Mesoscale Momentum Flux in a 7km Mesh Global Atmosphere Model

A two-year global nonhydrostatic atmosphere “nature run” simulation (G5NR) at 7km resolution is queried for one of its most unique strengths: What is the vertical momentum flux (u’w’ and v’w') by explicit air motions in the mesoscale (7-444 km) scale range? After motivating this classic question, especially in light of the hypothesis that organized convection can act as “negative viscosity” through upgradient flux, we address it comprehensively with the data. A global climatology indicates that these motions overall act as normal positive viscosity (damping the shear kinetic energy SKE), except perhaps for some grid points with steep topography and the associated gravity (internal) waves. However, many individual cases of positive SKE tendency are also seen. We drill down into space-time hyperslabs of full-resolution data for selected situations, to lay bare the nature of the calculation and the phenomena involved. Tropical cyclones are especially prodigious in producing convection-momentum interactions, of both signs.