Improving the condition number of estimated covariance matrices

TL;DR

This paper introduces and compares methods to improve the numerical stability of high-dimensional covariance matrices used in data assimilation, analyzing their theoretical properties and practical effects on variances, correlations, and assimilation accuracy.

Contribution

The paper develops new theoretical insights into two reconditioning methods—ridge regression and the minimum eigenvalue method—and compares them with variance inflation for covariance matrix stabilization.

Findings

Both methods increase variances for the same target condition number.

Ridge regression reduces off-diagonal correlations more effectively.

Reconditioning affects smaller eigenvalue scales more than variance inflation.

Abstract

High dimensional error covariance matrices and their inverses are used to weight the contribution of observation and background information in data assimilation procedures. As observation error covariance matrices are often obtained by sampling methods, estimates are often degenerate or ill-conditioned, making it impossible to invert an observation error covariance matrix without the use of techniques to reduce its condition number. In this paper we present new theory for two existing methods that can be used to 'recondition' any covariance matrix: ridge regression, and the minimum eigenvalue method. We compare these methods with multiplicative variance inflation. We investigate the impact of reconditioning on variances and correlations of a general covariance matrix in both a theoretical and practical setting. Improved theoretical understanding provides guidance to users regarding method selection, and choice of target condition number. The new theory shows that, for the same target condition number, both methods increase variances compared to the original matrix, with larger increases for ridge regression than the minimum eigenvalue method. We prove that the ridge regression method strictly decreases the absolute value of off-diagonal correlations. Theoretical comparison of the impact of reconditioning and multiplicative variance inflation on the data assimilation objective function shows that variance inflation alters information across all scales uniformly, whereas reconditioning has a larger effect on scales corresponding to smaller eigenvalues. The minimum eigenvalue method results in smaller overall changes to the correlation matrix than ridge regression, but can increase off-diagonal correlations. Data assimilation experiments reveal that reconditioning corrects spurious noise in the analysis but underestimates the true signal compared to multiplicative variance inflation.