Synoptic development

In this part, this study will analyze the synoptic-scale atmospheric conditions during the D18 event. During the D18 event, the horizontal wind field at the 925 hPa over central Vietnamese and South China Sea (SCS) characterized by a strong convergence wind zone between the strong northeasterly wind blowing from the northeast of China into the SCS and the central Vietnamese then and the east wind blowing from the Pacific Ocean into SCS (Fig. 3.3a). The wind direction is considerably similar to the climatological northeasterly monsoon wind (Fig. 3.3b). The wind speed over the northern part of SCS and central Vietnamese was over 10 m/s, which is stronger than the

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climatological wind speed. Besides, the horizontal wind anomaly field shows a convergence between the northeasterly wind with a wind speed of about 3-4 m/s and the southeasterly wind with a wind speed of around 4-5 m/s over central Vietnam during the D18 event. (Fig. 3.3c).

Analysis of the components of wind shows that during the D18 event when the meridional wind over mainland China propagate farther southward and reach 15°N with speed was 4 m/s (Fig. 3.4a). A sharp divergence of southeasterly wind anomaly was found over the southern part of the SCS, which reached the center of the SCS with wind speed was 2 m/s (Fig. 3.4b). Concurrently, the strong easterly wind began to strengthen and reached central Vietnam with wind speed was around 8 m/s (Fig. 3.4c). This easterly wind seems to prevent the northeasterly wind from propagating farther southward, and the southeasterly wind anomaly from propagating further northward at the center of the SCS, and then divert these winds into central Vietnam. Results in the formation of a convergence of these wind in this region.

The equivalent potential temperature field shows that a warm and moist tropical air mass existed southern part SCS with an equivalent potential temperature greater than 335k during the D18 event (Fig. 3.5). Furthermore, relative humidity over the central of SCS is higher than 80 percent in the lower troposphere (< 900-hPa), and the equivalent potential temperature decreases with altitude, indicating instability in the lower atmosphere (Fig. 3.6). Besides, stronger-than-usual of both southeasterly wind anomaly (refer Fig. 3.3c) and easterly wind transported relatively thermally unstable air into central Vietnamese and blocked by the Annamite range, leading to creating favorable environment conditions for activating orographic rainfall processes.

Furthermore, Fig. 3.7a, b, c, d showed that moisture was transported to mid-central Vietnam across Philippine by a strong and long-lasting moisture flux between the surface and the 200-hPa level. The transport band emerged from the western Pacific Ocean through the Philippines into SCS and mid-central Vietnam then between 1200 UTC 08 December and 1200 UTC 11 December. Strongest moisture transport occurred on 08 December with a maximum greater than 400 kgm^-1s^-1 and reducing in the next days.

Besides, the moisture flux further enhanced over the SCS, which was possibly related to a moisture uptake over SSTs above 27 ⁰C (Fig. 3.7e).

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Figure 3.8 showed that a rising motion occurred at the lower troposphere during the D18 event. In particular, large-scale rising motions occurred at 925 hPa, and 850 hPa over the whole central and the central coastal sea Vietnam with a maximum value exceed 0.35 Pas^-1 (Fig. 3.8a, b) and reduced to less than 0.15 Pas^-1 at 500 hPa level (Fig. 3.8c).

Besides, this figure shows that the Truong Son range plays a decisively important role in the development of heavy rain in central Vietnam. When the northeasterly wind and easterly wind at lower levels blew to central Vietnam and blocked by the Truong Son Range, which located along the border of Vietnam and Laos, leading to the formation of a forced rising motion over these areas (Fig. 3.8 a, b) and downward motion over Laos then.

Figure 3.9 showed that during the D18 event, occurred the convergence of moisture flux at 925 hPa with the values over two g/kg*s^-1 (Fig. 3.9a) and the divergence of moisture flux at 850 hPa level with two g/kg* s^-1 and reduced upper levels (Fig. 3.9b, c). As analyzed above, when the strong anomaly southeasterly wind and the easterly wind at low levels blow into central Vietnamese. They bring warm, high moist, unstable airs mass originated over the western Pacific Ocean and be enhanced over SCS into central Vietnam. And then was blocked by the Truong Son Range with the height is around 2000 m, leading to the formation of a forced convergence at lower levels and enhances upward motion vertically and then diverges at upper levels.

In summary, analysis of thermodynamics above indicating that central Vietnam was combined effect by factors as the strong northeasterly wind blowing from the Yellow Sea, the easterly wind blowing from the Pacific Ocean, and the strong southeasterly anomaly. Also, during this time, there is existing instability in the lower atmosphere. The SST in the central and southern part of the SCS is higher than 27 ⁰C, creating favorable environment conditions for supplying more moisture during the D18 event. These factors transported warm, high moist, unstable air mass originated over the western Pacific Ocean, which was enhanced over SCS into central Vietnam, and then was blocked by the Truong Son Range with the height is around 2000 m, leading to the formation of a forced convergence at lower levels and enhances upward motion vertically and then diverges at upper levels. These are favorable environmental conditions for activating orographic rainfall processes. Finally, as a consequence, the D18 event occurred then.

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