An example of rainbands associated with stationary longitudinal circulations in the planetary boundary layer

Abstract

Doppler radar and rawinsonde observations are presented showing the structure of some longitudinal circulations which extended downwind of hillsand gave rise to stationary rainbands. The circulations occurred within a low-level jet situated in a deep adiabatic boundary layer ahead of a moving cold front. They were several kilometres wide and up to lOOkm long with updraughts of about 30cm s -' . The layer of cloud was Jhallow in the present study so that the circulations gave rise to only light rain: however, in the presence of precipitation generated in the middle troposphere the airflow at low levels would be-moistened and seeded and it is suggested that stationary bands of heavy rain might then occur as a result of boundary layer circulations of the kind described in this study.

Two Examples of the Effect of Topography on Rainfall Distribution Patterns

“…Various authors (e.g. Browning and Harrold 1969, Hardman et al 1972, Browninget al 1973, Atkinson and Smithson 1974, Browning and Bryant 1975 discuss the nature of cells and bands of relatively heavy rainfall within mid-latitude depressions. These features are essentially mesoscale, having typical dimensions in the range 10-1000 km, unlike the microscale rainfall distribution patterns described above.…”