Slippery Thermals and the Cumulus Entrainment Paradox

AbstractIn numerical simulations of growing congestus clouds, the maximum upward velocities w typically occur in compact toroidal vortices or thermals. These maxima were tracked, and the momentum budget was analyzed within spherical regions centered on them with objectively determined radii approximately enclosing the vortex ring or pair. Such regions are proposed as an advantageous prototype for rising air parcels due to their prolonged identity as evident in laboratory flows. Buoyancy and other forces are generally less than 0.02 m s−2 (0.7 K). In particular, resolved mixing between thermals and their environment fails to produce the drag normally anticipated, often producing even a slight upward force, indicating that parcel models should allow for significantly different dilution rates for momentum than for material properties. A conceptual model is proposed to explain this as a result of the thermals' internal circulation and detrainment characteristics.The implications of momentum dilution for cumul...

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