**A Mission to Define Gravity Wave Sources, Propagation to Higher
Altitudes, and Influences in the MLTI on Earth (Poster 2B)**

**Dave Fritts**

**Colorado Research Associates/NWRA, Boulder, CO**

**Motivation:**

Small-scale gravity waves (GWs) play key roles in the dynamics of the
Earth\'s atmosphere extending well into the thermosphere. The dominant
sources of GWs on Earth are in the lower atmosphere and include
convection, topography, and jet streams (and related frontal systems and
wind shears). Dominant MLT effects include 1) transports of energy and
momentum, 2) turbulence and mixing, 3) mean-flow accelerations, 3)
mesospheric jet closure, and 4) a GW-driven residual circulation that
couples the summer and winter hemispheres, cools the summer mesopause to
below 130 K, warms the winter mesopase, and reverses the meridional
gradient of mean temperature at this altitude. Similar effects likely
also occur at higher altitudes (TI), including additional mean forcing
and triggering of plasma instabilities, though these are less certain at
this time. Understanding, accurate modeling, and parameterization of
these dynamics are essential to advance our predictive capabilities for
the Earth\'s lower and middle atmosphere in applications as diverse as
numerical weather prediction, climate change, orbital decay
calculations, space shuttle re-entry, and UAV flight risks.

Similar, and likely much larger, GW effects also occur in the upper
atmosphere of Mars. Indeed, fractional GW density perturbations at
aero-braking altitudes (\~ 100 km) are apparently much larger than on
Earth (often \~ 50% or larger, compared to \~ 10% near the mesopause on
Earth). On Mars, however, the majority of the small-scale GWs likely
arise initially due to strong surface flow over very significant
topography. Thus, it will be necessary to understand the dynamics
accompanying GWs generated by topography to a far greater extent than
can be inferred from Mars aero-braking measurements in planning for
future and more extensive Mars exploration. Such understanding is far
easier to acquire through both global satellite and correlative
ground-based studies on Earth, essentially employing Earth\'s atmosphere
as a \"natural laboratory\" for studies also relevant to modeling of GWs
and their effects on Mars.

**NASA Programmatic Relevance:**

A GW mission that would define lower atmosphere GW sources, propagation
to greater altitudes, and MLTI influences in the Earth\'s atmosphere
would direct relevance to NASA\'s Earth Science and Space Science
research themes and the Exploration Vision.

Earth Science relevance is addressed because GWs have large impacts that
need to be parameterized in order to achieve viable predictions of mean
structure and variability on time scales ranging from numerical weather
prediction to climate dynamics.

Space Science relevance is addressed because GWs have a controlling
influence on the dynamics of the Earth\'s MLTI and the coupling of the
lower atmosphere to higher altitudes. GWs almost surely have an even
greater role on Mars, given the small-scale, but large amplitude,
density perturbations at aero-braking altitudes, the strong surface
winds, and the very large topography relative to Earth.

The NASA Exploration Vision is addressed because future Mars missions
must understand the variability of the Martian atmosphere and the risks
to aero-braking, aero-capture, descent, and landing posed by
large-amplitude GWs and the turbulence arising from them. These needs
can best be met with a comprehensive measurement and modeling program
addressing GWs excited by topography (among other sources) on Earth.

**Science Drivers and Requirements:**

GWs are responsible for the dominant fluxes of energy and momentum from
the lower atmosphere into the MLTI. They arise primarily in the lower
atmosphere due to deep convection, air flow over topography, and jet
streams, frontal systems, and their related wind shears. These GW
sources are widely distributed in latitude, with convection dominant at
low latitudes and jet streams and significant topography extending from
middle to high latitudes. These various sources excite GWs having very
different character and anisotropy and encountering very different
propagation conditions. Because GW propagation is influenced primarily
by large-scale winds, these would also need to be measured from GW
source altitudes to higher altitudes.

The dominant MLTI effects occur for GWs having relatively high
frequencies and small horizontal scales (due to their large energy and
momentum fluxes and rapid vertical propagation). GWs increase
dramatically in amplitude with increasing altitude because atmospheric
density decreases by orders of magnitude from the surface to the MLTI.
Because of their small spatial scales, these GWs cannot be measured
systematically or comprehensively by any current or planned NASA
mission. These GWs also have smaller energy densities (as opposed to
energy fluxes) than GWs at lower frequencies and larger horizontal
scales. Hence, their quantification requires methods that take advantage
of their spatial structure with special viewing geometries.

To quantify GW propagation and effects as fully as possible, and to
account for these effects in predictive models for Earth (and in
extensions to Mars), a GW mission would also require a significant
modeling and parameterization effort. Especially important to include in
this capacity are tidal and planetary wave influences, equatorial
oscillations, and the dynamics and effects of nonlinear interactions.