Optimal grid resolution for the detection lead time of cyclogenesis in the North Indian ocean

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 five 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 atmospheric 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. Significant findings 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 suffice 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.