Spatial Propagation of Different Scale Errors in Meiyu Frontal Rainfall Systems

The spatial propagation of meso- and small-scale errors in a Meiyu frontal heavy rainfall event, which occurred in eastern China during 4-6 July 2003, is investigated by using the mesoscale numerical model MM5. In general, the spatial propagation of simulated errors depends on their horizontal scales. Small-scale (L < 100 km) initial error may spread rapidly as an isotropic circle through the sound wave. Then, many scattered convection-scale errors are triggered in moist convection zone that will spread abroathrough the isotropic, round-shaped sound wave further more. Corresponding to the evolution of the rainfall system, several new convection-scale errors may be generated continuously by moist convection within the propagated round-shaped errors. Through the above circular process, the small-scale error increases in amplitude and grows in scale rapidly. Mesoscale (100 km < L < 1000 km) initial error propagates up- and down-stream wavelike through the gravity wave, meanwhile migrating down-stream slowly along with the rainfall system by the mean flow. The up-stream propagation of the mesoscale error is very important to the error growth because it can accumulate error energy locally at a place where there is no moist convection and far upstream from the initial perturbation source. Although moist convection plays an important role in the rapid growth of errors, it has no impact on the propagation of meso- and small-scale errors. The diabatic heating could trigger, strengthen, and promote upscaling of small-scale errors successively, and provide "error source" to error growth and propagation. The rapid growth of simulated errors results from both intense moist convection and appropriate spatial propagation of the errors.