Predictability and Its Relationship to Scale Interaction Processes in Blocking

Abstract

This paper addresses that aspect of predictability which appears related to scale interaction processes in blocking. The basic approach consists of synoptic analysis and quasi-geostrophic diagnosis of the dynamical processes in the real versus forecast model atmosphere. Time?longitude plots of a blocking index, together with sequences of 500 mb height charts, are used to show the temporal and spatial relationships of circulation systems of three distinct wavebands; planetary, medium and short scale, defined as total wavenumbers 0?6, 7?12, and 13?30, respectively. The relevant quasi-geostrophic concepts and equations are formulated to permit explicit evaluation of the relative importance of barotropic versus baroclinic mechanisms and the contributions to each of scale interactive and non- (or self-) interactive processes. The case discussed here is the Atlantic/European blocking of January 1987, particularly the initial stage of development (3?12 January), and was drawn from the recent National Meteorological Center (NMC) experiment in Dynamical Extended Range Forecasting (DERF). The observed blocking pattern at 500 mb largely reflected superposition of medium-scale waves upon a planetary-scale background conducive to blocking. Overall, short-wave features modified the amplitude of the block somewhat, but were critical for its initial appearance in the full 500 mb height field. Barotropic processes, i.e., the horizontal advection of vorticity, were dominant in direct forcing of height tendencies at this level. Baroclinic mechanisms, i.e., thermal advection, differential vorticity advection, and latent heat release, were important indirectly through modifying the barotropic effects. Scale interactions were more important in the development of the block than noninteractive processes involving circulations of a given waveband alone. Forecasts generated during the DERF experiment with the R40 version of NMC's Medium Range forecast Model (MRF) failed totally to capture the initial development of the block beyond 3 days in advance. Diagnostic evaluation showed this reflected a feedback loop wherein errors on the subplanetary scales induced errors on the planetary circulation which then magnified subplanetary scale errors, etc. In experiments with the higher resolution T80 version of the model a blocking anticyclone was predicted even at 10 days and, concomitantly, the amplitude of this error feedback mechanism was markedly reduced.