下層ジェットのメカニズムに関する数値実験
A numerical experiment on the formation machanism of the low-level jet

氏名: 田中 小緒里
指導教官: 守田 治



 When heavy rainstorms occur during the late Baiu season, the low-level jet always appears, south of them, with the horizontal scale of 100〜200km. It is widely recognized that the low-level jet plays an important role in maintaining heavy rainstorms. However, its formation mechanism is not solved yet.

 The object of this study is to clarify the formation machanism of the low-level jet associated with heavy rainstorms in the Baiu season. For this purpose, numerical experiments are carred out in the north-south two-dimension, based on the hypothesis that the meridional circulation generated by convection develops and maintains the low-level jet via Coriolis torque.

 The model used is ARPS(Advanced Regional Prediction System), which is a quasi-elastic, non-hydrostatic model.

 Three experiments shown below are performed. Case1: convection only Case2: convection and convergence Case3: convection and convergence in the field that has a distribution of the east-west pressure gradient

 In all cases strong convection occurs, producing the direct and secondary circulations. The direct(secondary) circulation accelerates(decelerates) zonal wind in the low level, south of the convective region. The intensity of the direct circulation in Case2 is the largest, while that of the secondary circulation in Case3 is the largest. As a result, the acceleration of the zonal wind in Case2 is the largest. Although the strong convection and the associated circulations show some aspect of the environment around the low-level jet, no low-level jet appears; the acceleration of the zonal wind is not sufficient in the low level to produce the low-level jet. This is because the secondary circulation is too strong in the low level, south of the convection, which nesessarily occurs in this two-dimensional situation. Thus, it is suggested that interaction between mesoscale convection forced by large-scale fields and three dimensional, large-scale dynamical fields such as frontogenetic process is necessary for the formation mechanism of the low-level jet.