What is the basic structural unit of cumulus convection, and why does it matter?

Recent research suggests that cumulus updrafts often consist of a series of rising thermals, which contrasts with existing conceptual models of moist convection. In this work, a theory is developed for why this "thermal chain" structure occurs. A simple reduced-order model is developed from theoretical expressions for a passive tracer, buoyancy, and vertical velocity in axisymmetric moist updrafts. The theoretical model is compared directly to numerical axisymmetric cloud updraft simulations, and gives similar profiles of core updraft tracer, buoyancy, and vertical velocity over a wide range of conditions (varying environmental relative humidities and updraft radii). Both the theory and simulations show a pronounced transition of updraft structure from an isolated rising thermal, to a succession of rising thermals, to a rising plume as the environmental relative humidity and/or updraft width are increased. These results are supported by additional large eddy simulations of cloudy updrafts. Analyses from the theoretical model and simulations suggest that the thermal chain structure arises from enhanced lateral turbulent mixing associated with intrusions of dry environmental air below local vertical velocity maxima. This leads to local buoyancy minima in the updraft, and separation of the updraft flow above and below these levels of locally reduced buoyancy. The theoretical expressions indicate local enhancement of entrainment by a factor of up to 9/4 associated with these dry air intrusions. This contributes strongly to overall cloud core dilution in thermal chain-like updrafts, while other regions within the updraft can remain relatively undilute. Finally, from the theory we propose why ascending updraft towers tend to have radii less than a few km, except in special cases (e.g., supercells in flow with strong deep layer shear). This is evident from large eddy simulations, and may help explain the general prevalence of thermal chain-like updrafts given that this structure is favored for updraft radii smaller than a few km.