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Yuan Sun, Zhong Zhong, Hong Dong, Jian Shi, and Yijia Hu

Abstract

The Weather Research and Forecasting Model is employed to examine the sensitivity of simulated tropical cyclone (TC) motion and associated intensity of the western Pacific subtropical high (WSPH) to different heating and drying rates in the Betts–Miller–Janjić (BMJ) cumulus parameterization (CP) scheme. A case study of Tropical Cyclone Megi (2010) is performed. Results indicate that the simulated WPSH strengthens as the heating/drying effects of the BMJ decrease. A strong WPSH subsequently leads to changes in the large-scale steering flow in its southern edge and delays the northward turning of the simulated storm. The associated physical mechanism is revealed. As the heating/drying is overestimated in the BMJ, the model produces unrealistic drying below 500 hPa whereas the atmosphere becomes moist above 500 hPa. Drying in the lower troposphere hinders the activation of the microphysics while moistening in the upper troposphere facilitates the microphysics. As a result, the model generates extensive anvil clouds that extend far away from the TC center and reach the upper troposphere over the WPSH. This leads to a warming in the upper troposphere due to condensation in the anvil clouds, and a cooling in the lower troposphere due to precipitation evaporation below the anvil clouds. Subsequently, the WPSH weakens and the large-scale steering flow becomes anomalously northward, leading to an early recurvature of TC Megi.

Results of this study emphasize the importance of a correct representation of anvil clouds in simulating the WPSH and TC track. This study also implies that correcting the heating/drying can be an effective way to reduce errors in simulating the WPSH and TC motion.

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Yuan Sun, Zhong Zhong, Wei Lu, and Yijia Hu

Abstract

The Weather Research and Forecasting Model is employed to simulate Tropical Cyclone (TC) Megi (2010) using the Grell–Devenyi (GD) and Betts–Miller–Janjić (BMJ) cumulus parameterization schemes, respectively. The TC track can be well reproduced with the GD scheme, whereas it turns earlier than observations with the BMJ scheme. The physical mechanism behind different performances of the two cumulus parameterization schemes in the TC simulation is revealed. The failure in the simulation of the TC track with the BMJ scheme is attributed to the overestimation of anvil clouds, which extend far away from the TC center and reach the area of the western Pacific subtropical high (WPSH). Such extensive anvil clouds, which result from the excessively deep convection in the eyewall, eventually lead to a large bias in microphysics latent heating. The warming of the upper troposphere due to the condensation in anvil clouds coupled with the cooling of the lower troposphere due to precipitation evaporation cause a weakening of the WPSH, which in turn is favorable for the early recurvature of Megi.

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Yao Ha, Zhong Zhong, Yimin Zhu, and Yijia Hu

Abstract

The contribution of barotropic energy conversion to tropical cyclone (TC) activity over the western North Pacific (WNP) during warm and cold phases of El Niño–Southern Oscillation (ENSO) is investigated by separating TC vortices from reanalysis data and using a linearized eddy kinetic energy tendency equation. By comparing the characteristics of TC disturbances with synoptic-scale disturbances, it is found that the modulation of ENSO on the WNP TC intensity is presented more objectively by using TC kinetic energy (EKETC) than eddy kinetic energy (EKE). Barotropic energy conversion (KmKe) into TC disturbances (KmKeTC) is an effective indicator in detecting the barotropic energy source of low-level cyclone genesis and maintenance during the ENSO cycle. However, its dynamical processes play different roles. Shear in large-scale zonal wind and convergence in large-scale meridional wind provide direct barotropic energy source for TC genesis, but make effects in different regions of the WNP. In contrast, convergence in large-scale zonal and shear in large-scale meridional wind exert little influence on TC genesis during ENSO.

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Mei Hong, Ren Zhang, Yijia Hu, Yu Dandan, and Guorong Jiang

Abstract

Scholars have been showing great interest in revealing the mechanisms that govern the activities of the western Pacific subtropical high (WPSH). However, the problem currently remains unresolved. In this paper, a new model is developed to reveal the dynamical mechanism of the WPSH abnormal activities. Variables in the partial differential vorticity equations based on latent heat flux are separated in space and time using Galerkin methods. To overcome three deficiencies of the traditional highly truncated spectral method, spatial basis functions are reconstructed from observation field time series based on a combination of empirical orthogonal function decomposition and genetic algorithm. Based on the atmospheric vorticity equation, nonlinear ordinary differential equations of the WPSH can be obtained using an objective spatial basis function. Moreover, dynamical characteristics and actions, such as bifurcation and catastrophe induced by latent heat flux factors, are analyzed. Results show that the latent heat flux field in the Indian Ocean and the Bay of Bengal region determines the formation and rupture of the WPSH (such as the “double ridge” phenomenon). The enhancements of land–sea heating contrast and latent heat flux field in the South China Sea lead to WPSH abnormal activities and WPSH circulation anomalies, such as subtropical high northward jump and rapid westward extension. Linked with the real weather phenomena and the diagnostic analysis of the previous studies, the credibility of the bifurcation and catastrophe analysis is confirmed. This work documents new areas of research on the evolution mechanism of the WPSH under the action of latent heat flux from the view of chaotic dynamics.

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Yijia Hu, Yimin Zhu, Zhong Zhong, and Yao Ha

Abstract

The prediction of mei-yu onset date (MOD) in the middle and lower reaches of the Yangtze River valley (MLYRV) is an important and challenging task for those making seasonal climate predictions in China. In this paper, the atmospheric and oceanic conditions in the preceding winter and spring related to MOD are analyzed. It is found that the MOD is associated with the intensity of the Ural high and the East Asian trough in high latitudes, with the intensity of the upper-level westerly jet in middle latitudes, and with the contrast of land–sea temperature and pressure in the preceding winter and spring, which are proxies for the intensity of the East Asian winter monsoon (EAWM). It is suggested that the intensity of the EAWM is the most crucial factor affecting the MOD. Years with an early MOD usually correspond to strong EAWMs in the preceding winter, and vice versa. The EAWM can affect the MOD by influencing the East Asian summer monsoon (EASM) through tropical ocean–atmosphere and tropical–extratropical interactions. Based on the above analysis, a physics-based statistical forecast model is established using multivariable linear regression techniques. The hindcast of MOD during the 13 yr from 1998 to 2010 is carried out to evaluate the performance of this forecast model. The MOD can be predicted successfully in 8 out of the 13 yr. The forecast model predicts the MOD in the years with strong mei-yu intensity more accurately than in those with weak mei-yu intensity, especially for cases of extreme flooding. This is useful in the prevention of flooding disasters.

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Yao Ha, Zhong Zhong, Yijia Hu, and Xiuqun Yang

Abstract

This study investigates the influences of ENSO on tropical cyclone (TC) kinetic energy and its meridional transport in the western North Pacific (WNP) using the TC wind field obtained after a method for removing TC vortices from reanalysis data is applied. Results show that ENSO strongly modulates TC kinetic energy and its meridional transport in the WNP, but their effects and regions differ. The TC kinetic energy is positively correlated with the Niño-3.4 index in the entire WNP, and its poleward transport is positively (negatively) correlated with the Niño-3.4 index in the eastern WNP (the western WNP and the South China Sea); these correlations are statistically significant. The maximum TC kinetic energy is located around 25°N, 135°E (25°N, 125°E) in the warm (cold) year, showing an east–west pattern during different ENSO phases. The meridional transport of TC kinetic energy exhibits a dipole pattern over the WNP, with the poleward (equatorward) transport in the eastern (western) WNP. Both poleward and equatorward transports strengthen (weaken) and shift eastward (westward) in El Niño (La Niña) years. Therefore, El Niño has strong influences on TC kinetic energy and its meridional transport.

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