[1] The global-scale wave model (GSWM) is used to investigate mesospheric and lower thermospheric migrating and nonmigrating diurnal tidal components that propagate upward from the troposphere, where they are excited by latent heat release associated with deep tropical convection. Our diurnal tidal forcing parameterization is derived from a 7-year database of global cloud imagery. The GSWM migrating response is sufficiently large to modulate the dominant radiatively excited migrating diurnal tide in the middle and upper atmosphere during every month of the year. Five additional nonmigrating diurnal components, the eastward propagating zonal wave numbers 2 and 3, the westward propagating zonal wave number 2, and the standing oscillations, also introduce significant longitudinal variability of the diurnal tide in these regions. The comparative importance of the nonmigrating components evolves from month to month and varies with tidal field. Our GSWM investigation suggests that other dynamical models must account for the tropospheric latent heat source in order to make realistic predictions of the diurnal tide in the middle and upper atmosphere.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JD001236

Migrating and nonmigrating diurnal tides in the middle and upper atmosphere excited by tropospheric latent heat release

Migrating and nonmigrating semidiurnal tides in the upper atmosphere excited by tropospheric latent heat release: MIGRATING AND NONMIGRATING SEMIDIURNAL TIDES

[1] The seasonal and interannual variability of migrating (Sun-synchronous) and nonmigrating solar atmospheric tides at altitudes between 100 and 116 km are investigated using temperature measurements made with the SABER instrument on the TIMED spacecraft during 2002–2006. Quasi-biennial variations of order ±10–15% in migrating diurnal and semidiurnal tidal amplitudes are found, presumably due to modulation by the quasi-biennial oscillation (QBO) as the tides propagate from their troposphere and stratospheric sources to the lower thermosphere. A number of nonmigrating tidal components are found that have the potential to produce significant longitudinal variability of the total tidal fields. The most prominent of these, i.e., those that appear at amplitudes of order 5–10 K in a 5-year mean climatology, include the zonally symmetric (s = 0) diurnal tide (D0); the eastward propagating diurnal and semidiurnal tides with zonal wave numbers s = −2 (DE2 and SE2) and s = −3 (DE3 and SE3); and the following westward propagating waves: diurnal s = 2 (DW2); semidiurnal s = 1 (SW1), s = 3 (SW3), and s = 4 (SW4); and terdiurnal s = 5 (TW5). These waves can be plausibly accounted for by nonlinear interaction between migrating tidal components and stationary planetary waves with s = 1 or s = 2 or by longitudinal variations of tropospheric thermal forcing. Additional waves that occur during some years or undergo phase cancellation within construction of a 5-year climatology include DW5, SE1, SE4, SW6, TE1, TW1, and TW7. It is anticipated that the winds that accompany all of these waves in the 100–170 km region will impose longitudinal variability in the electric fields produced through the ionospheric dynamo mechanism, thereby modulating vertical motion of the equatorial ionosphere and the concomitant plasma densities. In addition to the wave-4 modulation of the equatorial ionosphere that has recently been discovered and replicated in modeling studies, the waves revealed here will generate wave-1 (SW1, SW3, D0, DW2), wave-2 (SW4, TW1), wave-3 (DE2, SE1), wave-4 (DE3, SE2, DW5, SW6, TE1, TW7), wave-5 (SE3), and wave-6 (SE4) components of this ionospheric variability, depending on year and time of year. However, the absolute and relative efficiencies with which these waves produce electric fields remains to be determined.

Tidal variability in the ionospheric dynamo region

[1] The SABER instrument on the TIMED satellite provides unprecedented geographical coverage for the determination and study of atmospheric tides. However, the slow local time precession rate of TIMED can cause longer-term temperature variations to alias into the tidal signals. A new method of analyzing satellite data for tides has been developed to circumvent this difficulty, but at the expense of temporal resolution of the tidal fields, i.e., 120-day mean tidal structures are obtained. In this work, we apply this method to SABER temperature data to derive a series of 120-day mean tidal structures, extending between 20 and 120 km altitude, 50°S–50°N latitude, and centered on each month from September 2003 to September 2004. In addition to the migrating (Sun-synchronous) diurnal and semidiurnal tides, a number of nonmigrating tides are revealed in the SABER measurements. Some of these waves are thought to originate via nonlinear coupling between the migrating tides and the stationary planetary wave with zonal wave number s = 1. Other nonmigrating tidal components appear to be forced by latent heating due to deep tropical convection. Of the latter, the eastward propagating diurnal tide with s = 3 is dominant and is as large as the migrating diurnal tide during some months. Of particular interest is the wave-4 structure with respect to longitude that characterizes both the diurnal and semidiurnal total tidal fields. This feature is a result of the predominant wave-4 topography/land-sea longitude dependence at the surface, which is reflected in the diurnal and semidiurnal components of the latent heating rates due to deep tropical convection. The ability of the global-scale wave model (GSWM) to approximate the observed tidal fields, including the wave-4 total tidal structures, is also assessed.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JA011504

Monthly tidal temperatures 20–120 km from TIMED/SABER

The transition between the middle atmosphere and the thermosphere is known as the MLT region (for mesosphere and lower thermosphere). This area has some char- acteristics that set it apart from other regions of the atmosphere. Most notably, it is the altitude region with the lowest overall temperature and has the unique characteristic that the temperature is much lower in summer than in winter. The summer-to-winter-tem- perature gradient is the result of adiabatic cooling and warming associated with a vigorous circulation driven primarily by gravity waves. Tides and planetary waves also contribute to the circulation and to the large dynamical variability in the MLT. The past decade has seen much progress in describing and understanding the dynamics of the MLT and the inter- actions of dynamics with chemistry and radiation. This review describes recent observa- tions and numerical modeling as they relate to understanding the dynamical processes that control the MLT and its variability. Results from the Whole Atmosphere Community Climate Model (WACCM), which is a comprehensive high-top general circulation model with interactive chemistry, are used to illustrate the dynamical processes. Selected observations from the Sounding the Atmosphere with Broadband Emission Radiometry (SABER) instrument are shown for comparison. WACCM simulations of MLT dynamics have some differences with observations. These differences and other questions and dis- crepancies described in recent papers point to a number of ongoing uncertainties about the MLT dynamical system.

/pdf/global-dynamics-of-the-mlt-27clum6d5c.pdf

K. Yamashita

Papers

Seasonal variation of non-migrating semidiurnal tide in the polar MLT region in a general circulation model

Variations of migrating and non-migrating tides simulated by the Middle Atmosphere Circulation Model at Kyushu University