BDA方案对台风背景高温天气预报的改进

Institute of Tropical and Marine Meteorology, CMA, Guangzhou 510080, China

中文关键词:  BDA方案，广州,高温，Grapes变分同化与模式，数值模拟

英文关键词:Bogus technique, BDA scheme, High temperature, Guangzhou, Grapes 3DVAR and Grapes Model, Numerical simulation

中文摘要:

      广州地区的高温天气主要是受副热带高压和台风外围下沉气流的影响所致。文中采用BDA（Bogus Data Assimilation）方法，探讨BDA方案对广州地区台风背景条件下高温预报的改进能力。选取2005年7月中旬广州地区出现的高温天气进行研究。这是比较典型的受副热带高压和台风（海棠）共同影响造成高温的天气过程。分析有无采用BDA方案的模式初始场，结果表明：采用BDA方案同化Bogus模型可以调整台风中心位置和强度，使所得到的初始场中心位置与观测更为接近，台风强度（气压梯度力、风速）比未用Bogus的情况强，与观测值更为接近。数值模拟的结果表明，采用了BDA方案的敏感试验可以更好地预报台风路径和台风中心强度变化，从而更好地预报高温天气，对高温区分布、日平均温度大小等的预报都有改进。文中对引起这种预报差异的原因进行了讨论，并探讨高温预报改进的可能机制。大气下沉运动的增强是高温预报改进的主要原因。敏感试验由于广州中低层大气的水汽减少，大气的下沉增强，致使天空的云量减少，对太阳短波辐射的阻挡减小，从而地面吸收热量增多，温度升高，输送给大气的感热增加，大气气温升高。采用BDA方案可以改进模式在台风“海棠”过程对广州高温的预报。

英文摘要:

      Summer high temperature in Guangzhou is mainly caused by the dominance of the subtropical high and the peripheral subsidence airflow of typhoon systems. This paper uses a BDA (Bogus Data Assimilation) scheme to investigate the ability of the BDA scheme to improve the summer high temperature forecasting during the typhoon period. A high temperature event in Guangzhou in middle July 2005 is selected as a typical case of summer high temperature caused by both the subtropical high and typhoon (Haitang). We found from comparative analyses of the initial fields of a sensitivity experiment with and a control experiment without the BDA scheme that the scheme is able to adjust the location and intensity of the typhoon c enter and to make the model initial field more consistent with observations. It is also found that once better forecasts of typhoon track and its intensity change are obtained, the BDA scheme is able to improve the forecasts of summer high temperature and its distribution and the daily mean temperature of Guangzhou. Further, the mechanism leading to this difference is discussed. The strengthening of subsidence is the main reason that contributes to the improvement of temperature forecast. In the sensitivity experiment, the amount of moisture in the mid to lower troposphere above Guangzhou is reduced, and the air subsidence is strengthened. Therefore, the cloud amount in the sky is reduced and more shortwave radiation reaches the ground surface, which results in the rising of surface temp erature. Consequently more sensible heat fluxes are transferred to the surface layer and the surface air temperature rises. It is concluded that using the BDA scheme is able to improve the summer high temperature forecast in Guangzhou during the period of typhoon Haitang.