On the modelling of warm orographic rain by the seeder-feeder mechanism

Abstract

A two-dimensional model of the seeder–feeder mechanism for the orographic enhancement of rain is presented. It satisfactorily reproduces observed rainfall rate distributions and is helpful in studying some aspects of the physics of orographic rain. The modelling approach is based on the principle of continuity of the water substance and on the two-dimensional theory of small perturbations of airflow over topographical ridges of modest dimensions. The model is significantly different from those found in the literature and is used to clarify two major points. Firstly, it emphasizes that there is a range of hill dimensions for which the seederfeeder mechanism plays a major role. That is, the model predicts that a change of regime occurs for larger hills (half length > 20 km) where the seeder-feeder mechanism is no longer dominant because of the possibility of rain development within the feeder cloud and also because the horizontal scale of rain drift becomes small compared with that of the topography. Secondly, it is also suggested that the difference of wind curvature below and above a low-level jet may be important in determining the intensity of orographic rain. The physical process behind the latter finding is closely linked to partial reflection of upward propagating gravity waves which may lead to ‘resonance’ of the orographically induced vertical velocity.