Convective momentum transport over the tropical Pacific

Abstract

The vertical transport of horizontal momentum by cumulus convection is investigated in observations and model simulations of the Tropical Ocean Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). Estimates of convective momentum transport (CMT) are obtained from the COARE momentum budget, aircraft dual-Doppler radar observations, cloud-resolving models, and CMT parameterizations. The role of CMIT in the large-scale circulation of the tropical Pacific is examined in the process.Budget estimates of CMT are shown to be highly uncertain, with no detectable signature of CMT above 850 mb. Below 850 mb, there is a slight tendency for downgradient transport. The pressure term is the major source of uncertainty in the budget. Radar CMT estimates are also only marginally significant, but show a tendency for countergradient transport above the time-mean low-level westerly jet. Countergradient transport is enhanced during westerly wind bursts, suggesting a role for CMT in the evolution of the Madden-Julian Oscillation. Convective updrafts accomplish nearly all of the CMT observed by the radars.A high-resolution 3D mesoscale model simulation of the December westerly wind burst also indicates the presence of countergradient transport by convective updrafts above the low-level jet. Momentum transport in the mesoscale model is much larger than either observational estimate, however. Differences in magnitude between the mesoscale model and the radar observations are attributed to the inability of the radar to adequately resolve vertical velocity features.The global model parameterization of Gregory et al. shows reasonable agreement with the mesoscale model at upper levels, but does not produce the countergradient layer above the low-level jet, suggesting the need for modification of simple mass flux representations of CMT. Further high-resolution modeling efforts are recommended.