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The unique role of the South China Sea summer monsoon (SCSSM) onset process in the development of the East Asian summer monsoon (EASM) is demonstrated in this study. The SCSSM onset process is examined in terms of the vertical linkage between the Western Pacific subtropical high (WPSH) and the South Asian high (SAH). A composite analysis is performed in order to adequately describe the vertical linkage in a synoptic timescale. The South China Sea (SCS) is a key region for the seasonal migrations of the WPSH and the SAH, with the former retreating northeastward, the latter advancing northwestward, and both taking place over the SCS during the SCSSM onset period. The SCSSM onset process is characterized by a significant change in the relative configuration of the ridge lines of the WPSH and the SAH. Just prior to the onset period, the ridge lines intersect vertically over the SCS, thus prohibiting convective activities. During the onset period, the ridge line intersection moves away from the SCS due to the retreating WPSH and the northward shift of the SAH ridge line. This coincides with the emergence of monsoonal convective activities over the SCS and the establishment of a moisture channel from the tropics, which in turn provides favorable conditions for the development of deep convective activity. The northeastward intrusion of the lower level southwesterlies and the moisture supplying channel are closely related to the development of a preexisting twin cyclone in the Bay of Bengal. The northeastward lower level southwesterlies form a monsoonal ascending motion in the SCS, which further merges upward into the northeasterlies to the south of the SAH ridge line. This is a signature of the establishment of the local Hadley circulation, which marks the beginning of the EASM. The frontal system is the most frequent attendant synoptic event during the SCSSM onset. From the viewpoint of synoptic process, the SCSSM undergoes a two-stage onset process which is characterized by the southward intrusion of the frontal system in the earlier stage and the outbreak of the tropical convection in the later stage. The frontal system may act as a trigger for the outbreak of the tropical convection in the later stage. The burst out of the monsoonal convection over the SCS is essential for the breakdown of the vertical intersection between the WPSH and the SAH therein.

In this study, PSU/NCAR MM5 is used to simulate the evolution of the 1998 East Asian summer monsoon onset over the South China Sea (SCS). Simulation outputs are further utilized to study the development of mesoscale convective systems (MCSs) and their interactions with the synoptic/large-scale environment in the SCS region. During the monsoon onset, over southern China and the SCS, high equivalent potential temperature, convective instability, and the southwesterly confluence in the lower troposphere provided favorable large-scale thermodynamic and dynamic conditions for the MCS development. Model simulated results reveal that, in a warm-moist and unstable atmosphere, the downdraft outflows as a result of the MCS development could become a possible mesoscale triggering mechanism for the formation of a new MCS. Sensitivity tests depict the effect of latent heat release playing a significant role in the establishment of monsoon circulation. The terrain effect is significant for local cloud formation over land. For well organized MCSs occurring over the SCS, the influence of the mesoscale wind field is more important. The latent heat release in MCSs heats the atmosphere, reduces the surface pressure, and therefore enhances the low-level wind convergence and cyclonic circulation. The mesoscale adjustment of wind field and mass field, on one hand, enhances the MCS development or initiates the formation of a new MCS, and the organization of MCSs, on the other hand, integrates the mesoscale disturbances into a synoptic structure and forms the local monsoon trough. Without the latent heat release, monsoon circulation cannot establish itself even if the model provides favorable large-scale boundary forcing.

Diabatic heating by latent heat release and longwave radiation and corresponding generation of available potential energy have been computed for a tornado-producing cyclone system. Also, the contributions of two subareas, encompassing the immediate cyclone vicinity and the convection area, to the cyclone system generation are examined applying the concepts of limited region baroclinic and barotropic generation. Heating values, which are dominated by convective latent heat release, are generally in good agreement with observed weather features and cyclone development. The resulting positive cyclone system generation is comparable with previous studies and is strongly influenced by the contributions made by the two subareas. In turn, the latter are dominated by their. barotropic components, indicating that the heating fields in these subareas contribute more effectively to increasing or maintaining the baroclinicity of the cyclone system than of the subareas themselves.

An available potential energy budget analysis is presented for an area and period (0000 GMT 3 April-1200 GMT 4 April 1974) spanning the lift cycle of a severe storm-producing extratropical cyclone which moved across the central United States. Included are diabatic heating estimates. Among these, total latent heat release and longwave radiation contribute significant positive and negative values, respectively. Total latent heat release is generally in good agreement with observed weather features and cyclone development. Budget results show that during the development of the extratropical cyclone the available potential energy within the region under investigation increases. Diabatic generation is a source of available potential energy, but its value is rather small when compared with other budget terms. Available potential energy generation primarily results from convective latent heat release ahead of the cold front in the warm sector. A large amount of total potential energy is released; howeve...