Investigating origin of the inadequate medium range predictability of the lower tropospheric ultra-long waves in tropics |
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Authors: | S De |
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Institution: | 1.Indian Institute of Tropical Meteorology,Pashan, Pune,India |
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Abstract: | The predictability of planetary or ultra-long scale waves is limited by the large growth of errors in these scales in almost
all the medium range forecast models. Understanding the cause for the enormous build up of error is, therefore, a necessary
task for improving the prediction of planetary waves. A diagnostic analysis of the systematic error energetics has been performed
in the Global Forecast System model to investigate the reasons for poor predictability of the lower tropospheric ultra-long
waves (wavenumber bands 1–4) in tropics using the analysis-forecast system of horizontal wind field at 850 hPa level during
the boreal summer period. For this purpose, systematic error energy is computed in spatial as well as in wavenumber domain.
Non-linear inter-scale transfer of error has been formulated and evaluated through energy exchanges among participating triads.
The study reveals that the error is generated in the prognostic model initially with a small magnitude at the different locations
around tropical convergence zone (TCZ) attributed to the inaccuracy in representing different physical processes like cumulus
convection applied in the model. At subsequent evolution of forecasts, error increases and spreads along the TCZ due to its
non-linear advection to the higher scales and eventually to the ultra-long scales attributed to the inherent dynamics of the
model evaluated through the process of wave-wave exchange of error energy in terms of the triad interactions. The continuous
generation and then, non-linear propagation of error up to the planetary scales in the course of prediction increase the uncertainty
in ultra-long scales which actually inhibit to predict accurately the planetary scale waves in tropics during medium range
forecasts. This work suggests caveats to the modeler’s community in the predictability study of tropical ultra-long waves. |
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