A case-study of a partially cloudy boundary layer

Abstract

A case-study of a partially cloudy boundary layer consisting of cumulus and stratocumulus is presented, and the main dynamical processes responsible for its evolution are identified. Measurements were made with the UK Meteorological Office tethered-balloon facility which was located in Hampshire, southern England, during the Doppler Radar Observation Project, September 1998. Results show that fair weather cumulus formed during the morning of the third and quickly overdeveloped into patches of stratocumulus. A characteristic of this day was a relatively weak capping inversion which allowed a significant amount of encroachment/entrainment to occur. Between 1100 and 1400 GMT this was calculated at 153 kg m−2. The ratio of entrapment to encroachment was found to be 0.6. Data analysis, together with a simple model, shows that encroachment/entrainment of warm air into the upper pan of the boundary layer significantly increased the stability there and caused a reduction in the cloud area. The data were used to assess the UK Meteorological Office cloud parametrization, and show that during the middle of the day cloud amount was underestimated.