• Baker, R. D., , B. H. Lynn, , A. Boone, , W-K. Tao, , and J. Simpson, 2001: The influence of soil moisture, coastline curvature, and land-breeze circulations on sea-breeze-initiated precipitation. J. Hydrol., 2 , 193211.

    • Search Google Scholar
    • Export Citation
  • Banta, R. M., and Coauthors, 1990: Atmospheric Processes over Complex Terrain. Meteor. Monogr. No. 45, Amer. Meteor. Soc., 323 pp.

  • Benjamin, B. T., 1968: Gravity currents and related phenomenon. J. Fluid Mech., 31 , 209248.

  • Carbone, R. E., , J. W. Wilson, , T. D. Keenan, , and J. M. Hacker, 2000: Tropical island convection in the absence of significant topography. Part I: Life cycle of diurnally forced convection. Mon. Wea. Rev., 128 , 34593480.

    • Search Google Scholar
    • Export Citation
  • Chen, G. T-J., 1983: Observational aspects of the Mei-Yu phenomenon in subtropical China. J. Meteor. Soc. Japan, 61 , 306312.

  • Chen, G. T-J., 1992: Mesoscale features observed in the Taiwan Mei-yu season. J. Meteor. Soc. Japan, 70 , 497516.

  • Chen, G. T-J., , and J-S. Yang, 1988: On the spatial and temporal patterns of heavy rainfall in Taiwan Mei-Yu season (in Chinese with English abstract). Atmos. Sci., 16 , 151162.

    • Search Google Scholar
    • Export Citation
  • Chen, G. T-J., , H-C. Chou, , J-S. Yang, , and S-Y. Chen, 1999: Case study of a spring squall line accompanied by hailstorm occurrence in northern Taiwan (in Chinese with English abstract). Atmos. Sci., 27 , 257271.

    • Search Google Scholar
    • Export Citation
  • Chen, G. T-J., , H-C. Chou, , T-C. Chang, , and C-S. Liu, 2001: Frontal and non-frontal convection over northern Taiwan in Mei-yu season (in Chinese with English abstract). Atmos. Sci., 29 , 3752.

    • Search Google Scholar
    • Export Citation
  • Chi, S-S., , and G. T-J. Chen, 1989: Case study of the MCSs and rainfall during TAMEX/Phase I (in Chinese with English abstract). Atmos. Sci., 17 , 5975.

    • Search Google Scholar
    • Export Citation
  • Chien, C-J., , and P-L. Lin, 2004: Numerical simulations of local circulations over complex terrain in southeastern Taiwan (in Chinese with English abstract). Proc. Eighth Conf. of Atmospheric Sciences, TaoYuan, Taiwan, National Science Council and Central Weather Bureau, 50–56.

  • Chu, L-P., , and C-H. Liu, 2001: Categories of convergence lines over eastern Taiwan. Preprints, Seventh National Atmospheric Sciences Conf., Taipei, Taiwan, National Science Council, 564–568.

  • Feng, J., , and Y-L. Chen, 1998: Evolution of katabatic flow on the Island of Hawaii on 10 August 1990. Mon. Wea. Rev., 126 , 21852199.

  • Frye, J. L., , and Y-L. Chen, 2001: Evolution of downslope flow under strong opposing trade winds and frequent trade-wind rainshowers over the Island of Hawaii. Mon. Wea. Rev., 129 , 956977.

    • Search Google Scholar
    • Export Citation
  • Garrett, A. J., 1980: Orographic cloud over the eastern slopes of Mauna Loa volcano, Hawaii, related to insolation and wind. Mon. Wea. Rev., 108 , 931941.

    • Search Google Scholar
    • Export Citation
  • Houze Jr., R. A., , S. G. Geotis, , F. D. Marks Jr., , and A. K. West, 1981: Winter monsoon convection in the vicinity of north Borneo. Part I: Structure and time variation of the clouds and precipitation. Mon. Wea. Rev., 109 , 15951614.

    • Search Google Scholar
    • Export Citation
  • Johnson, R. H., , and J. F. Bresch, 1991: Diagnosed characteristics of precipitation systems over Taiwan during the May–June 1987 TAMEX. Mon. Wea. Rev., 119 , 25402557.

    • Search Google Scholar
    • Export Citation
  • Jorgensen, D. P., , and M. A. LeMone, 1989: Vertical velocity characteristics of oceanic convection. J. Atmos. Sci., 46 , 621640.

  • Jorgensen, D. P., , M. A. LeMone, , and B. J-D. Jou, 1991: Precipitation and kinematic structure of an oceanic mesoscale convective system. Part I: Convective line structure. Mon. Wea. Rev., 119 , 26082637.

    • Search Google Scholar
    • Export Citation
  • Jou, B. J-D., 1994: Mountain-originated mesoscale precipitation system in northern Taiwan: A case study 21 June 1991. Terr. Atmos. Oceanic Sci., 5 , 169197.

    • Search Google Scholar
    • Export Citation
  • Jou, B. J-D., , and S-L. Shieh, 2001: A brief summary of GIMEX operation. Proc. Int. Conf. on Mesoscale Meteorology and Typhoon in East Asia, Taipei, Taiwan, National Science Council, 252–257.

  • Keenan, T. D., , and R. E. Carbone, 1992: A preliminary morphology of precipitation systems in tropical northern Australia. Quart. J. Roy. Meteor. Soc., 118 , 283326.

    • Search Google Scholar
    • Export Citation
  • Kingsmill, D. E., 1995: Convection initiation associated with a sea-breeze front, a gust front, and their collision. Mon. Wea. Rev., 123 , 29132933.

    • Search Google Scholar
    • Export Citation
  • Knupp, K. R., , and W. R. Cotton, 1985: Convective cloud downdraft structure: An interpretive survey. Rev. Geophys., 23 , 183215.

  • Kousky, V. E., 1980: Diurnal rainfall variation in northeastern Brazil. Mon. Wea. Rev., 108 , 488498.

  • Kuo, Y-H., , and G. T-J. Chen, 1990: The Taiwan Area Mesoscale Experiment (TAMEX): An overview. Bull. Amer. Meteor. Soc., 71 , 488503.

  • Leu, Y-S., , and P-L. Lin, 2004: Characteristics of local circulation and boundary layer over southeastern Taiwan. Proc. Eighth Conf. of Atmospheric Sciences, TaoYuan, Taiwan, National Science Council and Central Weather Bureau, 43–49.

  • Li, J., , and Y-L. Chen, 1999: A case study of nocturnal rain showers over the windward coastal region of the Island of Hawaii. Mon. Wea. Rev., 127 , 26742692.

    • Search Google Scholar
    • Export Citation
  • Lin, P-L., , and Y-V. Sheng, 1990: A preliminary study on the characteristics of sea and land breezes in Taiwan area during TAMEX period (in Chinese with English abstract). Proc. Conf. on Weather Analysis and Forecasting, Taipei, Taiwan, Central Weather Bureau, 133–144.

  • Mahrer, Y., , and R. A. Pielke, 1977: The effects of topography on sea and land breezes in a two-dimensional numerical model. Mon. Wea. Rev., 105 , 11511162.

    • Search Google Scholar
    • Export Citation
  • Mapes, B. E., , T. T. Warner, , and M. Xu, 2003: Diurnal patterns of rainfall in northwestern South America. Part III: Diurnal gravity waves and nocturnal convection offshore. Mon. Wea. Rev., 131 , 830844.

    • Search Google Scholar
    • Export Citation
  • Meyer, J. H., 1971: Radar observations of land breeze fronts. J. Appl. Meteor., 10 , 12241232.

  • Ohara, T., , I. Uno, , and S. Wakamatsu, 1989: Observed structure of a land breeze head in the Tokyo metropolitan area. J. Appl. Meteor., 28 , 693704.

    • Search Google Scholar
    • Export Citation
  • Purdom, J. F. W., 1976: Some uses of high-resolution GOES imagery in the mesoscale forecasting of convection and its behavior. Mon. Wea. Rev., 104 , 14741483.

    • Search Google Scholar
    • Export Citation
  • Raga, G. B., , J. B. Jensen, , and M. B. Baker, 1990: Characteristics of cumulus band clouds off the coast of Hawaii. J. Atmos. Sci., 47 , 338355.

    • Search Google Scholar
    • Export Citation
  • Rotunno, R., , J. B. Klemp, , and M. L. Weisman, 1988: A theory for strong, long-lived squall lines. J. Atmos. Sci., 45 , 463485.

  • Schoenberger, L. M., 1984: Doppler radar observation of a land-breeze cold front. Mon. Wea. Rev., 112 , 24552464.

  • Simpson, J. E., 1969: A comparison between laboratory and atmospheric density currents. Quart. J. Roy. Meteor. Soc., 95 , 758765.

  • Simpson, J. E., , and R. E. Britter, 1979: The dynamics of a head of a gravity current advancing over a horizontal surface. J. Fluid Mech., 94 , 477495.

    • Search Google Scholar
    • Export Citation
  • Simpson, J. E., , and R. E. Britter, 1980: A laboratory model of an atmospheric mesofront. Quart. J. Roy. Meteor. Soc., 106 , 485500.

  • Smith, R. K., , and M. J. Reeder, 1988: On the movement and low-level structure of cold fronts. Mon. Wea. Rev., 116 , 19271944.

  • Smolarkiewicz, P. K., , R. M. Rasmussen, , and T. L. Clark, 1988: On the dynamics of Hawaiian cloud bands: Island forcing. J. Atmos. Sci., 45 , 18721905.

    • Search Google Scholar
    • Export Citation
  • Stull, R. B., 1988: An Introduction to Boundary Layer Meteorology. Kluwer Academic, 666 pp.

  • Sun, W. Y., , and J-D. Chern, 1993: Diurnal variation of lee vortices in Taiwan and the surrounding area. J. Atmos. Sci., 50 , 34043430.

    • Search Google Scholar
    • Export Citation
  • Takahashi, T., 1977: A study of Hawaiian warm rain showers based on aircraft observation. J. Atmos. Sci., 34 , 17731790.

  • Takahashi, T., 1988: Long lasting trade-wind rain showers in a three-dimensional model. J. Atmos. Sci., 45 , 33333353.

  • Takahashi, T., , K. Yoneyama, , and Y. Tsubota, 1989: Rain duration in Hawaiian trade-wind rainbands—Aircraft observations. J. Atmos. Sci., 46 , 937955.

    • Search Google Scholar
    • Export Citation
  • Wakimoto, R. M., 1982: The life cycle of thunderstorm gust fronts as viewed with Doppler radar and rawinsonde data. Mon. Wea. Rev., 110 , 10601082.

    • Search Google Scholar
    • Export Citation
  • Wakimoto, R. M., , and N. T. Atkins, 1994: Observations of the sea-breeze front during CaPE. Part I: Single-Doppler, satellite, and cloud photogrammetry analysis. Mon. Wea. Rev., 122 , 10921114.

    • Search Google Scholar
    • Export Citation
  • Wang, J-J., , and Y-L. Chen, 1998: A case study of trade-wind rainbands and their interaction with the island-induced airflow. Mon. Wea. Rev., 126 , 409423.

    • Search Google Scholar
    • Export Citation
  • Weisman, M. L., , J. B. Klemp, , and R. Rotunno, 1988: Structure and evolution of numerically simulated squall lines. J. Atmos. Sci., 45 , 19902013.

    • Search Google Scholar
    • Export Citation
  • Wilson, J. W., , and D. L. Megenhardt, 1997: Thunderstorm initiation, organization, and lifetime associated with Florida boundary layer convergence lines. Mon. Wea. Rev., 125 , 15071525.

    • Search Google Scholar
    • Export Citation
All Time Past Year Past 30 Days
Abstract Views 0 0 0
Full Text Views 21 21 4
PDF Downloads 21 21 4

Radar Observations of the Diurnally Forced Offshore Convective Lines along the Southeastern Coast of Taiwan

View More View Less
  • 1 Department of Atmospheric Sciences, Chinese Culture University, Taipei, Taiwan
  • | 2 Department of Atmospheric Sciences, National Taiwan University, Taipei, Taiwan
© Get Permissions
Restricted access

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).

Corresponding author address: Dr. Cheng-Ku Yu, Department of Atmospheric Sciences, Chinese Culture University, 55, Hwa-Kang Rd., Yang-Ming-Shan, Taipei 111, Taiwan. Email: yuku@faculty.pccu.edu.tw

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).

Corresponding author address: Dr. Cheng-Ku Yu, Department of Atmospheric Sciences, Chinese Culture University, 55, Hwa-Kang Rd., Yang-Ming-Shan, Taipei 111, Taiwan. Email: yuku@faculty.pccu.edu.tw

Save