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Abstract-
Cumulus clouds experience off-source heat addition beyond the cloud base due to
latent heat release brought about by condensation of water vapor. This
off-source heat addition drastically affects the entrainment behavior of the
rising cloud. A laboratory analogue consisting of a volumetrically off-source
heated jet was constructed to study this phenomenon. Both experimental (using
PIV, LIF, and whole field thermometry) and numerical (DNS) tools have been
applied to better understand the role of buoyancy addition on free-shear flows,
and the results are supplemented by new theoretical models. In this talk, we
will focus on the experimental results, and present a conceptual model of
entrainment for cumulus clouds.The characteristics of a volumetrically heated
jet are found to be very different from both a normal jet and a plume. The large
eddies residing at the edge of the jet are disrupted by heat injection,
resulting in a reduction of the scalar width. However, the velocity width and
mass flux increases rapidly at the beginning of the heat injection zone, and
subsequently tends to flatten out. The entire heat injection zone can be
divided into three sub-zones with sharp differences in the flow properties, and
mechanisms of entrainment. The streamwise velocity profile changes from a
Gaussian to a flat-top Gaussian, allowing a larger transport of both momentum
and mass flux. A twin-peak Gaussian profile for temperature has been identified
as the reason for the change in the streamwise velocity profile. The
cross-stream velocity profile shows an absence of outflow regions near the jet
axis; this is attributed to acceleration of the jet due to the added buoyancy.
The insights gained from the heated jet experiments are successfully extended to
a conceptual model for entrainment in cumulus clouds. The model describes a
system with a constant mass flux, but a varying momentum flux. Cumulus clouds
may have a constant mass flux because a redistribution of mass between the up
and downdrafts is possible, unlike a volumetrically heated jet, which has only
regions of positive moving velocity corresponding to updrafts.
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