Extraction of wind and temperature information from hybrid 4D-Var assimilation of stratospheric ozone using NAVGEM
Authors: Allen Douglas R. , Hoppel Karl W. , Kuhl David D. 
Published: March 2018

Abstract. Extraction of wind and temperature information from stratospheric ozone
assimilation is examined within the context of the Navy Global Environmental
Model (NAVGEM) hybrid 4-D variational assimilation (4D-Var) data assimilation (DA) system. Ozone can improve
the wind and temperature through two different DA mechanisms: (1) through the
“flow-of-the-day” ensemble background error covariance that is blended
together with the static background error covariance and (2) via the ozone
continuity equation in the tangent linear model and adjoint used for
minimizing the cost function. All experiments assimilate actual conventional
data in order to maintain a similar realistic troposphere. In the
stratosphere, the experiments assimilate simulated ozone and/or radiance
observations in various combinations. The simulated observations are
constructed for a case study based on a 16-day cycling truth experiment (TE),
which is an analysis with no stratospheric observations. The impact of ozone
on the analysis is evaluated by comparing the experiments to the TE for the
last 6 days, allowing for a 10-day spin-up. Ozone assimilation benefits the
wind and temperature when data are of sufficient quality and frequency. For
example, assimilation of perfect (no applied error) global hourly ozone data
constrains the stratospheric wind and temperature to within
∼ 2 m s−1 and ∼ 1 K. This demonstrates that there is
dynamical information in the ozone distribution that can potentially be used
to improve the stratosphere. This is particularly important for the tropics,
where radiance observations have difficulty constraining wind due to
breakdown of geostrophic balance. Global ozone assimilation provides the
largest benefit when the hybrid blending coefficient is an intermediate value
(0.5 was used in this study), rather than 0.0 (no ensemble background error
covariance) or 1.0 (no static background error covariance), which is
consistent with other hybrid DA studies. When perfect global ozone is
assimilated in addition to radiance observations, wind and temperature error
decreases of up to ∼ 3 m s−1 and ∼ 1 K occur in the
tropical upper stratosphere. Assimilation of noisy global ozone (2 %
errors applied) results in error reductions of ∼ 1 m s−1 and
∼ 0.5 K in the tropics and slightly increased temperature errors in
the Northern Hemisphere polar region. Reduction of the ozone sampling
frequency also reduces the benefit of ozone throughout the stratosphere, with
noisy polar-orbiting data having only minor impacts on wind and temperature
when assimilated with radiances. An examination of ensemble
cross-correlations between ozone and other variables shows that a single
ozone observation behaves like a potential vorticity (PV) “charge”, or a
monopole of PV, with rotation about a vertical axis and vertically oriented
temperature dipole. Further understanding of this relationship may help in
designing observation systems that would optimize the impact of ozone on the
dynamics.