Controlling instabilities along a 3DVar analysis cycle by assimilating in the unstable subspace: a comparison with the EnKF

TL;DR

This paper introduces a hybrid data assimilation scheme combining 3DVar with Assimilation in the Unstable Subspace (AUS) and compares its performance to 3DVar and EnKF in a quasi-geostrophic model, showing promising results especially during forecasting.

Contribution

The study presents a novel hybrid 3DVar-AUS scheme that effectively assimilates observations in the unstable subspace, outperforming EnKF during forecast stages in an idealized model.

Findings

3DVar-AUS outperforms 3DVar alone in the model.

3DVar-AUS performs better than EnKF during forecast.

The hybrid scheme is easy to implement in idealized conditions.

Abstract

A hybrid scheme obtained by combining 3DVar with the Assimilation in the Unstable Subspace (3DVar-AUS) is tested in a QG model, under perfect model conditions, with a fixed observational network, with and without observational noise. The AUS scheme, originally formulated to assimilate adaptive observations, is used here to assimilate the fixed observations that are found in the region of local maxima of BDAS vectors (Bred vectors subject to assimilation), while the remaining observations are assimilated by 3DVar. The performance of the hybrid scheme is compared with that of 3DVar and of an EnKF. The improvement gained by 3DVar-AUS and the EnKF with respect to 3DVar alone is similar in the present model and observational configuration, while 3DVar-AUS outperforms the EnKF during the forecast stage. The 3DVar-AUS algorithm is easy to implement and the results obtained in the idealized conditions of this study encourage further investigation toward an implementation in more realistic contexts.