Development of a unified representation of boundary layer clouds and turbulence in the NASA GEOS AGCM

Abstract:

The representation of boundary layer clouds remains a key source of uncertainty in weather and climate prediction. Unification of turbulence, dry and shallow convection processes into a single parameterization has long been recognized as a necessary condition for remedying these model errors.In this study, we summarize our experience implementing such a unified parameterization in the Goddard Earth Observing System (GEOS) atmospheric general circulation model. Our scheme combines two well-known methods, the eddy diffusivity-mass flux (EDMF) and high-order closure-assumed distribution(ADHOC) approaches. The local component of EDMF mixing/transport is modeled using the Mellor-Yamada-Nakanishi-Niino (MYNN) level-2.5 turbulence closure, while the non-local EDMF component uses a multiple mass flux scheme developed at NASA JPL. The ADHOC method is applied via the assumption that the jointv ariability of heat, moisture, and momentum in a grid cell has a doubleGaussian distribution, with one component Gaussian distribution quantifying variability within the mass flux scheme’s updraft ensemble while the other quantifies variability inside the environment of the ensemble. Unlike conventional EDMF schemes, the second-order moments of heat, moisture, and momentum are consistently partitioned between these two parts of the grid cell, thereby determining the shape of the double Gaussian distribution without requiring a predictive or diagnostic equation for third-order moments. Rather, the mass flux scheme implicitly determines such skewnesses via its entraining plume equations. Moreover, turbulent kinetic energy (TKE)is consistently partitioned in the same way, eliminating spurious sources and sinks of energy due to double counting of buoyant production/destruction.Instead, organized TKE associated with the rising updraft plumes and subsiding environment interacts with TKE within the environment via entrainment, detrainment, and subsidence. Results will be presented for single column model simulations of several standard marine boundary layer cloud cases using our new parameterization in GEOS. Considerations and challenges associated with energetic consistency and numerical stability will be discussed as well as considerations for future development and testing in three-dimensional simulations of GEOS.