Dynamics of Rotation and Scale Selection in Deep Convective Storms

This research is focussed on study of deep convective storm dynamics with emphasis on three related yet scientifically distinct problems. The first concerns the tendency for some severe (supercell) storms to develop a succession of mesocyclones at fairly regular intervals. This 'cyclic mesocyclogenesis' has been observed in real storms and noted in some numerical simulations. It is proposed to identify the conditions (both environmental and storm-induced) that delineate cyclic from non-cyclic storms. In addition, the Principal Investigators will extend their previous work down to the tornado scale to study cyclic tornadogenesis with emphasis on multi-scale linkages between the tornado vortex circulation and that of the parent mesocyclone.

The second study seeks to understand the dynamics of storms and storm systems which, during all or part of their lifetime, move though environments comprising large horizontal and/or temporal variations in shear and/or stability. Although environmental variability is known to play a key role in storm lifecycles, previous cloud model simulations have utilized a horizontally uniform base state environment. The Principal Investigators will extend prior research by further investigating the dynamics associated with storm environmental transitions and, by combining variations in instability and shear as well as imposing temporal variations in the environment. Additionally, comparisons will be drawn between model results and real storms, which demonstrated clear changes in behavior due to environmental variability.

Finally, the Principal Investigators will build upon their recent work in helicity dynamics and the turbulent nature of both shallow and deep convection in an effort to understand the dynamics of scale selection, organization and predictability within deep convective storms. In prior research, simulated deep convection at various scales of initial forcing were used to determine whether convective storms evolve at the spatial scale of the initial forcing or instead evolved to some other preferred scale. These 'scale-forced' simulations were compared to other results in which convection was initiated by a field of random disturbances. Preliminary findings, based on spectral and other analysis techniques, confirm that updrafts in the most supercell storms tend to be considerably larger in size then their less severe counterparts. When the initial storms are forced at a particular spatial scale, they tend to retain a memory of that scale, with the retention time proportional to the scale of forcing. Even when forced at scales much larger than the 'natural or preferred scale,' the supercell storms eventually settle into a scale larger than storms forced in weaker or zero shear. The Principal Investigators will expand the simulation data set with emphasis on identifying the most significant theoretical length scales of the flow and relating their structure and energetics to simulated storm behavior. Further, the Principal Investigators will seek to understand fundamental limits to storm and storm system predictability via a systematic analysis of error growth and propagation among scales. The latter is of particular importance to future operational prediction models.