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- Author or Editor: Andrzej A. Wyszogrodzki x
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Abstract
A Cartesian, small- to mesoscale nonhydrostatic model is extended to a rotating mountainous sphere, thereby dispensing with the traditional geophysical simplifications of hydrostaticity, gentle terrain slopes, and weak rotation. The authors discuss the algorithmic design, relative efficiency, and accuracy of several different variants (hydrostatic, nonhydrostatic, implicit, explicit, elastic, anelastic, etc.) of the global model and prepare the ground for a future “global cloud model”—a research tool to study effects of small- and mesoscale phenomena on global flows and vice versa. There are two primary threads to the discussion: (a) presenting a novel semi-implicit anelastic global dynamics model as it naturally emerges from a small-scale dynamics model, and (b) demonstrating that nonhydrostatic anelastic global models derived from small-scale codes adequately capture a broad range of planetary flows while requiring relatively minor overhead due to the nonhydrostatic formulation of the governing equations. The authors substantiate their theoretical discussions with a detailed analysis of numerous simulations of idealized global orographic flows and climate states.
Abstract
A Cartesian, small- to mesoscale nonhydrostatic model is extended to a rotating mountainous sphere, thereby dispensing with the traditional geophysical simplifications of hydrostaticity, gentle terrain slopes, and weak rotation. The authors discuss the algorithmic design, relative efficiency, and accuracy of several different variants (hydrostatic, nonhydrostatic, implicit, explicit, elastic, anelastic, etc.) of the global model and prepare the ground for a future “global cloud model”—a research tool to study effects of small- and mesoscale phenomena on global flows and vice versa. There are two primary threads to the discussion: (a) presenting a novel semi-implicit anelastic global dynamics model as it naturally emerges from a small-scale dynamics model, and (b) demonstrating that nonhydrostatic anelastic global models derived from small-scale codes adequately capture a broad range of planetary flows while requiring relatively minor overhead due to the nonhydrostatic formulation of the governing equations. The authors substantiate their theoretical discussions with a detailed analysis of numerous simulations of idealized global orographic flows and climate states.
Abstract
Three-dimensional simulations of the daytime thermally induced valley wind system for an idealized valley–plain configuration, obtained from nine nonhydrostatic mesoscale models, are compared with special emphasis on the evolution of the along-valley wind. The models use the same initial and lateral boundary conditions, and standard parameterizations for turbulence, radiation, and land surface processes. The evolution of the mean along-valley wind (averaged over the valley cross section) is similar for all models, except for a time shift between individual models of up to 2 h and slight differences in the speed of the evolution. The analysis suggests that these differences are primarily due to differences in the simulated surface energy balance such as the dependence of the sensible heat flux on surface wind speed. Additional sensitivity experiments indicate that the evolution of the mean along-valley flow is largely independent of the choice of the dynamical core and of the turbulence parameterization scheme. The latter does, however, have a significant influence on the vertical structure of the boundary layer and of the along-valley wind. Thus, this ideal case may be useful for testing and evaluation of mesoscale numerical models with respect to land surface–atmosphere interactions and turbulence parameterizations.
Abstract
Three-dimensional simulations of the daytime thermally induced valley wind system for an idealized valley–plain configuration, obtained from nine nonhydrostatic mesoscale models, are compared with special emphasis on the evolution of the along-valley wind. The models use the same initial and lateral boundary conditions, and standard parameterizations for turbulence, radiation, and land surface processes. The evolution of the mean along-valley wind (averaged over the valley cross section) is similar for all models, except for a time shift between individual models of up to 2 h and slight differences in the speed of the evolution. The analysis suggests that these differences are primarily due to differences in the simulated surface energy balance such as the dependence of the sensible heat flux on surface wind speed. Additional sensitivity experiments indicate that the evolution of the mean along-valley flow is largely independent of the choice of the dynamical core and of the turbulence parameterization scheme. The latter does, however, have a significant influence on the vertical structure of the boundary layer and of the along-valley wind. Thus, this ideal case may be useful for testing and evaluation of mesoscale numerical models with respect to land surface–atmosphere interactions and turbulence parameterizations.
Abstract
After extensive efforts over the course of a decade, convective-scale weather forecasts with horizontal grid spacings of 1–5 km are now operational at national weather services around the world, accompanied by ensemble prediction systems (EPSs). However, though already operational, the capacity of forecasts for this scale is still to be fully exploited by overcoming the fundamental difficulty in prediction: the fully three-dimensional and turbulent nature of the atmosphere. The prediction of this scale is totally different from that of the synoptic scale (103 km), with slowly evolving semigeostrophic dynamics and relatively long predictability on the order of a few days.
Even theoretically, very little is understood about the convective scale compared to our extensive knowledge of the synoptic-scale weather regime as a partial differential equation system, as well as in terms of the fluid mechanics, predictability, uncertainties, and stochasticity. Furthermore, there is a requirement for a drastic modification of data assimilation methodologies, physics (e.g., microphysics), and parameterizations, as well as the numerics for use at the convective scale. We need to focus on more fundamental theoretical issues—the Liouville principle and Bayesian probability for probabilistic forecasts—and more fundamental turbulence research to provide robust numerics for the full variety of turbulent flows.
The present essay reviews those basic theoretical challenges as comprehensibly as possible. The breadth of the problems that we face is a challenge in itself: an attempt to reduce these into a single critical agenda should be avoided.
Abstract
After extensive efforts over the course of a decade, convective-scale weather forecasts with horizontal grid spacings of 1–5 km are now operational at national weather services around the world, accompanied by ensemble prediction systems (EPSs). However, though already operational, the capacity of forecasts for this scale is still to be fully exploited by overcoming the fundamental difficulty in prediction: the fully three-dimensional and turbulent nature of the atmosphere. The prediction of this scale is totally different from that of the synoptic scale (103 km), with slowly evolving semigeostrophic dynamics and relatively long predictability on the order of a few days.
Even theoretically, very little is understood about the convective scale compared to our extensive knowledge of the synoptic-scale weather regime as a partial differential equation system, as well as in terms of the fluid mechanics, predictability, uncertainties, and stochasticity. Furthermore, there is a requirement for a drastic modification of data assimilation methodologies, physics (e.g., microphysics), and parameterizations, as well as the numerics for use at the convective scale. We need to focus on more fundamental theoretical issues—the Liouville principle and Bayesian probability for probabilistic forecasts—and more fundamental turbulence research to provide robust numerics for the full variety of turbulent flows.
The present essay reviews those basic theoretical challenges as comprehensibly as possible. The breadth of the problems that we face is a challenge in itself: an attempt to reduce these into a single critical agenda should be avoided.
