing longwave radiation, cloudiness, and precipitation are examined in the CCM2. The fields of radiative, sensible, and latent beating are similarly analyzed. These results are subjectively compared with available data.
Equatorial diurnal surface pressure tides are fairly well simulated by CCM2. The model successfully reproduces the semidiurnal surface pressure tides; however, this may result in part from reflection of wave energy at the upper boundary. The CCM2 large-scale diurnal OLR is generally consistent with observations. The moist-convective scheme in the model is able to reproduce the diurnally varying cloudiness and precipitation patterns associated with land-sea contrasts; however, the amplitudes of CCM2 diurnal continental convective cloudiness are weaker than observations. The CCM2 boundary-layer sensible heating is consistent with a very limited set of observations, and with estimates obtained from simple models of diffusive heating. Although the CCM2 tropospheric solar radiative heating is similar in magnitude to previous estimates, there are substantial differences in the vertical structures. A definitive assessment of the validity of the CCM2 diurnal cycle is precluded by the lack of detailed observations and the limitations of our CCM2 sample. Nevertheless, the authors conclude that the global-scale components of the CCM2 diurnal heating are useful proxies for the true diurnal forcing of the tides.
