Observations of intermittent cumulus convection in the boundary layer

Abstract

AbstractDuring the 1978 JASIN experiment radiosonde and tethered balloon measurements were performed from the R. V. Meteor. On 1 September, two cloud layers (cumulus below stratocumulus) were observed below an inversion at about 900m. Such a boundary layer situation occurred relatively often during the experiment. Cumulus clouds were not present all the time but alternately appeared and disappeared. When they occurred on 1 September, a relatively large vertical shear (about 0.006 s−1) together with an inflection point in the profile of the wind component normal to the mean wind direction was observed in the cumulus cloud layer. the airsea temperature difference was small (δT = −0.5 K) and there was a relatively large dynamical contribution to cloud generation. This may suggest the idea of an interaction between large‐scale processes generating a favourable mean wind shear profile and dynamically forced convection.Significant vertical transport of heat and momentum in the cloud layer occurred only in connection with cumulus clouds. Although in parts of some clouds the transports were locally as large as several 100W m−2 and 1N m−2, there were only a few events with such large excursions so that the transports averaged over the whole field of cumulus clouds were small.The relative air flow in and around active cumulus clouds was characterized by updraughts within the cloud and downdraughts at the cloud edges. the thermodynamic properties of updraughts and downdraughts are discussed.