C-band Doppler weather radar observations during the passage of tropical cyclone ‘Ockhi’

Abstract

Recently, a C-band polarimetric Doppler weather radar (DWR) was installed and commissioned at Thumba (8.5° N, 77° E), a west coastal station in the southern peninsula of India known as ‘Gateway of Indian Summer Monsoon’, for monitoring severe weather events and tropical cyclones. The DWR operates in the frequency range of 5.6 to 5.65 GHz with a peak transmitting power of 250 kW and employs state-of-the-art technology for both transmitter and receiver subsystems. Apart from providing regular base products such as reflectivity (Z_h), radial velocity (V) and spectral width (σ), the DWR being equipped with dual polarization capability provides additional products such as differential reflectivity (Z_dr), differential phase shift (ϕ_dp) and correlation coefficient (ρ_hv), which are used to study the microphysical properties of rain bearing clouds. The ‘Ockhi’ cyclone was the most recent one from 29th November to 2nd December 2017, which caused large devastation in terms of both economic and human life in Kerala coastal places. The C-band polarimetric DWR provides an opportunity to investigate the spatial and vertical structure of ‘Ockhi’ cyclone over the south-west coast of Kerala. The cyclone ‘Ockhi’ developed near the west of Srilanka and later advanced and stretched out near south-east of Kanyakumari through the Arabian Sea and impacted the coastal places of Kerala on 1st December 2017. During its formative stage, the eyewall was not clearly visible. The cyclone eye was clearly seen at the well-developed stage of cyclone, when it was in the vicinity of DWR range. The overall spatial extent of cyclone is ~ 1000 km as seen in INSAT-3D images. The maximum surface wind speed was observed to be around 30 m/s near to the Kerala coast. The outer fringes of the cyclone were located above the Trivandrum and caused heavy rainfall on 30 November 2017 around 17 h. The C-band DWR has captured the various stages of cyclone ‘Ockhi’ and its primary bands. Observations of C-band polarimetric DWR indicates that the vertical cross section of reflectivity along the cyclone eye showed that the size of cyclone is around 20 km and the precipitating cloud top heights reaching as high as 10–12 km. The cyclone’s outer rainbands are slanted outward and were more intense than the inner rainbands. The area surrounding the cyclone eye is the eyewall, characterized by very strong wind and torrential rainfall. The large differential reflectivity values ranging from 2 to 3 dB are found in the eyewall and rainbands regions. The outer rainband is characterized by large Z_dr values with high reflectivity above the melting layer. The different values of Z_dr are observed in the eyewall and outer rainband regions, which suggest that different microphysical characteristics prevail. As time progress, the cyclone system gradually intensified and the eyewall became more intense compared to the outer ones. The present study utilizes the C-band polarimetric DWR observations to characterize the spatial and vertical structure of cyclone ‘Ockhi’ and provides the ample information on the polarimetric signatures of cyclone structure to improve the better understating on the microphysical characteristics of tropical cyclones.