长江中下游地区旱涝急转的阈值诊断及危险性评估

选择与长江中下游地区夏季旱涝异常有关的15项大气环流指数作为自变量,以旱涝急转指数为因变量,采用非线性非参数的分类与回归树方法(CART)预测旱涝急转成灾的危险性等级。CART不仅能够提取出主要致灾因子,同时可以诊断不同影响阈值条件下,旱涝异常的类型及危险性等级。研究结果显示,夏季和春季北极涛动指数、春季亚洲径向环流指数,以及春季亚洲区极涡面积指数4项指标是旱涝突变成灾的主要影响因子,对旱涝急转成灾的危险程度具有很好的表征作用。当夏季北极涛动指数大于1.11时,更容易发生涝转旱事件(危险性等级为1级),而当夏季北极涛动指数小于或等于1.11,同时春季北极涛动指数大于或等于-1.11时,更容易发生旱转涝事件(危险性等级为6级),其他危险性等级的条件也可从模型中直接判读出来。采用2011~2013年的实测数据和模型预测结果进行对比,两者非常接近,验证了模型的可靠性。采用的CART方法为长江中下游地区旱涝急转致灾的等级预测提供了一种新的思路。

THRESHOLD DIAGNOSIS AND HAZARD DANGEROUSNESS EVALUATION FOR THE DISASTER OF DROUGHT-FLOOD ABRUPT ALTERNATION IN THE MIDDLE AND LOWER REACHES OF THE YANGTZE RIVER

Based on the related impact factors of the precipitation anomaly in summer in the Middle and Lower Reaches of the Yangtze River, 15 atmospheric circulation indicators as the independent variables and the hazard dangerousness level of drought-flood abrupt alternation index as the dependent variable were selected to construct the nonlinear and non-parametric classification and regression tree (CART) model for the hazard evaluation. The time series of data is from 1954 to 2010. Results showed that Arctic Oscillation Indices in summer (AOI_SU) and spring (AOI_SP), Asia Meridional Circulation Index in spring (AMCI_SP), and Asia Polar Vortex Area Index in spring (APVAI_SP) were the four main impact factors, which were proved to be suitable for the hazard evaluation of disaster about drought-flood abrupt alternation through the CART model. The type of the disaster (flood to drought or drought to flood) and the predicted dangerousness level under different diagnosis conditions can also be obtained from the model. The diagnosis thresholds and dangerousness level as follows: 1) When AOI_SU more than 1.11, the disaster of the quick turn from flood to drought is more apt to happen (dangerousness level is 1). 2) When the AOI_SU is less than 1.11 and the AOI_SP is more than -1.11, the disaster of the abrupt alternation from drought to flood will occur (dangerousness level is 6). 3) When the AOI_SU is less than 1.11, the AOI_SP less than -1.11, and the AMCI_SP more than 61, the same situation will happen as the one before this, and the predicted level is 5.5. 4) When the AOI_SU is less than 1.11, the AOI_SP less than -1.11, and the AMCI_SP less than 57, the abrupt alternation from flood to drought will happen, and the predicted level is 1.29. 5) When the AOI_SU less than 1.11, the AOI_SP less than -1.11, the AMCI_SP between 57 and 61, and the APVAI_SP less than 172, the abrupt alternation from flood to drought will happen, and the predicted level is 2.1. The corresponding indicators from 2011 to 2013 were selected to verify the final model through the comparison between the predicted values and actual levels, and the model was proved to be reliable for the close values. The CART proposed in this study provides a new method that can predict the hazard dangerousness level from the disaster of drought-flood abrupt alternation in the middle and lower reaches of the Yangtze River.