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Yi-Leng Chen

Abstract

A diagnostic analysis of the net cloud mass flux for the mean state of Phase III is presented. In the upper troposphere, the environmental mass flux is shown to be slightly larger than the adiabatic sinking motion required by radiative cooling. At the outflow layer of tall clouds, if the moistening effect due to detrainments is balanced primarily by the drying caused by adiabatic sinking in the environment, the computed net detrainment rate would be underestimated. It is hypothesized that a considerable portion of the moisture detrained from tall clouds is transported horizontally toward higher latitudes by horizontal eddy fluxes.

The diagnosed environmental subsidence has a maximum in the midtroposphere and is less than 1.5 times the sinking motion required by the radiative cooling in low levels. The vertical profile of the diagnosed net detrainment rate suggests three primary detrainment layers: below 800 mb, between 700 and 500 mb, and above 300 mb.

It is shown that overprediction of the cloud liquid water content from a one-dimensional entrainment cloud model will lead to spurious shallow cloud population and an overestimate of environmental sinking motion in low levels. However, the computed detrainment rate in middle and high levels and the computed net cloud mass flux in high levels are not sensitive to the predicted cloud liquid water content.

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Yi-Leng Chen

Abstract

Although the term “Mei-Yu” is widely used to describe the early-summer rainy season of Taiwan, it is shown that during TAMEX (Taiwan Area Mesoscale Experiment), most of the surface fronts over southern China differ from the classical model of a “Mei-Yu front” found in the earlier literature. These fronts occur before the seasonal transition in mid-June, during which the upper-level south Asian anticyclone advances northward with considerable tropospheric warming over the China plain. In contrast to Mei-Yu fronts over central China, which occur after. the seasonal transition, most of these fronts possess baroclinic characteristics.

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Yi-Leng Chen

Abstract

During the entire period of GATE, all oceanic squall lines over the A/B array developed in the vicinity of the near-equatorial convergence zone (NECZ) when a strong midtropospheric easterly jet along the leading and southern borders of a Saharan air outbreak was observed at the Acad. Korolov. The peak magnitude of the jet, averaged between 700 and 600 mb, exceeded 16 m s−1 for all squall line occurrences. Phase III easterly wave activity was the most pronounced of the three phases. Every wave of Phase III with a Saharan air outbreak preceding the trough-produced squall lines. It is suggested that the presence of the Saharan air ahead of the trough is favorable for the development of squall lines in the vicinity of NECZ over the eastern Atlantic.

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Jiuhua Feng and Yi-Leng Chen

Abstract

Numerical experiments are performed with the fifth generation Mesoscale Model to study the evolution of island airflow, thermodynamic fields, and clouds over the island of Hawaii at night. The model has successfully simulated the major observed features associated with the nocturnal flow regime. These include the formation of nocturnal inversion, development, gradual offshore extension and deepening of the katabatic flow, shifting of the overall cloudy areas from the windward slopes to the ocean, and generation of clouds within the simulated offshore convergence zone. Furthermore, it is also shown that rain evaporative cooling affects the depth, strength, and offshore extension of the katabatic flow.

In the early evening, the nocturnal cooling provides the land–sea thermal contrasts for the development of the simulated downslope flow on the windward slopes. With continued nocturnal cooling and rain evaporation, the simulated katabatic flow extends toward the coast. The simulated convergence zone and overall cloudy areas also move seaward. Throughout the night, the simulated cold air near the surface moves down the slope to the coast within the katabatic flow layer. The simulated offshore flow deepens and extends farther over the ocean. In the early morning, the simulated offshore extent of the katabatic flow reaches 20 km with a relatively deep (∼300 m) offshore flow. Clouds frequently form within the offshore convergence zone. They move westward in the model and weaken over the deep offshore flow before reaching the coast.

In the model, the noctural cooling not only affects the near-surface airflow over the slope surface and coastal areas, but also the airflow aloft and upstream. With the development of the descending katabatic flow over the slope surface and coastal areas because of nocturnal cooling, stronger easterly winds are simulated aloft with much weaker incoming trade winds upstream in low levels than without nocturnal cooling. The simulated low-level flow deceleration is most significant early in the morning. At that time, the simulated offshore flow has the largest horizontal extent with a maximum depth.

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Yoshi Ogura and Yi-Leng Chen

Abstract

No abstract available.

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Yi-Leng Chen and Jiuhua Feng

Abstract

Both the daily rainfall on the windward side of the island of Hawaii and the early morning (0200 HST—Hawaiian standard time) trade-wind inversion height at Hilo varied substantially during the Hawaiian Rainband Project (HaRP). In general, the inversion was higher during the passage of a cold front to the north or during the approach of tropical disturbances from the east to the south. The daily rainfall on the windward side is highly correlated with the 0200 HST inversion height at Hilo with a maximum correlation greater than 0.7 in the Hilo area. The daily daytime (1100–1900 HST) rainfall on the windward slopes, nocturnal (1900–0300 RST) showers an the windward lowland, and coastal precipitation in the predawn and early morning hours (0300–1100 HST) are also highly correlated with the inversion height with a maximum correlation greater than 0.7, 0.7, and 0.5, respectively. For the low- (high-) inversion days, the median daily rainfall on the windward side is about one-half (more than twice) of the HaRP median daily rainfall.

On the high-inversion days, the afternoon clouds around the island wore closer to the summits of Mauna Loa and Mauna Kea than the low-inversion days because the upslope flow can bring the moist air from low levels to high elevations. The differences in the mean daily resultant winds between the low- and the high-inversion days are characterized by a weak northerly wind component on the windward lowland and an easterly wind component over the northern part of the island and at the southern tip of the island. It appears that slightly more air is forced to move around the island when the inversion is lower. For both the high- and low-inversion days, the surface airflow on the windward side is dominated by the daytime upslope flow and the nighttime downslope flow. For the ten high-inversion days, the modifications of the surface airflow by the effects of precipitation on the windward side are more significant than the ten low-inversion days because the rainfall amount is usually larger when the inversion is higher.

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Jiuhua Feng and Yi-Leng Chen

Abstract

The data collected on 10 August 1990 from the Hawaiian Rainband Project were analyzed to study the evolution of katabatic flow. Results from this study show that for a relatively dry case thermal forcing is important to account for the onset, evolution, depth, and offshore extension of the katabatic flow on the windward side of the island of Hawaii where the mean winds are weak because of island blocking.

During the evening transition, the initiation of katabatic flow on the windward slopes was mainly driven by the diurnal heating cycle. At the windward coast, the wind shift was caused by the arrival of the drainage front from the windward lowlands. After the arrival of the drainage front at Hilo, slopewise instability was generated because of rain showers along the coast and diminishing orographic clouds and showers on the slopes. Along the coast, because of the reduced radiative cooling due to cloud cover and vertical mixing associated with rains, a warm period was observed at Hilo. On the slopes, the katabatic flow dissipated the orographic clouds and produced strong surface radiative cooling. As a result, the surface air on the lower slopes became potentially colder than the surface air along the coast and continued to move downward toward the coast. Furthermore, because of surface radiative cooling, the surface air at higher elevations moved to lower elevations along the isentropic surfaces but remained within the katabatic flow. With continued cold air advection down the slope under opposing trade winds, the nocturnal inversion was deeper along the coast than on the windward slopes.

Throughout the night, the katabatic flow extended gradually over the ocean. A narrow updraft of ∼0.5 m s−1 was observed along the leading edge of the offshore flow, which resembled a density current. The location of the convergence zone between the offshore flow and the incoming trade winds is related to the offshore extension of katabatic flow and is not solely determined by the upstream Froude number.

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Yi-Leng Chen and Jun Li

Abstract

The large-scale processes responsible for development of heavy precipitation during 20–23 May 1987 along the southeastern China coast are studied. There are two distinct rainfall peaks around 0000 UTC 20 May and 0000 UTC 22 May. Prior to the heavy rain event, strong low-level southwesterly flow developed ahead of a 850-hPa trough and transported warm, moist air from the south into southeastern China to generate the convective instability. For the first rainfall period, positive vorticity advection by thermal winds was observed ahead of the 850-hPa trough. Upper-level divergence as a result of the imbalance between winds and geopotential height fields was found in the different airflow region on the northeastern quadrant of the south. Asian anticyclone prior to the first rainfall period. For the second rainfall period, an upper-level trough deepened in the lee side of the Tibetan Plateau and moved to the southeastern China coast. Strong upper-level frontogenesis caused by horizontal deformation was found along the trough axis. The heaviest precipitation occurred along the low-level warm, moist tongue when the low-level baroclinic forcing was coupled with the upper-level one as the upper-level trough approached. These results are in contrast to a CISK (conditional instability of the second kind) process proposed by Chen and Chang for a Mei-Yu case in mid-June over southern China.

Analyses of the large-scale heat and moisture budgets show a vertical separation of Q 1 and Q 2 peaks during the first rain period, suggesting that the rainfall is convective in nature. For the heaviest rain period, the Q 1 peak increases and shifts upward. The Q 2 profile shows a double-peak structure with the upper-level peak coinciding with the Q 1 peak but with a much smaller magnitude, indicating that the precipitation mainly falls from convective clouds and the anvils associated with them.

Over the Taiwan area, heavy rain did not develop as forecasted, because of the weakening of both the upper-level and low-level troughs and the lack of coupling between the upper-level and low-level forcings. When the low-level southwesterly flow interacted with the island of Taiwan, the low-level flow moved around the island with a windward ridge-leeside trough pressure pattern. Along the western coast, the wind component parallel to the central mountain range increased northward down the pressure gradient and had a maximum value (≥14 m s−1) at approximately 1.5 km above sea level over the northwestern coast. With the turning of the large-scale low-level flow from south-southwest to west-southwest and the strengthening of the southwesterly flow, the surface pressure ridge became stronger and extended northward along the western coast of Taiwan. The deflection and flow deceleration upstream were also more significant.

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Jun Li and Yi-Leng Chen

Abstract

Nocturnal rain showers over the windward side of the island of Hawaii were investigated from the late afternoon of 2 August to the early morning of 3 August 1990 during the Hawaiian Rainband Project (HaRP). Three types of rainbands produce rainfall peaks over the lowland/coastal region during the evening, after midnight, and along the coast in the early morning.

In the early evening, an inland rainband develops over the lower slopes as a result of orographic lifting and low-level forcing along the drainage front. As the drainage front progresses toward the coast, new rain cells continue to develop along the drainage front. These cells move westward with the trade winds aloft and dissipate. After the drainage front moves to the Hilo coast, new cell generation along the drainage front ceases. It appears that in the absence of orographic lifting, the shallow (∼0.2 km) katabatic flow offshore is not deep enough to lift the low-level air to saturation. Thus, the rainband weakens and dissipates. It lasts about 6 h and produces the heaviest rainfall over the lowland/coastal region during the analysis period.

During the evening transition, rain evaporative cooling aloft deepens the cool pool behind the drainage front over land. After the drainage front moves off the coast around 2300 HST, cold air continues to move from lower slopes and lowlands toward the coast. The offshore flow gradually deepens. By ∼0207 HST, the depth of the katabatic flow over the coastal region reaches 0.48 km, which is above the level of free convection. A rainband develops along the drainage front offshore. As the rainband moves westward over the deepened katabatic flow, the katabatic flow is replaced by easterly winds. The rainband weakens and dissipates over the lower slopes.

During the early morning, two groups of trade wind rain showers move into the coastal region. They are enhanced offshore in the convergent zone between the offshore flow and the trade winds. When the first group of rain showers moves over the katabatic flow, the outflow associated with the rain showers deepens the offshore flow (∼0.6 km) east of the rain showers. The second group of rain showers is more intense than the first group as the rain showers move into the convergent zone and interact with the deep offshore flow. Nevertheless, they weaken rapidly over the deep offshore flow and produce little rainfall along the coast.

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Yoshmitsu Ogura and Yi-Leng Chen

Abstract

A simple automated objective analysis scheme is developed to analyze upper air sounding data from the National Severe Storm Laboratory mesonetwork. This scheme uses a combination of Cressman' successive correction technique and cubic spline curve fitting.

This scheme is applied to a squall line case that occurred on 8 June 1966 along the confluent line where and air from the desert southwest was brought into juxtaposition with moist air from the Gulf of Mexico. The first radar echoes were detected at about 1600 CST and a nearly continuous band was formed by 1830. At 2000 the intense squall line became disorganized and it dissipated by 2300. Serial sounding were started at 1400 and continued until 2300.

The analysis results indicate that a well–defined narrow band of convergence (and consequently upward motion) with a width of about 100 km was present at low levels prior to the appearance of first radar echoes. The location and orientation of the line of maximum upward motion and of the subsequent squall line formation agree well. This low–level upward motion continued to increase in intensity until 1830 with its peak value at 800 mb. A well–defined mixed layer also developed in low levels because of the strong solar heating at the earth's surface. The height of the mixed layer increased and became very close to the lifting condensation level of near–surface air by early afternoon. The low–level convergence then provided the final push needed to release the potential instability.

Shortly after 1700, the development of a second maximum in the vertical velocity was observed in the 450–400 mb layers. This second maximum reached its peak value of about 1.7 m s−7 at 2000 and apparently was a manifestation of the development of deep penetrative Invective clouds. The mesoscale heat and moisture budgets are also investigated by calculating the apparent beat source and moisture sink for the mature stage of the squall line. The horizontal distribution of accumulated rainfall estimated from the apparent moisture sink agrees fairly well with that observed. The decay of the squall line occurred when the low–level horizontal moisture convergence became disorganized.

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