Understanding Moist Convection From a Lagrangian Perspective

Abstract

Moist convection plays an important role in the climate system by redistributing heat, moisture, momentum, as well as chemical species, and by perturbing the radiation budget. However, due to our limited knowledge of convection, its poor representation in climate models introduces the most uncertainty in climate projections. In this dissertation work, the dynamics of both shallow and deep convection are investigated from a Lagrangian perspective. By tracking Lagrangian particles in the cloud field and casting them into a spectral plume representation, a relationship between fractional entrainment rate, vertical velocity and cloud radius was proposed. It was found that the fractional entrainment rate per unit height can be represented by the inverse product of vertical velocity and cloud radius multiplied by a coefficient. This coefficient has a turbulent velocity scale that might scale with the magnitude of turbulent kinetic energy. It is almost constant through the bulk of the clouds, which is estimated to be ~ 0.23 m/s for shallow convection and ~ 2.2 m/s for deep convection. This result provides a new way to parameterize entrainment rate in convection and has the potential for a unified convective parameterization. A parallel effort was taken to understand the vertical velocity distribution within convection. Both buoyancy and pressure gradient term play important roles in the vertical momentum budget. Furthermore we illustrate that the effective buoyancy and dynamic perturbation pressure can be approximated to a good extent by a simple cylindrical updraft model given the cloud radius, and this has implications for more objectively evaluating vertical velocity in convective parameterization and allowing for a tighter coupling between cloud dynamics and microphysics. The similar analysis framework was employed to understand the mechanism that underlies different convective sensitivities between the lower and upper troposphere, a behavior that is important to the dynamics of large-scale moist flows, such as convectively coupled waves. We found that both the differences in updraft buoyancy and vertical velocity play a significant role, with the vertical velocity being the more important, and the effect of liquid water content is only secondary compared to the other two factors.