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T639 Ensemble Forecast System

20-06-2014Source:China Meteorological News Press

T639 high-resolution numerical global-model (typhoon) 1-15-day ensemble forecast system solves the application of BGM (Breeding of Growing Mode) method in global spectrum model and adds physical process perturbation. Meanwhile, based on the features of BGM perturbation background field, typhoon ensemble forecast vortex initialization solution and flow are designed; and integrated operation of ensemble forecast and typhoon ensemble forecast is realized. This system consists of 15 forecast members, including 7 pairs of perturbation forecasts and a control forecast. To upgrade the typhoon and global ensemble forecast system to a system based on T639 assimilation and model is an effective method to solve the current problems and raise the level of typhoon and ensemble forecast.  

Process of integration with T639 meso-scale ensemble forecast/typhoon ensemble forecasting

 

Compared with the present T213 global ensemble forecast system, the establishment process of T639 global-model (typhoon) ensemble forecast system realizes improvements and breakthroughs in three aspects: firstly, the current advanced technological achievements for typhoon ensemble forecast are embedded in T639 ensemble forecast system. An integrated system is conducive to intensive development of the numerical service system; secondly, physical process perturbation technology is adopted. It is consistent with the technical strategy of major international service institutions and helps improve forecasting; thirdly, the post-processing module for processing period by period is enriched with new products and solves the problems of operation period and data storage caused by increase of resolution. In addition, the forecast period is also extended from 10 days to 15 days.

 

T639 ensemble forecast system acquires an assimilation background field from T639 deterministic forecast in real time and conducts perturbation calculation with the 6-hour forecast field generated from the previous forecast cycle of the system. If a typhoon is formed, then vortex repositioning will be conducted in the 6-hour forecast field of each member before perturbation, and the intensity will be adjusted after initial perturbation. After a model initial field is formed, the system enters the model integration module. 00,12utc is integrated 15 days. After every six hours' integration, the system will concurrently invoke the post-processing program and complete the processing of different forecast periods and product fabrication and distribution.

 

At present, the ensemble forecast system products of European Center of Medium-Range Weather Forecasts (ECMWF), National Center for Environment Prediction (NCEP) and Canadian Meteorological Centre (CMC) have been preliminarily applied in Chinese state-level and some provincial-level forecast platforms and are being promoted. The performance of Chinese T639 ensemble forecast system products in major weather processes becomes a main issue cared about by ensemble product users. T639 ensemble forecast effects on the processes of three typical weather cases are selected to evaluate the application performance of this ensemble forecast system. These processes include a process of typical heavy precipitation in August 2012, a process of typhoon precipitation in November 2013 and a process of typical heavy precipitation on May 17~19, 2014 in South China.

 

Case 1: A weather process of precipitation in the east of Sichuan Basin from August 31 to September 2, 2012. 

a, 500hPa height field and 850hPa wind field at 20:00 of August 31, 2012

b, daily precipitation at 8:00 of September 1, 2012

 

The following figure shows a zero field at 500hPa geopotential height and 20: 00, August 31, and 24h, 48h and 72h ensemble mean product. 

500hPa geopotential height (black), 24h (red), 48h (green) and 72h (blue)

 

In respect to precipitation level, the precipitation level of ensemble mean and that of deterministic forecast are both obviously smaller than the actual condition (Figure 3a). The probability-matched product is adjusted upward on the basis of ensemble mean (Figure 3b) and indicates the northeastern part of the basin has precipitation of above 100mm, playing a warning role to weather forecasters. 

Figure 3a

Figure 3b

 

Figure 4 shows probability forecast of heavy rain and rainstorm in three periods. The axis of the large value domain of 36h probability forecast is basically consistent with the actual condition, but the position significantly deviates to the north and the information of eastward extension is provided. 

Figure 4

 

Case 2: Typhoon "Haiyan" process inspection and analysis

 

In respect to large situation fields, such as: intensity of subtropical high positions and the positions of Xinjiang low-trough system, the overall performance of T639 ensemble forecast system in various periods is stable. 

500hPa geopotential height field at the initial time (black), integration average of 24h (red) and 48h (green) height fields and 24h Height field dispersion (shadow)

 

The forecast average maximum precipitation is above 100mm. The forecast on the position of the rain band of downpour is accurate. It slightly deviates to the north compared with the actual condition.

 

The probability forecasts in three periods all show a trend of eastward adjustment, whether 25mm or 50mm. The forecast of the model on typhoon movement speed is accelerated when the period is drawing near. 

 

24h precipitation of 84h (above)、60h (middle) and 36h (below) forecast >= 25mm (left)、50mm (right) probability forecast (color filling)

 

Case 3: A weather process of precipitation in South China on May 17~19, 2014.

 

The 24h and 48h forecasts of T639 ensemble forecast system are close to the zero field and satisfactorily provide the low-trough situations in the east of the northwestern region and the south of the lower reaches of the Yangtze River.

 

Compared with the result of deterministic forecast, the level of deterministic forecast in the south of the lower reaches of the Yangtze River is stronger, while the forecast of the ensemble product is close to the actual condition. This indicates the ensemble product plays a certain role in helping the forecaster correct precipitation level under the condition that the deterministic forecast effectively positions the rain band.

 

The 36h forecast position is approximate to the actual condition. The position and range of large value domain of probability where heavy rain appears (>70%) are basically consistent with the actual condition. In respect to the area where rainstorm appears in the north of South China, the forecast gives a probability of above 60% and the large value domain shows a trend of westward adjustment compared to the deterministic forecast.

 

24h precipitation of  84h (above)、60h(middle) and 36h (below) forecast >= 25mm(left)、50mm(right) probability forecast (color filling)

 

In comparison with EC ensemble forecast products: EC ensemble forecast system is more stable than T639 in terms of forecast on precipitation area, but T639 ensemble forecast system has certain advantage over EC in the aspect of forecast on precipitation level.

 

24h precipitation of 84h (above)、60h (middle) and 36h (below) forecast >= 25mm (left)、50mm (right) probability forecast  (color filling)

 

Editor Kong Yan and Hao Jing