Is the Monsoon climatology observed over the National Capital Region, Delhi indicative of an Urban -Monsoon linkage on rainfall modification?

The Indian Summer Monsoon (ISM) plays a vital role in the lives of the National Capital Region (NCR) - Delhi, the second biggest urban settlement globally. This study attempt to assess the spatio-temporal characters and decadal changes of monsoonal features in detail. The climatology of Monsoonal rainfall and Monsoon Low Level Jet (MLLJ) are investigated. NCR Delhi exhibits a - 28.5% in the recent decade and - 19.7% in the 1997-06 decade reduction in rainfall is noticed compared with 1987-96 decade. The study reveals that the precipitation reduction is alongside the weakening of the MLLJ and marginal reduction of core height over the region. Also, the magnitude of surface wind declined with the directional shift of both MLLJ and surface winds notable in southerlies in the 2007-16 decade. Rainfall islands parallel to the urban heat islands (UHI) are seen likely as coupled monsoon-urban induced effects under the weakened synoptic regime evidenced through MLLJ. The Land Surface Temperature difference (UHI) of - 2.5 degrees C and more than 3.5 degrees C is observed from the urban centre to the surrounding cropland during the months of August (core monsoon) and September, respectively. In addition to the rainfall islands during August, islands of rainfall in both the upwind and downwind directions of the urban centre are noted in September. The rainfall patterns point out the signal of monsoon-rainfall modification due to NCR urbanization. Further, the influence of crosswinds with Aravali ridges and the presence of Himalayan foothills are also notable in the NCR rainfall patterns.