Convective Momentum Transport and Perturbation Pressure Field from a Cloud-Resolving Model Simulation

Abstract

This study uses a 2D cloud-resolving model to investigate the vertical transport of horizontal momentum and to understand the role of a convection-generated perturbation pressure field in the momentum transport by convective systems during part of the Tropical Ocean and Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) Intensive Observation Period. It shows that convective updrafts transport a significant amount of momentum vertically. This transport is downgradient in the easterly wind regime, but upgradient during a westerly wind burst. The differences in convective momentum transport between easterly and westerly wind regimes are examined. The perturbation pressure gradient accounts for an important part of the apparent momentum source. In general it is opposite in sign to the product of cloud mass flux and the vertical wind shear, with smaller magnitude. Examination of the dynamic forcing to the pressure field demonstrates that the linear forcing representing the interaction between the convective updrafts and the large-scale wind shear is the dominant term, while the nonlinear forcing is of secondary importance. Thus, parameterization schemes taking into account the linear interaction between the convective updrafts and the large-scale wind shear can capture the essential features of the perturbation pressure field. The parameterization scheme for momentum transport by Zhang and Cho is evaluated using the model simulation data. The parameterized pressure gradient force using the scheme is in excellent agreement with the simulated one. The parameterized apparent momentum source is also in good agreement with the model simulation. Other parameterization methods for the pressure gradient are also discussed.