Initiation of Norʹwester in relation to mid-upper and low-level water vapor patterns on METEOSAT-5 images

Water vapor (WV) and infrared (IR) imageries from METEOSAT-5 are used to study the mid-upper and lower atmospheric water vapor patterns pertinent to the initiation of convective cells of Norʹwester, a severe thunderstorm during pre-monsoon season (March, April, and May) in northeastern part of Indian subcontinent. WV images are used to diagnose the mid-upper atmospheric dryness, while low-level water vapor patterns are monitored using IR images. Six important combinations of mid-upper and lower atmospheric moisture patterns are identified and their role in convective initiation for a variety of Norʹwesters (A, B, and D type Norʹwesters) is investigated. The detailed image analysis reveals that, in general, convective cells of Norʹwester have a strong tendency to be initiated along/near the edges of mid-upper atmospheric dryness passing over a low-level moist layer of atmosphere. Moisture patterns associated with dryline activity play a crucial role in convective initiation for A type Norʹwester while for B and D type Norʹwesters they are totally absent. The features of combinations of moisture patterns are discussed in the context of a case study. The conceptual considerations describe the possible physical processes behind the phenomena. WV and IR images provide indications of convective instability and mesoscale lifting processes. These observations, at a higher temporal and spatial resolutions, could be very useful for forecasting Norʹwesters.