Limitations of wind extraction from 4D-Var assimilation of ozone D. R. Allen1, K. W. Hoppel1, G. E. Nedoluha1, D. D. Kuhl1, N. L. Baker2, L. Xu2, and T. E. Rosmond3 1Remote Sensing Division, Naval Research Laboratory, Washington, DC, USA 2Marine Meteorology Division, Naval Research Laboratory, Monterey, CA, USA 3Science Applications International Corporation, Forks, WA, USA
Abstract. Time-dependent variational data assimilation allows the possibility of
extracting wind information from observations of ozone or other trace gases.
Since trace gas observations are not available at sufficient resolution for
deriving feature-track winds, they must be combined with model background
information to produce an analysis. If done with time-dependent variational
assimilation, wind information may be extracted via the adjoint of the
linearized tracer continuity equation. This paper presents idealized
experiments that illustrate the mechanics of tracer–wind extraction and
demonstrate some of the limitations of this procedure. We first examine
tracer–wind extraction using a simple one-dimensional advection equation. The
analytic solution for a single trace gas observation is discussed along with
numerical solutions for multiple observations. The limitations of tracer–wind
extraction are then explored using highly idealized ozone experiments
performed with a development version of the Navy Global Environmental Model
(NAVGEM) in which globally distributed hourly stratospheric ozone profiles
are assimilated in a single 6 h update cycle in January 2009. Starting
with perfect background ozone conditions, but imperfect dynamical conditions,
ozone errors develop over the 6 h background window. Wind increments are
introduced in the analysis in order to reduce the differences between
background ozone and ozone observations. For "perfect" observations
(unbiased and no random error), this results in root-mean-square (RMS) vector
wind error reductions of up to ~4 m s−1 in the winter hemisphere
and tropics. Wind extraction is more difficult in the summer hemisphere due
to weak ozone gradients and smaller background wind errors. The limitations
of wind extraction are also explored for observations with imposed random
errors and for limited sampling patterns. As expected, the amount of wind
information extracted degrades as observation errors or data voids increase.
In the case of poorly specified observation error covariances, assimilation
of ozone data with imposed errors may result in increased RMS wind error,
since the assimilation is constrained too tightly to the noisy observations.
Citation: Allen, D. R., Hoppel, K. W., Nedoluha, G. E., Kuhl, D. D., Baker, N. L., Xu, L., and Rosmond, T. E.: Limitations of wind extraction from 4D-Var assimilation of ozone, Atmos. Chem. Phys., 13, 3501-3515, doi:10.5194/acp-13-3501-2013, 2013.