PNNL

Abstract

Previous studies show that for nonlinear mesoscale convective systems (MCSs), convective momentum transport (CMT) is always downgradient but for linear MCSs, line-normal and line-parallel components in general have upgradient and downgradient transports, respectively. There is, however, a lack of understanding of the scale dependency of CMT, especially towards the gray-zone resolutions. Here, we explore the scale dependency of mass fluxes and CMT using 3-D cloud-resolving model (CRM) simulations of three MCSs. We find that updraft and downdraft mass fluxes and CMTs have strong scale dependency in temporal evolution and vertical structure. The prior understanding of CMT characteristics is applicable to updrafts but not downdrafts across different grid spacings. The updraft CMT transport with respect to wind shear does not change with grid spacings but downdraft CMT can have different CMT transport characteristics between large and small grid spacings. For non-linear MCSs, the downdraft CMT is downgradient (upgradient) at large (small) grid spacings. For linear MCS, the downdraft CMT is mostly downgradient for both line-parallel and line-normal components at both small and large spacings. This result is explained by the fact that downdrafts tend to be less organized than updrafts, and thus can be very sensitive to grid spacing. We also compare CRM-calculated and approximated CMT by the one-updraft and three-updraft approaches. The one-updraft approach is unable to reproduce the vertical profiles and scale dependency of updraft CMT and significantly underestimates the magnitude, but the three-updraft approach represents CRM-calculated CMT effectively across scales because it accounts for the large inter-draft variability.