Motivation:

        Small-scale gravity waves play a central role in forcing the large-scale
circulation and the thermal and constituent structures in the mesosphere and
lower thermosphere (MLT) due to their vertical fluxes of energy and momentum
and the induced mixing accompanying wave instability. This forcing is highly
dependent on gravity wave sources at lower altitudes and on their interactions
with mean and lower-frequency motions at larger scales. As a result, gravity
wave influences are highly variable spatially and temporally and poorly understood
at present. The goal of this TIMED IDS effort is to quantify these gravity wave
influences in the MLT of greatest relevance to TIMED energetics needs.

IDS Research Tasks:

        Studies of gravity wave influences will include three components:

        1) direct numerical simulations of wave instability processes and of wave-wave
        and wave-mean flow interactions to quantify gravity wave forcing with varying
        source and filtering conditions;

        2) simulation of the effects of gravity wave forcing of the large-scale circulation and
        thermal structure in a spherical geometry; and

        3) correlative use of TIMED and ground-based data to constrain mean and variable
        gravity wave forcing and to quantify gravity wave influences and energetics in global
        models of the MLT.
 

1. Direct Numerical Simulations

        Direct numerical simulations will use incompressible and anelastic
models to examine gravity wave interactions, instability processes, and
spectral evolutions in representative and measured mean and tidal wind and
thermal structures in the MLT. These simulations will be designed to assess the
mean and variable gravity wave forcing and the implications for the mean and
tidal structures and energetics of the MLT. Where possible, the simulations
will be designed to duplicate measurements of gravity waves in the mean and
tidal environment measured using comprehensive instrument suites expected to
be available as a part of the TIMED/CEDAR ground-based collaboration efforts.
Our current codes are pseudo-spectral and highly optimized on the Cray T3E and
SGI Origin 2000 supercomputer platforms and allow for a variety of wave forcing
conditions, environmental specifications, and boundary conditions.

         Previous simulations of gravity wave breaking (click here) have addressed wave
instability processes and turbulence dynamics in various environments, including
high-frequency motions with and without transverse shear and low-frequency
motions with and without high-frequency motions superposed. Previous studies of
Kelvin-Helmholtz (KH) instability and transition to turbulence (click here) have
examined the instability dynamics driving the turbulence transition, the vortex
dynamics underlying the turbulence cascade, and the implications of KH instability
and turbulence for mixing, turbulence statistics, and radar backscatter and energy
dissipation rate estimates. For a listing of all of our papers on wave breaking, shear
instability, and turbulence dynamics, click here.
 

2. Spherical Simulations of Gravity Wave Forcing

        Simulations of gravity wave effects in a spherical geometry will
address the mean meridional circulation accompanying gravity wave momentum
flux variations with altitude, local time, and season, as well as those imposed by
spatial and temporal variability of gravity wave sources at lower levels.
These studies will employ our spherical anelastic code and will describe
gravity wave forcing as a body force that varies spatially and temporally,
based on our direct propagation and interaction studies, in order to describe
the responses to such variable forcing in the most efficient manner. A primary
focus of this effort will be the implications of the induced meridional
circulation for MLT energetics and departures from radiative equilibrium thermal
structures for forcing and environments derived as fully as possible from the
TIMED and CEDAR ground-based measurements. To understand the implications
of the induced meridional circulation for TIMED energetics, it will be essential
to compare the modelled circulation with that inferred from combined satellite
and ground-based techniques, as neither alone can fully describe this circulation.

        As guidance for our spherical modeling studies, we have developed analytic
Boussinesq and compressible initial-value and forced descriptions of the inviscid,
linearized Navier-Stokes equations in a cartesian geometry. These have been used
to develop an understanding of dependence of the mean and gravity wave responses
to sources with various geometries and intervals of forcing. For further details of
these studies, click here.
 

3. Correlative Ground-Based Studies

        This IDS research effort will also include the PI's participation in
correlative TIMED/CEDAR ground-based wind and temperature measurements,
which will be used to constrain the statistical inputs of gravity wave energy and
momentum fluxes in calculations of MLT forcing. Particularly important in this
regard will be measurements indicative of spatial and temporal variability in
MLT forcing, correlations between gravity wave energy and momentum flux
variations and mean and tidal wind structures, and measurements enabling an
accurate determination of the wave-induced circulation and its variations in
space and time. Tidal structures will be inferred from both TIMED and
ground-based measurements in order to characterize both the global structure
and the short-time variability. Sensitivity to gravity wave forcing will rely
rely on ground-based measurements using comprehensive instrument suites and
distributed sensors of wave structures at MLT altitudes.

Specific IDS Deliverables:

1. Quantification of the effects of gravity wave propagation and tidal
interactions on both the vertical fluxes of energy and momentum by the wave
spectrum and on the tidal and mean wind fields. TIMED/CEDAR measurements and
model results will be used to construct maps of mean and variable wave forcing
for use in estimating the (anticipated large) influence of gravity waves on MLT
energetics in the TIMED core region.

2. Analyses and model comparisons of gravity wave propagation, filtering, and
tidal interactions accompanying comprehensive measurements of these processes
using data from those ground-based sites providing comprehensive measurement
capabilities.

3. Extrapolation of detailed analyses and modeling studies (item 2 above)
to global forcing based on use of measured global wind fields and gravity wave
statistics inferred from distributed ground-based wave observations.

4. Inference of the meridional circulation arising from gravity wave forcing
during the TIMED mission and estimation of the contributions of this circulation
to thermal influences and energetics in the MLT. Specific comparisons will be
made between the meridional circulation inferred from TIMED and correlative
ground-based measurements and from our spherical model of the residual
circulation arising from inferred global wave forcing in the MLT.