Effect of Vertical Wind Shear on Numerically Simulated Multicell Storm Structure

Robert G. Fovell Department of Atmospheric Sciences, University of Illinois, Urbana, Illinois

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Yoshi Ogura Department of Atmospheric Sciences, University of Illinois, Urbana, Illinois

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Abstract

A strictly two-dimensional cloud model was used to gauge the effect of vertical wind shear on the mature phase behavior of model-simulated multicellular storms, extending the previous work of the authors. We specifically examined the propagation speed, quasi-equilibrium behavior, storm scale and updraft orientation of the model storms as a function of shear intensity. We also considered the precipitation efficiencies of our. model storms and applied density current and Rotunno–Klemp–Weisman theories to our results.

Our previous work revealed that model storms could achieve a mature phase consisting of repetitive multicellular development when certain numerical obstacles were overcome. This was referred to as a “quasi-equilibrium state.” We found herein that this state was also reached by model storms even when subjected to a very wide range of low-level wind shear intensities, although the temporal behavior during this stage was clearly dependent on the shear. We also found a very systematic relationship between the storm speed and the shear strength. Therefore, small shear values produced slowly moving storms which generally exhibited simple oscillations with time, fitting the classic multicell model. Larger shears resulted in complex oscillations similar to what has been termed “weak evolution,” culminating in a nearly unicellular storm in the most extreme case.

The transition between the strongly and weakly evolving modes was abrupt in the wind shear spectrum, and the temporal behavior of the precipitation production was quite different between the two regimes. Yet, we also found that the precipitation efficiencies of these model storms were roughly constant among the simulations, irrespective of the low-level shear. The larger shear storms typically produced more precipitation, because they were processing water vapor at faster rates due to their more rapid propagation speeds, but were neither identifiably more nor less efficient in doing so. The rear inflow current feature, present in each case, appeared to play a major role in creating the colder subcloud cold pools which helped the storms formed in larger shear to move faster.

An important result is that none of the model storms suffered a terminal decaying phase, certainly not within a reasonable period of time. This suggests that the storm itself does not sow the seeds of its own demise, at least for the favorable, homogeneous environmental conditions considered and the simple, strictly two-dimensional framework adopted for this study.

Abstract

A strictly two-dimensional cloud model was used to gauge the effect of vertical wind shear on the mature phase behavior of model-simulated multicellular storms, extending the previous work of the authors. We specifically examined the propagation speed, quasi-equilibrium behavior, storm scale and updraft orientation of the model storms as a function of shear intensity. We also considered the precipitation efficiencies of our. model storms and applied density current and Rotunno–Klemp–Weisman theories to our results.

Our previous work revealed that model storms could achieve a mature phase consisting of repetitive multicellular development when certain numerical obstacles were overcome. This was referred to as a “quasi-equilibrium state.” We found herein that this state was also reached by model storms even when subjected to a very wide range of low-level wind shear intensities, although the temporal behavior during this stage was clearly dependent on the shear. We also found a very systematic relationship between the storm speed and the shear strength. Therefore, small shear values produced slowly moving storms which generally exhibited simple oscillations with time, fitting the classic multicell model. Larger shears resulted in complex oscillations similar to what has been termed “weak evolution,” culminating in a nearly unicellular storm in the most extreme case.

The transition between the strongly and weakly evolving modes was abrupt in the wind shear spectrum, and the temporal behavior of the precipitation production was quite different between the two regimes. Yet, we also found that the precipitation efficiencies of these model storms were roughly constant among the simulations, irrespective of the low-level shear. The larger shear storms typically produced more precipitation, because they were processing water vapor at faster rates due to their more rapid propagation speeds, but were neither identifiably more nor less efficient in doing so. The rear inflow current feature, present in each case, appeared to play a major role in creating the colder subcloud cold pools which helped the storms formed in larger shear to move faster.

An important result is that none of the model storms suffered a terminal decaying phase, certainly not within a reasonable period of time. This suggests that the storm itself does not sow the seeds of its own demise, at least for the favorable, homogeneous environmental conditions considered and the simple, strictly two-dimensional framework adopted for this study.

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