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TC Forecast

We develop a combined statistical and dynamical theoretical model to predict seasonal tropical cyclone (TC) activity over the western North Pacific (WNP). The model has three modules: TC genesis, TC track and TC intensity.

Theoretical Model System

We develop a combined statistical and dynamical theoretical model to predict seasonal tropical cyclone (TC) activity over the western North Pacific (WNP). The model has three modules: TC genesis, TC track and TC intensity.

TC genesis module predicts TC genesis frequency and TC genesis position. Two statistical regression models are constructed by the relationship between the observed TC genesis frequency and sea surface temperature (SST) / Genesis potential index (GPI). GPI quantified the influences of these large-scale environmental parameters on TC genesis, which includes the effects of 850 hPa absolute vorticity, 700 hPa relative humidity, vertical wind shear between 850 and 200 hPa, and the maximum potential index. The predicted large-scale environmental factors are input to statistical regression models to predict TC genesis frequency. TC genesis position is predicted based on the time-space distributed GPI density function, which also provides the initial condition in TC track module.

 TC track module is based on trajectory model by Wu and Wang (2004), which suggested that TC motion is controlled essentially by large-scale steering and Beta drift. We consider the effect of synoptic scale disturbance on TC track in the model. In TC track module, the initial integral point is TC genesis position predicted by TC genesis module, and then TC track is calculated based on the steering wind vector. The output track provides the integral path for TC intensity module.

TC intensity module is based on the Coupled Hurricane Intensity Prediction System (CHIPS) by Emanuel (2004). CHIPS is an axisymmetric coupled model, formulated in potential radius coordinates, that yields high resolution (of the order of 1 km) in the eyewall of the storm. The evolution of TC intensity is calculated based on enthalpy fluxes from the ocean to the atmosphere. The CHIPS model also uses a parameterization of the deleterious effect of environmental wind shear on TC, developed empirically so that the model’s real-time predictions match observed TC  intensity (Emanuel et al ., 2004; Emanuel et al ., 2008 ). TC track, SST and vertical wind shear are input to TC intensity module, and then TC intensity is computed by running intensity model along TC track.

 Seasonal Forecasting of Tropical cyclone Activity

Theoretical model system has participated prediction operation of China’s National Climate Center during the period 2011- 2015. Our predicted results have been applied in the form of consultation and special report. We further evaluate predicted results during the period 2011- 2014 and find a better predict ability for the seasonal TC activity over the WNP. Theoretical model system provides scientific theoretical basis to improve the long-term forecast of TC activity.