class="E-Title2">3.3. Vapor, Energy and Dynamic Conditions
By using the NCEP/GFS analysis data to calculate the water vapor flux divergence at 850 hPa, we can see that the southeast periphery of the subtropical high and the southerly flows from the strong tropical storm Saola formed a southeast wind jet, which delivered a steady flow of water vapor from the sea to Damrey and produced a strong water vapor flux convergence in the east of North China, contributing to the occurrence of heavy rainfall (Figure 4(a)). Sufficient water vapor supply provided the water vapor conditions for the rainstorm areas, at the same time, the release of latent heat of condensation fuelled the system and enabled it to last long. According to the NCEP temperature advection at 500 hPa in 06:00 UTC on August 3rd (Figure 4(b)), we can see that there are large value centers for the temperature over the dimensions of both Damrey and Sula. The two centers were connected to each other, and there was an interaction between them so that Damrey can maintain it. Affected by the Damrey periphery, it was warm and wet at 500 hPa in east of North China, and the strong precipitation occurred on the warm and wet side of the temperature gradient.
When the water vapor conditions and energy conditions are met, the dynamic conditions are triggered by the unstable stratification, triggering the rising movement of precipitation necessary conditions. At 18:00 UTC on the August 3rd, the wind distribution of 850 hPa (Figure 5(a)) shows that there is convergence of the southerly and northerly winds in east of North China. It is the convergence zone of the wind speed and the upward movement zone. The ascending speed reached 2 Pa/s just like July 21st heavy rain in 2012 in Beijing  . The
Figure 3. Synoptic chats at 500 hPa at (a) 00:00 UTC and (b) 12:00 UTC on August 3, 2012.
Figure 4. (a) Water vapor flux divergence at 850 hPa at 18:00 UTC and (b) Temperature advection at 500 hPa at 06:00 UTC on August 3, 2012.
Figure 5. (a) Wind and wind divergence at 850 hPa at 00:00 UTC and (b) K index at 18:00 UTC on August 3, 2012.
K-Index is a measure of thunderstorm potential in meteorology. According to the National Weather Service, USA, the index harnesses measurements such as vertical temperature lapse rate, moisture content of the lower atmosphere, and the vertical extent of the moist layer  . It is generally accepted that k-index greater than 35˚C is the basic condition for heavy rain events, which is also confirmed over North China in this case (Figure 5(b)).
Rainstorms are one of the most important meteorological disasters in China, especially heavy storms, and their continued heavy rainfall can lead to frequent natural disasters such as flash floods and debris flows, causing huge losses to people’s lives and property. During the typhoon, Dami was in the vicinity of Tianjin in the eastern part of North China and Hebei Province. On August 3, 2012, heavy rainfall occurred in Qinhuangdao, Hebei Province and Tangshan City, Hebei Province.
1) The causes of the torrential rains include the south-facing subtropical high and the south-easterly jet stream around the Sula typhoon, a water vapor and energy belt that allow the Daveilu to maintain long-term water vapor and heat in the eastern part of North China. The structure of the Davidsia caused a strong ascending updraft in the eastern part of North China, with a K-index greater than 35˚C, which provided the impetus for the formation of heavy rainfall.
2) The study also found that the typhoon storm in North China is the result of the interaction between the westerly, subtropical and tropical systems. Subtropical and tropical systems have a clear interaction during this heavy rain.
3) The situation of circulation at 500 hPa before the regional heavy rainstorms and the meridional features of the typhoon at low latitudes are outstanding. At the same time, the typhoon and the cold air in the middle and high latitudes have the characteristics of the opposite movement, that is, the typhoon exists, which is favorable for the cold air in the middle and high latitudes to move eastward or southward.
This work was supported by the Tianjin Municipal Meteorological Administration Doctor’s Fund (2017bsjj01) of China.
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