| The NCEP/NCAR reanalysis data, China 740 stations precipitation data, and the TBB data from Meteorological Institute Research of Japan are employed to study the structure and variation law of subtropical high (SH) in the period of double ridges. By combining together case diagnosis and Statistic analysis, some new findings of the SH double ridges are obtained. The main results are as follows:1. Advance of conception of "the double SH ridges"Analysis finds that under specific external environment and surrounding circulations, a northern and a southern line are available at the same longitude in subtropical area on a synchronous basis, though the subtropical high has characteristic of single ridge in most areas at most of time. According to this phenomenon, a new conception of "the double SH ridges" is proposed. In virtue of this conception, the structure and variation law of SH in the period of double ridges are systemically investigated in this study.2. Rebuilding of the double SH ridgesBased on the case of double ridges during the breaking interval of plum-season rainfall in the summer of 1998, it is indicated that the SH ridge would remarkably locate much more southward than its normal position after the phenomenon of double SH ridge is over. Furthermore, this southward migration of SH ridge is not sudden but results from the southward rebuilding of the double SH ridges, that is, the northern (original) ridge gets enfeebled and disappeared, in contrast, the southern counterpart becomes intensified enough and steadily persists. Therefore, the phenomenon of double SH ridge which consecutively impenetrates during its rebuilding provides the important omen for the discontinuous southward withdrawal of the SH ridge. In addition, the similar results are also obtained from other several typical cases of the double SH ridges.3. Geographical and seasonal distributions of the double SH ridgesAccording to the statistics of 41-year data, it is found that the appearance of such double ridges is not peculiar to 1998 only but universal, and it has characteristic of the notably geographical and seasonal distributions. The events of double SHridges occur most frequently in the summer and autumn and mostly exist from the mid-east of Indian Ocean to the middle of the Pacific. In comparison, they seldom happen in the spring and the winter.4. Structures of the SH during double ridgesThe structures of the SH during double ridges are investigated. It is shown that the circulation features and temperature/humidity structures differ greatly between the intervals of single ridge and double ridges (a northern and a southern) available, with the characteristics around the northern ridge more similar to those of the conventional single ridge but the southern component displaying the features typical of low-latitude systems.5. Styles and possibly formative mechanism of the double SH ridgesBy virtue of the different appearance of double SH ridges, the double SH ridges are divided into two styles, one resulting from the transfiguration of single SH periphery, and the other from the synchronous emergence of a southern and a northern SH monomer over the western Pacific. Based on this conclusion, the possibly formation mechanism of the double SH ridges is discussed. The results show that the formation process of double SH ridge is discrepant for the different style. The first style occurs as a result of the northward movement, the through-get and consolidation of the equatorial buffer band with the WPSH, and the second one is due to the strength, western extension, and then through-get and consolidation of the mid-Pacific with the equatorial buffer band.6. Relationship between the double SH ridges and the rainbands in the East China in summerA station data-based investigation is undertaken, discovering the close relationship between the double-ridge maintenance upon the rainfall pattern of eastern China, also indicating that the single-related vapor passage is changed during the doub...
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