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A Partially Prognostic Third-Order Closure Model for Modeling the Boundary Layer

Cheng, A.C.(a) and Xu, K.-M.(b), Center for Atmospheric Sciences, Hampton University (a), Atmospheric Sciences, NASA Langley Research Center (b)
Fourteenth Atmospheric Radiation Measurement (ARM) Science Team Meeting

A new partially prognostic third-order closure (TOC) model is developed to model boundary-layer clouds in this study. The model assumes joint double Gaussian distributions of vertical velocity, temperature and moisture. The first and second moments of all variables as well as the third moments of vertical velocity, liquid-water potential temperature and total water mixing ratio are predicted to determine a proper probability density function (PDF). Once the PDF is known, the rest of the third moments and all fourth moments are diagnosed. The model differs from a fully prognostic model in three major aspects: 1) it is computationally less expensive, 2) it is internally physically consistent, and 3) there is a lack of unrealistically strong liquid-water oscillations. Compared to a minimally-prognostic model, which only predicts the triple correlation of vertical velocity, the partially-prognostic model uses more third moments to determine the PDF and thus removes a few assumptions in the former model. Five cases, clear convective boundary layer, stratocumulus, marine tradewind cumulus, continental shallow cumulus, and transition-regime cloud, are chosen to test the new model. Results from large-eddy simulations (LESs), the fully prognostic, the minimally prognostic, and the new TOC models are compared. Observational data are provided for some of the cases. The new model shows an improved ability to simulate the shallow cumuli and the transition-regime cloud. There are no major differences among the three TOC models in simulating the clear convective boundary layer and the stratocumulus regime. Another important result is that a good agreement between the partially-prognostic model and LESs is obtained for the five diverse cases without the use of any case-specific adjustments.

Note: This is the poster abstract presented at the meeting; an extended version was not provided by the author(s).