| Nonsingular implementation of the Mellor-Yamada level 2.5 scheme in the NCEP Meso model - :11409 | National Weather Service (NWS)
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Nonsingular implementation of the Mellor-Yamada level 2.5 scheme in the NCEP Meso model
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    "The Mellor and Yamada Level 2.5 scheme is analyzed in order to (i ) identify the minimum conditions that enable satisfactory performance of the scheme in the full range of atmospheric forcing, and (ii) develop a robust, consistent, accurate and affordable computational procedure for application in synoptic and mesoscale models. In order to achieve the first goal it is sufficient to impose an appropriate upper limit on the master length scale in addition to requiring that the turbulent kinetic energy (TKE) and the master length scale be positive. This upper limit is proportional to the square root of TKE and a function of buoyancy and shear of the driving flow. In the unstable range this function is defined from the requirement that the TKE production be nonsingular in the case of growing turbulence, and in the stable range the function is derived from the requirement that the ratio of the vertical velocity deviation variance and TKE cannot be smaller than that corresponding to the regime of vanishing turbulence. Thus, within the PBL the master length scale is estimated using the usual diagnostic formula, and above the PBL it is computed as a fraction of the vertical grid size. The values of the master length scale are then modified if necessary in order to satisfy the described constraint"--Summary, paragraph 1.

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