寒潮过程和海洋锋面影响南海西北部大气波导演变的个例分析

文章利用2012年冬季南海西北部的航次探空资料, 研究了寒潮过程和海洋锋面对大气波导特征演变的共同影响。文中观测发现, 航次期间的大气波导以悬空波导为主, 平均波底高度约738.64m, 平均厚度约185.17m, 平均强度10.21M单位。观测前期, 天气形势稳定, 东北季风较弱, 在锋面暖水区一侧的悬空波导较为深厚, 且高度较低。其主要成因是大气边界层顶部925hPa至850hPa高度左右存在深厚的逆温层, 且具有显著的日变化特征。航次中期的寒潮过程导致东北季风大幅增强, 使得大气边界层顶部的逆温层被破坏, 从而导致悬空波导显著变薄变弱。而锋面冷水区一侧, 低水温抑制湍流发展导致大气修正折射率(M)的负梯度扰动较弱, 较难形成稳定且有一定强度的波导层, 且无显著日变化。但当东南暖湿气流覆盖锋面冷水区上空时, 容易形成较稳定的表面波导。

A case study of the influence of the cold surge and ocean front on the evolution of atmospheric ducts in the northwestern South China Sea

Using the GPS sonde data collected at the northwestern part of the South China Sea in the winter of 2012, we studied the influences of cold surge and ocean front on the characteristics of the atmospheric duct. In the investigation period, the main atmospheric ducts were elevated ducts, with an average bottom height of 738.64 m, an average thickness about 185.17 m, and an average strength of 10.21 M-unit. In the early stage of investigation, the weather was stable, and the northeast monsoon was weak in the study area, and the elevated ducts on the warm side of the front were relatively deep and strong but not very high. The main reason for the elevated duct layer is the temperature inversion layer on top of the atmospheric boundary layer from 925 hPa to 850 hPa, which also has significant diurnal characteristics. In the middle of the investigation, a cold surge significantly strengthened the northeast monsoon which destroyed the inversion layer at the top of the atmospheric boundary layer. As a result, the height of the elevated ducts rises significantly, and the duct layer becomes thinner and weaker. Meanwhile, the disturbance of the negative atmospheric modified refractive index gradient was weak due to the depressed turbulence over the cold side of the front. Thus, forming a stable and robust duct layer is difficult, and there is no significant diurnal variation. However, when the warm and humid air flows southwesterly covering the cold side of the front, it is highly possible to form a stable surface duct.