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- Author or Editor: A. A. M. Holtslag x
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Abstract
The results of a local and a nonlocal scheme for vertical diffusion in the atmospheric boundary layer are compared within the context of a global climate model. The global model is an updated version of the NCAR Community Climate Model (CCM2). The local diffusion scheme uses an eddy diffusivity determined independently at each point in the vertical, based on local vertical gradients of wind and virtual potential temperature, similar to the usual approach in global atmospheric models. The nonlocal scheme determines an eddy-diffusivity profile based on a diagnosed boundary-layer height and a turbulent velocity scale. It also incorporates nonlocal (vertical) transport effects for heat and moisture.
The two diffusion schemes are summarized, and their results are compared with independent radiosonde observations for a number of locations. The focus herein is on the temperature and humidity structure over ocean, where the surface temperatures are specified, since the boundary-layer scheme interacts strongly with the land-surface parameterization. Systematic differences are shown in global-climate simulations, with CCM2 using the two schemes. The nonlocal scheme transports moisture away from the surface more rapidly than the local scheme, and deposits the moisture at higher levels. The local scheme tends to saturate the lowest model levels unrealistically, which typically leads to clouds too low in the atmosphere.
The nonlocal scheme has been chosen for CCM2 because of its more comprehensive representation of the physics of boundary-layer transport in dry convective conditions.
Abstract
The results of a local and a nonlocal scheme for vertical diffusion in the atmospheric boundary layer are compared within the context of a global climate model. The global model is an updated version of the NCAR Community Climate Model (CCM2). The local diffusion scheme uses an eddy diffusivity determined independently at each point in the vertical, based on local vertical gradients of wind and virtual potential temperature, similar to the usual approach in global atmospheric models. The nonlocal scheme determines an eddy-diffusivity profile based on a diagnosed boundary-layer height and a turbulent velocity scale. It also incorporates nonlocal (vertical) transport effects for heat and moisture.
The two diffusion schemes are summarized, and their results are compared with independent radiosonde observations for a number of locations. The focus herein is on the temperature and humidity structure over ocean, where the surface temperatures are specified, since the boundary-layer scheme interacts strongly with the land-surface parameterization. Systematic differences are shown in global-climate simulations, with CCM2 using the two schemes. The nonlocal scheme transports moisture away from the surface more rapidly than the local scheme, and deposits the moisture at higher levels. The local scheme tends to saturate the lowest model levels unrealistically, which typically leads to clouds too low in the atmosphere.
The nonlocal scheme has been chosen for CCM2 because of its more comprehensive representation of the physics of boundary-layer transport in dry convective conditions.
Abstract
A mass flux parameterization scheme for shallow cumulus convection is evaluated for a case based on observations and large eddy simulation (LES) results for the Barbados Oceanographic and Meteorological Experiment (BOMEX). The mass flux scheme is embedded in a one-column model with prescribed large-scale forcings. Comparing the findings of the latter with the LES results, it is found that the mass flux scheme is too active. As a result, the scheme is mixing too much heat and moisture between the cloud layer and the inversion layer, giving rise to erroneous moisture and temperature profiles for the trade wind region. This is due to an underestimation of the lateral exchange rates. LES results show that for shallow cumulus cloud ensembles (lateral) entrainment and detrainment rates are typically one order of magnitude larger than values used in most operational parameterization schemes and that the detrainment rate is systematically larger than the entrainment rate. When adapting them enhanced rates, the mass flux scheme produces realistic mass fluxes and cloud excess values for moisture and heat and is therefore capable of maintaining the stationary state as observed during BOMEX.
Abstract
A mass flux parameterization scheme for shallow cumulus convection is evaluated for a case based on observations and large eddy simulation (LES) results for the Barbados Oceanographic and Meteorological Experiment (BOMEX). The mass flux scheme is embedded in a one-column model with prescribed large-scale forcings. Comparing the findings of the latter with the LES results, it is found that the mass flux scheme is too active. As a result, the scheme is mixing too much heat and moisture between the cloud layer and the inversion layer, giving rise to erroneous moisture and temperature profiles for the trade wind region. This is due to an underestimation of the lateral exchange rates. LES results show that for shallow cumulus cloud ensembles (lateral) entrainment and detrainment rates are typically one order of magnitude larger than values used in most operational parameterization schemes and that the detrainment rate is systematically larger than the entrainment rate. When adapting them enhanced rates, the mass flux scheme produces realistic mass fluxes and cloud excess values for moisture and heat and is therefore capable of maintaining the stationary state as observed during BOMEX.
Abstract
The interaction of the atmospheric boundary layer with the heterogeneous pine forest in HAPEX-MOBILHY on a scale of order 10 km is studied. A state-of-the-art, coupled atmosphere-soil-vegetation model is used and is run for 16 June 1986 in a stand-alone mode using prescribed dynamics. Published values for the effective roughness lengths of heat and momentum from different origins are used to show the impact on the surface fluxes and the boundary layer development. The modal simulations indicate that the coupled atmosphere-vegetation system is rather sensitive to the value for the roughness length of heat. This affects in particular the sensible heat flux and, as a consequence, the boundary layer height and profiles of mean quantities in the boundary layer. The model results are compared with observations made at a forest town, with radiosonde profiles, and with aircraft data. The best overall agreement for the boundary layer quantities is obtained by using a roughness length of heat that is three orders of magnitude smaller than the roughness length of momentum.
Abstract
The interaction of the atmospheric boundary layer with the heterogeneous pine forest in HAPEX-MOBILHY on a scale of order 10 km is studied. A state-of-the-art, coupled atmosphere-soil-vegetation model is used and is run for 16 June 1986 in a stand-alone mode using prescribed dynamics. Published values for the effective roughness lengths of heat and momentum from different origins are used to show the impact on the surface fluxes and the boundary layer development. The modal simulations indicate that the coupled atmosphere-vegetation system is rather sensitive to the value for the roughness length of heat. This affects in particular the sensible heat flux and, as a consequence, the boundary layer height and profiles of mean quantities in the boundary layer. The model results are compared with observations made at a forest town, with radiosonde profiles, and with aircraft data. The best overall agreement for the boundary layer quantities is obtained by using a roughness length of heat that is three orders of magnitude smaller than the roughness length of momentum.
Abstract
The daytime interaction of the land surface with the atmospheric boundary layer (ABL) is studied using a coupled one-dimensional (column) land surface–ABL model. This is an extension of earlier work that focused on modeling the ABL for 31 May 1978 at Cabauw, Netherlands; previously, it was found that coupled land–atmosphere tests using a simple land surface scheme did not accurately represent surface fluxes and coupled ABL development. Here, findings from that earlier study on ABL parameterization are utilized, and include a more sophisticated land surface scheme. This land surface scheme allows the land–atmosphere system to respond interactively with the ABL. Results indicate that in coupled land–atmosphere model runs, realistic daytime surface fluxes and atmospheric profiles are produced, even in the presence of ABL clouds (shallow cumulus). Subsequently, the role of soil moisture in the development of ABL clouds is explored in terms of a new relative humidity tendency equation at the ABL top where a number of processes and interactions are involved. Among other issues, it is shown that decreasing soil moisture may actually lead to an increase in ABL clouds in some cases.
Abstract
The daytime interaction of the land surface with the atmospheric boundary layer (ABL) is studied using a coupled one-dimensional (column) land surface–ABL model. This is an extension of earlier work that focused on modeling the ABL for 31 May 1978 at Cabauw, Netherlands; previously, it was found that coupled land–atmosphere tests using a simple land surface scheme did not accurately represent surface fluxes and coupled ABL development. Here, findings from that earlier study on ABL parameterization are utilized, and include a more sophisticated land surface scheme. This land surface scheme allows the land–atmosphere system to respond interactively with the ABL. Results indicate that in coupled land–atmosphere model runs, realistic daytime surface fluxes and atmospheric profiles are produced, even in the presence of ABL clouds (shallow cumulus). Subsequently, the role of soil moisture in the development of ABL clouds is explored in terms of a new relative humidity tendency equation at the ABL top where a number of processes and interactions are involved. Among other issues, it is shown that decreasing soil moisture may actually lead to an increase in ABL clouds in some cases.
Abstract
To describe the heat and scalar fluxes in the convective boundary layer, we propose expressions for eddy diffusivities and countergradient terms. The latter expressions can be used in a modified flux-gradient approach, which takes account for nonlocal convective vertical exchange. The results for heat are based on a derivation similar to that of Deardorff by utilizing the turbulent heat-flux equation, but the closure assumptions applied to the heat-flux budgets are different. As a result, the physical interpretation for the countergradient term differs; our countergradient term results from the third-moment transport effect, while Deardorff's results from the buoyancy production term. On the basis of our analysis, we are able to calculate an eddy diffusivity for heat, using large-eddy simulation results. The results are presented in the form of a similarity profile, using the convective velocity scale w * and the inversion height zi . It is shown that the latter profile is well behaved and that it matches the results of surface-layer theory. Using the top-down and bottom-up decomposition, we have generalized our findings for any scalar, such as the moisture field or an air pollution contaminant. We show that the eddy diffusivity profile for scalar C is sensitive to the entrainment–surface flux ratio of C. Therefore, a different scalar field should have a different eddy-diffusivity profile. The proposed expressions for the eddy diffusivities and the countergradient terms are easy to apply in (large-scale) atmospheric and diffusion models.
Abstract
To describe the heat and scalar fluxes in the convective boundary layer, we propose expressions for eddy diffusivities and countergradient terms. The latter expressions can be used in a modified flux-gradient approach, which takes account for nonlocal convective vertical exchange. The results for heat are based on a derivation similar to that of Deardorff by utilizing the turbulent heat-flux equation, but the closure assumptions applied to the heat-flux budgets are different. As a result, the physical interpretation for the countergradient term differs; our countergradient term results from the third-moment transport effect, while Deardorff's results from the buoyancy production term. On the basis of our analysis, we are able to calculate an eddy diffusivity for heat, using large-eddy simulation results. The results are presented in the form of a similarity profile, using the convective velocity scale w * and the inversion height zi . It is shown that the latter profile is well behaved and that it matches the results of surface-layer theory. Using the top-down and bottom-up decomposition, we have generalized our findings for any scalar, such as the moisture field or an air pollution contaminant. We show that the eddy diffusivity profile for scalar C is sensitive to the entrainment–surface flux ratio of C. Therefore, a different scalar field should have a different eddy-diffusivity profile. The proposed expressions for the eddy diffusivities and the countergradient terms are easy to apply in (large-scale) atmospheric and diffusion models.
Abstract
The modeling of vertical mixing by a turbulence scheme on the basis of prognostic turbulent kinetic energy (E) and a diagnostic length scale (l) is investigated with particular emphasis on the representation of entrainment. The behavior of this E–l scheme is evaluated for a stratocumulus case observed in the Atlantic Stratocumulus Transition Experiment, and a comparison is made with the results of large eddy simulation models for the same case. It appears that the E–l model is well capable of reproducing the main features of vertical mixing and entrainment. This is the case with a high vertical grid spacing of 25 m and a short time step of 1 s, even with a relatively simple formulation for the turbulent length scale. However, the model results degenerate rapidly on coarse temporal and spatial resolution. For time steps on the order of 1 min it is shown that the process-splitting time integration scheme (in which the tendencies due to turbulent diffusion and radiation are computed independently) results in a too cold cloud top, a too large buoyancy flux, and a too high entrainment rate. For a vertical grid spacing of the order of 200 m (as commonly used in operational models) the model does not behave well either. At such resolution, entrainment appears to be predominantly related to the Eulerian (gridbox) representation of the cloud and not to the physics of the turbulence scheme. This gridbox representation of the cloud prevents the cloud from descending due to prevailing large-scale subsiding motion, and therefore generates an entrainment rate that balances the subsidence rate. An unphysical dependency of the entrainment rate on the subsidence rate results. A general conceptual model for this behavior is presented. Finally, the relevance of these results for large-scale atmospheric models is discussed.
Abstract
The modeling of vertical mixing by a turbulence scheme on the basis of prognostic turbulent kinetic energy (E) and a diagnostic length scale (l) is investigated with particular emphasis on the representation of entrainment. The behavior of this E–l scheme is evaluated for a stratocumulus case observed in the Atlantic Stratocumulus Transition Experiment, and a comparison is made with the results of large eddy simulation models for the same case. It appears that the E–l model is well capable of reproducing the main features of vertical mixing and entrainment. This is the case with a high vertical grid spacing of 25 m and a short time step of 1 s, even with a relatively simple formulation for the turbulent length scale. However, the model results degenerate rapidly on coarse temporal and spatial resolution. For time steps on the order of 1 min it is shown that the process-splitting time integration scheme (in which the tendencies due to turbulent diffusion and radiation are computed independently) results in a too cold cloud top, a too large buoyancy flux, and a too high entrainment rate. For a vertical grid spacing of the order of 200 m (as commonly used in operational models) the model does not behave well either. At such resolution, entrainment appears to be predominantly related to the Eulerian (gridbox) representation of the cloud and not to the physics of the turbulence scheme. This gridbox representation of the cloud prevents the cloud from descending due to prevailing large-scale subsiding motion, and therefore generates an entrainment rate that balances the subsidence rate. An unphysical dependency of the entrainment rate on the subsidence rate results. A general conceptual model for this behavior is presented. Finally, the relevance of these results for large-scale atmospheric models is discussed.
Abstract
In this paper a summary is given of observations and modeling efforts on surface fluxes, carried out at Cabauw in The Netherlands and during MESOGERS-84 in the south of France. Emphasis is put on those aspects that are important from a modeling point of view, e.g., surface roughness lengths for momentum and heat, stomatal resistance for evaporation, and related quantities. Special attention is paid to the problem of subgrid surface inhomogeneities up to horizontal scales of a few kilometers. A qualitative explanation is given for the apparent low values of the roughness length for heat. Simple flux parameterizations are compared with observations, and empirical closure functions are proposed to model the transfer coefficients between the surface and the first model layer.
Abstract
In this paper a summary is given of observations and modeling efforts on surface fluxes, carried out at Cabauw in The Netherlands and during MESOGERS-84 in the south of France. Emphasis is put on those aspects that are important from a modeling point of view, e.g., surface roughness lengths for momentum and heat, stomatal resistance for evaporation, and related quantities. Special attention is paid to the problem of subgrid surface inhomogeneities up to horizontal scales of a few kilometers. A qualitative explanation is given for the apparent low values of the roughness length for heat. Simple flux parameterizations are compared with observations, and empirical closure functions are proposed to model the transfer coefficients between the surface and the first model layer.
Abstract
A comparison is made between two methods for determining the surface fluxes of sensible and latent heat during daytime. The first method, known as the Penman-Monteith approach, incorporates a more complete description of the physics. However, it needs a relatively large number of input parameters, which is inconvenient in many applications. The second method is a modification of the Priestley-Taylor evaporation model, which needs only net radiation, air temperature and an indication of the moisture condition at the surface. Both models are compared on the basis of hourly micro-meteorological data above short grass obtained in the Netherlands during the summer of 1977. The experiments were performed under predominantly unstable conditions [0 ≥ z/L 0 ≥ −0.3z = (mean) measuring height, L 0 = Obukhov length] with weak or no advection. It appears that, under these environmental conditions, the models have a similar skill. Therefore, the simple parameterization is preferred for practical purposes. It reveals that this result can be partially explained by the fact that the so-called equilibrium latent heat flux density LE EQ and vapor pressure deficit are correlated. The method requires further verification for different climatological conditions.
Abstract
A comparison is made between two methods for determining the surface fluxes of sensible and latent heat during daytime. The first method, known as the Penman-Monteith approach, incorporates a more complete description of the physics. However, it needs a relatively large number of input parameters, which is inconvenient in many applications. The second method is a modification of the Priestley-Taylor evaporation model, which needs only net radiation, air temperature and an indication of the moisture condition at the surface. Both models are compared on the basis of hourly micro-meteorological data above short grass obtained in the Netherlands during the summer of 1977. The experiments were performed under predominantly unstable conditions [0 ≥ z/L 0 ≥ −0.3z = (mean) measuring height, L 0 = Obukhov length] with weak or no advection. It appears that, under these environmental conditions, the models have a similar skill. Therefore, the simple parameterization is preferred for practical purposes. It reveals that this result can be partially explained by the fact that the so-called equilibrium latent heat flux density LE EQ and vapor pressure deficit are correlated. The method requires further verification for different climatological conditions.
Abstract
This paper gives the outline of a “meteorological preprocessor” for air pollution modeling. It is shown how significantly more information can be extracted from routinely available measurements than the traditional Pasquil stability classes and power law wind speed profiles. Also it is shown how additional special measurements—if available—can be accommodated. The methods are primarily intended for application in generally level, but not necessarily homogeneous terrain. The improved characterization of the state of the planetary boundary layer allows a more modern and probably more accurate description of diffusion. The paper is an extended version of an introductory paper presented during the “Workshop on Updating Applied Diffusion Models” Clearwater, Florida, January 1984.
Abstract
This paper gives the outline of a “meteorological preprocessor” for air pollution modeling. It is shown how significantly more information can be extracted from routinely available measurements than the traditional Pasquil stability classes and power law wind speed profiles. Also it is shown how additional special measurements—if available—can be accommodated. The methods are primarily intended for application in generally level, but not necessarily homogeneous terrain. The improved characterization of the state of the planetary boundary layer allows a more modern and probably more accurate description of diffusion. The paper is an extended version of an introductory paper presented during the “Workshop on Updating Applied Diffusion Models” Clearwater, Florida, January 1984.
Abstract
In this paper a semiempirical scheme is proposed which relates the nocturnal surface fluxes of sensible heat, latent heat, and momentum to routine weather data. The main components of the surface radiation and energy balance over land are described on a half-hourly basis. Observations over a grass-covered surface at Cabauw are used to investigate topics proposed in the literature, and to develop new parameterizations. The input data of the scheme are total cloud cover, wind speed, air temperature, and specific humidity deficit at single heights in the atmospheric surface layer. A semiempirical expression is proposed for the estimation of the soil heat flux. Also the relation between the surface radiation temperature and the temperature at the level of the roughness length is described semiempirically. It is found that their difference is considerable, especially for low wind speeds. The output of the scheme is presented in terms of the main forcing terms. On average, the agreement of the model quantities with observations is reasonable. For instance, for clear skies with total cloud cover N ≦ 0.25, it appears that root mean square errors are at best 9 W m−2 for sensible heat flux, 6 W m−2 for latent heat flux, 9 W m−2 for soil heat flux, 13 W m−2 for net radiation, and 1.8 K for surface radiation temperature. The temperature profile up to 80 m is well described by the present scheme. The difference of the scheme with previous methods in literature is discussed.
Abstract
In this paper a semiempirical scheme is proposed which relates the nocturnal surface fluxes of sensible heat, latent heat, and momentum to routine weather data. The main components of the surface radiation and energy balance over land are described on a half-hourly basis. Observations over a grass-covered surface at Cabauw are used to investigate topics proposed in the literature, and to develop new parameterizations. The input data of the scheme are total cloud cover, wind speed, air temperature, and specific humidity deficit at single heights in the atmospheric surface layer. A semiempirical expression is proposed for the estimation of the soil heat flux. Also the relation between the surface radiation temperature and the temperature at the level of the roughness length is described semiempirically. It is found that their difference is considerable, especially for low wind speeds. The output of the scheme is presented in terms of the main forcing terms. On average, the agreement of the model quantities with observations is reasonable. For instance, for clear skies with total cloud cover N ≦ 0.25, it appears that root mean square errors are at best 9 W m−2 for sensible heat flux, 6 W m−2 for latent heat flux, 9 W m−2 for soil heat flux, 13 W m−2 for net radiation, and 1.8 K for surface radiation temperature. The temperature profile up to 80 m is well described by the present scheme. The difference of the scheme with previous methods in literature is discussed.
