The Structure and Formation Mechanism of Transversal Cloud Bands Associated with the Japan-Sea Polar-Airmass Convergence Zone

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  • EITO Hisaki
    Meteorological Research Institute, Japan Meteorological Agency
  • MURAKAMI Masataka
    Meteorological Research Institute, Japan Meteorological Agency
  • MUROI Chiashi
    Numerical Prediction Division, Forecast Department, Japan Meteorological Agency
  • KATO Teruyuki
    Meteorological Research Institute, Japan Meteorological Agency
  • HAYASHI Syugo
    Meteorological Research Institute, Japan Meteorological Agency
  • KUROIWA Hiroshi
    Support Center for Advanced Telecommunications Technology Research, Foundation
  • YOSHIZAKI Masanori
    Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology

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Abstract

During a cold-air outbreak, a broad cloud band is occasionally observed over the Japan-Sea Polar-Airmass Convergence Zone (JPCZ) that forms over the Sea of Japan from the base of the Korean Peninsula to the Japanese Islands. On 14 January 2001, a broad cloud band associated with the JPCZ (JPCZ cloud band) extended in a southeastward direction from the base of the Korean Peninsula to Wakasa Bay, and it stagnated for half a day. The JPCZ cloud band consisted of two cloud regions: one was a long cloud band extending along its southwestern edge (a developed convective cloud band), and the other was the region consisting of cloud bands normal to a wind direction of winter monsoon (transversal cloud bands). The structure and formation mechanism of the transversal cloud bands were examined on the basis of observations (e.g., satellite images, in situ measurement and cloud-pro.ling radar data from an instrumented aircraft and upper-air soundings from observation vessels) and simulation results of a cloud-resolving model with a horizontal resolution of 1 km.<br>The transversal cloud bands had the following characteristic structures; they extended along a northeast-southwest direction, which was parallel to the direction pointed by the vertical shear vector of horizontal wind in the mixed layer, they mainly consisted of convective clouds, which slanted with height toward the down-shear side, and they widened and deepened toward southwest, as the depth of the mixed layer increased. An examination of simulation results presented that the transversal cloud bands were accompanied by roll circulations. The axes of rolls were oriented nearly parallel to the direction of the vertical shear vector in the mixed layer. An analysis of the eddy kinetic energy budget indicated that the roll circulations derived most of its energy from the mean vertical shear and the buoyancy.

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