Mesoscale Signatures within the Tropics Generated by Physical Initialization

Abstract

This paper presents some recent results on physical initialization from the use of a very high resolution global model. Fundamentally this procedure improves the model-based initial rainfall, surface fluxes, and diagnostic cloud amount. Physical initialization is a useful procedure for the nowcasting of rainfall. Correlation between model-based initialized rain and satellite/rain gauge-based rain over the Tropics (for 6-h averages and averaged over transform grid squares) is of the order 0.85. This compares with a correlation of around 0.3 for models that do not include physical initialization. The day 1 tropical rainfall forecast skill is also relatively high for the physically initialized experiments; the correlation is of the order 0.55. It should be noted that the lifetime of mesoconvective systems is approximately 1 day, whereas more organized tropical disturbances may last substantially longer. A major portion of the tropical rainfall is associated with these short-lived systems, hence the skill beyond 1 day degrades somewhat. However, the model does seem to capture the 1-day passage of mesoconvective systems and their coupling to the large-scale, synoptic environment. The mesoconvective systems illustrated exhibit a robust vertical structure of divergence, heating, and vertical motion, which is absent without physical initialization. The organization of mesoconvective systems (advected by the large-scale circulations and coalescence of the mesoscale elements) appears to play an important role in the formation of tropical storms. The vorticity associated with these mesoscale elements, however, does not exhibit any interesting organization during the forecast as the storms form. The Florida State University atmospheric global circulation model at the resolution T213 discerns the tight central circulation features and the outer rainbands of Hurricane Andrew (1992), which appear similar to the radar imagery; however, the storm as seen from the model is not on the exact scale as that of the radar that is shown. Further enhancement of resolution is needed to model tropical storms on a more realistic scale, which is well known in the modeling community. Overall this study demonstrates that mesoconvective elements are in fact simulated by very high resolution global models. It appears that very high resolution models with an augmented analysis using satellite data may soon aid in resolving the formation issue associated with tropical cyclones and cyclogenesis.