On the influence of small‐scale topography on precipitation

A simple model of the influence of small‐scale topography on precipitation via the ‘seeder‐feeder’ mechanism is presented. In the model the large‐scale stratiform seeder cloud and the subcloud layer are modified by the airflow perturbations caused by small‐scale topography. Two separate scales of motion are considered; the large scale, with the updraught maintaining the seeder cloud, and the small scale tied to topography. In the diagnostic numerical model the interaction of these two scales is simulated. Two basic situations are considered: rain with a ground temperature of 12°C and snow with –5°C. Results of the model application to a bell‐shaped solitary hill are presented and compared with previous theoretical studies and field observations in the Welsh hills. The influence of orographic airflow perturbations on snow growth above the melting level may explain the observed strong dependence of rain enhancement on the horizontal wind speed. Simple considerations for the complex topography are also presented with reference to observations made near Uppsala, Sweden. It is suggested that the observed rain distribution may again result from influences on snow growth, with both the amplitude of airflow perturbations near the melting level and its phase relative to the topography beneath being very important.