Vol.:(0123456789) 1 3 Meteorology and Atmospheric Physics https://doi.org/10.1007/s00703-019-00695-4 ORIGINAL PAPER Vertical structure and evolution of monsoon circulation as observed by 205‑MHz wind profler radar Ajil Kottayil 1  · Prince Xavier 2  · K. Satheesan 1,3  · K. Mohanakumar 1  · V. Rakesh 1 Received: 26 February 2019 / Accepted: 4 September 2019 © Springer-Verlag GmbH Austria, part of Springer Nature 2019 Abstract A wind profler radar at 205 MHz is operational since January 2017, at Cochin ( 10.04 ◦ N; 76.33 ◦ E), a region lying in the west coast of Southern Peninsular India, which also is the entry point of the Indian summer monsoon. Using the radar wind profles obtained during April to September, the detailed vertical structure of wind during the pre-monsoon and monsoon period was studied for the years 2017 and 2018. The gradual transition from pre-monsoon to monsoon season as manifested by the development of monsoon circulations in the lower and upper troposphere is well captured by the radar observations. Parameters which characterize the strength of monsoon circulations have been derived which are shown to be potential predictors for declaring the monsoon onset over Kerala in an objective manner. The monsoon circulation during the year 2018 was studied in detail in the backdrop of extreme heavy rainfall over Kerala. It is observed that there is an anomalous decrease in the core height, but with high core speed in the Low-level Jet stream (LLJ) during 2018 as compared to year 2017. Owing to this unique placement of LLJ, it can be concluded that intense orographic lifting could have played a role in causing heavy rainfall over Kerala in 2018. The transitions in LLJ prior to heavy rainfall over south-west coast are aptly captured by the radar observations which opens up the possibility of predicting heavy rainfall events through continuous monitoring of monsoon circulation using radar. 1 Introduction The Indian summer monsoon (ISM) is a much speculated annual phenomenon which is being ofcially forecasted using an ensemble of statistical and dynamical models. Though, dynamical models have been observed to out-per- form the conventionally used statistical methods and may soon replace the latter in real-time prediction of monsoon rainfall (DelSole and Shukla 2012), they are mostly reliable for short-range forecasts. Declaration of the monsoon onset over Kerala (MOK) by India Meteorological Department (IMD) ofcially marks the beginning of the summer mon- soons. MOK prediction is usually done a week in advance and is mostly accurate, though there have been instances when they have failed, like in the year 2002. It was always upheld that the onset was purely a synoptic process, but sub- sequent studies point to its teleconnection with other phe- nomena like El Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), Madden–Julian Oscillation (MJO), etc. (Kane 1998; Ashok et al. 2001; Gadgil et al. 2004; Xavier et al. 2014). Likewise, the declaration of MOK also assumes a great deal of importance and timely prediction of MOK is still a challenge. Several studies in the past have sought to study the pre- disposing factors behind the onset. Raju et al. (2005) using 52 years of reanalysis data show that the strength of circula- tion increase rapidly at the time of summer monsoon. They also observed the development of a zone of fux convergence of heat and moisture over the region of Bay of Bengal which intensifes through onset and post-onset. Joseph et al. (2006) recorded large-scale atmospheric changes around South Asian and Indian regions ahead of MOK. They also establish that large-scale convection and SST gradients pull up jets from very low latitudes to Kerala latitudes leading to MOK. Earlier works have attempted to defne MOK through empirical or objective means (Wang et al. 2009; Pai and Responsible Editor: A.-P. Dimri. * Ajil Kottayil ajilsat@gmail.com 1 Advanced Centre for Atmospheric Radar Research, Cochin University of Science and Technology, Cochin, India 2 Met Ofce Hadley Centre, Exeter, UK 3 Department of Atmospheric Sciences, Cochin University of Science and Technology, Cochin, India