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Tom H. Zapotocny, Fred M. Reames, R. Bradley Pierce, Donald R. Johnson, and Bart J. Wolf


The main goals of this paper are 1) to demonstrate the feasibility of incorporating a prognostic equation for water vapor and diabatic processes in the University of Wisconsin θσ model discussed in Part I, 2) to document methods applied to overcome difficulties stemming from the inclusion of moist processes and 3) to present results illustrating the effects of latent heat release on baroclinic development. The results confirm earlier studies that a prognostic equation for water vapor and the diabatic component of latent heat release may be included in a hybrid model. However, the modifications made in this study were important for eliminating spurious supersaturation and release of latent heat within grid volumes emerging and submerging through the interface between sigma and isentropic model domains. The results demonstrate the hybrid model's robust nature and potential for use in prediction.

For this demonstration, model simulations of an analytically specified synoptic-scale wave that amplified baroclinically under dry and moist conditions are compared. Simulations with and without a hydrological component show that the overall effect of latent heat release is to markedly enhance cyclo- and frontogenesis. The resultant pattern of precipitation is coherent, and the structure of the developing wave is consistent with the concepts of self-development. No detrimental effects are evident in either the structure or processes resulting from the inclusion of moist processes and the presence of an interface between sigma and isentropic model domains.

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R. Bradley Pierce, Fred M. Reames, Tom H. Zapotocny, Donald R. Johnson, and Bart J. Wolf


In a validation experiment of a hybrid isentropic–sigma coordinate primitive equation model developed at the University of Wisconsin (the UW θσ model), an initial value technique is used to investigate numerically the normal-mode characteristics of baroclinically amplifying disturbances over a spectrum of meteorologically significant wavelength. The experiments are designed to determine the impact of coupling an isentropic-coordinate free atmospheric domain to a sigma-coordinate planetary boundary layer (PBL) on the normal-mode characteristics. The growth rate and phase speed spectra of the most unstable normal modes are obtained for an analytically prescribed zonal flow field. The evolution and vertical structure of the kinetic energy, energy conversions, growth rates, and geopotential fields are investigated.

Several modifications have been made to earlier versions of the UW θσ model to overcome noise introduced by adjustments associated with emerging and submerging grid volumes at the sigma–isentropic interface. With these modifications, the hybrid model accurately simulates the evolution and structure of normal-mode baroclinic disturbances for all wavenumbers considered except for wavenumber two. The normal-mode growth rate and phase speed spectra compare well with previous studies using standard sigma coordinate models. There is no evidence of aliasing the baroclinic normal-mode characteristics due to the coupling of isentropic and sigma domains.

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