Abstract
The background error covariance structure influences a variational data
assimilation system immensely. The simulation of a weather phenomenon like
monsoon depression can hence be influenced by the background correlation
information used in the analysis formulation. The Weather Research and
Forecasting Model Data assimilation (WRFDA) system includes an option for
formulating multivariate background correlations for its three-dimensional
variational (3DVar) system (cv6 option). The impact of using such a
formulation in the simulation of three monsoon depressions over India is
investigated in this study. Analysis and forecast fields generated using this
option are compared with those obtained using the default formulation for
regional background error correlations (cv5) in WRFDA and with a base run
without any assimilation. The model rainfall forecasts are compared with
rainfall observations from the Tropical Rainfall Measurement Mission (TRMM) and
the other model forecast fields are compared with a high-resolution analysis
as well as with European Centre for Medium-Range Weather Forecasts (ECMWF)
ERA-Interim reanalysis. The results of the study indicate that inclusion of
additional correlation information in background error statistics has a
moderate impact on the vertical profiles of relative humidity, moisture
convergence, horizontal divergence and the temperature structure at the
depression centre at the analysis time of the cv5/cv6 sensitivity
experiments. Moderate improvements are seen in two of the three depressions
investigated in this study. An improved thermodynamic and moisture structure
at the initial time is expected to provide for improved rainfall simulation.
The results of the study indicate that the skill scores of accumulated
rainfall are somewhat better for the cv6 option as compared to the cv5 option for at
least two of the three depression cases studied, especially at the higher
threshold levels. Considering the importance of utilising improved
flow-dependent correlation structures for efficient data assimilation, the
need for more studies on the impact of background error covariances is
obvious.
Citation
ID:
252253
Ref Key:
dhanya2016annalesimpact