Mechanisms influencing the distribution of precipitation within baroclinic disturbances

Abstract An analysis of the flow within baroclinic disturbances over and near the British Isles is made assuming that the wet bulb potential temperature (θ w is a conserved property of the flow. Conservation of mass and, when the flow is dry, of mixing ratio, are also used as constraints in the analysis. A model of the flows significant for the production and distribution of precipitation is derived. It is shown that most of the precipitation reaching the surface initially forms within a single well‐defined flow, labelled the ‘conveyor‐belt’ which is typically a few hundred km wide and a few km deep and which flows parallel to and immediately ahead of the surface cold front. The speed of ascent above the warm frontal zone is of the order of 10 cm s −1 . Ascent often begins within the warm sector so that there is no well‐marked discontinuity between the precipitation within the warm sector and that in advance of the surface warm front. This flow has been identified as the most significant flow producing widespread precipitation in a variety of synoptic situations, including some which would be analysed conventionally as non‐frontal. Potential instability is continually generated by differential advection in advance of the cold front as a result of over‐running of the low level flow by mid‐tropospheric air of lower θ w . The instability is gradually released as the low‐level flow ascends. The leading edge of the warm‐frontal precipitation at the surface is eroded by the evaporation within a descending flow beneath the warm‐frontal zone. When the melting level is low, or the air entering the system on its forward side is dry, the evaporation causes the width of the area of precipitation at the surface to be several hundred km less than that aloft. Some of the moisture which is evaporated is eventually re‐precipitated farther north as the air ascends close to the surface warm front. The mesoscale variability of the airflow within the large‐scale flow modifies the distribution of precipitation. Various structures which have been identified in many studies are summarized and a model of the mesoscale airflow within the conveyor belt is deduced.