15 August 2018

A Sharper View of Wind Changes by Large Storms

A new study strengthens understanding of how momentum is changed by vertical motions of storms across scales.

Science

To capture both large- and small-scale Earth system processes, global and regional models run in both fine and coarse modes, with grid spacing ranging from a few kilometers to hundreds of kilometers. Scientists remain unclear about how vertical motions in storm systems change transport of air masses and momentum across scales. Limited understanding of this process—referred to as convective momentum transport, or CMT—introduces large uncertainty into how these components are represented at different model resolutions. A research team led by scientists at the U.S. Department of Energy’s Pacific Northwest National Laboratory studied wind momentum transport by convection at different scales and used their new understanding to evaluate and improve a parameterization for large-scale models. Results showed for the first time a strong scale dependence of the time evolution and vertical structure of wind momentum transport in storms. 

Impact

CMT has a strong influence on global atmospheric circulations and Earth system processes, but model formulations often oversimplify its effects. This study provides a better understanding of CMT at different scales and new insights for improving simulations of circulations in global models. 

Summary

Using 3-D cloud-resolving model simulations of two midlatitude mesoscale convective systems (intermediate-scale thunderstorm clusters) from the Midlatitude Continental Convective Clouds Experiment (MC3E)—a DOE ARM supported field campaign—and a simple statistical ensemble method, researchers studied the scale dependency of CMT and CMT-related properties. They also evaluated a widely used representation for convection-induced pressure changes. Results showed that air mass fluxes and CMT exhibited strong scale dependency in time evolution and vertical structure. Consistent with previous studies, the CMT characteristics for updrafts were generally similar between small and large model grid spacings, but they could be different for downdrafts across wide-ranging grid spacings.

For small to intermediate grid spacings (about 4-64 kilometers), a widely used CMT scheme reproduced some aspects of the scale dependency of convection-induced pressure changes except for underestimating variations of pressure change rate. For large grid spacings (about 128-512 kilometers), the scheme underestimated convection-induced pressure changes because it omitted the contribution from nonlinear changes of wind and buoyancy. Further diagnosis of cloud-resolving model results suggested that including nonlinear wind shear changes improved simulations with large grid spacings. Compared to the original single-plume approach, a modified CMT scheme with a three-plume approach to represent different flux intensities helped better capture variations in convection-induced pressure changes as grid spacing decreased.

Contact
Jiwen Fan
Pacific Northwest National Laboratory (PNNL)
Publications
Liu, Y, J Fan, K Xu, and G Zhang.  2018.  "Analysis of Cloud-Resolving Model Simulations for Scale Dependence of Convective Momentum Transport."  Journal of the Atmospheric Sciences 75(7): 2445-2472.  https://doi.org/10.1175/jas-d-18-0019.1.