Structure and mechanism of precipitation and the effect of orography in a wintertime warm sector

摘要: A case study is presented showing the three-dimensional structure and evolution of precipitation upwind, over, downwind south Wales hills during passage a wintertime warm sector that gave rather heavy prolonged rainfall. The was synthesized from network weather radars autographic raingauges; it interpreted within dynamical framework derived routine upper air soundings supplemented by serial ascents upwind hills. The characterized fast-moving airstream with potential instability (PI) not only at low levels due to its over sea but also in middle troposphere. middle-level PI occurred extensively played significant role determining distribution amount precipitation. It has received much attention until now, mainly because limitations humidity element conventional radiosondes tend cause magnitude be underestimated. differential thermal advection an intense nearly vertical baroclinic zone extended ahead surface cold front levels. Large-scale ascent slight but, nevertheless, realized even scattered ‘Mesoscale Precipitation Areas’ (MPAs) which travelled rapidly speed winds troposphere (about 120 km hr−1). Once experienced orographic uplift, fresh outbreaks convection between existing MPAs. were observed first as echoes up 100 hills, indicating aloft began far hills. Thus some generated reached despite drift strong winds; there seeded low-level cloud rain Some convective result ground thereby displacing extent commonly-observed shadow. The importance seeding for releasing rainfall long been recognized. Sometimes achieved large scale ascent; however, present suggests certain circumstances may able generate their own seeds when they are being ascent. In occurrence warm-sector appears have favoured existence which, although generally ascent, required small local realize it. forecasting techniques predict on basis forecast large-scale fail identify important situations unless interaction orography realistically taken into account.