基于高分辨数值模式和资料同化的南海北部中尺度涡可预报性研究

The northern South China Sea (SCS) is one of the strongest active regions for mesoscale eddy processes. The mesoscle eddy has an important effect on the development of typhoon and the occurring of coastal weather system and the biogeochemical cycles, thus the quantitative study of the predictability of the mesoscale eddy in the northern SCS will provide the rationale for the realistic forecasting. In the past, the most studies have focused on the statistical characterization of the eddy number and their space distributions in the SCS, or qualitative discussed the occurring and maintenance mechanism of the eddy in some example analysis. However, there is still not a sufficiency attention to the predictability of the mesoscale eddy. Overseas, there are just minor researches related with the predictability of mesoscale eddy which located at the Loop Current in the Gulf of Mexico, and a very few study focus on the northern SCS. In this study, many hindcast experiments will be done. Based on the high-resolution ocean model and data assimilation system, this study will initialize each forecast by assimilating many difference kinds of observation data. As a measure of the forecast skill, we will quantitative evaluate the forecast error of the mesoscale eddy in the northern SCS, by many targets (eddy intensity, eddy radius and eddy trajectory). This study will investigate the variation of the forecast error along the forecasting time, and know the dependence on season. By many compared hindcast experiments, this study will investigate the effects of two factors: the accuracy of the forecasted atmosphere forcing and the tide process on the increasing and propagating of the forecast error. It will improve the understanding of the occurrence and maintenance mechanism of mesoscale eddies in the northern SCS.

南海北部是中尺度涡现象最活跃的区域之一。中尺度涡不仅对台风的发展，而且对沿海天气系统以及海洋生态均有重要影响，因此定量研究南海北部中尺度涡的可预报性可为实际预报提供理论基础。以往研究大多针对南海中尺度涡个数和空间分布进行统计，或在个例分析中定性讨论涡旋发生和维持机制，对于中尺度涡的可预报性问题关注不足；而少数国外的海洋中尺度涡可预报性研究也仅集中在墨西哥湾绕流海域，很少针对南海北部。本研究将基于高分辨率的海洋模式和资料同化系统，通过同化多种观测资料进行后报试验,从多个指标（涡的强度、半径以及移动路径）定量估计南海北部中尺度涡的预报误差随预报时间的变化，及其季节依赖性。通过设置多种对比后报试验，研究大气强迫场准确性、有无潮汐过程等因素对中尺度涡预报误差的增长和传播过程的影响，深化对中尺度涡生成和演化控制机理的理解.

中尺度涡旋是南海最活跃的海洋学现象之一，不仅对海洋内部物质和能量交换，而且对沿海天气和海洋生态都有重要影响。以往研究大多针对中尺度涡个数和空间分布特征等进行分析，或在个例中讨论涡旋发生和维持机制，而对于模式中尺度涡的可预报性问题关注不足。.本研究完成了南海高分辨率海洋环流模式的优化和模拟检验，可以较好模拟出中尺度涡旋和锋面等中尺度海洋学现象。发展了一个原理先进且可以在有潮汐强迫的海洋环流模式中同化沿轨高度计的海洋资料同化系统，其一个创新点在于通过采用动态更新的样本生成方式减小采样误差从而有效同化中尺度信息。基于大量系统参数优化试验和改进之后，1993-2011年南海高分辨率模式的沿轨高度计连续同化再分析结果被用于中尺度涡旋数量和空间尺度的可预报性研究。结果表明：同化沿轨SLA观测之后每年可分辨的中尺度涡旋数量达到80个，即中尺度涡旋预报数量增加了18.7%,其中气旋（反气旋）式涡旋年平均涡旋数量增加了18.8%（18.2%)。逐月总涡旋数量分布对比结果显示，不同月份能模拟涡旋数量接近观测的程度存在显著差异。总体而言，19年每月总涡旋数量是145个，同化观测之后涡旋数量达到126个（即增加了18.6%）。 最后，中尺度涡旋在冬季（12和1月）的轨迹分布进一步表明：反气旋式涡旋数量在12月和1月相对于没有同化结果低估之误差并没有改进；尤其在13N附近靠近菲律宾海域的气旋式中尺度涡旋数量被严重低估，当前高度计同化结果几乎没有预报模拟能力的提升。平均中尺度涡旋的水平直径约70km，模式本身对涡旋尺度有明显高估，同化后涡旋平均直径误差减小50％。上述成果是我国首个对中尺度涡旋数值同化试验长期模拟预报水平的定量评估分析，对于未来合理布局海洋业务化观测网，提升业务化海洋学预报水平具有积极的参考和指导。