ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
Causes and Focus Points of Forecasting the Scattered Rainstorm of Typhoon Soudelor on Day 2 After Landing
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Abstract:
Conventional observation data, quantitative precipitation forecast (QPF) data of numerical weather prediction (NWP) models and NMC forecasters, and NCEP FNL analysis data are employed to verify the QPF and study the causes and forecast focus of the scattered rainstorm of Typhoon Soudelor on Day 2 after landing. On Day 2, rainstorm of Typhoon Soudelor was relatively scarred. QPF of NWP models all had obvious errors. Although 24 h QPF produced by NMC got many good corrections compared with numerical models, it still showed false heavy rainfall alarm or missed rainstorm in detail. Because of the forecasted location of rainstorm was more southerly than observation, there exited false alarm on the south side and missed rainstorm on the north side of the heavy rainfall. Impacted by environmental conditions and asymmetric structure of the typhoon, the favorable dynamical and water vapor conditions distributed in the north side of the typhoon and the eastern coastal areas. There were two strong convergence belts in lowlevel troposphere in northeast quadrant of typhoon. Between the two convergent belts, weak lowlevel divergence and downward motion were seen leading to much weaker rainfall in this area. Forecasters underestimated the asymmetry of typhoon structure and the inhomogeneity of wind field. So, for the weak rainfall area around the typhoon center and between two convergence belts, they obviously overestimated. Induced by topographic effect, heavy rainfall emerged continuously in Zhejiang coastal areas. And then the rainstrom evolved into a spiral rainband along tangential direction and extended northward. Forecasters omitted the tangential evolution and outward transportation of spiral rainband forced by terrain continuously, which resulted in the missing of rainstorm in the north of Zhejiang. The forecast key points of the Day 2 scattered rainstorm included typhoon’s asymmetric structure, nonhomogeneous distribution of wind, evolution of spiral rainband and terrain effect, etc. Largerscale heavy rainfall distributions were decided by the asymmetric structure of typhoon, while for refined locations of rainstorm, the evolution of spiral rainband influenced by nonhomogeneous lowlevel convergence and location of lowlevel jet was very important. Thus, terrain can not only amplify rainfall in local areas, but also have effects on downstream heavy rainfall.
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