Diagnosing model performance in the tropics
´agar, Nedjeljka ; Blaauw, Marten ; Jesenko, Bla¸ ; Magnusson, Linus
Tropical circulation is an important component of weather and climate models. Tropical features of organised convection, such as the Madden–Julian Oscillation (MJO) and the El Niño–Southern Oscillation (ENSO), are believed to have a strong impact on extra-tropical circulation and are critical for extended-range predictions. Waves in tropical circulation can be excited by disturbances caused by organised convection, and these tropical waves can propagate far from their sources and spread the impact of the convection. Error properties in forecasts of tropical circulation are more difficult to establish than in the extra-tropics, where the geopotential height often provides sufficient information about the large-scale circulation. This is a consequence of a strong coupling between the geopotential height and winds in the extra-tropics, known as quasi-geostrophic balance. In the tropics, such balance is often weak and, therefore, the flow has to be decomposed into a balanced component, which is represented by the equatorial Rossby waves, and an unbalanced component that consists of inertio-gravity (IG) waves of many scales. Such a 'modal decomposition' can provide a deeper understanding of the circulation in the tropics, and of the sources of deficiencies in analyses and forecasts. The recently developed MODES tool provides such a decomposition into balanced (Rossby) and unbalanced (IG) modes. It has been applied to analyses and forecasts produced by ECMWF’s Integrated Forecasting System (IFS) to discuss the scale-dependent energy content of Rossby and IG waves in the IFS and to analyse model performance in the tropics. Key findings include that IG modes make up a significant part of tropical circulation and are associated with significant analysis uncertainties and forecast errors.
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