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Chung-Chieh Wang

Abstract

As one of the most widely used skill scores for model quantitative precipitation forecast (QPF) verification and evaluation, the equitable threat score ETS differs from the threat score TS in that the random hits R are removed from its calculation. In practice, however, when applied to a set of verification points confined to small areas, the random assumption often becomes increasingly questionable or even invalid in larger events. As a result, the random hits are overestimated and the ETS becomes biased and not indicative of model skills. In this paper, such an issue is explored and demonstrated through the example of Taiwan with steep topography from Typhoon Morakot (2009) and mei-yu heavy-rainfall cases. It is found that the ETS is affected more seriously and scaled down by at least about 0.1 compared to the TS whenever the rain area occupies roughly 20% or more of the total verification area (if the random assumption of R is invalid). As such conditions often occur for small areas, it is vital to estimate R as correctly as possible for the ETS to work properly. A simple solution is offered by using all gridpoint values from the entire model domain, rather than just a small subset falling into the verification area, to estimate the random hit rate in the forecast. While the ETS remains unaltered in its definition, the proposed method yields the best estimates of R available by using the largest sample size from the model and subsequently better-behaved ETS values and is, therefore, recommended for all applications of ETS for small verification areas.

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Chung-Chieh Wang
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Chung-Chieh Wang

Abstract

A strong dependency of model performance in quantitative precipitation forecasts (QPFs) as measured by scores such as the threat score (TS) on rainfall amount (i.e., the better the model performs when there is more rain), is demonstrated through real-time forecasts by the 2.5-km Cloud-Resolving Storm Simulator (CReSS) for 15 typhoons in Taiwan in 2010–12. Implied simply from the positive correlation between rain-area sizes and scores, this dependency is expected to exist in all regions, models, and rainfall regimes, while for typhoon QPFs in Taiwan it is also attributed to the model’s capability to properly handle (within 72 h) the processes leading to more rain, which are largely controlled by the typhoon’s track, size, structure, and environment, and the island’s topography. Because of this dependency, the performance of model QPFs for extreme events can be assessed accurately only when forecasts targeted for periods of comparable rainfall magnitude are included for averaging. For the most-rainy 24 h of the top-5 typhoons, the 0–24-h QPFs by CReSS have mean TS of 0.67, 0.67, 0.58, 0.51, and 0.32 at thresholds of 25, 50, 130, 200, and 350 mm, and 0.64, 0.57, 0.37, 0.33, and 0.22 from 48–72-h QPFs, respectively, suggesting superior performance even 2–2.5 days in advance. These scores are strikingly high, precisely because Taiwan can receive extreme rainfall from typhoons. For smaller (nonhazardous) events, the mean scores are progressively lower, but also unimportant and less representative statistically. Therefore, it is inappropriate to average scores over multiple forecasts as those for less-rainy periods would contaminate the result for key periods. The implication for forecasters is also discussed.

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Chung-Chieh Wang and George Tai-Jen Chen

Abstract

An observational study was performed on a leeside mesolow case that occurred 8 June 1987 during the Taiwan Area Mesoscale Experiment (TAMEX). The case developed as prevailing west-southwest winds strengthened and interacted with the terrain of Taiwan, with a Froude number (Fr = U/Nh) increasing from below 0.3 to over 0.5. A quasi-stationary mesolow formed to the southeast of Taiwan with no closed circulation through most of its 13-h life stages until passage of the Mei-yu front. A stationary and localized mesovortex also formed about 90 km southwest of the low center, but little adjustment was observed between the mesolow and the vortex.

Results suggest that airflow at lower levels was blocked and moved around the terrain of the southern Central Mountain Range (CMR). This led to the formation of low-level jets (LLJs) both to the northwest and southeast of Taiwan. The latter branch provided shear vorticity in the background region of the vortex. Air parcels at higher levels, on the other hand, tended to climb over the mountain and caused precipitation on the windward slope, then subsided at the lee side. The subsidence produced the mesolow through adiabatic warming and drying, which was strongest between 1 and 2 km. Latent heat release at the windward side was estimated to contribute a maximum of about 55%–60% of the total warming. Eventually, as the Mei-yu front moved southward along the eastern coast of Taiwan, the mesolow merged with the front and transformed into a migratory mesocyclone along the front.

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Chung-Chieh Wang and Jeffrey C. Rogers

Abstract

General characteristics of the dynamical and thermal structure and evolution of strong explosive cyclones in the northwestern Atlantic near North America (18 cases) and the extreme northeastern Atlantic near Iceland (19 cases) are compared and contrasted through a composite study. Twice-daily gridded analyses from the European Centre for Medium-Range Weather Forecasts at 2.5° resolution from January 1985 to March 1996 are used. In the process of case selection, it is found that the frequency of rapid cyclogenesis in the Greenland–Iceland region is higher than previously thought, and some of the events can be extremely violent.

Many dynamically consistent differences are found when composite cyclones in the two sectors of the North Atlantic are compared. The upper-level forcing that triggers the development in the northeast Atlantic (NEA) is no less intense at the onset of rapid deepening. The NEA cyclones are also associated with lower static stability and locally concentrated but shallower thermal gradient, with less overall environmental baroclinicity. These factors lead to rapid depletion of available potential energy and result in a faster evolution and a shorter life cycle. Therefore, low-level thermal gradient and upper-level forcing components all weaken immediately after rapid deepening. The low-level incipient low in the NEA composite is also stronger, with a distinct potential vorticity (PV) anomaly visible at least 24 h prior to most rapid deepening, and the development produces a more pronounced warm core seclusion. Explosive cyclones in the northwest Atlantic, on the other hand, tend to have a higher stability and a greater amount of environmental baroclinicity, with temperature gradients in a broader area and deeper layers. These factors correspond to slower evolution and a longer life cycle.

For cases in the NEA near Iceland, it appears that both upper-level forcing and initial system strengths affect the maximum deepening rate. The close proximity of this region to the high PV reservoir in the lower stratosphere is helpful in the generation of very strong forcing and a violent development under favorable synoptic conditions, when a “parent cyclone” with appreciable strength exists to the north/northeast of the incipient system.

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George Tai-Jen Chen, Chung-Chieh Wang, and An-Hsiang Wang

Abstract

During 8–14 June 2000, a 500-hPa blocking event occurred over Mongolia and northern China (near 45°N, 108°E), which was the only case over this region in June since 1981. As the block developed, the initially weak low-level mei-yu front over southern China evolved into a system with strong baroclinity and subsequently moved south. The frontal passage over Taiwan caused temperatures to drop by 10°C, the largest in June over two decades. Using gridded analyses, manually analyzed weather maps, and satellite and surface data, the present study investigates the evolution of this mei-yu front under the influence of the block. The 925-hPa frontogenetical function is computed and effects of different processes are discussed. As the blocking event developed, concurrent ridge–trough amplification in the lower–midtroposphere produced a reversed thermal pattern. The lower-tropospheric high moved southward, and large-scale confluence and deformation were enhanced between the northerly flow and the prefrontal southwesterly flow. The location of the block, to the west-southwest of the Okhotsk Sea area, allowed it to affect the front over southern China and caused it to penetrate inside 20°N, unusual for the month of June. The distribution of the frontogenetical function indicated that the mei-yu frontogenesis and the maintenance of the front were attributed to both deformation and convergence. These two processes together counteracted the strong frontolysis along the frontal zone from diabatic effects, caused by evaporative cooling of frontal precipitation on the warm side and stronger sensible heat transfer (and daytime heating over less cloudy areas) on the cold side of the front. When deformation, convergence, and diabatic effects were all combined, the net total frontogenesis peaked slightly ahead of the frontal zone, thus contributing to the southward propagation of the front in addition to the advection by postfrontal cold air in the present case. When the front moved into the South China Sea, the cross-frontal thermal gradient diminished rapidly, mainly due to the frontolytic effect from sensible heat flux over warm waters.

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Jong-Hoon Jeong, Dong-In Lee, and Chung-Chieh Wang

Abstract

In this study, an extreme rainfall-producing quasi-stationary mesoscale convective system (MCS) associated with the Changma front in southeastern South Korea is investigated using numerical simulations and sensitivity tests. A record-breaking rainfall amount was recorded in response to repeated initiation of new cells (i.e., back-building) over the same area for several hours. The aim of this study is to realistically simulate and analyze this extreme rainfall event to better understand an impact of the cold pool that leads to the quasi-stationary MCS over southeastern South Korea by using a convection-allowing-resolution (2 km) nonhydrostatic atmospheric model.

The control experiment (CNTL) was successfully performed, yielding the quasi-stationary, back-building MCS at approximately the correct location and time. In the CNTL run, diabatic cooling due to evaporation of raindrops was responsible for the formation of the cold pool. The development of the cold pool was responsible for the deceleration of the propagating convective line, which played a role in the stalling of the MCS over southeastern South Korea. Moreover, new convective cells were repeatedly initiated in the region where an oncoming warm inflow met the leading edge of the cold pool and was uplifted. In an experiment without evaporative cooling (NOEVA), the simulated precipitation pattern was shifted to the northeast because the MCS became nonstationary without the cold pool. The cold pool had an essential role in the stationarity of the MCS, which resulted in extreme rainfall over the Busan metropolitan area.

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Chung-Chieh Wang, George Tai-Jen Chen, and Shin-Yi Huang

Abstract

In this study, the heavy-rainfall event over central Taiwan during the mei-yu season on 8 June 2007 is investigated, with an emphasis on the triggering mechanism for the deep convection that produced the rain. Observations indicate that there existed two lines of forcing with convection prior to the rain: one over the northern Taiwan Strait along the mei-yu front and the other over the southern Taiwan Strait. Yet, the convection in question developed over the central strait between these two lines, in an unstable environment with strong westerly vertical wind shear. This motivated the authors to carry out the present study.

The Cloud-Resolving Storm Simulation (CReSS) of Nagoya University was used and the event was reproduced at a horizontal grid size of 2 km, including the initiation of new convection over the central strait at the correct location and time. The model results suggest a crucial role played by the series of active, persistent, and propagating storms in the southern strait (along the aforementioned second forcing line). On their back (northern) side, these storms repeatedly produced pulses of cold outflow that traveled toward the north-northeast with positive pressure perturbation. With characteristics of gravity waves, the perturbation propagated faster than the cold air and the associated increase in forward-directed (horizontal) pressure gradient force led to northward acceleration of near-surface flow (by up to 4–5 m s−1 h−1). The stronger southerly flow in turn enhanced downstream convergence, and the deep convection was triggered in the central strait near the arrival of the gravity wave ahead of the cold air. When the convection moved eastward over Taiwan, heavy rainfall resulted. The mechanism presented here for remote triggering of convection over the ocean has not been documented near Taiwan during the mei-yu season. With a better understanding about the behavior of convection, these results can contribute to the improvement of quantitative precipitation forecasts and hazard prevention and reduction.

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Chung-Chieh Wang, George Tai-Jen Chen, and Kuok-Hou Ho

Abstract

After advancing southward across Taiwan and becoming quasi stationary, a mei-yu front moved north again and led to a second period of significant rainfall during 13–14 June 2012. Associated with this frontal retreat, a meso-α-scale low pressure developed to the southwest of Taiwan, in the proximity of organized mesoscale convective systems (MCSs) along and south of the front over the northern South China Sea. In this study, using mainly the European Centre for Medium-Range Weather Forecasts gridded analyses, the physical mechanisms of this frontal retreat are investigated and diagnosed, with a focus on the initial retreat and the role played by the deepening frontal low.

The diagnoses employing the vorticity equation and frontogenetical function both indicate that the appearance of southerly winds, and thus the retrogression of cold air north of the front was the cause of the initial frontal retreat, consistent with earlier studies. The potential vorticity diagnosis using the piecewise inversion technique further confirms that the deepening low over the southern Taiwan Strait provided the southerly winds east of Taiwan where the retreat started, while the low itself intensified in response to the persistent latent heating by the active and organized MCSs. Thus, the northerly winds on the cold side of the front near Taiwan were replaced by southerly winds, and the mei-yu front in the present case retreated and essentially became a warm front. While mei-yu frontal retreats near Taiwan are more frequent than previously recognized, the present case was the most significant event in three seasons during 2012–14.

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George Tai-Jen Chen, Chung-Chieh Wang, and Li-Fen Lin

Abstract

During 7–8 June 1998, an organized mesoscale convective system (MCS) formed within the mei-yu frontal cloud band and moved northeastward to produce heavy rain over the island of Taiwan. During this period, the section of the mei-yu front east of Taiwan moved northward, most significantly for about 300 km over 12 h. Meanwhile, a low-level jet (LLJ) developed within the environmental southwesterly flow to the south of the mei-yu front and the MCS.

Observations revealed that the front retreated as low-level meridional wind components over the postfrontal region shifted from northerly to southerly. Using European Centre for Medium-Range Weather Forecasts (ECMWF) analyses with piecewise potential vorticity (PV) inversion technique and other methods, a diagnostic study was carried out to investigate the northward frontal movement and the formation of the LLJ.

Results indicated that diabatic latent heating from the MCS, large enough in scale, generated positive PV and height fall at low levels. The enhanced height gradient induced northwestward-directed ageostrophic winds and the LLJ formed southeast of the MCS through Coriolis torque. The southwesterly flow associated with this diabatic PV perturbation led to rapid retreat of the frontal segment east of Taiwan at a speed of about 25 m s−1, while the movement was dominated by horizontal advection in the present case. During this process of readjustment toward geostrophy, a thermally indirect circulation also appeared over and south of the front, and the LLJ formed within its lower branch at 850 hPa. The enhanced southwesterly winds reached LLJ strength because they were superimposed upon a background monsoon flow at the same direction. To the lee of Taiwan, the topography also played the role in enhancing local wind speed at lower levels and contributed toward the frontal retreat at nearby regions.

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