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Abstract
The purpose of this article is to introduce a new diagnostic measure of the time-integrated diabatic (thermal) forcing of a valley–plain system. This measure can be used to synchronize the evolution of thermally induced valley winds with respect to their forcing. Differences among numerical models or model configurations originating from diabatic forcing versus those originating from the model dynamics (e.g., turbulence scheme, dynamical core, etc.) can then be distinguished.
Abstract
The purpose of this article is to introduce a new diagnostic measure of the time-integrated diabatic (thermal) forcing of a valley–plain system. This measure can be used to synchronize the evolution of thermally induced valley winds with respect to their forcing. Differences among numerical models or model configurations originating from diabatic forcing versus those originating from the model dynamics (e.g., turbulence scheme, dynamical core, etc.) can then be distinguished.
Abstract
In convection-permitting simulations, the spectrum of resolved motions is truncated near scales where convection is active. An “energy gap” between resolved and unresolved motions does not exist, such that the upscale and downscale fluxes of energy across the spectrum are affected by the representation of turbulence as well as (implicit and explicit) numerical diffusion. In the current study, a systematic analysis is undertaken of the role of explicit numerical diffusion in simulations of diurnal convection over a large Alpine region, using the Consortium for Small Scale Modeling (COSMO) mesoscale model. Results are explored by using energy spectra and by diagnosing the physical and dynamical contributions to the bulk mesoscale heat budget. In addition, a linear analytical model is employed to assess different formulations of numerical diffusion.
Consistent with previous studies the authors find that diffusion may strongly affect the energy spectrum and the formation of precipitation. Besides the direct impact on convective intensity and cloud distribution, they demonstrate that diffusion has an upscale influence and ultimately affects the mesoscale dynamics. Diffusion reduces the bulk Alpine net heating on a scale of O(100 km). It is hypothesized that this upscale influence is primarily due to the following factor: multiple triggering of orographic convection over a complex mountain range leads to mountain-scale diurnal signals in vertical velocity that are sensitive even to scale-selective diffusion.
The simulations show that, in agreement with linear stability theory of convective growth, convective amplification is most sensitive to numerical diffusion of buoyancy and horizontal momentum components on near-surface model levels. If horizontal diffusion is not accomplished by a physically based parameterization and if the application of noise-reducing (e.g., monotonic) advection schemes proves to be insufficient to obviate the amplification of numerical noise, a necessary minimum of explicit diffusion is found to improve (i.e., decrease) the upscaling of energy to the mesoscale.
Abstract
In convection-permitting simulations, the spectrum of resolved motions is truncated near scales where convection is active. An “energy gap” between resolved and unresolved motions does not exist, such that the upscale and downscale fluxes of energy across the spectrum are affected by the representation of turbulence as well as (implicit and explicit) numerical diffusion. In the current study, a systematic analysis is undertaken of the role of explicit numerical diffusion in simulations of diurnal convection over a large Alpine region, using the Consortium for Small Scale Modeling (COSMO) mesoscale model. Results are explored by using energy spectra and by diagnosing the physical and dynamical contributions to the bulk mesoscale heat budget. In addition, a linear analytical model is employed to assess different formulations of numerical diffusion.
Consistent with previous studies the authors find that diffusion may strongly affect the energy spectrum and the formation of precipitation. Besides the direct impact on convective intensity and cloud distribution, they demonstrate that diffusion has an upscale influence and ultimately affects the mesoscale dynamics. Diffusion reduces the bulk Alpine net heating on a scale of O(100 km). It is hypothesized that this upscale influence is primarily due to the following factor: multiple triggering of orographic convection over a complex mountain range leads to mountain-scale diurnal signals in vertical velocity that are sensitive even to scale-selective diffusion.
The simulations show that, in agreement with linear stability theory of convective growth, convective amplification is most sensitive to numerical diffusion of buoyancy and horizontal momentum components on near-surface model levels. If horizontal diffusion is not accomplished by a physically based parameterization and if the application of noise-reducing (e.g., monotonic) advection schemes proves to be insufficient to obviate the amplification of numerical noise, a necessary minimum of explicit diffusion is found to improve (i.e., decrease) the upscaling of energy to the mesoscale.
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
Numerical simulations of flow over steep terrain using 11 different nonhydrostatic numerical models are compared and analyzed. A basic benchmark and five other test cases are simulated in a two-dimensional framework using the same initial state, which is based on conditions during Intensive Observation Period (IOP) 6 of the Terrain-Induced Rotor Experiment (T-REX), in which intense mountain-wave activity was observed. All of the models use an identical horizontal resolution of 1 km and the same vertical resolution. The six simulated test cases use various terrain heights: a 100-m bell-shaped hill, a 1000-m idealized ridge that is steeper on the lee slope, a 2500-m ridge with the same terrain shape, and a cross-Sierra terrain profile. The models are tested with both free-slip and no-slip lower boundary conditions.
The results indicate a surprisingly diverse spectrum of simulated mountain-wave characteristics including lee waves, hydraulic-like jump features, and gravity wave breaking. The vertical velocity standard deviation is twice as large in the free-slip experiments relative to the no-slip simulations. Nevertheless, the no-slip simulations also exhibit considerable variations in the wave characteristics. The results imply relatively low predictability of key characteristics of topographically forced flows such as the strength of downslope winds and stratospheric wave breaking. The vertical flux of horizontal momentum, which is a domain-integrated quantity, exhibits considerable spread among the models, particularly for the experiments with the 2500-m ridge and Sierra terrain. The differences among the various model simulations, all initialized with identical initial states, suggest that model dynamical cores may be an important component of diversity for the design of mesoscale ensemble systems for topographically forced flows. The intermodel differences are significantly larger than sensitivity experiments within a single modeling system.
Abstract
Numerical simulations of flow over steep terrain using 11 different nonhydrostatic numerical models are compared and analyzed. A basic benchmark and five other test cases are simulated in a two-dimensional framework using the same initial state, which is based on conditions during Intensive Observation Period (IOP) 6 of the Terrain-Induced Rotor Experiment (T-REX), in which intense mountain-wave activity was observed. All of the models use an identical horizontal resolution of 1 km and the same vertical resolution. The six simulated test cases use various terrain heights: a 100-m bell-shaped hill, a 1000-m idealized ridge that is steeper on the lee slope, a 2500-m ridge with the same terrain shape, and a cross-Sierra terrain profile. The models are tested with both free-slip and no-slip lower boundary conditions.
The results indicate a surprisingly diverse spectrum of simulated mountain-wave characteristics including lee waves, hydraulic-like jump features, and gravity wave breaking. The vertical velocity standard deviation is twice as large in the free-slip experiments relative to the no-slip simulations. Nevertheless, the no-slip simulations also exhibit considerable variations in the wave characteristics. The results imply relatively low predictability of key characteristics of topographically forced flows such as the strength of downslope winds and stratospheric wave breaking. The vertical flux of horizontal momentum, which is a domain-integrated quantity, exhibits considerable spread among the models, particularly for the experiments with the 2500-m ridge and Sierra terrain. The differences among the various model simulations, all initialized with identical initial states, suggest that model dynamical cores may be an important component of diversity for the design of mesoscale ensemble systems for topographically forced flows. The intermodel differences are significantly larger than sensitivity experiments within a single modeling system.
