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Lorenzo Dell'Osso

Abstract

The development of mesoscale features in numerical weather forecasts and their evolution as a function of the resolution of the numerical model are studied for the case of Alpine cyclogenesis of 5 March 1982. The results suggest that the European Centre for Medium Range Weather Forecasts (ECMWF) model with a reduced grid of about 0.47° (37.0 km at latitude 45°) is capable of improving the forecast and in particular the detail in the surface wind and precipitation patterns. The inadequacy of a coarse mesh analysis for the representation of the mesoscale structure of the fields is discussed and the need for a fine-mesh analysis to initialize and verify fine mesh models is stressed.

The use of an “envelope” type orography to parameterize the effect of mountains on the atmospheric circulation is shown to improve the forecast of the ECMWF Model with a 1.875° grid. The height he, of the “envelope” orography is computed from the United States Navy data ad (resolution 0.167°), using the expression he = h + eσ, where h and σ are the mean and standard deviation of the height in a model grid square and e is a constant. Experimentation with this envelope orographic shows that e = 1 provides a good parameterization for subgrid-scale orography forcing at resolution 1.875°, but e should be decreased with increasing resolution. The separate effect, as well as the combined effect, of resolution and orography is examined.

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Shou-Jun Chen and Lorenzo Dell'osso

Abstract

The relative importance of latent beat release and surface sensible heat flux in a case of East Asian coastal cyclogenesis were investigated by performing numerical experiments with the ECMWF limited-area model. In the control experiment that included all physical processes, the cyclone developed rapidly in a way similar to that observed. In the experiment without latent beat feedback, only a shallow low appeared when the upper short-wave trough approached the inverted surface trough situated on the coast, but no further development took place. This suggests that the baroclinic forcing was enhanced by the feedback of physical process.

An increasingly unbalanced subtropical jet streak, an ageostrophic low-level jet and the associated vertical indirect circulation prior to the major development were well simulated in the Control but they were not simulated in the “dry” experiment (without latent heating). The latent heating had a profound impact on the amplifying jet streak circulation and the vertical coupling within the system which appeared to prime the rapid cyclogenesis along the coast.

The sensible beating contributed newly 18% to the surface development. It helped to build a potential temperature contrast along the coast below 900 mb. Without sensible heating, the model latent heat release was reduced. The results from the experiment without sensible and latent beating indicated that, the impact of sensible heating was partly through the moist process rather than direct beating.

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