Impact of ocean mixed‐layer depth initialization on the simulation of tropical cyclones over the Bay of Bengal using the WRF‐ARW model

Viswanadhapalli Yesubabu,Hari Prasad Dasari,Naresh Krishna Vissa,Vijaya Bhaskara Rao Sarangam,Vijaya Kumari Kattamanchi

doi:10.1002/met.1862

Abstract

AbstractThe sensitivity of the simulated tropical cyclone (TC) intensity and tracks to the different ocean mixed‐layer depth (MLD) initializations is studied using coupled weather research and forecasting (WRF) and ocean mixed‐layer (OML) models. Four sets of numerical experiments are conducted for two TCs formed during the pre‐ and post‐monsoon. In the control run (CONTROL), the WRF model is initialized without coupling. In the second experiment, the WRF‐OML model is initialized by prescribing the MLD as a constant depth of 50 m (MLD‐CONST). In the third experiment, the spatial varying MLD obtained from the formulation of depth of the isothermal layer (MLD‐TEMP) is used. For the fourth experiment (MLD‐DENS), the model is initialized with the density‐based MLD obtained from ARMOR‐3D data. The results indicate that the CONTROL exhibits an early intensification phase with a faster translation movement, leading to early landfall and the production of large track deviations. The coupled OML simulations captured the deepening phase close to the observed estimates, resulting in the reduction of errors in both the vector and along the tracks of the storm. The initialization of the different estimates of the MLD in the WRF‐OML shows that the TC intensity and translation speed are sensitive to the initial representation of the MLD for the post‐monsoon storm. The gradual improvements in the intensity and translation speed of the storm with the realistic representation of the OML are mainly due to the storm‐induced cooling, which in turn alters the simulated enthalpy fluxes supplied to the TC, leading to the better representation of secondary circulation and the rapid intensification of the storm.

Highlights

Tropical cyclones (TCs) that form over warm tropical oceans are one of the most destructive weather phenomena, with a high potential to damage coastal installations due to its extreme winds, heavy rainfall, flooding and storm surges (Emanuel, 2003; Samala et al, 2013)
It is primarily due to the simulated ocean mixed-layer (OML) response, and the resultant TC-induced cooling strongly depends on both initial mixed-layer depth (MLD) as well the characteristics of the TCs passing over the basin
The track variations in the weather research and forecasting (WRF)-OML are due to variations in the simulated mixed layer, which induce the differences in the simulated sea surface temperature (SST) cold wakes in the right forward sectors and shift the model storm to move towards the warmer SSTs

Summary

Introduction

Tropical cyclones (TCs) that form over warm tropical oceans are one of the most destructive weather phenomena, with a high potential to damage coastal installations due to its extreme winds, heavy rainfall, flooding and storm surges (Emanuel, 2003; Samala et al, 2013). Gray, 1975; Emanuel, 2003; Yu and McPhaden, 2011; Vissa et al, 2013) postulate that the warm oceans play a critical role in the genesis, intensification and maintenance of TCs by supplying the required energy through enthalpy fluxes. Coupled global models advocate that, along with the SST, the OMLD plays a crucial role in the prediction of TC intensity The upper ocean responds during and after the passage of a TC; turbulent and vertical (diapycnal) mixing are the primary plausible mechanisms that cool the sea's surface. This negative feedback of wind-induced ocean cooling on the intensity of the storm has already been reported (e.g. Chang and Anthes, 1979; Schade and Emanuel, 1999), it has not been incorporated into storm-prediction models both regionally and globally until recently (Kim and Hong, 2010; Dare and McBride, 2011; Samala et al, 2013)

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