DART/CAM Inertio-Gravity wave exploration
This project aims at quantifying the impact of various motion types in
analysis and forecast fields by using normal modes. The DART/CAM is the
main analysis system used in the project.
The first question addressed is about how large part of the atmospheric
energy is associated with the inertio-gravity motions, an important part
of the global circulation primarily because of their role in the tropical
system.
The methodology is based on the three-dimensional normal-mode function
(NMF) expansion. The representation of field in terms of NMFs enables both
quantification of energy in each mode (defined by the zonal wave number k,
meridional mode n and vertical eigenstructure m) and the representation of
motions in terms of the balanced (ROT) and the inertio-gravity (IG)
contributions.
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Figure 1 presents the energy distribution among various motion types in
DART/CAM analyses for July 2007. It shows that about 88% of the wave
energy is in the balanced (ROT) motion and about 12% in the eastward- and
westward-propagating (EIG and WIG, respectively) IG motions. About 1% of
the wave energy in this system in July 2007 was associated with the mixed
Rossby-gravity (MRG) waves and about 2.7% with the Kelvin wave (KW)
motion, which is the most energetic among all IG waves.
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The time evolution of the Kelvin wave at two model levels in the DART/CAM
anayses between 6 and 31 July 2007 can be observed below.

This result shows
that the vertical structure of the "real" Kelvin wave agrees with the
representation of this wave employed in the simple models for the tropics,
with the wind and geopotential perturbations having opposite signs in the
lower and the upper troposphere.
The normal-mode expansion is also employed to study the dynamics of the
short-range forecast errors, an important issue for the
data assimilation modeling. Questions asked are about the characteristic
structures of the forecast errors in time and space. By comparing the
error structures in the NMF space a relative contributions of the balanced
and unbalanced modes to the error fields can be quantified. Another
application of the NMFs is the diagnosis of the systematic tendencies of
the analysis systems to place the observations into specific motions,
usually identified as the analysis system bias.
Ongoing publications:
- Zagar, N., J. Tribbia, J. L. Anderson and K. Raeder:
Diagnosis of model biases by using DART.
Poster presentation on the 13th Annual CCSM Workshop,
Breckenridge, 17-19 June 2008.
- Zagar, N., J. Tribbia, J. Anderson and K. Raeder:
Quantification of inertio-gravity energy in atmospheric analyses by using normal modes.
Part I: Intercomparison of four analysis systems.
Submitted to Mon. Wea. Rev.
- Zagar, N., J. Tribbia, J. Anderson and K. Raeder:
Quantification of inertio-gravity energy in atmospheric analyses by using normal modes.
Part II: Large-scale equatorial waves.
Submitted to Mon. Wea. Rev.