Cut off and Weakening Processes of an Upper Cold Low

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Author(s)

Abstract

Near the tropopause over the North Pacific in summer an isolated low pressure system (Upper Cold Low, UCL) is often generated by the deepening and cutting off of a trough in the mid-latitude westerlies. The tracks and structures of these UCLs have been investigated in previous studies, but understanding of the cut off and weakening processes remains poor. In this paper, the tracks of UCLs generated in the 1999 summer are analyzed using ECMWF data. The physical processes occurring in one ofthese systems are investigated in detail using the ECMWF data, and the meso-scale model MM5. We focus particularly on cut off and weakening processes, and on the structure of the vertical velocity in the UCL.<br>The summer of 1999 was hot over Japan, and part of the Tibetan high pressure, around 200 hPa, was shifted northward. This allowed some UCLs to approach Japan in July and August. A UCL on August 19th is selected for detailed analysis, and was generated in the following process. Positive vorticity in a westerly wave at 200 hPa was extended by a northeast wind in the upper layer only. The positive vorticity was cut off by non-liner effects and upper level divergence, associated with convective clouds, generating the isolated UCL. The structure of the cyclonic circulation and the warm and cold cores were similar to those in previous studies. The structure of the vertical motion of the moving UCL was explained by dry dynamics and there was upward motion on the front side of the UCL, in the direction of movement. Upper level clouds in the UCL strengtJhened this upward motion. Convective clouds were seen in the system. The latent heat of these convective clouds played an important role in weakening the cold core of the UCL.

Journal

  • Journal of the Meteorological Society of Japan. Ser. II

    Journal of the Meteorological Society of Japan. Ser. II 83(5), 817-834, 2005-10-24

    Meteorological Society of Japan

References:  16

Codes

  • NII Article ID (NAID)
    110002535439
  • NII NACSIS-CAT ID (NCID)
    AA00702524
  • Text Lang
    ENG
  • Article Type
    ART
  • ISSN
    00261165
  • NDL Article ID
    7492264
  • NDL Source Classification
    ZM43(科学技術--地球科学--気象)
  • NDL Call No.
    Z54-J645
  • Data Source
    CJP  NDL  NII-ELS  J-STAGE  NDL-Digital 
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