Application of the Regional Air-sea Coupled Model WON to the Simulation of a Heavy Precipitation Event over the Low Latitude Highland in Southeast Asia

Abstract

To improve the simulation of precipitation and the accuracy of rainfall forecast over the low-latitude highland in Southeast Asia （LLHSA）， a regional air-sea coupled model is developed with the Weather Research and Forecast （WRF） Model （version 4.2） and ocean general circulation model NEMO （version 3.4）， using the coupler OASIS3-MCT.This new regional air-sea coupled model WRF-OASIS-NEMO is herein referred as WON.Both the atmospheric and oceanic components were configured into the same Arakawa-C grid with a horizontal spatial resolution of 0.25° and a coupling frequency of 1 hour， which are suitable for facilitating the mesoscale coupling between the atmosphere and ocean models.The evaluation of the WON model is based on the heavy precipitation event from August 16 to 18， 2020， where the simulation of WON model is compared with the standalone WRF model.The WON and WRF models simulated large precipitation over the northeastern LLHSA， the central and western LLHSA with daily precipitation around 20 mm·d-1， which is generally consistent with the observation.The WON model ameliorated the underestimation bias of precipitation over the southern LLHSA and the overestimation bias of precipitation over the northwestern LLHSA and the western LLHSA in the WRF model.The WON model improved the simulation of the dynamic conditions of precipitation， with enhanced cyclonic circulation over the central and southern LLHSA and enhanced anticyclonic circulation over the western LLHSA.Hence， the WON model ameliorated underestimation of precipitation over the southern LLHSA and overestimation of precipitation over the northwestern LLHSA.Both WRF and WON models could reproduce the development characteristics of vertical helicity， that is， positive vertical helicity in the lower-mid troposphere and negative vertical helicity in the upper troposphere.However， the simulated vertical helicity is too strong near 400 hPa layer over the western rain belt， but too weak at 600~700 hPa layer.Compared with the WRF model， the WON model shows improvements mainly in the central-western part of the rain belt.The water vapor sources of this heavy rainfall include the water vapor transport from the southwest of the Bay of Bengal and the water vapor transport from the South China Sea.Both WRF model and WON model reproduced the spatial characteristics of water vapor flux.In the WRF model， the water vapor convergence is too strong over the northern Bay of Bengal， but too weak over South China Sea.The improvement of WON model is mainly over South China Sea.The simulation improvement of the WON model is mainly because the surface heat flux exchange over the Bay of Bengal caused the mid-lower levels of troposphere to become cooler and drier.The atmospheric convection was weakened， associated with a low-level anticyclonic bias over the northern Bay of Bengal.This anticyclonic bias improved the simulation of atmospheric dynamics and water vapor conditions for this heavy precipitation event.

Keywords

• regional air-sea coupled model
• low latitude highland in southeast asia
• heavy rain
• vertical helicity
• atmospheric circulation