No abstract

The present study investigates the characteristics of low-level jets (LLJs) (≥12.5 m s−1) below 600 hPa over northern Taiwan in the mei-yu season and their relationship to heavy rainfall events (≥50 mm in 24 h) through the use of 12-h sounding data, weather maps at 850 and 700 hPa, and hourly rainfall data at six surface stations during the period of May–June 1985–94. All LLJs are classified based on their height, appearance (single jet or double jet), and movement (migratory and nonmigratory). The frequency, vertical structure, and spatial and temporal distribution of LLJs relative to the onset of heavy precipitation are discussed. Results on the general characteristics of LLJs suggest that they occurred about 15% of the time in northern Taiwan, with a top speed below 40 m s−1. The level of maximum wind appeared mostly between 850 and 700 hPa, with highest frequency at 825–850 hPa. A single jet was observed more often (76%) than a double jet (24%), while in the latter case a barrier jet usually existed at 900–925 hPa as the lower branch. Migratory and nonmigratory LLJs each constituted about half of all cases, and there existed no apparent relationship between their appearance and movement. Migratory LLJs tended to be larger in size, stronger over a thicker layer, more persistent, and were much more closely linked to heavy rainfall than nonmigratory jets. They often formed over southern China between 20° and 30°N and moved toward Taiwan presumably along with the mei-yu frontal system. Before and near the onset of the more severe heavy rain events (≥100 mm in 24 h) in northern Taiwan, there was a 94% chance that an LLJ would be present over an adjacent region at 850 hPa, and 88% at 700 hPa, in agreement with earlier studies. Occurrence frequencies of LLJs for less severe events (50–100 mm in 24 h) were considerably lower, and the difference in accumulative rainfall amount was seemingly also affected by the morphology of the LLJs, including their strength, depth, elevation of maximum wind, persistence, proximity to northern Taiwan, source region of moisture, and their relative timing of arrival before rainfall. During the data period, about 40% of all migratory LLJs at 850 or 700 hPa passing over northern Taiwan were associated with heavy rainfall within the next 24 h. The figure, however, was much lower compared to earlier studies, and some possible reasons are offered to account for this deficit.

During the morning hours on 23 May 2002, a convective line associated with a mei-yu front brought heavy rainfall along the coast of central Taiwan under favorable synoptic conditions of warm air advection and large convective available potential energy (CAPE) of over 3000 m 2 s Ϫ2 . Doppler radar observations indicated that deep convection was organized into a linear shape with a northeast-southwest orientation along the front about 70 km offshore from Taiwan over the northern Taiwan Strait. The system then moved toward Taiwan at a slow speed of about 4 m s Ϫ1 . In the present study, the effects of Taiwan topography on this convective line and subsequent rainfall distribution were investigated through numerical modeling using the Nagoya University Cloud-Resolving Storm Simulator (CReSS) at a 2-km horizontal grid size. Experiments with different terrain heights of Taiwan, including full terrain (FTRN), half terrain (HTRN), and no terrain (NTRN), were performed. The control run using full-terrain and cold rain explicit microphysics realistically reproduced the evolution of the convective line and the associated weather with many fine details.Two low-level convergence zones were found to be crucial in the development of this convective line and the subsequent rainfall distribution over Taiwan. The first was along the mei-yu front and forced mainly by the front, but was terrain enhanced off the northwestern coast of Taiwan due to the blocking of air on the windward side of the Central Mountain Range (CMR). After formation, convective cells along this zone propagated southeastward and produced rainfall over the northwestern coast. As the front moved closer to Taiwan, a second arc-shaped convergence zone with a nearly north-south orientation along about 120°E formed ahead of the front between the prevailing flow and near-surface offshore flow induced by the blocking. This second zone was terrain induced, and convection initiated near its northern end was found to be responsible for the rainfall maximum observed near the coast of central Taiwan. Its intensity and position were highly sensitive to terrain height. In the HTRN run where the terrain was reduced by half, a weaker zone closer to the CMR (by about 50 km) was produced, and the rain fell mostly over the windward slope of the terrain instead of over the coastal plain. When the terrain was removed in the NTRN run, no such zone with the correct orientation formed. It was also found that the frontal movement near northern Taiwan was slightly delayed with the presence of terrain, and this affected the timing and distribution of local rainfall during the later stages of this event.

The present study has used the Geostationary Meteorological Satellite (GMS) IR brightness temperature observations to investigate the regional and intraseasonal variability of east Asian warm-season cloud/precipitation episodes (in distance–time space) due to land–sea contrast and latitudinal effects. The data period was May–August 1998–2001, and harmonic analysis was employed as the major tool for analysis. The full domain of study (20°–40°N, 95°–145°E) was divided into northern and southern zones, and into eastern and western sectors, and statistics of episodes in each subregion were derived and compared. For latitudinal effects, episodes were found to be significantly larger in span and duration in northern (30°–40°N) than in southern (20°–30°N) zones. In the northern zone, the propagation characteristics were also stronger and remain evident even in midsummer, while episodes south of 30°N reversed in direction and traveled westward in July and August. For land–sea contrast, the May–August transition over land (western sector, 95°–120°E) was mainly characterized by an increase in diurnal activities, while that over ocean (eastern sector, 120°–145°E) was characterized by decreased overall activities instead. Over the land itself, significant regional variability also existed, with strongest diurnal signals over the eastern Tibetan Plateau near 100°E, and increased diurnal activities over mountain areas in southeastern China since June. Between the two bands, near 107°E, semidiurnal signals were relatively strong and became dominant in June. This double-peaked structure in the diurnal cycle resulted from overlying signals of convection propagating eastward off the plateau with those induced locally in late afternoon, and the phenomenon was more evident in May–June. Over the ocean, on the other hand, both diurnal and semidiurnal waves had small amplitudes, and the regional variability was much weaker. For intraseasonal transition, the number of large episodes was reduced from May through July, as was mean propagation speed. In August, however, some larger events started to reappear over east Asia.

Abstract. During 11-12 June 2012, quasistationary linear mesoscale convective systems (MCSs) developed near northern Taiwan and produced extreme rainfall up to 510 mm and severe flooding in Taipei. In the midst of background forcing of low-level convergence, the back-building (BB) process in these MCSs contributed to the extreme rainfall and thus is investigated using a cloud-resolving model in the case study here. Specifically, as the cold pool mechanism is not responsible for the triggering of new BB cells in this subtropical event during the meiyu season, we seek answers to the question why the location about 15-30 km upstream from the old cell is still often more favorable for new cell initiation than other places in the MCS.With a horizontal grid size of 1.5 km, the linear MCS and the BB process in this case are successfully reproduced, and the latter is found to be influenced more by the thermodynamic and less by dynamic effects based on a detailed analysis of convective-scale pressure perturbations. During initiation in a background with convective instability and near-surface convergence, new cells are associated with positive (negative) buoyancy below (above) due to latent heating (adiabatic cooling), which represents a gradual destabilization. At the beginning, the new development is close to the old convection, which provides stronger warming below and additional cooling at mid-levels from evaporation of condensates in the downdraft at the rear flank, thus yielding a more rapid destabilization. This enhanced upward decrease in buoyancy at low levels eventually creates an upward perturbation pressure gradient force to drive further development along with the positive buoyancy itself. After the new cell has gained sufficient strength, the old cell's rear-flank downdraft also acts to separate the new cell to about 20 km upstream. Therefore, the advantages of the location in the BB process can be explained even without the lifting at the leading edge of the cold outflow.