Naslov
Influence of the quasi-biennial oscillation on the ECMWF model short-range forecast errors in the tropical stratosphere

Autori
Žagar, Nedjeljka ; Andersson, Erik ; Fisher, Michael ; Untch, Agathe

Izvornik
ECMWF Technical Memorandum #534

Vrsta, podvrsta
Ostale vrste radova, ekspertiza

Godina
2007

Ključne riječi
deep convection; equatorial waves; inertio-gravity waves; Kelvin waves; mass-wind coupling; Rossby-gravity waves; stratosphere; 3D-Var assimilation

Sažetak
This paper addresses the impact of the quasi-biennial oscillation (QBO) on the background-errorcovariances 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 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 east- erly QBO phase, the percentage of error variance in Kelvin waves is significantly increased in comparison with 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 incre- ments, 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.

Izvorni jezik
Engleski

Znanstvena područja
Geologija

Napomena
Nedjeljka Žagar (rođ. Brzović) vodi se u evidenciji znanstvenika MZOŠ-a pod brojem 221165.

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