Cumulus Friction in the Asian Monsoon of a Global Model with 7 km Mesh

Vertical transport of horizontal momentum by convective eddies (CMT) in the 7–400 km size range is examined comprehensively in data from the GEOS-5 Nature Run (G5NR), a 2-year global simulation with a 7 km horizontal mesh. This diagnosis is possible because NASA offers a coarse-grained dataset of the quadratic flux terms wu and wv in addition to the model velocity variables u, v, w. We assess the time tendency of large-scale vertically integrated shear kinetic energy (SKE) due to CMT. Negative values of a few tenths of \(1W m^{-2}\) prevail on average over warm tropical oceans, indicating that explicit convection on these scales exerts a viscous or frictional or downgradient transport effect on wind shear. However, positive as well as negative values do occur locally, based on spatial correlations u\(^\prime \)w\(^\prime \) and v\(^\prime \)w\(^\prime \) in the arrangement (“organization”) of convective motions. In the Asian monsoon, where convection and shear are both strong, the viscosity can be characterized by a regression coefficient with values of about 5% cm\(^{-1}\), meaning that convection which yields 1 cm of precipitation decrements SKE by about 5%. Adjustment of balanced monsoon flow to such a viscous effect implies adiabatic ascent to the north of existing convection, a mechanism that may be relevant to northward-propagating large-scale variability.