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Fang-Ching Chien
and
Ben Jong-Dao Jou

Abstract

This study presents precipitation verification of individual members and ensemble means in the Taiwan area for a real-time mesoscale ensemble prediction system, during the 2000, 2001, and 2002 early summer convective (mei-yu) seasons. The ensemble system, using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) as a forecast model, consists of six members, each run with a different combination of moisture physics schemes. Precipitation forecasts within the 15-km domain were verified against the observational data from the 342 rain gauge stations on the island. In general the model that utilized the Grell cumulus parameterization scheme (CPS) and the Reisner I microphysics scheme (the GR model) had the best forecast skill among the six members. This physics combination is, therefore, recommended for MM5 rainfall simulations in the Taiwan area during the mei-yu season. The Kain–Fritsch CPS dominated the rainfall process and generally underforecast rainfall at high rainfall thresholds. The Betts–Miller CPS overforecast rainfall, especially at high thresholds. The equitable threat scores of the ensemble mean were not the highest, but were, in general, above the average among all members.

Several other methods were examined for determining an ensemble mean (or weighted mean) rainfall forecast. WT1, which calculated rainfall by giving each member weightings determined by model performance of the member in rainfall forecast of the A period (0–12 h), generally outperformed the ensemble mean and every single member in the B period (12–24 h). This advantage did not extend to the C period (24–36 h), because the relation of model performance between the C and the A periods became weaker. WT2, in which weightings were determined according to the performance of each member in rainfall forecasts of the preceding year, performed slightly worse than WT1 in the B period, while it did better than WT1 in the C period. Another method that utilized the multiple linear regression technique to calculate weightings also showed positive impact on improving the rainfall forecast at medium to heavy precipitation thresholds. Unfortunately, its weightings appeared to be inadequate for another year's rainfall forecasts. The probability matching method helped reduce the bias problem inherent in the ensemble mean.

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Cheng-Ku Yu
and
Ben Jong-Dao Jou

Abstract

This study documents offshore convective lines along the southeastern coast of Taiwan, a frequent but poorly understood mesoscale phenomenon that influences coastal weather during the Taiwan mei-yu season. Doppler radar and surface observations were gathered from a specially chosen period (11–15 May 1998) when the offshore convective lines were active off the southeastern coast of Taiwan. These observations were used to show the basic character, structure, and possible formative processes of offshore convective lines. The synoptic environment accompanying these events was found to be relatively undisturbed and featured uniformly prevailing southerly/south-southeasterly winds in the boundary layer with southwesterlies/westerlies aloft. Examination of radar data during the study period indicates that the lines generally occurred ∼10–30 km offshore and were characterized by an elongated narrow zone (∼5–10 km wide) of heavy precipitation. The lines were oriented roughly parallel to the coastline and generally did not move significantly. The intensity of the radar reflectivity associated with the lines exhibited a marked diurnal variation and was closely related to the coastal offshore flow developing at night.

Detailed analyses of an event on 14–15 May 1998 further show the important physical link between the offshore flow and the development of the line. The offshore line was found to be located near and immediately ahead of the seaward extent of the offshore flow. Particularly, a very narrow zone (∼2 km) of low-level heavy precipitation (40–45 dBZ) coincided with regions of strong updrafts and convergence, where the prevailing southerly onshore flow encountered the cool offshore flow nearshore. This offshore flow–induced convergence, given a stable thermodynamic condition in the lowest ∼1 km in the inflow region, was a crucial low-level forcing that provided lifting to trigger moist deep convection in this case. The line’s precipitation tilt eastward was confined primarily to the warmer inflow side rather than feeding the offshore flow to the west of the line. No consistent upshear tilt of updrafts throughout the storm layer was observed, which is consistent with the presence of a strong westerly shear in the line’s environment. Both of these observations explain a relatively strong (weak) modification of low-level onshore (offshore) flow by precipitation. Additionally, a combination of surface and Doppler radar observations indicates that the leading edge of the offshore flow moved seaward very slowly at 0.7 m s−1 and possessed a frontal character with notable discontinuities in near-surface wind and temperature (instead of pressure and dewpoint temperature).

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Yu-Cheng Kao
and
Ben Jong-Dao Jou

Abstract

Using coastal radar and surface observations of Taiwan, an investigation of intensity and structure variations in the inner core of Typhoon Haitang (0505) is conducted. Within a 3-h period (1933-2233 UTC 17 July 2005), Haitang experienced intensity vacillation with merging of the eyewall with a rainband induced by coastal barrier jet (CBJ), concentric eyewall breakdown and weakening, and eyewall recovery, contraction, and re-intensification. The northerly flow of the CBJ converged with the southerly flow of a leeside meso-low to form a west-east line of convection south of the storm when the storm was still 100 km offshore. The rainband propagated radially inward and triggered eyewall–rainband interaction. The interaction resulted in approximately 30% amplification of precipitation and 20% decrease in the axisymmetric tangential wind. Barotropic instability is speculated to be the underlying dynamic process. The recovery of the eyewall, following nearshore eyewall axisymmetrization and contraction, resulted in a 40% intensity increase before landfall.

Open access
Cheng-Ku Yu
,
Ben Jong-Dao Jou
, and
David P. Jorgensen

Abstract

Airborne Doppler radar and flight-level measurements were used to document the three-dimensional thermodynamic structure of a convective line that developed off the southeastern coast of Taiwan on 16 June 1987 during the Taiwan Area Mesoscale Experiment. During the period of aircraft observation the convective line appeared to be quasi-stationary in a region ∼90 km away from the coast. Calculations of perturbation pressure and buoyancy retrieved from the Doppler-radar-derived wind fields show that a high pressure perturbation associated with negative buoyancy existed in the region of heavy precipitation in the lowest level. In low levels, an elongated zone of low pressure was documented along the line, which was closely related to the distributions of buoyancy. These retrieved features are basically consistent with those calculated from in situ observations. In mid- to upper levels, the high (low) pressure perturbation lay around the updraft cores on the upshear (downshear) of environmental shear. The quantitative diagnosis for the perturbation pressure indicates that the positive buoyancy associated with the in-cloud latent heat release as well as evaporative cooling occurring near cloud base and subcloud regions is crucial to determine the low-level perturbation pressure distributions. However, dynamic forcing contributing to the pressure perturbation cannot be ignored in middle levels, because of updraft–shear interaction due to the presence of pronounced cross-line wind shear and corresponding updrafts reaching maximum values near midtroposphere.

In distinct contrast to squall lines, which are rapidly moving, the near-surface temperature gradient across this quasi-stationary line was very weak, and at low levels no gust front, rear inflow, or dynamically induced high pressure was evident near the leading edge of the line. Analyses presented here suggest that the persistent, stationary convergence, in association with the deceleration of low-level southeasterly flow as it approached the nearshore blocked flow, played an important role in maintaining and organizing the deep convection in this case.

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Cheng-Ku Yu
,
Ben Jong-Dao Jou
, and
Bradley F. Smull

Abstract

The formative stage of a long-lived mesoscale cyclonic vortex was captured by the NOAA P-3 aircraft as it investigated a developing mesoscale convective system (MCS) near the southeastern coast of Taiwan on 16 June 1987 during the Taiwan Area Mesoscale Experiment. The supporting environment of the mesovortex was characterized by an exceptionally moist atmosphere and moderate ambient vertical shear through a deep layer from the near surface to ∼6 km, with much weaker shear and winds aloft. In addition, a pronounced low-level mesoscale shear/convergence zone, which resulted from the interaction of southeasterly flow with northeasterly flow confined to the near-coast region, existed in the vicinity of the observed mesovortex. Composite three-dimensional wind fields derived via pseudo-dual-Doppler synthesis show the vortex had a horizontal diameter expanding from ∼40 km to ∼70 km in the lower to midtroposphere, respectively, and exhibited considerable tilt through this layer. Contrary to previously documented mesovortices, which have generally been fully developed and observed in the stratiform region of mature-to-decaying MCSs, the present vortex was intimately coupled to convective precipitation within this developing MCS. This study provides unique observational evidence that under appropriate environmental conditions a long-lasting mesovortex may originate in the convective region of an MCS.

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David P. Jorgensen
,
Margaret A. LeMone
, and
Ben Jong-Dao Jou

Abstract

The precipitation, thermodynamic, and kinematic structure of an oceanic mesoscale convective system is studied using airborne Doppler and in situ (flight-level) data collected by the NOAA P-3 aircraft. The system, a well-organized, stationary, north-south convective line, was located near the east coast of Taiwan. In Part I, the basic structure of the line is documented with both datasets, a procedure revealing the strengths and weakness of both approaches.

The Doppler data reveal that the warm, moist air feeding the line enters from the east side. Most updrafts associated with the leading edge of the convective line tilt westward below 5 km and then eastward above 5 km. This change of tilt corresponds to a change in the sign of the vertical flux of east-west momentum. To the east of the leading edge, a 10-km-wide zone of strong mesoscale descent is seen. The band is not a complete barrier to the low-level southeasterly flow, and at times and places along the line the inflowing air can move through the band with little or no upward acceleration. The minimum pressures at low levels lie east of the highest reflectivity and also underneath the tilted updraft at upper levels, in agreement with the tilt of the updraft, the buoyancy distribution, and the interaction of the updraft with the vertical shear of the horizontal wind. The Doppler data show very few convective-scale downdrafts and no low-level gust front that would organize the convection as in propagating squall lines, although lack of resolution in the pseudo-dual-Doppler data at the lowest levels may mask features with horizontal scales <5 km. Vertical incidence Doppler observations show only a few relatively weak convective-scale downdrafts within the heavy rainfall region of the convective line.

The in situ data confirm that warm, moist air feeds the convective line from the east side, but they show a larger fraction of air coming into the convection from the boundary layer than do the Doppler data. They confirm that the line is not an effective barrier to the flow: some air from the east of the line, including boundary-layer air, passes through the line without joining the updrafts. Again, some weak convective-scale downdrafts are evident, but a gust front was not detected. However, at low levels, a pool of low-θ e , air lies 10–20 km to the west of the line, outside the dual-Doppler domain. This cool air apparently originated to the north (beneath an extensive stratiform area, but preexisting baroclinicity associated with a front may have also contributed to the cool air) and advected southward. Vertically incident Doppler data confirm the upper-level downdraft zone to the east of the updraft. Above 2 km, the pressure and vertical velocity fields are consistent, with low pressure lying beneath the tilting updrafts in both datasets. Below 2 km, the in situ data reveal a mesolow beneath the westward-tilting updraft that was not captured by the Doppler data, apparently because of contamination of the very lowest levels by ground clutter.

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Hsiao-Wei Lai
,
Christopher A. Davis
, and
Ben Jong-Dao Jou

Abstract

This study examines a subtropical oceanic mesoscale convective vortex (MCV) that occurred from 1800 UTC 4 June to 1200 UTC 6 June 2008 during intensive observing period (IOP) 6 of the Southwest Monsoon Experiment (SoWMEX) and the Terrain-influenced Monsoon Rainfall Experiment (TiMREX). A dissipating mesoscale convective system reorganized within a nearly barotropic vorticity strip, which formed as a southwesterly low-level jet developed to the south of subsiding easterly flow over the southern Taiwan Strait. A cyclonic circulation was revealed on the northern edge of the mesoscale rainband with a horizontal scale of 200 km. An inner subvortex, on a scale of 25–30 km with maximum shear vorticity of 3 × 10−3 s−1, was embedded in the stronger convection. The vortex-relative southerly flow helped create local potential instability favorable for downshear convection enhancement. Strong low-level convergence suggests that stretching occurred within the MCV. Higher θe air, associated with significant potential and conditional instability, and high reflectivity signatures near the vortex center suggest that deep moist convection was responsible for the vortex stretching. Dry rear inflow penetrated into the MCV and suppressed convection in the upshear direction. A mesolow was also roughly observed within the larger vortex. The presence of intense vertical wind shear in the higher troposphere limited the vortex vertical extent to about 6 km.

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Pao-Liang Chang
,
Ben Jong-Dao Jou
, and
Jian Zhang

Abstract

A tropical cyclone (TC) eye tracking (TCET) algorithm is presented in this study to objectively identify and track the eye and center of a tropical cyclone using radar reflectivity data. Twelve typhoon cases were studied for evaluating the TCET algorithm. Results show that the TCET can track TC centers for several hours. The longest tracking time is about 35 h. Eye locations estimated from different radars showed consistency with a mean distance bias of about 3.5 km and a standard deviation of about 1.5 km. The TCET analysis shows decreasing eye radius as TCs approach land, especially within 50 km of the coastline.

The TCET algorithm is computationally efficient and can be automated by using the TC center in the previous volume or the estimated center from satellite images as an initial guess. The TCET may not accurately find the TC center when a TC is weak or does not have an enclosed eyewall or when it does have highly noncircular eyes. However, the algorithm is still suitable for operational implementation and provides high spatial and temporal resolution information for TC centers and eye radii, especially for intense TCs.

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Fang-Ching Chien
,
Yi-Chin Liu
, and
Ben Jong-Dao Jou

Abstract

This paper presents an evaluation study of a real-time fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) mesoscale ensemble prediction system in the Taiwan area during the 2003 mei-yu season. The ensemble system consists of 16 members that used the same nested domains of 45- and 15-km resolutions, but different model settings of the initial conditions (ICs), the cumulus parameterization scheme (CPS), and the microphysics scheme (MS). Verification of geopotential height, temperature, relative humidity, and winds in the 15-km grid shows that the members using the Kain–Fritsch CPS performed better than those using the Grell CPS, and those using the Central Weather Bureau (CWB) Nonhydrostatic Forecast System (NFS) ICs fared better than those using the CWB Global Forecast System (GFS) ICs. The members applying the mixed-phase MS generally exhibited the smallest errors among the four MSs. Precipitation verification shows that the members using the Grell CPS, in general, had higher equitable threat scores (ETSs) than those using the Kain–Fritsch CPS, that the members with the GFS ICs performed better than with the NFS ICs, and that the mixed-phase and Goddard MSs gave relatively high ETSs in the rainfall simulation. The bias scores show that, overall, all 16 members underforecasted rainfall. Comparisons of the ensemble means show that, on average, an ensemble mean, no matter how many members it contains, can produce better forecasts than an individual member. Among the three possible elements (IC, CPS, and MS) that can be varied to compose an ensemble, the ensemble that contains members with all three elements varying performed the best, while that with two elements varying was second best, and that with only one varying was the worst. Furthermore, the first choice for composing an ensemble is to use perturbed ICs, followed by the perturbed CPS, and then the perturbed MS.

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Pin-Fang Lin
,
Pao-Liang Chang
,
Ben Jong-Dao Jou
,
James W. Wilson
, and
Rita D. Roberts

Abstract

The spatial and temporal characteristics and distributions of thunderstorms in Taiwan during the warm season (May–October) from 2005 to 2008 and under weak synoptic-scale forcing are documented using radar reflectivity, lightning, radiosonde, and surface data. Average hourly rainfall amounts peaked in midafternoon (1500–1600 local solar time, LST). The maximum frequency of rain was located in a narrow strip, parallel to the orientation of the mountains, along the lower slopes of the mountains. Significant diurnal variations were found in surface wind, temperature, and dewpoint temperature between days with and without afternoon thunderstorms (TSA and non-TSA days). Before thunderstorms occurred, on TSA days, the surface temperature was warmer (about 0.5°–1.5°C) and the surface dewpoint temperature was moister (about 0.5°–2°C) than on non-TSA days. Sounding observations from northern Taiwan also showed warmer and higher moisture conditions on TSA days relative to non-TSA days. The largest average difference was in the 750–550-hPa layer where the non-TSA days averaged 2.5°–3.5°C drier. These preconvective factors associated with the occurrences of afternoon thunderstorms could be integrated into nowcasting tools to enhance warning systems and decision-making capabilities in real-time operations.

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