SA53B-01
Tropical tides and thermal structure in MLT region observed by lidars and SABER at Arecibo, Puerto Rico and Maui, Hawaii
A complete range-resolved seasonal climatology of the nocturnal thermal structure in the MLT region (80-105 km) is characterized for the first time at a tropical site Arecibo, PR (18.35°N, 66.75°W) with groundbased nighttime observations from 2004 to 2006 using the Arecibo K Doppler lidar. Several unique features emerge, which are very different from the climatologies obtained by the University of Illinois Na wind and temperature lidar at a mid-latitude site SOR, NM (35°N, 106.5°W) and a sub-tropical site Maui, HI (20.7°N, 156.3°W). First, semiannual variations dominate the seasonal variations at Arecibo with maxima around the equinoxes and minima near the solstices, in contrast to the annual periodicity dominating the SOR seasonal variations. Second, the Arecibo mesopause altitude never drops below 90 km, unlike Maui and SOR that both show a low summer mesopause altitude around 86.5 km. Third, the Arecibo mesospheric inversion layers (MIL) are much weaker than those for SOR and Maui most of the year except summer, when the Arecibo MIL has similar amplitude to SOR and Maui. Fourth, the Arecibo lower thermosphere (100-105 km) exhibits summer cooling, in contrast to the mid- and high-latitude summer warming. Fifth, the Arecibo semidiurnal tides show a nearly 180 degree of phase shift from winter to equinox, which is not observed at SOR and Maui. With assistance from TIMED-SABER temperature data, we investigate to answer the following questions: (1) What effect does the tidal phase have on the observed MIL, and whether the MIL is caused by local gravity wave - tide interactions or by decay of global scale planetary waves? (2) How does the undersampling of the diurnal tides affect the observed mesospheric semi-annual oscillation (MSAO)? (3) Whether the summer cooling in the tropical lower thermosphere is caused by the MSAO-induced cooling/heating or is it an aliasing effect from the semiannual and annual variations of the diurnal tides? (4) Whether the observed temperature differences between Arecibo and Maui--two sites separated only by 2.4 degrees in latitude but by 90 degrees in longitude -- are caused by non-migrating tidal effects.
SA53B-02 INVITED
Simultaneous Wind and Temperature Observations of Tides in the Mesosphere and Lower Thermosphere (MLT) From Orbit
The TIMED satellite, launched in December 2001, has acquired a unique set of synoptic observations from space
of both neutral winds and neutral temperatures in the mesosphere and lower thermosphere (MLT) with the TIDI
and SABER instruments, respectively. Though the measurements are simultaneous, the fields of view of the two
instruments are sufficiently different that the limb airglow is sampled at different Local Solar Times at any
common Universal Time. Given measurements over a complete yaw cycle, it is possible to reconstruct an
‘average-day' weather map of temperature and neutral wind for the MLT throughout the mission. The orbit of
TIMED is repeatable on an annual basis providing a capability to study the differences in the MLT weather over a
5-year period. The maps show the dominance of the diurnal tide at low latitudes, peaking in strength at the
equinoxes, and the variation of the amplitude of the diurnal tide from one year to the next roughly following the
phase of the stratospheric Quasi-Biennial Oscillation (QBO).
Similar observations in the MLT were conducted during the UARS mission by the HRDI experiment. This
instrument had sufficient resolution to extract both neutral wind and temperature from individual limb airglow
spectra. The HRDI temperature data have not been widely used but show similar tidal and interannual patterns to
those derived from TIMED-SABER. Combining HRDI wind and temperature data into TIMED-like yaw cycles
permits the evolution of the MLT to be displayed for the period 1992 to present on a common basis.
This paper will compare the MLT neutral temperature/wind patterns obtained by the UARS and TIMED satellites. It
is important to document the basic state parameters of the MLT over the past 15 years to ensure that changes in
the upper atmosphere are considered in the context of available data sets. Only by combining observations over
several satellite missions is it possible to show that the MLT exhibits strong seasonal patterns that are mixed
with longer period oscillations.
http:tidi.engin.umich.edu
SA53B-03
A Mechanism for Seasonal and Interannual Variability of the Migrating Diurnal Tide
The amplitude of the migrating diurnal tide in the mesosphere and lower thermosphere varies seasonally with a semi-annual pattern that peaks during equinox, often with largest amplitude during March. The amplitude is also significantly correlated with the quasi-biennial oscillation (QBO) within the stratosphere. This talk will present mechanistic model simulations that suggest that these amplitude variations are controlled directly by the structure of the zonal mean background near the stratopause, where the semi-annual oscillation (SAO) is strongly modulated by the QBO. The model experiments employ background zonal-mean winds and temperatures from the UARS reference atmosphere that extend from surface to the thermosphere and represent the variability observed from 1992-1995 by UARS. Tidal heating is calculated from monthly NVAP water vapor and UARS ozone measurements for the same period. Model tide amplitudes calculated using only tropospheric heating have only minor variability due to seasonal changes in the heating and mean wind structures. However, the tide amplitude variations observed by HRDI are reproduced by the model when the ozone tidal heating is also included. It is found that the zonal mean background in the ozone heating source region significantly affects the tidal response to this heating. The modulation of the ozone tide by variations in the background can be explained in terms of the dependence of the Green's functions for the tidal equations on the static stability. The SAO- modulated ozone tide alternates between constructive and destructive interference with the tide response to troposphere heating, producing significant amplitude modulation of the combined response in the MLT.
SA53B-04
Solar Diurnal Tides in the MLT Horizontal Winds as Observed by WINDII/UARS and Simulated by CMAM
The global experimental observations of the tides were not possible until the advent of satellite measurements at altitudes where the tidal signal is sufficiently large in comparison to other sources of atmospheric variability. This analysis explores the solar diurnal tides in the MLT (mesosphere and lower thermosphere) region of 90-110 km by using the WINDII/UARS horizontal wind measurements taken during November 1991 through May 1997, and compares the observational results with the simulations of the extended Canadian Middle Atmosphere Model (CMAM). Both datasets show great similarities, but some significant differences are also found. In addition to the well-known westward propagating migrating tide with zonal wave number s=1 (W1), our analysis also revealed the most prominent non-migrating diurnal tidal components in the MLT region: the eastward propagating diurnal tide with the wave number s=3 (E3), the standing diurnal oscillation with s=0 (D0), and the westward propagating diurnal tide with s=2 (W2). The W1 mode has the largest annual mean meridional amplitudes of 60 m/s around 20N and 20S and at 90 and 95 km levels, respectively, in the WINDII data, but at 93 and 102 km in the CMAM. The W1 meridional amplitudes show significant semi-annual variation with magnitudes of 15 m/s and northward maxima occurred around early March and late August. The D0 and W2 modes are more or less similar to the W1 mode. They have maximum annual mean meridional amplitudes of 16-20 m/s between 95-100 km at 20N and 20S, and show little annual/semi-annual variations, except for the W2 meridional amplitudes between 90-105 km at 20N, where the annual variation of 10 m/s exists. The strongest E3 mode occurs primarily around the equator at the altitudes between 95-110 km, with maximum annual mean zonal amplitude of 10-22 m/s for both WINDII and CMAM data. The E3 zonal amplitudes are dominated by annual variation, which has the largest magnitude of 12 m/s at altitudes between 95 and 110 km around the equator in the WINDII data, but of 6 m/s at 100 km near 20N and 20S in the CMAM data. The maximum eastward winds are seen around July/August in both hemispheres in the WINDII data, but around December/January in the CMAM. The combination of E3, D0, and W2 with W1 gives rise to significant longitude variations in the diurnal tide between 40S and 40N latitude.
SA53B-05
The CAWSES Global Observing Campaign on Tides: Current Status
The CAWSES Global Tidal Campaign was initiated to encourage collaboration between satellite and ground based observations and to identify features in various observation types consistent with specific components. This project is one of several sponsored under Theme 3, Atmospheric Coupling Processes, of the international Climate and Weather of the Sun Earth System program (CAWSES, a SCOSTEP sponsored program). The overall goal of the campaign is to provide global data sets for several concentrated time periods over the next few years which includes coordinated ground-based and satellite measurements and modeling efforts. To unambiguously resolve the tidal components present in the Earth's atmosphere requires spatial and temporal sampling sufficient to resolve wavenumbers up to at least 5 and periods down to 4.8 hours every two to three days. Neither satellite or ground based observations on their own are capable of achieving these goals. Interpretation of tidal signatures in different observables (for example wind and temperature) is complicated by the fact the the associated latitudinal structures are typically different. A global network is required to allow these structures to be examined. Three campaign periods have been sceduled to date. The first tidal campaign took place from September 1 to October 31, 2005 and this year two campaigns, March 1 to April 31, and June 1 to August 15 are planned. The first of these latter campaigns will concentrate on the global tidal structures during equinox and their evolution and variability during this time period. The second of these campaigns will address the tidal structures during solstice conditions. Strong hemispheric asymmetries are know to develop in the structure of the migrating diurnal tide and it is of interest to determine the form of other components. These campaigns will allow the characterization of the heating sources, tidal components (migrating and nonmigrating), and tidal effects from the surface of the Earth to the ionosphere, and support and stimulate the use of models to simulate the conditions during these campaigns. Radar, microwave, optical, and ionospheric observations and satellite data are essential to the success of these campaigns and are now starting to be analysed. In this paper, we describe the organization of and motivation for this effort, plans for the incorporation of various observation types, and early results from the campaigns.
SA53B-06
Semidiurnal tides from the Extended Canadian Middle Atmosphere Model (CMAM) and comparisons with TIMED Doppler Interferometer (TIDI) and meteor radar observations
The Extended Canadian Middle Atmosphere Model (extended CMAM) is a general circulation model which extends from the surface to about 210 km. This high upper boundary allows dynamical processes to be studied from the ground to the lower thermosphere without the influence of sponge layers, which are often inserted in the mesosphere. The extended CMAM includes realistic tidal forcing due to radiative heating, convective adjustment and latent heat release and uses the gravity wave breaking parameterization of Hines. In this paper, spatial complex spectral analysis is applied to horizontal winds simulated by the extended CMAM to obtain semidiurnal tidal amplitudes and phases (from e5 to w5) in the mesosphere and lower thermosphere (MLT) region. The dominant w2 migrating component and the presence of nonmigrating tides (w3, e1, e2) in the mid-latitudes are identified. The migrating semidiurnal tide (w2) has amplitudes reaching 20 m/s for both zonal and meridional winds in mid-latitude region. The amplitudes of non-migrating semidiurnal tides are also non- negligible compared to the migrating semidiurnal tides, the amplitudes for w3 exceeds 12 m/s and e2 reaches 8 m/s. Comparisons are made with the TIMED Doppler Interferometer (TIDI) wind measurements, which are analyzed to obtain 6 nonmigrating tidal components (w4, w3, w1, s0, e1, e2) between 85 km and 105 km altitude and between 45oS and 45oN latitude. Overall, the modeled semidiurnal components agree very well with TIDI observations. The 11 semidiurnal components from the model are then superimposed to get the total semidiurnal winds which are compared to two equatorial MWR radar stations (Jakarta and Kototabang). The comparisons between CMAM and two radar stations show that the amplitudes and phases have generally good agreement for semidiurnal tide, with Jakarta station agreeing much better than Kototabang station.
SA53B-07
The Terdiurnal Tide Simulated by the Extended Canadian Middle Atmospheric Model (CMAM)
The diurnal (24-hour) tide, which is dominated in the subtropics, and the semidiurnal (12-hour) tide, which is larger at high latitudes, have been extensively studied from the observations and models. However, our understanding of the terdiurnal (8-hour) tide remains limited, partly because of it being the third harmonic in the wind decomposition. Horizontal winds simulated from the extended Canadian Middle Atmosphere Model (CMAM) are analyzed to delineate diurnal, semidiurnal and terdiurnal tidal structures and stationary planetary waves. Each frequency component is then subjected to Fast Fourier Transform (FFT) to perform the zonal wavenumber decomposition for s = -5 to s = 5. In this paper, the seasonal-latitudinal and height structures of these 11 terdiurnal tide components are now revealed. The migrating terdiurnal component is dominated over other components at middle latitudes with significant amplitudes (wind speed over 15 m/s) in the lower thermosphere (90 -110 km). The amplitudes vary strongly with season below 95 km and are maximum during winter; however above 95 km, the seasonal variation is not as obvious. Other components tend to peak at Polar Regions with amplitudes between 2 - 6 m/s. There is uncertainty about the origin of the terdiurnal tides. Solar heating, convective heating and latent heat release in the model at March and June for terdiurnal tidal components are utilized to aid in interpreting their behaviors and ascertaining their origins.
SA53B-08
Gravity wave propagation in a non-isothermal atmosphere with varying background wind
We present a gravity wave propagation equation for a compressible and non-isothermal atmosphere with a variable background wind profile. Impact of all the gradient terms on the vertical wavenumber depends only on the phase velocity and the state of the background atmosphere. For the background wind variation, any one of the linear first order derivative, second order derivatives, and the square of the first order derivative terms can be the dominate term under different conditions. For temperature variation, only the linear first order derivative is important for waves having a slow phase velocity. Our equation indicates that the effect of wind shear on the vertical wavenumber is opposite to that predicted by the Taylor-Goldstain equation, which assumes an incompressible fluid. We also discuss gravity wave ducting in the mesosphere using the new dispersion relation.