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Abstract

A case study is presented showing the three-dimensional structure and evolution of precipitation upwind, over, and downwind of the south Wales hills during the passage of a wintertime warm sector that gave rather heavy and prolonged rainfall. The precipitation structure was synthesized from a network of weather radars and autographic raingauges; it is interpreted within a dynamical framework derived from routine upper air soundings supplemented by serial ascents upwind and downwind of the hills.

The warm sector was characterized by a fast-moving airstream with potential instability (PI) not only at low levels due to its passage over a warm sea but also in the middle troposphere. The middle-level PI occurred extensively and played a significant role in determining the distribution and amount of precipitation. It has not received much attention until now, mainly because the limitations of the humidity element in conventional radiosondes tend to cause its magnitude to be underestimated. The middle-level PI was due to differential thermal advection in an intense and nearly vertical baroclinic zone that extended ahead of the surface cold front at middle levels.

Large-scale ascent was slight in the warm sector but, nevertheless, middle-level PI was realized even over the sea in scattered ‘Mesoscale Precipitation Areas’ (MPAs) which travelled rapidly at the speed of the winds in the middle troposphere (about 120 km hr−1). Once the airstream experienced orographic uplift, fresh outbreaks of middle-level convection occurred extensively between existing MPAs. The fresh outbreaks were observed first as middle-level precipitation echoes over the sea up to 100 km upwind of the hills, indicating that the orographic ascent aloft began far upwind of the hills. Thus some of the precipitation generated by orographic ascent in the middle levels reached low levels over the hills despite the drift of the precipitation in the strong winds; there it seeded low-level orographic cloud which gave heavy rain over the hills. Some of the convective precipitation generated aloft as a result of the middle-level PI also reached the ground downwind of the hills, thereby displacing to some extent the commonly-observed rain shadow.

The importance of seeding from aloft for releasing heavy orographic rainfall has long been recognized. Sometimes the seeding is achieved by precipitation generated aloft by large scale ascent; however, the present study suggests that in certain circumstances the hills may be able to generate their own seeds when they are not being generated by large scale ascent. In the case study the occurrence of heavy warm-sector rainfall over the hills appears to have been favoured by the existence of middle-level PI which, although not being realized generally by large scale ascent, required only a small amount of local orographic ascent to realize it. Thus forecasting techniques that predict rainfall on the basis of the forecast large-scale vertical ascent may fail to identify some important situations of orographic warm sector rain unless the interaction of PI and orography is realistically taken into account.