Observations of the impact of orographic cumulus clouds on the ambient flow

Cumulus towers penetrating above the boundary layer typically are relatively short-lived, and thus the downstream enrichment of ambient air with boundary-layer air is due to both detrainment of cloudy air during the lifespan of the tower and to convective “debris”, i.e. air left behind in the wake of an evaporating cloud. This talk describes this enrichment process by means of aircraft in-situ data, mm-wavelength radar data, time lapse photography, and close-proximity radiosonde data for one flight in the 2006 Cumulus Photogrammetric, In-situ and Doppler Observations (CuPIDO-06) campaign, on Aug 08th, 2006. On this day a series of Cu mediocris clouds formed on the upwind side of the target mountain, and decayed on the downwind side. It has been postulated that new Cu towers grow more readily in the enriched air where the erosive effect of entrainment is lessened. We test this hypothesis, known as the moisture-convection feedback hypothesis, for orographic convection. The main finding is that a clear enrichment signature is present in the downwind environment. This enrichment reduces potential instability but increases ambient humidity, thus it may render the environment more conducive to penetrative convection, although the mid-tropospheric plume of modified air usually is blown off the mountain before new convection can bubble up. Thus the moisture-convection feedback mechanism does not explain the commonly observed gradual deepening of daytime orographic convection.