Ensemble Sensitivity Analysis

CHAPTER 2. DATA AND METHOD

2.2. Method

2.2.6. Ensemble Sensitivity Analysis

As mention above, an ensemble forecast is a set of forecasts produced by many separate forecasts with differences in initial conditions, respectively. Moreover, as we know, the numerical weather forecasts are sensitive to small changes in initial conditions.

Hence, we have to sensitivity analysis to examine how a forecast variable responds to changes in initial conditions, we have to sensitivity analysis. Sensitivity analysis is considered a measure to improve forecasts through targeting observations. In this study, ensemble sensitive is computed by the formula:

𝜕𝑅

This study examined the sensitivity between an ensemble forecast of the 24-h rainfall and the initial condition state variables. The results are present in chapter 4.

Spread = √

^∑^𝑛^𝑖=1^(𝑥^𝑖^−𝜇^𝑥⁾²

𝑛−1

CHAPTER 3. SYNOPTIC CONDITIONS DURING THE D18 EVENT

3.1. An overview of the D18 event

During 9-12 December 2018, a record-breaking rainfall occurred along the mid-central Vietnamese coast between Quang Binh and Quang Ngai provinces. Figure 3.1 shows the accumulated rainfall in 3 days (1200 UTC 8 –1200 UTC 11 December) by the VnGP data, the observed rainfall data, and the TRMM data. In which, TRMM showed that the highest rainfall sums occurred along the mid-central shoreline and over the coastal sea between Quang Tri and Quang Ngai provinces with precipitation amount reached at 150 mm (Fig. 3.1, right). Besides, the observational data shows that particularly heavy rainfall fall in provinces: Thua Thien Hue, Quang Nam, Da Nang city, and the northern part of Quang Ngai, with the maximum accumulated rainfall from 1200 UTC 08 Dec to 1200 UTC 11 Dec exceeding 800 mm (Fig. 3.1, center). This rainfall was hundreds of times larger than the climatological mean daily precipitation in comparison (refer Fig. 3.1, left and center).

Furthermore, at Da Nang station (16.0°N, 108.2°E) recorded precipitation amounts in 24 hours greater than 600 mm on Dec 09 and over 300 mm on the next day while the rainfall climatology is just less than 10 mm in both of days (Fig. 3.2). This record-breaking rainfall event led to catastrophic flooding and landslides, which resulting in 13 deads, many destructions in the environment, downstream cities, and causing many other economic losses.

3.2. 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

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).

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.

3.3. Evolution of precipitable clouds

In this part, this study will analyze the local thermodynamic conditions that led to the D18 event. Figure 3.10 shows that at 1200 UTC 8 Dec, the equivalent potential temperature at 925 hPa over central Vietnam and the SCS is commonly between 325 K and 340 K, the relative humidity is over 90% exists between 106E and 110E and reached the 800 hPa level (Fig. 3.10b). The strongest moisture air vertical upward occurs around 108E (Fig. 3.10c). The satellite image also shows a series of convective cells (red, white, black colors) first formed over the study area's northern and central parts. These convective clouds are composed of several big and small isolated cells. Their distribution separates into two main directions. One direction is northeast-southwest over the northern part, and the other direction is east-west over the central part of the study area. Over the next 6h, most small isolated cells were weakened significantly and dissipated after 1800 UTC while moving slowly offshore. The life span of these small convective clouds cells was from 1 to 3 hours. On the contrary, the large convection cloud in the north of the study area appears to be intensified and reach better structured at 2000 UTC, then weakened significantly and dissipated after 0100 UTC 9. These convective cells' life span was about 12 hours—however, this large cell distributed over the ocean. After 18 UTC 8 to 11 UTC 9 Dec, new small cells continued to appear and scattered develop along the coast and move slowly offshore. Besides, there is a remarkable convection cloud, forming over the coastline of the southern part of the study area at 2300 UTC 8 Dec. This convective cell's development is very vigorous and matured at 0400 UTC 9 Dec, then gradually weakened while moving slowly eastward over the coastal sea (Fig. 3.11).

Besides developing these convective cells, this area is also covered by the Stratus clouds (green colors), Including the nimbostratus clouds, the Altostratus cloud, etc. These clouds first formed over northern parts of the study area at 1200 UTC 8, and then grows and expands southward. Especially along the coast. This cloud also covered the entire study area from 2300 UTC 8 to 1100 UTC 9.

Furthermore, the Column Maximum radar reflectivity (Cmax) over the Central Vietnamese area with 1-h intervals shows that the first rainfall occurred along the coast from 1200 UTC to 1700 UTC 8. After that, the rain zone has extended to both the coastal sea and inland. In particular, during 2000 UTC 8 to 0200 UTC 9, Cmax reached the highest value (40 dBZ), which shows that the rainfall intensity was the greatest during

this period. After 0200 UTC, the intensity of rain tends to decrease gradually. In general, the Cmax value of December 8 fluctuated quite steadily between 15 dBZ and 35 dBZ, indicating that rainfall was not too intense. However, steady rain falls over many hours, leading to the cumulative rainfall of this day is very large (Fig. 3.12). At 1200 UTC 9, these thermodynamic conditions of the previous day tend to strengthen. Particularly, the high equivalent potential temperature (>340 k) expands from the southern part to the northern part of the SCS (Fig. 3.13a). The easterly wind expands to 750 hPa (Fig. 3.13b).

The high relative humidity (>90%) cover from 104E to 112E and reached 700 hPa level converges with equivalent potential temperature decreases with altitude, indicating the area that occurs instability in saturated air is larger than it in the previous day. Besides, the strong moisture air vertical upward occurs not only around 108E as a previous day but also expand to 111E. The strong vertical downward occurs around 106E (Laos) (Fig.

3.13c). These thermodynamic conditions have led to the formation of precipitable clouds along the coastline and coastal sea, observed by satellite imagery. Concretely, figure 3.14 shows the infrared enhanced color images of the Himawari satellite over the Central Vietnamese area from 1200 UTC 9 December 2018 to 1100 UTC 10 December 2018. It clear to see that the most prominent is the formation and strong development of the Stratus clouds (green colors). This cloud has covered the study area during 1200 UTC 9 to 2300 UTC 9, then gradually disintegrated. Besides, it clear to see that a small convective cell formed and developed over the area of Da Nang (Da Nang is one of the main extreme rainfall centers of the D18 event) at 1200 UTC 9. The life span is about 5 hours (1200 UTC 9 to 1600 UTC 9), and a few cells have formed and developed over the southern part of the study area. In which some cells formed from the previous day and existed until this day. Some of them appear on this day. All these convection cells' common features are formed and developed in the coastal area, then gradually weakening and disintegrating while moving to the sea. (Fig. 3.14). From another point of view, the Cmax data show that the rain occurred 24 hours of this day. However, Intense rain occurs mainly from 1200 UTC 9 to 2100 UTC 9. after that, the rain gradually decreased in both intensity and area. Besides, the Cmax data also indicating that rainfall is mainly distributed over the coastal plain and coastal sea (Fig. 3.15).

At 1200 UTC 10 Dec, the vertical motion tends to decrease in intensity compared to the previous day. High humidity (>90%) areas also narrow significantly in the same

comparison. The easterly wind at 750 hPa level is replaced by southeasterly wind (Fig.

3.16). These changes reduce the development of convection clouds, which have been observed by satellite imagery (Fig. 3.18). At 1200 UTC 11 Dec, the vertical upward motion still exists over central Vietnam. However, relative humidity has decreased considerably more than in the previous day, especially at upper levels (> 500 hPa). The southeasterly wind at 750 hPa is replaced by southeasterly wind. As a result, environmental conditions are unfavorable for the development of convection clouds in the area, resulting in reduced rainfall (Fig. 3.17). Particularly, figure 3.18 shows that the precipitable cloud has been decreased significantly or only exists at sea. Furthermore, the number of convective cells has been greatly reduced compared to the previous two days.

Only two small convection cells were formed and developed in the study area's coastline.

One cell appeared over the central coastline of the study area at 1400 UTC 10, then gradually weaken while moving eastward to the SCS and dissipated after 1800 UTC 10.

Another cell formed at 2000 UTC 10 over the southern coastline. This cell grew very vigorously and matured at 2200 UTC 10, then weakened significantly and completely disintegrate at 0200 UTC 11. However, the lifetime of these convective cells on land is only 1-3 hours. The size of these cells is very small. The decrease of the precipitable cloud leads to a significant decrease in rainfall in this day. This can be seen in Figure 3.19.

Particularly, Cmax data show that rainfall occurs mainly from 1 hour to 3 hours, then decreased significantly with time.

In general, during the D18 event, due to the interaction of the low-level cold surge, originating in China, with the low-level easterlies wind over the South China Sea (SCS), led to the formation of a strong low-level convergence and then local deep convections.

Besides, the strong easterly and strong southeasterly anomaly winds also played an important role in transporting moisture from the tropics across the SCS toward central Vietnam. As analyzed above, these conditions, combined with the local thermodynamic conditions, led to the formation of precipitable clouds, including the nimbostratus clouds and convective cells. However, these convective cells are not evenly distributed. They are concentrated mainly in the northern sea or the southern coastline of the study area.

The size of these cells is also very different, composed of several big and small isolated cells. Small cells' lifespan is very short (from 1 to 3 hours), while several big cells can exist up to over 10 hours. Most of these convection cells tend to move slowly offshore.

This movement is due to strong southwest or westerly winds at higher pressure levels (refer Fig. 3.6 and Fig. 3.8c). Besides, it is noteworthy that these convection cells mainly form and develop during the nighttime. Meanwhile, the formation and strong development of the Stratus clouds during the D18 event led to steady rains over many hours.

CHAPTER 4. RESULTS

In this chapter, all results of the daily time-lagged forecasts executed by the CReSS for the D18 event are present, and these results will be compared with observations, validated, to investigate the predictability of CReSS model for the D18 event.

4.1. 24-h accumulated rainfall for individual days and three days 4.1.1. 24-h accumulated rainfall for 9 Dec 2018

With four 8-day forecasts per day. In which, the first forecast performed on 1200 UTC 3 Dec 2018, and the last forecasts are on 1200 UTC 8 Dec 2018. Fig. 4.1 showed 21 scenarios of possible future states of the atmosphere (24 hours of rainfall and wind surface fields) for 9 Dec 2018 by CReSS. In general, most scenarios predicted 24-h of rainfall closer to observation. Particularly, these scenarios indicate 24-h rainfall up to over 500 mm (shading by green color), which is the same in observation. However, the location and spatial distribution of the 24-h rainfall are very different from the observation data and scenarios. Along the central Vietnamese coast, where the observed rainfall exceeds 500 mm, most scenarios predicted rainfall below 100 mm. These differences may be due to the model having mispredicted other atmospheric variables, such as wind surface field.

In Fig. 4.1, some scenarios mispredicted the wind direction over the SCS and mid-central Vietnam. For example, scenarios on 7 Dec 2018. Some scenarios have incorrectly predicted the magnitude of the wind field, such as scenarios on 3 Dec 2018. The accuracy of forecasts decreases with lead times. At the shortest-range prediction (1200 UTC 08), the direction and magnitude of the surface wind field are more similar to the reanalyzed data than other predictions at other times. At the longest-range prediction (1200 UTC 03), The direction and magnitude of the surface wind field are not the same as the reanalysis data. The skill scores in Fig. 4.2a also shows that the member that was run at the shortest-range prediction (1200 UTC 8) has the highest score (0.66). The member that was run at the longest-range prediction (1200 UTC 3) has the lowest score (0.04). These scores are consistent with the previous analysis.

4.1.2. 24-h accumulated rainfall for 10 Dec 2018

Figure 4.3 presents 25 possible scenarios of 24-h rainfall and average wind surface fields for 10 Dec 2018. The first forecast runs on 1200 UTC 3 Dec 2018, and the last forecast ran on 1200 UTC 9 Dec 2018. It can be clearly seen that several members have not only a very good 24-h rainfall forecast but also a good forecast for the location and spatial distribution of rainfall. Namely, members executed on the 8th and the 9th of December. Moreover, there is an impressive member that ran at 1800 UTC on 4 December. This member has a good forecast of rainfall even though the forecast made at a long forecast range before the target date (approximately five days before the target date). A common feature among these members is that they both well predict the direction and magnitude of the surface wind field. Besides, many members cannot forecast rainfall.

Most of them executed at forecast periods longer than two days before the target date. In general, most of them cannot predict the direction or magnitude or both of these characteristics of a surface wind field. As well as the analysis for the previous day, this may be the reason why they cannot predict the rain accumulation field of 24 hours. The fraction skill scores in Fig. 4.2b also indicated members ran on the 8th and the 9th of December, and at 1800 UTC 4 December reach scored higher than the rest of 25 members.

4.1.3. 24-h accumulated rainfall for 11 Dec 2018

Figure 4.4 show 29 possible scenarios of 24 hours rainfall and wind surface fields for 11 December 2018, executed between 1200 UTC 3 December and 1200 UTC 10 December. It is clear to see that most scenarios forecast quite good the quantitative rainfall in 24-h, although the 24-h rainfall observed of December 11 has decreased significantly compared to the previous two days. However, most of the scenarios predicted the spatial distribution of inland rainfall is smaller than the observed data, which may be related to the fact that members inadequately predicted the direction and magnitude of the wind field, resulting in the incorrect prediction of moisture transport and moisture convergence.

For example, members executed on December 3. These members did not predict the surface wind field; as a result, they could not predict the accumulative rain field of December 11. Besides, the fraction skill sore in Fig. 4.5a also shows that the score of each member in 25 members is low. The member has the highest score (approximately 0.4),

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