A Modeling Case Study of Interaction between Heavy Precipitation and a Low-Level Jet over Japan in the Baiu Season

Abstract

This paper presents results of simulations for a case of heavy precipitation that occurred on 23 July 1982 over western Japan. Special emphasis is placed on synoptic- and subsynoptic-scale processes that led to heavy precipitation and also its linkage with the low-level jet (LLJ). The model result recaptures the major observed features of this event reasonably well. The sequence of events revealed by the model starts with the formation of a localized surface warm front caused by the deformation field that is associated with an eastward traveling, nondeepening meso-αscale low. It is followed by the initiation of both concentrated convective precipitation at the surface front and stratiform grid-scale precipitation along the sloping frontal surface. The simulations with different model physics reveal significant roles that diabatic heating processes play in the linkage between heavy precipitation and the LLJ. While condensation heating produces a cyclonic circulation with failing pressure manifested as a mesoscale trough over the front, evaporative cooling from stratiform raindrops generates a marked frontogenetic forcing and creates a cold pool beneath the sloping frontal surface. An anticyclonic outflow from the cold pool is accompanied by convergence on its southwestern flank, which further enhances and concentrates the convective activity and the mesoscale trough. The supergeostrophic LLJ is formed in this situation, where an air parcel crosses height contours into low pressure with large angles due to a combination of an alongfront flow in the southwestern part of the anticyclonic outflow anomaly induced by the evaporative cooling and a cross-front flow in the upper branch of the direct secondary circulation associated with the warm-frontogenetical processes. Isentropic and isobaric ageostrophic motion diagnoses show that the inertial advective component, mainly arising from the horizontal displacement, is the dominant part in the ageostrophic wind in the entrance region of the LLJ, supporting the conclusion that the rapid parcel acceleration itself occurs almost adiabatically through the horizontal displacement crossing into low pressure.