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- Author or Editor: H. A. R. De Bruin x
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Abstract
The behavior of the Priestley-Taylor parameter α is investigated using a simplified atmospheric boundary-layer model coupled to the Penman-Monteith equation. It is found that under conditions typical for a sunny summer day in the mid-latitudes, α is primarily determined by the surface resistance r 3, (α ≈ 1.3 when r 3 = 0, α ≈ 1 when r 3 ≈ 60 s m−1, and α ≈ 0.6 when r 3 ≈ 250 s m−1). This is in good agreement with, experimental values reported in literature. The model is an extension of the models by McNaughton (1976) and Perrier (1980).
Abstract
The behavior of the Priestley-Taylor parameter α is investigated using a simplified atmospheric boundary-layer model coupled to the Penman-Monteith equation. It is found that under conditions typical for a sunny summer day in the mid-latitudes, α is primarily determined by the surface resistance r 3, (α ≈ 1.3 when r 3 = 0, α ≈ 1 when r 3 ≈ 60 s m−1, and α ≈ 0.6 when r 3 ≈ 250 s m−1). This is in good agreement with, experimental values reported in literature. The model is an extension of the models by McNaughton (1976) and Perrier (1980).
Abstract
Using heuristic arguments, Businger presented a derivation of the so-called Dyer–Businger formulas for nondimensional wind and temperature profiles. In this note it is shown that by slightly changing this “derivation,” different expressions are obtained that almost coincide with the original ones for 0 < −z/L < −1, but that behave differently in the free-convection region. The new formula for temperature appears to have a correct free-convection behavior; that is, it becomes proportional to (−z/L)−1/3. This note is primarily “historical” in nature.
Abstract
Using heuristic arguments, Businger presented a derivation of the so-called Dyer–Businger formulas for nondimensional wind and temperature profiles. In this note it is shown that by slightly changing this “derivation,” different expressions are obtained that almost coincide with the original ones for 0 < −z/L < −1, but that behave differently in the free-convection region. The new formula for temperature appears to have a correct free-convection behavior; that is, it becomes proportional to (−z/L)−1/3. This note is primarily “historical” in nature.
Abstract
By combining the empirical model of Priestley and Taylor (1972) and the well-known Penman equation, a simple expression is obtained for evaporation from a shallow lake. This model is tested for a large lake in the Netherlands with an area of about 460 km2 and an average depth of 3 m. The estimated values of evaporation of 1, 5, 10 and 20 days are compared with energy-budget measurements. The comparison indicates that the proposed model gives good results for periods of 10 days or more.
Abstract
By combining the empirical model of Priestley and Taylor (1972) and the well-known Penman equation, a simple expression is obtained for evaporation from a shallow lake. This model is tested for a large lake in the Netherlands with an area of about 460 km2 and an average depth of 3 m. The estimated values of evaporation of 1, 5, 10 and 20 days are compared with energy-budget measurements. The comparison indicates that the proposed model gives good results for periods of 10 days or more.
Abstract
Ice covers have an important influence on the hydrology of surface waters. The growth of ice layer on stationary waters, such as lakes or canals, depends primarily on meteorological parameters like temperature and humidity of the air, windspeed and radiation balance. The more complicated ice formation in rapidly flowing rivers is not considered in this study. A model is described that simulates ice growth and melting utilizing observed or forecast weather data. The model includes situations with a snow cover. Special attention is given to the optimal estimation of the net radiation and to the role of the stability of the near-surface air. Since a major practical application in the Netherlands is the use of frozen waters for recreation skating, the model is extended to include artificial ice tracks.
Abstract
Ice covers have an important influence on the hydrology of surface waters. The growth of ice layer on stationary waters, such as lakes or canals, depends primarily on meteorological parameters like temperature and humidity of the air, windspeed and radiation balance. The more complicated ice formation in rapidly flowing rivers is not considered in this study. A model is described that simulates ice growth and melting utilizing observed or forecast weather data. The model includes situations with a snow cover. Special attention is given to the optimal estimation of the net radiation and to the role of the stability of the near-surface air. Since a major practical application in the Netherlands is the use of frozen waters for recreation skating, the model is extended to include artificial ice tracks.
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
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.
Abstract
The applicability of the model of Priestley and Taylor (1972) for evaporation of saturated surfaces is examined for the former Lake Flevo (The Netherlands). This lake had an area of about 460 km2 and an average depth of 3 m. Daily values of evaporation in the period July–September 1967, determined with the energy-budget method, are compared with the corresponding estimated values obtained by the Priestley-Taylor model. The agreement appears to be satisfactory. The diurnal variation of the parameter α of the Priestley-Taylor model is found to be pronounced. From standard meteorological observations at Oostvaardersdiep, a station at the perimeter of the lake, and an energy-budget model of Keijman (1974) an indirect extension of the available time series is obtained. In this way energy-budget data for the period April–October 1967 became available. Analysis of this data set leads to the preliminary conclusion that α has a seasonal variation. This is due to the fact that there is a linear relation between the daily latent heat flux LE and the equilibrium latent heat flux LEEQ with a nonzero intercept.
Abstract
The applicability of the model of Priestley and Taylor (1972) for evaporation of saturated surfaces is examined for the former Lake Flevo (The Netherlands). This lake had an area of about 460 km2 and an average depth of 3 m. Daily values of evaporation in the period July–September 1967, determined with the energy-budget method, are compared with the corresponding estimated values obtained by the Priestley-Taylor model. The agreement appears to be satisfactory. The diurnal variation of the parameter α of the Priestley-Taylor model is found to be pronounced. From standard meteorological observations at Oostvaardersdiep, a station at the perimeter of the lake, and an energy-budget model of Keijman (1974) an indirect extension of the available time series is obtained. In this way energy-budget data for the period April–October 1967 became available. Analysis of this data set leads to the preliminary conclusion that α has a seasonal variation. This is due to the fact that there is a linear relation between the daily latent heat flux LE and the equilibrium latent heat flux LEEQ with a nonzero intercept.
Abstract
Several authors have determined the sensitivity of transpiration to different environmental parameters using the Penman-Monteith equation. In their studies the interaction between transpiration and, for example, the humidity of the air is ignored: the feedback with the planetary boundary layer (PBL) is not accounted for. Furthermore, surface-layer (SL) feedback (e.g., stability effects in the surface layer) is often neglected.
In our study, both PBL feedback and SL feedback are accounted for by coupling the big-leaf model to a detailed model for the PBL. This study provides an analysis of the sensitivity of transpiration to net radiation calculated after an albedo change, aerodynamic resistance calculated after a change in the aerodynamic roughness, and surface resistance. It is shown that PBL feedback affects the sensitivity of transpiration to the tested variables significantly. The sensitivity of transpiration to surface resistance and to aerodynamic resistance, or aerodynamic roughness, is decreased by the PBL feedback. In contrast, PBL feedback enlarges the sensitivity of transpiration to the net radiation, or albedo, and appears to be highly dependent on the specific conditions, especially on the aerodynamic roughness of the vegetation. It is recommended that future sensitivity studies for prognostic use account for PBL feedback.
Abstract
Several authors have determined the sensitivity of transpiration to different environmental parameters using the Penman-Monteith equation. In their studies the interaction between transpiration and, for example, the humidity of the air is ignored: the feedback with the planetary boundary layer (PBL) is not accounted for. Furthermore, surface-layer (SL) feedback (e.g., stability effects in the surface layer) is often neglected.
In our study, both PBL feedback and SL feedback are accounted for by coupling the big-leaf model to a detailed model for the PBL. This study provides an analysis of the sensitivity of transpiration to net radiation calculated after an albedo change, aerodynamic resistance calculated after a change in the aerodynamic roughness, and surface resistance. It is shown that PBL feedback affects the sensitivity of transpiration to the tested variables significantly. The sensitivity of transpiration to surface resistance and to aerodynamic resistance, or aerodynamic roughness, is decreased by the PBL feedback. In contrast, PBL feedback enlarges the sensitivity of transpiration to the net radiation, or albedo, and appears to be highly dependent on the specific conditions, especially on the aerodynamic roughness of the vegetation. It is recommended that future sensitivity studies for prognostic use account for PBL feedback.
Abstract
In this study, the authors develop and validate an approach to calculate a canopy conductance that can successfully be implemented in an atmospheric model. The approach is based on plant physiology approaches that have developed recently. However, it includes a new analytic formulation to scale the conductance up from leaf to canopy. Furthermore, a new expression that accounts for the effect of soil moisture on the canopy conductance is proposed. The parameters are not optimized locally; rather, values are assigned to them as a function of vegetation type. This approach is validated for three experimental sites: the First International Satellite Land Surface Climatology Project Field Experiment (FIFE)–Kansas area, the Hydrological Atmospheric Pilot Experiment–Modélisation du Bilan Hydrique (HAPEX–MOBILHY) site, and the Cabauw area. The vegetation at these sites is representative for large areas with low vegetation in the world. The results of the plant physiological approach are based on a distinction in C3 and C4 plant types, and these results are found to be better (FIFE–Kansas) and more consistent (HAPEX–MOBILHY) than estimates obtained by a traditional Jarvis–Stewart approach. The parameters of the latter are also obtained from a vegetation classification. For the Cabauw area, both approaches perform comparably. Furthermore, the new soil moisture content response function is found with work well, as compared with previous formulations.
Abstract
In this study, the authors develop and validate an approach to calculate a canopy conductance that can successfully be implemented in an atmospheric model. The approach is based on plant physiology approaches that have developed recently. However, it includes a new analytic formulation to scale the conductance up from leaf to canopy. Furthermore, a new expression that accounts for the effect of soil moisture on the canopy conductance is proposed. The parameters are not optimized locally; rather, values are assigned to them as a function of vegetation type. This approach is validated for three experimental sites: the First International Satellite Land Surface Climatology Project Field Experiment (FIFE)–Kansas area, the Hydrological Atmospheric Pilot Experiment–Modélisation du Bilan Hydrique (HAPEX–MOBILHY) site, and the Cabauw area. The vegetation at these sites is representative for large areas with low vegetation in the world. The results of the plant physiological approach are based on a distinction in C3 and C4 plant types, and these results are found to be better (FIFE–Kansas) and more consistent (HAPEX–MOBILHY) than estimates obtained by a traditional Jarvis–Stewart approach. The parameters of the latter are also obtained from a vegetation classification. For the Cabauw area, both approaches perform comparably. Furthermore, the new soil moisture content response function is found with work well, as compared with previous formulations.
Abstract
In this study different parameterizations for land surface models currently employed in meteorological models at ECMWF [Tiled ECMWF Surface Scheme for Exchange Processes over Land (TESSEL)] and NCEP (Noah) are evaluated for a semiarid region in Ghana, West Africa. Both schemes utilize the Jarvis–Stewart approach to calculate canopy conductance as the critical variable for partitioning the available energy into sensible and latent heat flux. Additionally, an approach within Noah is tested to calculate canopy conductance based on plant physiology (A-gs method), where the photosynthetic assimilation is coupled to the leaf stomatal conductance.
All parameterizations were run offline for a seasonal cycle in 2002/03 using observations as forcings at two test sites. The two locations are in the humid tropical southern region and in the drier northern region. For the purpose of forcing and evaluation, a new set of data has been utilized to include surface fluxes obtained by scintillometry. The measurements include the rapid wet-to-dry transition after the wet season at both sites.
As a general trend, it has been found that during the wet period of a season net radiation is described well by all parameterizations. During the drying process the errors in modeled net radiation increased at both sites. The models perform poorly in simulating soil heat fluxes with larger errors for TESSEL for both sites. The evolution in time for sensible heat flux and latent heat flux was tackled in different ways by the utilized parameterizations and sites with enhanced model performance for the more southern site. Soil moisture in the upper soil layers is modeled with small errors for the different parameterizations.
Key adjustments for reducing net radiation during the dry period of a season are discussed. In particular, the ratio of roughness length of momentum and heat was found to be an important parameter, but will require seasonal adjustments.
Abstract
In this study different parameterizations for land surface models currently employed in meteorological models at ECMWF [Tiled ECMWF Surface Scheme for Exchange Processes over Land (TESSEL)] and NCEP (Noah) are evaluated for a semiarid region in Ghana, West Africa. Both schemes utilize the Jarvis–Stewart approach to calculate canopy conductance as the critical variable for partitioning the available energy into sensible and latent heat flux. Additionally, an approach within Noah is tested to calculate canopy conductance based on plant physiology (A-gs method), where the photosynthetic assimilation is coupled to the leaf stomatal conductance.
All parameterizations were run offline for a seasonal cycle in 2002/03 using observations as forcings at two test sites. The two locations are in the humid tropical southern region and in the drier northern region. For the purpose of forcing and evaluation, a new set of data has been utilized to include surface fluxes obtained by scintillometry. The measurements include the rapid wet-to-dry transition after the wet season at both sites.
As a general trend, it has been found that during the wet period of a season net radiation is described well by all parameterizations. During the drying process the errors in modeled net radiation increased at both sites. The models perform poorly in simulating soil heat fluxes with larger errors for TESSEL for both sites. The evolution in time for sensible heat flux and latent heat flux was tackled in different ways by the utilized parameterizations and sites with enhanced model performance for the more southern site. Soil moisture in the upper soil layers is modeled with small errors for the different parameterizations.
Key adjustments for reducing net radiation during the dry period of a season are discussed. In particular, the ratio of roughness length of momentum and heat was found to be an important parameter, but will require seasonal adjustments.
