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- Author or Editor: J. W. M. Cuijpers x
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Abstract
A simple parameterization of cloud water related variables has been developed which can be used in meteorological models that use a prognostic equation for the turbulent kinetic energy. Based on the results of large-eddy simulations (LES), expressions are derived for the liquid water flux, the partial cloudiness, and the cloud water content as a function of the normalized saturation deficit. With these relations, which are based on LESs ranging from situations with small to moderate amount of cumulus clouds to a stratocumulus case, no further knowledge about distribution functions or skewness is needed.
Abstract
A simple parameterization of cloud water related variables has been developed which can be used in meteorological models that use a prognostic equation for the turbulent kinetic energy. Based on the results of large-eddy simulations (LES), expressions are derived for the liquid water flux, the partial cloudiness, and the cloud water content as a function of the normalized saturation deficit. With these relations, which are based on LESs ranging from situations with small to moderate amount of cumulus clouds to a stratocumulus case, no further knowledge about distribution functions or skewness is needed.
Abstract
A large-eddy simulation (LES) model has been utilized to study nonprecipitating Shallow Convective clouds such as observed during the undisturbed BOMEX period in the trade wind areas. By choosing a realistic large-scale forcing the authors have been able to simulate shallow convective clouds under quasi-steady-state conditions over a long period of 7 hours. This is a necessary condition to investigate diagnostic cumulus parameterization schemes since such schemes usually assume steady-state conditions. The response of the model to the applied large-scale forcing compares well with budget study results of BOMEX. In addition, the LES model delivers detailed information concerning the dynamics of shallow convective clouds. This is used to verify basic parameterizations of turbulent fluxes and entrainment and detrainment rates used in large-scale models. The most important conclusions are (i) the fractional entrainment and detrainment rates used in present large-scale atmospheric models are one order of magnitude too small, confirming previous results obtained by Esbensen, and (ii) estimates of turbulent fluxes by bulk cloud updrafts and environmental downdrafts give an underestimation of 20% to 50% depending on the variable that is transported. Implications of these results for cumulus parameterizations will be discussed.
Abstract
A large-eddy simulation (LES) model has been utilized to study nonprecipitating Shallow Convective clouds such as observed during the undisturbed BOMEX period in the trade wind areas. By choosing a realistic large-scale forcing the authors have been able to simulate shallow convective clouds under quasi-steady-state conditions over a long period of 7 hours. This is a necessary condition to investigate diagnostic cumulus parameterization schemes since such schemes usually assume steady-state conditions. The response of the model to the applied large-scale forcing compares well with budget study results of BOMEX. In addition, the LES model delivers detailed information concerning the dynamics of shallow convective clouds. This is used to verify basic parameterizations of turbulent fluxes and entrainment and detrainment rates used in large-scale models. The most important conclusions are (i) the fractional entrainment and detrainment rates used in present large-scale atmospheric models are one order of magnitude too small, confirming previous results obtained by Esbensen, and (ii) estimates of turbulent fluxes by bulk cloud updrafts and environmental downdrafts give an underestimation of 20% to 50% depending on the variable that is transported. Implications of these results for cumulus parameterizations will be discussed.
Abstract
A large eddy simulation (LES) model, used for studying the dry convective boundary layer, has been extended with an equation for the total water specific humidity and a condensation scheme to simulate the partly cloudy convective boundary layer. A simulation has been made based on the observations gathered near Puerto Rico on 15 December 1972. Starting from a clear air situation, the model evolves to a situation with small cumulus clouds. Vertical profiles of variances and fluxes show satisfactory agreement with the experimental data. It will be shown that layer-averaged fluxes and variances within the cloud layer are related to the amount of cloud water.
Abstract
A large eddy simulation (LES) model, used for studying the dry convective boundary layer, has been extended with an equation for the total water specific humidity and a condensation scheme to simulate the partly cloudy convective boundary layer. A simulation has been made based on the observations gathered near Puerto Rico on 15 December 1972. Starting from a clear air situation, the model evolves to a situation with small cumulus clouds. Vertical profiles of variances and fluxes show satisfactory agreement with the experimental data. It will be shown that layer-averaged fluxes and variances within the cloud layer are related to the amount of cloud water.
Abstract
Large-eddy simulation (LES) results of three prototype convective boundary layers (CBL) are used to study flux budgets and to test simple expressions for scalar and buoyancy fluxes. The simulated CBLs differ in their characteristics of skewness of the vertical velocity field and the scalar flux gradients. This is accomplished by applying a surface heat flux or radiative cooling at the boundary layer top. In this way boundary layers with positive, zero, and negative skewness are simulated.
Useful approximations for the transport and buoyancy terms in the flux budgets are achieved by using a generalized convective scaling. This directly provides an expression for scalar and buoyancy fluxes that combines downgradient and nonlocal effects on the fluxes. The nonlocal effects are due to vertical velocity variance and the integrated flux over the boundary layer. This result is compared with an alternative expression in which the nonlocal flux is related to the skewness of the vertical velocity field and the scalar flux gradient.
Overall it appears that the nonlocal flux is mostly related to the vertical velocity variance and the integrated flux and not so much to the skewness. The final result is a simple expression for scalar and buoyancy fluxes, that can be used in mesoscale and global models.
Abstract
Large-eddy simulation (LES) results of three prototype convective boundary layers (CBL) are used to study flux budgets and to test simple expressions for scalar and buoyancy fluxes. The simulated CBLs differ in their characteristics of skewness of the vertical velocity field and the scalar flux gradients. This is accomplished by applying a surface heat flux or radiative cooling at the boundary layer top. In this way boundary layers with positive, zero, and negative skewness are simulated.
Useful approximations for the transport and buoyancy terms in the flux budgets are achieved by using a generalized convective scaling. This directly provides an expression for scalar and buoyancy fluxes that combines downgradient and nonlocal effects on the fluxes. The nonlocal effects are due to vertical velocity variance and the integrated flux over the boundary layer. This result is compared with an alternative expression in which the nonlocal flux is related to the skewness of the vertical velocity field and the scalar flux gradient.
Overall it appears that the nonlocal flux is mostly related to the vertical velocity variance and the integrated flux and not so much to the skewness. The final result is a simple expression for scalar and buoyancy fluxes, that can be used in mesoscale and global models.
Abstract
A simple method is proposed to extend a low-order turbulence scheme including a subgrid-scale cloudiness scheme to represent not only nonconvective (stratiform) cloudiness and turbulence but also shallow, nonprecipitating cumulus convection by utilizing an appropriate subgrid-scale distribution function. A simple approach is chosen that avoids the knowledge of the skewness parameter. All cloud water-related variables (cloud water content, partial cloudiness, liquid water flux) are computed by interpolating linearly as a function of the saturation deficit between two limit cases: the stratocumulus case, which can be well represented by a Gaussian distribution function, and the trade wind cumulus case, characterized by a positively skewed distribution function with a skewness of 2.
Comparisons of the scheme with a third-order turbulence scheme and large-eddy simulations (LES) for the Puerto Rico Field Experiment show satisfying results. It is shown that the liquid water flux term, which is strongly dependent on the distribution chosen, is the crucial parameter of the scheme.
Abstract
A simple method is proposed to extend a low-order turbulence scheme including a subgrid-scale cloudiness scheme to represent not only nonconvective (stratiform) cloudiness and turbulence but also shallow, nonprecipitating cumulus convection by utilizing an appropriate subgrid-scale distribution function. A simple approach is chosen that avoids the knowledge of the skewness parameter. All cloud water-related variables (cloud water content, partial cloudiness, liquid water flux) are computed by interpolating linearly as a function of the saturation deficit between two limit cases: the stratocumulus case, which can be well represented by a Gaussian distribution function, and the trade wind cumulus case, characterized by a positively skewed distribution function with a skewness of 2.
Comparisons of the scheme with a third-order turbulence scheme and large-eddy simulations (LES) for the Puerto Rico Field Experiment show satisfying results. It is shown that the liquid water flux term, which is strongly dependent on the distribution chosen, is the crucial parameter of the scheme.
Abstract
This study has determined energy spectra of turbulent variables in large eddy simulations of the penetrating dry convective boundary layer (microscale convection). The simulated domain has a large aspect ratio, the horizontal size being roughly 16 times the boundary layer depth. It turns out that both the turbulent velocities and the potential temperature exhibit “classic” energy spectra, which means that the dominant contribution to the variance originates from a scale of the order of the boundary layer height.
Surprisingly, the authors find that energy spectra of passive scalars in the convective boundary layer can behave completely differently from the velocity and temperature spectra. Depending on the boundary conditions of the scalar, that is, the surface flux and the entrainment flux, the spectrum is either classical in the aforementioned sense or it is dominated by the smallest wavenumbers, implying that the fluctuations are dominated by the largest scales. Loosely speaking the results can be summarized as follows: if the scalar entrainment flux is a negative fraction (about −½) of the surface flux, the scalar fluctuations are dominated by relatively small scales (∼ boundary layer depth), whereas in most other cases the scalar fluctuations tend to be dominated by the largest scales resolved (∼ tenths of kilometers, i.e., mesoscales). The latter result is rather peculiar since neither the velocity components nor the temperature field contains these large-scale fluctuations.
Abstract
This study has determined energy spectra of turbulent variables in large eddy simulations of the penetrating dry convective boundary layer (microscale convection). The simulated domain has a large aspect ratio, the horizontal size being roughly 16 times the boundary layer depth. It turns out that both the turbulent velocities and the potential temperature exhibit “classic” energy spectra, which means that the dominant contribution to the variance originates from a scale of the order of the boundary layer height.
Surprisingly, the authors find that energy spectra of passive scalars in the convective boundary layer can behave completely differently from the velocity and temperature spectra. Depending on the boundary conditions of the scalar, that is, the surface flux and the entrainment flux, the spectrum is either classical in the aforementioned sense or it is dominated by the smallest wavenumbers, implying that the fluctuations are dominated by the largest scales. Loosely speaking the results can be summarized as follows: if the scalar entrainment flux is a negative fraction (about −½) of the surface flux, the scalar fluctuations are dominated by relatively small scales (∼ boundary layer depth), whereas in most other cases the scalar fluctuations tend to be dominated by the largest scales resolved (∼ tenths of kilometers, i.e., mesoscales). The latter result is rather peculiar since neither the velocity components nor the temperature field contains these large-scale fluctuations.
