Journal of Atmospheric and Solar–Terrestrial Physics 204 (2020) 105289 Available online 22 April 2020 1364-6826/© 2020 Elsevier Ltd. All rights reserved. Research Paper Optimal grid resolution for the detection lead time of cyclogenesis in the North Indian ocean Jiya Albert, Prasad K. Bhaskaran * Department of Ocean Engineering and Naval Architecture, Indian Institute of Technology Kharagpur, Kharagpur, 721 302, India A R T I C L E INFO Keywords: Okubo-weiss parameter Bay of bengal Early detection Cyclogenesis WRF model ABSTRACT Early detection of disturbances in the atmospheric column, its spatio-temporal behavior and evolution prior to tropical cyclogenesis over warm ocean surface is a challenging problem. High-resolution atmospheric models help to better understand the atmospheric dynamical conditions and instability mechanism triggered by intense air-sea interaction leading to the formation of tropical cyclogenesis. Present study performed a series of model- based sensitivity experiments to evaluate tropical cyclogenesis lead detection time for fve cyclone cases with varying grid resolutions ranging between 27 km and 12 km in the north Indian Ocean. High-resolution Weather Research and Forecast (WRF) model examined the evolution of pre-cyclonic eddy vortices in the vertical at- mospheric column associated with Aila, Thane, Mora, Ockhi, and the recent Vayu cyclones. Detection technique was based on Okubo-Weiss (OW) parameter that is very sensitive to varying grid resolutions. Study examined the sensitivity of pre-cyclogenesis lead detection time to WRF model grid resolutions. Signifcant fndings from the study indicate a good skill in genesis prediction with approximately four days (~90 h) lead time in all cases irrespective of pre- and post-monsoon seasons and the domain size. First part of this study reveals characteristic difference in the eddy detection between pre- and post-monsoon cyclones and its lead detection time. The lead detection time using a grid resolution of 27 km resulted in a lag-time of nearly one day (~24 h) for pre-monsoon cyclogenesis as compared to the post-monsoon cases. Interestingly, the pre-cyclonic eddies are detected at higher atmospheric levels (450–650 hPa) for the pre-monsoon unlike post-monsoon developing cases in the lower tropospheric levels (800–950 hPa). Secondly, a 15 km grid resolution with OW technique would suffce early tropical cyclogenesis detection found optimum in context to lead detection time (~15 h) that better resolves the structure of pre-cyclonic eddies useful for advanced prediction. 1. Introduction Tropical Cyclogenesis (TCG) characteristics, its evolution and early detection are challenging problems in tropical meteorology investigated during the past fve decades or more. Though several studies were focused to understand TCG characteristics for the global ocean basins the skill level of prediction still remains questionable. The inherent physical processes that are quite complex involves both kinematics and thermodynamics interacting at different spatio-temporal scales through the boundary layer in the air-sea interface having a mutual feedback in the ocean-atmosphere system. The overall process is controlled by the atmospheric dynamical conditions at different vertical levels as well as the water mass characteristics extending below mixed layer depths and that play an important role in the overall fux exchange thereby modi- fying the characteristics of the atmospheric strata conducive for development of pre-cyclonic cyclogenesis. Precisely there is a lack of clarity and proper knowledge that primes the initiation and sustainment of mesoscale vortex circulation in TCG (Farfan and Zehnder, 1997). There has been consistent efforts to evaluate and better understand the background environmental physical processes and favorable conditions that enhances the atmospheric humidity levels in the mid-atmospheric levels that initiates the development of pre-cyclonic vortex in the mid- and lower atmospheric column (Yoo et al., 2015). The initiation stage of cyclogenesis involves the development of at- mospheric Mid-Level Vortices (MLVs) having horizontal scales ~ 100 km and rotating convective towers having horizontal scales ~ 10 km. The MLVs can persist in the atmosphere for duration of several days whereas the convective towers persist for few hours. Also it is exten- sively documented in literature that the maxima of mesoscale vorticity feld are the key element for TCG detection scheme (Sinclair, 1997; * Corresponding author. E-mail address: pkbhaskaran@naval.iitkgp.ernet.in (P.K. Bhaskaran). Contents lists available at ScienceDirect Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp https://doi.org/10.1016/j.jastp.2020.105289 Received 31 October 2019; Received in revised form 5 March 2020; Accepted 11 April 2020