The Development Of The GRAPES-ECOM-si Regional Coupled Atmosphere-ocean Model And Its Application On Tropical Cyclone Intensity Prediction

In the last two decades, in spite of a steady improvement in tropical cyclone (TC) trackforecasting, there still appears to be little skill in predicting TC intensity changes. As tonumerical models, an obvious disadvantage in the forecast of TC intensity is that theinteractions between tropical cyclones and ocean on TC intensity are still not clearlyunderstood. This paper focuses on the development of a regional coupledatmosphere-ocean model based on regional GRAPES typhoon model (GRAPES_TYM) and itsapplication on the intensity prediction of tropical cyclone. The motivation for this researchis to provide a referene for GRAPES typhoon prediction.Firstly the research focused on the test of an ideal TC developed in ideal environmentalfileds using GRAPES_TYM. The results indicated that the change of TC intensity weredifferent on an f-plane， a β-plane and with a uniform flow. The intensification anddevelopment of the TC on an f-plane without uniform flow were consistent with the existingresearch results. The magnitude of warm core structure and the associated subsidence werefound reasonable and the symmetric structure of the TC was kept good. Compared with thecases of no uniform flow, the TC in a uniform flow intensified more rapidly in the period ofrapidly intensification, while the intensity intensified slower in the latter period. On theother hand, a fairly symmetric TC structure was simulated on an f-plane. A TC on a β-planealso intensified slower than one on an f-plane. All of the results indicated that theGRAPS_TYM model could calculate the basic characterize of a TC development which wasconsistent with existing research efforts. The simulated results of different resolutions ofthe model were different. The experiment of the model with15km resolution was muchcloser to the existing results. Different cumulus parameterizations have different obviouseffects on the development of TC intensity.In order to better simulate the surface flow field and temperature, we improved thevertical Coordinate σ of ocean model to hybrid coordinate (Coordinate S), using CoordinateZ on the upper layer and Coordinate σ on the middle and lower layers Thus, the verticallayering of sea surface became denser, and the ocean model could better simulate the surface flow field and temperature. Experiments with typhoon Phoenix and Sinlaku showedthat the sea surface temperature (SST) distributions were much closer to the satellite datawith the new hybrid coordinate. The initialization scheme for ECOM-si applies theinterpolations of gradual high-precision SST, salinity, flow velocity and sea surface heightprovided by assimilated calculation products of HYCOM (Hybrid Coordinate Ocean Model)global model to ECOM-si model. Experiments with typhoon Muifa and Nanmadol showedthat the model could give quite accurate SST initialize distributions.A regional coupled atmosphere-ocean model based on GRAPES_TYM and ECOM-siestuary ocean model was developed. The coupling between the model components wasimplemented by using the coupler OASIS3.0. The ocean model obtains sea surface windstress, heat flux, and vapor flux computed in the typhoon model. The new SST calculated bythe ocean model is then used in the typhoon model. Numerical prediction experiments wererun with and without inclusion of the coupling for the case of typhoon Muifa in theNorthwest Pacific. The results indicated that the coupled model improved the intensityforecast significantly with the mean absolute error of the maximum wind speed within48hand72h reduced by32%and20%respectively. The coupled model improved intensityoverpredictions of typhoon Muifa predicted by GRAPES_TYM. In different developmentstages of Muifa, due to various oceanic mixing layer depths, the influences of the coupledmodel on the intensity were different. The coupled model had much obvious effe ct on thelater stage of Muifa, but there was no obvious effect on the earlier stage. The SST predictedby the coupled model decreased about5-6℃at most after the typhoon passing, which wasmuch closer to satellite data. According to the analysis on sea surface heat flux, wet staticenergy of boundary layer, atmospheric temperature and precipitation forecasted by thecoupled model and GRAPES_TYM, the calculation results of this coupled atmosphere-oceanmodel could reasonably reflect the main mechanisms of the interactions between tropicalcyclone and ocean. The coupled model has a good development potential in the aspect offorecasting typhoon intensity.To improve the higher SST simulated by GRAPES--ECOM-si in the East China Sea andTaiwan Srait, a new tide current-shelf circulation coupled way was introduced in GRAPES－ECOM-si. Numerical prediction experiments were run with the coupled model inclusion ofthe tide effect for the case of typhoon Muifa. The results indicated that the coupled modelwith tide effect improved overpredicted SST and the intensity forecast of Muifa.Numerical prediction experiments were conducted with the establishedGRAPES-ECOM-si coupled model for all the typhoons generated in Northwest Pacific in2012and the verifications were made between GRAPES_TYM and the coupled model. The resultsindicated that the coupled model improved the intensity forecast significantly with the mean absolute error of minimum sea surface pressure and maximum wind speed reduced by12.6%and10%respectively. The coupled model improved TC intensity overpredictionspredicted by GRAPES_TYM while the intensity underpredictions were not even bad. Theimprovement of the coupled model compared with GRAPES_TYM was different with theintensity of TC. On average, the coupled model improved the intensity forecasts for stormswith a minimum sea level pressure greater than930hpa. The most significant improvements,about27.6%, were achived for the TCs with the intensity in a range of930-940hpa. Theverifications of12landing typhoons show that the coupled model also has reference valuefor landing typhoon intensity prediction.