Island Effects on Mei-Yu Jet/Front Systems and Rainfall Distribution during TIMREX IOP#3
Yi-Leng Chen and Chuan-Chi Tu
Department of Meteorology
SOEST, University of Hawaii
5/31 1400 LST (0600UTC)
I. The island/orographic effects on the complex
Mei-Yu jet/front system and rainfall distribution a b
during the South-West Monsoon Experiment
(SoWMEX)/Terrain-induced Rainfall Experiment
(TIMREX) Intensive Observing Period (IOP) #3 Taiwan
in 2008. Strait
2) 5/30 1800-5/31 1400 (LST), an enhanced NE-SW
orientated Mei-Yu frontal convection line northwest of
Taiwan moved onshore over northwestern Taiwan
and then propagated southward producing heavy
rainfall over northwestern Taiwan
c d H
1) 5/30 0900-1800 (LST), in the 3) 5/31 1400- 2000, after the passage of the Figure 3. (a) 1100-1400 LST e
prefrontal SW flow regime, Mei-Yu front over the Taiwan Strait at the lightening, (b) mosaic radar images H
the convection cells are surface, the Mei-Yu frontal cyclone moved (Courtesy of Central Weather
Bureau), (c) WRF model simulated
enhanced by the terrain. eastward across the Central Mountain
925-hPa water vapor mixing ratio (kg
Orographic lifting of the pre- Range. Orographic lifting of the convective kg-1, shaded) and theta (K,
frontal SW flow and a sea cells by the SW flow ahead of the cyclone, contoured), (d) 925-hPa winds (barbs,
breeze contributed to heavy combined with sea breezes, brought in 1 full barb is 10 m s-1 with wind
rainfall over the western heavy rainfall on the SW slopes of the speed shaded) and geopotential
Central Mountain Range. mountains. height (gpm, contoured) and (e) 10m
winds and sea level pressure (hPa)
Figure 1. Daily rainfall accumulation (mm) over Taiwan on 30 and 31 May, 2008 at 1400 LST using GFS 0.5-deg
(LST) (Courtesy of Central Weather Bureau). resolution data as initial and
boundary conditions (18-h forecast
with a 9-km horizontal resolution).
5/30 1800 LST (1000UTC)
The northeasterly (NE) winds in the southeast quadrant of the pressure ridge along the southeastern
A Mei-Yu frontal mesocyclone was over southeastern China. The NE-SW orientated Mei- China coast brought in relatively dry, cold air to the Taiwan Strait. The NE flow was deflected by the
Yu front associated with the frontal cyclone was off the northwestern Taiwan coast (Fig. 2d). CMR over northern Taiwan. The NE flow in the Taiwan Strait is strengthened by the coastal ridge
The cold air behind the front over southeastern China was retarded by the hilly terrain along southeastern China and the frontal mesocyclone over southwestern Taiwan (Figs. 3c-e).
along the coast but was advected southward along the coast by the strengthened northerly
winds, similar to the cases presented by Chen and Hui (1990; 1992) (Figs. 2c-d). The Central Mountain Range (CMR) split the frontal low into two, with one over western Taiwan
and the other merged with the lee vortex to the east (Fig. 3d). Deep convective cells formed over the
Ahead of the Mei-Yu front, the prefrontal southwesterly (SW) flow over the Taiwan Strait west coast of central Taiwan in the early morning, ahead of the mesocyclone between the deflected
was strengthened due to the presence of the frontal cyclone to the west and an southerly wind along the western coast of Taiwan and the southwesterly flow in the southeastern
orographically induced high to the east over southwestern Taiwan (Fig. 2d). The quadrant of the mesocyclone (not shown). Orographic lifting of convective cells and the
strengthened SW flow upstream of Taiwan was blocked and deflected by the orographically southwesterly winds ahead of the low over western Taiwan, combined with sea breezes at the surface,
induced high with upstream flow splitting over southwestern Taiwan and relatively strong brought in heavy rainfall over the western slope of the southern CMR in the afternoon hours.
winds off the western coast (Li and Chen, 1998).
The convergence along the Mei-Yu front was orographically enhanced by the strengthened
SW flow ahead and NE flow behind the front with a well defined convective line (Fig. 2b).
The convection line brought in heavy rainfall and lightning activity (Fig. 2a) to II. 5/31 Effects of the CMR on the propagating Mei-Yu
northwestern Taiwan as it advanced southward and moved inland. frontal cyclone
a. 12-h run (0800 LST) b. 15-h run (1100 LST) c. 24-h run (2000 LST)
Sensitivity test with terrain removed
c d H d. Model terrain e. 15-h run (1100 LST) f. 24-h run (2000 LST)
Figure 2. (a) 1800-1900 LST lightning, (b) mosaic radar images at 1830 LST (Courtesy of Figure 4. WRF model simulated 925-hPa winds (barbs, 1 full barb is 10 m s-1 with wind speed shaded ) and
Central Weather Bureau), (c) WRF model simulated 925-hPa water vapor mixing ratio (kg geopotential height (gpm, contoured) with 9km horizontal resolution at (a) 0800, (b) 1100 and (c)2000 UTC
kg-1, shaded) and theta (K, contoured) and (d) 925-hPa winds (barbs, 1 full barb is 10 m s-1 31 May 2008, (d) model terrain height (m), model simulation without terrain run at (e) 1100 and (f) 2000 UTC.
with wind speed shaded ) and geopotential height (gpm, contoured) at 1800 LST using GFS
0.5 deg resolution data as initial and boundary conditions (22-h forecast with a 9-km
Acknowledgments Chen, Y.-L., and N. B.-F. Hui, 1990: Analysis of a shallow front during TAMEX. Mon. Wea. Rev., 118, 2607-2623.
Chen, Y.-L., and N. B.-F. Hui, 1992: Analysis of a relatively dry front during the Taiwan Area Mesoscale Experiment.
Thanks to the Central Weather Bureau (CWB) for providing an archived data set of
Mon. Wea. Rev., 120, 2442-2468.
rainfall, radar, lightning, satellite IR images for the SoWMEX/TIMREX field experiment at
Li, J., and Y.-L. Chen, 1998: Barrier jets during TAMEX. Mon. Wea. Rev., 126, 959-971.