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Based on the observation data from the coastal island stations in Beibu Gulf and the fifth generation ECMWF reanalysis (ERA5) data, the probability of occurrence of mixed-type (that is, high-pressure rear type to southwest inverted trough type) sea fog processes in Beibu Gulf is analyzed. Furthermore, the typical mixed-type sea fog process from 15 March to 23 March 2016 is selected for in-depth analysis. The results show the following: (1) Compared with other types of sea fog processes, the probability of occurrence of the mixed-type sea fog processes was the highest during 2015-2020, with a cumulative number of 15 times (58 days), accounting for 34.9% (39.5%). (2) The typical mixed-type sea fog process from 15 March to 23 March 2016 was characterised by long duration and wide range. In the early stage, the Beibu Gulf was influenced by the easterly flow behind the high pressure entering the sea. In the later period, the plateau trough developed and moved eastward, which guided the warm and humid south-western airflow to expand inland, and the easterly airflow gradually turned to the southerly airflow at the edge of the south-west inverted trough in Beibu Gulf. (3) In terms of thermal conditions, the strong warm and humid transport of the low-level jet stream increased the specific humidity over the Beibu Gulf, and then the water vapour saturated layer extended from the near-surface layer to 800 hPa on the one hand. It also contributed to the establishment of the temperature inversion layer on the other hand. And the inversion layer cooperated with the high humidity region below to reach a stable state. As for the dynamical conditions, the wind shear in the vertical direction maintained a certain intensity, which was favourable to the turbulent mixing in the boundary layer. The cooperation of the wind shear with the baroclinic atmosphere was the configuration that favoured the maintenance of a stable layer in the low-layer. In addition, the weak upward motion below 800 hPa was conducive to water vapour uplift and condensation in the low-layer, which promoted the development and maintenance of the sea fog. (4) During the occurrence of the typical mixed-type sea fog, the air temperature was mostly higher than the sea surface temperature in each sea area. And the difference between the air temperature and sea surface temperature in the early stage (late stage) was between 0 and 1.5 ℃ (1.5 and 2.5 ℃).

Based on the methods of Fourier decomposition, correlation analysis and Liang-Kleeman information flow, the interaction between intensity and rainfall of Typhoon Rammasun (1409) is studied using the best track data of Shanghai Typhoon Institute (STI) of China Meteorological Administration (CMA), Tropical Rainfall Measuring Mission (TRMM) satellite 3B42 rainfall estimation data of National Aeronautics and Space Administration (NASA) and ERA5 reanalysis data of European Centre for Medium-Range Weather Forecasts (ECMWF) with 0.25°×0.25° grids. The results show that the rainfall of Typhoon Rammasun (1409) has obvious characteristics of asymmetry, and it is mainly located in the west side of the typhoon center. The rainfall of Typhoon Rammasun (1409) increases significantly twice during the whole life cycle, which corresponds to the intensification period. The information flow analysis shows that the rainfall of Typhoon Rammasun (1409) is affected by the intensity of typhoon itself, whereas the rainfall can also feedback on the latter. Compared with the influence of typhoon intensity on rainfall, the information flow from rainfall to typhoon intensity decreases by nearly an order of magnitude, indicating that the typhoon intensity plays a dominant role in the interaction relationship. The possible mechanism by which the typhoon intensity affects rainfall are analyzed by diagnosing the water vapor and dynamic conditions respectively. In terms of water vapor conditions, the convergence area of vertical integral of moisture flux corresponds well to the rainfall. The intensification (reduction) of Typhoon Rammasun (1409) partly results in the enhanced convergence (divergence) of vertical integral of moisture flux in the southwest of the typhoon center, which brings more (less) rainfall in this region. In addition, the South China Sea and the Western Pacific water vapor transport channel have obvious response to the changes of Typhoon Rammasun (1409) intensity. In terms of dynamic conditions, the strong center of vertical helicity in the middle and lower layers is mainly located in the west side of the typhoon center, indicating the rainfall area. The intensification (reduction) of Typhoon Rammasun (1409) leads to the increase (decrease) of absolute vertical helicity on the west side of the typhoon center, thus promotes (inhibits) the development of upward movement to a certain extent, resulting in more (less) water vapor condensation and rainfall.

Tropical cyclones entering coastal areas adversely affect southern China. However, changes in the frequency of tropical cyclones affecting the west of southern China remain unclear. Our study reveals the possible impact of the thermal forcing anomaly over the southeast Tibetan Plateau (TP) on the frequency of tropical cyclones affecting Guangxi formed within the west of 120° E during boreal summer. Further analysis indicates that the cooling over the southeast TP is accompanied by local descending motions over southeastern TP and compensating ascending motions over eastern Indochina Peninsula and results in a reduced 850–200 hPa vertical wind shear over the north of 15° N in South China Sea (SCS), which is conducive to the westward development of tropical cyclones and favorable conditions for the formation of TCs affecting Guangxi over the SCS. Finally, the results from a linear baroclinic model experiment also verify that the changes in the 850–200 hPa vertical wind shear over southern SCS and compensating vertical motions over eastern Indochina Peninsula are associated with the thermal forcing anomaly over the southeast TP. Our results imply that in summer the thermal forcing anomaly over TP should be emphasized when interpreting and predicting the frequency of tropical cyclones affecting local areas in southern China. [ABSTRACT FROM AUTHOR]