Influence of the Quasi‐Biennial Oscillation on the ECMWF model short‐range‐forecast errors in the tropical stratosphere

Abstract

AbstractThis paper addresses the impact of the Quasi‐Biennial Oscillation (QBO) on the background‐error covariances in the tropical atmosphere of the ECMWF model. The tropical short‐range‐forecast‐error covariances are represented in terms of equatorial waves coupled to convection.By comparing the forecast‐error proxy data from two different phases of the QBO, it is shown that the phase of the QBO has an effect on the distribution of tropical forecast‐error variances between various equatorial waves. The influence of the QBO is limited to the stratospheric levels between 50 hPa and 5 hPa. In the easterly QBO phase, the percentage of error variance in Kelvin waves is significantly greater than in the westerly phase. In the westerly phase, westward‐propagating inertio‐gravity waves become more important, at the expense of Kelvin modes, eastward‐propagating mixed Rossby‐gravity waves and inertio‐gravity modes. Comparison of datasets from two easterly phases shows that the maxima of stratospheric error variance in various equatorial modes follow the theory of the interaction of waves with descending shear zones of the horizontal wind.Single‐observation experiments illustrate an impact of the phase of the QBO on stratospheric analysis increments, which is mostly seen in the balanced geopotential field. Idealized 3D‐Var assimilation experiments suggest that background‐error statistics from the easterly QBO period are on average more useful for the multivariate variational assimilation, as a consequence of a stronger mass‐wind coupling due to increased impact of Kelvin waves in the easterly phase.By comparing the tropical forecast errors in two operational versions of the model a few years apart, it is shown here that recent model improvements, primarily in the model physics, have substantially reduced the errors in both wind and geopotential throughout the tropical atmosphere. In particular, increased wind‐field errors associated with the intertropical convergence zone have been removed. Consequently, the ability of the applied background‐error model to represent the error fields has improved. Copyright © 2007 Royal Meteorological Society