Western disturbance is defined as a low on the surface or an upper air cyclonic circulation in the westerly wind regime seen over northern parts of the Indian subcontinent. These systems originate over Mediterranean Sea, Caspian Sea or Black Sea and move eastward across north India. The precipitation over the northern parts of India during the winter months of January and February could be attributed to western disturbance. Ten western disturbances which resulted in fairly widespread, moderate to heavy rainfall over north India have been studied. The data used were obtained from NCEP and IMD. The systems were traced from West Asia to Western Himalayas. It is observed that Western disturbance causes 3–4 days of scattered light to moderate precipitation and one or 2 days of moderate to heavy precipitation. The specific humidity reaches a maximum value of greater than 10 g kg −1 at 1000 hPa between 0600 and 1200 UTC, a day prior or on the day of heavy rainfall. Positive vorticity advection takes place as a WD approaches the Indian region. The maximum value of vorticity advection reaches in the range of 1–2 × 10 −7 s −1 between 500 and 250 hPa. A strong differential vorticity advection is observed between lower and middle troposphere on the day of most intense and widespread precipitation. It is also observed that the updraught maximum coincides with the differential vorticity advection maximum. When the WD is active, warm and moist air advection takes place over north India, which increases with height, reaching its maximum values of 7–12 × 10 −4 K s −1 at levels between 300 and 200 hPa.

中文翻译：

---

北印度冬季涡度平流和热平流在西部扰动发展中的作用

西部扰动被定义为在印度次大陆北部看到的西风状态下的低表面或高空气旋环流。这些系统起源于地中海、里海或黑海，然后向东移动穿过印度北部。1 月和 2 月冬季期间印度北部地区的降水可归因于西部干扰。对导致印度北部相当广泛的中到强降雨的十次西部干扰进行了研究。使用的数据来自 NCEP 和 IMD。这些系统从西亚追溯到喜马拉雅西部。据观察，西部扰动导致3-4天的零星小到中降水和1到2天的中到强降水。在 UTC 时间 0600 和 1200 之间、大雨前一天或当天，在 1000 hPa 下，比湿度达到最大值，大于 10 g kg -1。当 WD 接近印度地区时会发生正涡度平流。涡度平流最大值在 500 至 250 hPa 之间达到 1-2 × 10 -7 s -1 范围内。在降水最强烈和最广泛的那天，在对流层中低层之间观察到强烈的差异涡度平流。还观察到上升气流最大值与微分涡度平流最大值一致。当 WD 活跃时，印度北部发生暖湿空气对流，随着高度增加，在 300 到 200 hPa 之间达到最大值 7–12 × 10 -4 K s -1。