Evolution of the vertical structure of the atmosphere over North-West India during winter season

Abstract

 This paper examines the structural changes of the atmosphere in relation to the passage of Western Disturbances (WD) over Delhi, during winter season using data for the period December 2001 to February 2002. Various thermodynamic and dynamic parameters such as Convective Available Potential Energy (CAPE), Convective Inhibition Energy (CINE), precipitable water content and vorticity over Delhi have been computed based on model analysis fields of India Meteorological Department. Although, a clear increase in the value of CAPE is noticed on most of the days of occurrence of rainfall over the station, this is not a necessary condition for the occurrence of rainfall nor is the amount of rainfall very well correlated to the value of CAPE. The vertical profile of the daily specific humidity reveals that each spell of disturbed weather is preceded and accompanied by deep positive moisture anomalies over the station. The equivalent potential temperature profile shows that this sudden increase in moisture associated with the passage of WD is due to advection of moisture over the station. This again is validated by the synoptic charts, which demonstrate the moisture feed in the boundary layer from the Arabian Sea. The rainfall amount is not very well correlated to the thermodynamic parameters considered. However, rainfall amount is found to have a stronger dependence on positive vorticity field of the lower troposphere. This reflects the stronger dynamic forcing of the rainfall activity over the station. The northward tilt of the ridge line in the lower troposphere in association with rainfall demonstrates that the rainfall activity over the station is primarily caused by the induced surface low in association with the upper level troughs of a WD.  Hence the trigger for the rainfall activity over the station and hence this region, during this season is primarily dynamic in nature and local convective structure of the atmosphere does not play a very determining role in the entire process.