Diurnal variation of rainfall in the presence of large- and small-scale precipitating systems during different monsoon seasons over a complex terrain (Gadanki) region

Abstract

The diurnal cycle of rainfall by large-scale systems (LSS) and small-scale systems (SSS) has been studied over a complex terrain region (Gadanki) in southern peninsular India using eight years of data from a network of 36 rain gauges. The diurnal cycle of accumulated rainfall by LSS and SSS shows peaks at 2200 and 1900 LT, respectively, during the southwest monsoon (SWM) and at 1900 and;1700 LT during the northeast monsoon (NEM). Irrespective of the season and system size, the diurnal mode is the dominant mode of variation; it explains;60% of variance during the SWM and;54% during theNEMin LSS presence and explains;43% of variance during theSWMand;36% during theNEMin SSS presence. The correlation structure of rainfall is anisotropic with an axis ratio of;1.5 for LSS and;1.4 for SSS. Propagating systems are prevalent (80%-90% of times produce rain) in the presence of LSS during both seasons and play a dominant role in altering the diurnal cycle of rainfall over the Gadanki region. The conducive environment, like the presence of large relative humidity, updrafts in the lower and midtroposphere, and large lower and small midtropospheric shears, favors convective initiation and propagation of precipitating systems during LSS in SWM and NEM. The atmosphere favors convective initiation between 1800 and 2000 LT. The dry midtroposphere and weak upward motion in the midtroposphere inhibit mesoscale organization and form SSS during the SWM. During the NEM, a somewhat drier midtroposphere than in LSS and small wind shear in the lower troposphere ('‘L-shear’') inhibit the convective organization and form SSS between 1500 and 1800 LT.