Role of vorticity advection and thermal advection in the development of western disturbance during North Indian winter

Abstract

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.