Impact assessment of simulated Doppler wind lidars with a multivariate variational assimilation in the tropics

Žagar, N., A. Stoffelen, G.-J. Marseille, C. Accadia and P. Schluessel
in print in Monthly Weather Review
(pdf)

Abstract

This paper deals with the dynamical aspect of variational data assimilation in the tropics and the role of the background-error covariances in the observing system simulation experiments for the tropics. This is studied by using a model which describes the horizontal structure of the potential temperature and wind fields in regions of deep tropical convection.
The assimilation method is three- and four-dimensional variational assimilation. The background-error covariance model for the assimilation is a multivariate model which includes the mass-wind couplings representative of equatorial inertio-gravity modes, equatorial Kelvin and mixed Rossby-gravity modes, in addition to the balanced equatorial Rossby waves. Spectra of the background errors based on these waves are derived from the tropical forecast errors of the ECMWF model.
Tropical mass-wind (im)balances are illustrated by studying the potential impact of the space-borne Doppler wind lidar (DWL) ADM-Aeolus which measures horizontal line-of-sight (LOS) wind components. Several scenarios with two DWLs of ADM-Aeolus type are compared under different flow conditions and using different assumptions about the quality of the background-error covariances.
Results of three-dimensional variational assimilation (3D-Var) illustrate the inefficiency of multivariate assimilation in the tropics. The consequence for the assimilation of LOS winds is that the missing part of the wind vector can hardly be reconstructed from the mass-field observations and applied balances as in the case of the mid-latitudes.
Results of four-dimensional assimilation (4D-Var) show that for large-scale tropical conditions and using reliable background-error statistics, differences among various DWL scenarios are not large. As the background-error covariances becomes less reliable, horizontal scales become smaller and the flow becomes less zonal, importance of obtaining information about the wind vector increases.
The added value of another DWL satellite increases as the quality of the background-error covariances deteriorates and it can be more then twice larger then in the case of reliable covariances.

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