Roughness Lengths For Heat Research Articles

Determination of surface turbulent fluxes over leads in Arctic sea ice

We present a new model to compute turbulent surface heat and momentum fluxes over leads in the Arctic sea ice. The momentum roughness length uses a sea state parameterization which is fully consistent with the surface turbulent flux parameterization. The flux parameterization accounts for the fetch limitation of the airflow over a lead. The surface roughness length for heat is determined from an application of surface renewal theory to the air‐sea interface. The modeled fluxes are compared with in situ observations of lead fluxes. We also compare our model results with a bulk flux algorithm which has been commonly used to evaluate surface heat fluxes from leads. We perform sensitivity studies to examine the role of the surface renewal timescale, the importance of the cool skin, and impact of wave age dependence on the momentum roughness length. We have computed integral heat fluxes as a function of lead width/fetch for various atmospheric states to determine the magnitude of heat flux in a mesoscale model grid in which a lead is present.

Impact of Atmospheric Surface-layer Parameterizations in the new Land-surface Scheme of the NCEP Mesoscale Eta Model

We tested three atmospheric surface-layer parameterization schemes (Mellor-Yamadalevel 2, Paulson, and modified Louis), both ina 1-D mode in the new NCEP land-surface scheme against long-term FIFE and HAPEX observations, and in a coupled 3-D mode withthe NCEP mesoscale Eta model. The differences inthese three schemes and the resulting surface exchange coefficients do not, in general, lead to significant differences in model simulated surface fluxes, skin temperature, andprecipitation, provided the same treatment of roughness length for heat is employed.Rather, the model is more sensitive to the choice of the roughness length for heat. To assess the latter, we also tested two approaches to specifythe roughness length for heat: 1) assuming the roughness length for heat is a fixed ratio of the roughness length for momentum, and 2) relating this ratio to the roughness Reynolds number as proposed by Zilitinkevich.Our 1-D column model sensitivity tests suggested that the Zilitinkevich approach can improve the surface heat fluxand skin temperature simulations. A long-term test with the NCEP mesoscaleEta model indicated that this approach can also reduce forecast precipitation bias. Based on these simulations, in January 1996 we operationally implemented the Paulsonscheme with the new land-surface scheme of the NCEP Eta model, along with the Zilitinkevich formulation to specify the roughness length for heat.

On the role of roughness lengths in flux parameterizations of boundary-layer models

Flux-profile relationships based on surface-layer similarity theory are used to derive relationships between the Monin-Obukhov stability parameter ξ = z/L and the bulk Richardson number Ri b . In contrast to previous studies, the roughness length for heat, z 0h ,is assumed unequal to the roughness length for momentum, z 0m .For the stable case, an analytic expression of ζ in terms of Ri b can be derived and in the unstable case, the solution is obtained through a simple iterative process. Errors introduced from the simplification of z 0h = z 0m are evaluated and are shown to be very significant in most cases. Thus, this error in many practical applications may invalidate the intended solution.

Parameterization of the surface-layer exchange coefficients for atmospheric models

A new surface-flux parameterization is presented and its impact on climate simulations with the Canadian Centre for Climate Modelling and Analysis (CCCMA) general circulation model (GCM) is discussed. The parameterization is based on the Monin-Obukhov similarity theory using well established flux-profile relationships for the unstable conditions. However, recently proposed new relationships are used in stable conditions. The new formulation allows different roughness lengths for heat and momentum, and gives transfer coefficients that are in agreement with Monin-Obukhov similarity theory. It also includes a parameterization for the free-convective boundary layer, which often occurs over warm surfaces within light winds. In circumstances where the surface layer is not neutrally stratified the proposed flux parameterization yields surface transfer coefficients that are different from those resulting from the standard surface flux formulation used in the GCM. The most marked effects of implementing the new formulation in the GCM are found over land and adjacent oceanic regions in winter where significant differences are found in the surface heat and moisture fluxes and surface temperatures.

Modelling the surface processes and the atmospheric boundary layer for semi-arid conditions

A simple parameterization of land-surface processes is tested for application in semi-dry areas. The surface scheme is included in the physical package of the Meteo-France atmospheric model ARPEGE. Its calibration using the EFEDA91 data set was necessary prior to mesoscale integrations of selected days of the field experiment. In order to model correctly the water budget and the energy balance over vineyards under dry soil conditions, the surface scheme was improved by parameterizing the vapor phase transfer in the soil and using two distinct roughness lengths for momentum and heat transfer. The introduction of the roughness length for heat is included in a surface boundary layer scheme, avoiding the classical iterative calculations of the heat transfer coefficient. Favourable results of tests of these modifications were found both for a time period of one month in stand-alone mode using an observed atmospheric forcing and for a diurnal cycle using a single column atmospheric model fully interactive with the surface scheme. Predictions of the surface temperature and of the sensible heat flux are significantly improved when a value of the roughness length for heat twenty times lower than the roughness length for momentum is used. Finally, the sensitivity of the predictions of the vertical development of the convective boundary layer to the value of the roughness lengths is examined.

Air‐sea exchange of water vapor and sensible heat: The Humidity Exchange Over the Sea (HEXOS) results

Surface layer fluxes of sensible heat and water vapor were measured from a fixed platform in the North Sea during the Humidity Exchange over the Sea (HEXOS) Main Experiment (HEXMAX). Eddy wind stress and other relevant atmospheric and oceanic parameters were measured simultaneously and are used to interpret the heat and water vapor flux results. One of the main goals of the HEXOS program was to find accurate empirical heat and water vapor flux parameterization formulas for high wind conditions over the sea. It had been postulated that breaking waves and sea spray, which dominate the air‐sea interface at high wind speeds, would significantly affect the air‐sea heat and water vapor exchange for wind speeds above 15 m/s. Water vapor flux has been measured at wind speeds up to 18 m/s, sufficient to test these predictions, and sensible heat flux was measured at wind speeds up to 23 m/s. Within experimental error, the HEXMAX data do not show significant variation of the flux exchange coefficients with wind speed, indicating that modification of the models is needed. Roughness lengths for heat and water vapor derived from these direct flux measurements are slightly lower in value but closely parallel the decreasing trend with increasing wind speed predicted by the surface renewal model of Liu et al. [1979], created for lower wind speed regimes, which does not include effects of wave breaking. This suggests that either wave breaking does not significantly affect the surface layer fluxes for the wind speed range in the HEXMAX data, or that a compensating negative feedback process is at work in the lower atmosphere. The implication of the feedback hypothesis is that the moisture gained in the lower atmosphere from evaporation of sea spray over rough seas may be largely offset by decreased vapor flux from the air‐sea interface.

Simulation of Surface Fluxes and Boundary Layer Development over the Pine Forest in HAPEX-MOBILHY

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...

The Roughness Length for Heat of Sparse Vegetation

Abstract A dual-source model that solves the energy balance over vegetation and soil separately can be inverted to obtain the roughness length for heat z0h of a single-source model. Model parameters for the dual-source model were taken from previous analysis of data from a sparse canopy in semiarid terrain. In these circumstances, the value of z0h, is shown to be dependent on the humidity deficit, the available energy, the vegetation fraction, and the surface resistance of the soil and the vegetation.

A modified parameterization of flux-profile relationships in the surface layer using different roughness length values for heat and momentum

The surface-layer flux-profile formulae of Louis (1979), used in many atmospheric models, are modified in a simple way to allow for different values of the roughness lengths for heat and momentum. The modified set of formulae simplifies the calculation of surface-layer fluxes over most natural land surfaces, where the roughness length for momentum can be almost an order of magnitude greater than that for heat.

Modelling sensible heat fluxes from a wheat canopy: an evaluation of the resistance energy balance model

Sensible heat fluxes measured over a growing wheat crop are used to evaluate the performance of the resistance energy balance model for both sparse and full canopy covers. Aerodynamic temperatures (for heat and momentum) are derived from measured sensible heat fluxes and then compared to canopy radiometric temperatures which were measured using infra-red radiometers and a pyrgeometer. Agreement between measured radiometric and aerodynamic temperatures is excellent for the more dense, fully-closed wheat crop, and the resistance energy balance model also performed adequately for this canopy. However, very large differences between radiometric and aerodynamic temperatures were observed for the sparse wheat crop, especially at intermediate leaf area indexes (LAIs) (less than 2). Large differences were even found between the infra-red radiometer-based and pyrgeometer-based canopy temperatures. Under such conditions of sparse canopy cover the model performs poorly. At these times greater agreement between aerodynamic and canopy temperatures was found when the latter was determined as a `composite temperature', combining both infra-red radiometer and pyrgeometer measurements. Modelling of sensible heat flux using the resistance energy balance model is improved with the use of this composite canopy temperature. Measured values for kB −1 were extremely variable and so a unique value for the ratio of roughness lengths for heat and momentum could not be determined.

Surface energy balance estimates at local and regional scales using optical remote sensing from an aircraft platform and atmospheric data collected over semiarid rangelands

Remotely sensed data in the visible, near‐infrared, and thermal‐infrared wave bands were collected from a low‐flying aircraft during the Monsoon '90 field experiment. Monsoon '90 was a multidisciplinary experiment conducted in a semiarid watershed. It had as one of its objectives the quantification of hydrometeorological fluxes during the “monsoon” or wet season. The remote sensing observations along with micrometeprological and atmospheric boundary layer (ABL) data were used to compute the surface energy balance over a range of spatial scales. The procedure involved averaging multiple pixels along transects flown over the meteorological and flux (METFLUX) stations. Average values of the spectral reflectance and thermal‐infrared temperatures were computed for pixels of order 10−1 to 101 km in length and were used with atmospheric data for evaluating net radiation (Rn), soil heat flux (G), and sensible (H) and latent (LE) heat fluxes at these same length scales. The model employs a single‐layer resistance approach for estimating H that requires wind speed and air temperature in the ABL and a remotely sensed surface temperature. The values of Rn and G are estimated from remote sensing information together with near‐surface observations of air temperature, relative humidity, and solar radiation. Finally, LE is solved as the residual term in the surface energy balance equation. Model calculations were compared to measurements from the METFLUX network for three days having different environmental conditions. Average percent differences for the three days between model and the METFLUX estimates of the local fluxes were about 5% for Rn, 20% for G and H, and 15% for LE. Larger differences occurred during partly cloudy conditions because of errors in interpreting the remote sensing data and the higher spatial and temporal variation in the energy fluxes. Minor variations in modeled energy fluxes were observed when the pixel size representing the remote sensing inputs changed from 0.2 to 2 km. Regional scale estimates of the surface energy balance using bulk ABL properties for the model parameters and input variables and the 10‐km pixel data differed from the METFLUX network averages by about 4% for Rn, 10% for G and H, and 15% for LE. Model sensitivity in calculating the turbulent fluxes H and LE to possible variations in key model parameters (i.e., the roughness lengths for heat and momentum) was found to be fairly significant. Therefore the reliability of the methods for estimating key model parameters and potential errors needs further testing over different ecosystems and environmental conditions.

Heat and water vapour fluxes and scalar roughness lengths over an Antarctic ice shelf

We present eddy-correlation measurements of heat and water vapour fluxes made during the Antarctic winter. The surface layer was stably stratified throughout the period of observation and sensible heat fluxes were always directed downwards. However, both upward and downward water vapour fluxes were observed. Their magnitude was generally small and the latent heat flux was not a significant fraction of the surface energy budget. The variation of heat and water vapour fluxes with stability is well described by Monin-Obukhov similarity theory but the scalar roughness lengths for heat and water vapour appear to be much larger than the momentum roughness length. Possible explanations of this effect are discussed.

Estimation of effective roughness length for heat and momentum from FIFE data

Using aircraft and surface data from the 1987 FIFE experiment in Kansas, we estimate the roughness length for momentum to be 0.19 m (with an error range 0.10–0.35 m), and the ratio of roughness length for momentum to that for heat to be about 16 (with an error range of 7–35).

Spatial variability of turbulent fluxes and roughness lengths in HAPEX-MOBILHY

Surface-based and aircraft measured fluxes over the heterogeneous surface in HAPEX-MOBILHY are analyzed for the ten flight days when cloud cover above the boundary layer was minimal. The fair-weather climatology of the spatial variation of surface fluxes is estimated to provide an assessment of the generality of previous case studies appearing in the literature. For the 10-day averages, greater heating over the forest generates a forest breeze which leads to rising motion and a modest increase of boundary-layer cloud cover at the forest edge. The exchange coefficients and effective roughness lengths are computed for local averages (15 km scale) and for regional averages (100 km scale) intended to represent a range of grid sizes in numerical models of the atmosphere. The effective roughness length for momentum over the mixed agricultural region for both scales is on the order of 1 m, apparently due to bluff roughness effects associated with scattered trees, edges of small woods and other obstacles. This roughness length value is an order of magnitude larger than values used in numerical models for the same region, which are based on the dominant vegetation type. The spatially varying effective roughness length for heat is computed for use in those models which use surface radiation temperature to estimate surface heat flux. The effective roughness lengths for heat are found to be smaller than those typically used in numerical models of the atmosphere.

Comments on “The Roughness Length for Heat and Other Vegetation Parameters for a Surface of Short Grass”

Comments on “The Roughness Length for Heat and Other Vegetation Parameters for a Surface of Short Grass”

The Roughness Length for Heat and Other Vegetation Parameters for a Surface of Short Grass

Abstract Observations are presented that were made in the lower 2 m of the atmosphere and in the soil near the Cabauw mast in the Netherlands. The surroundings of the mast are horizontally homogeneous and the soil is covered with short grass. In the air, the wind, temperature, specific humidity, and different radiation fluxes were measured, whereas in the soil, observations of the temperature and soil heal flux were made. From the observations, the ratio of the roughness length for heat and momentum has been calculated and is shown to depend strongly upon the friction velocity u*. From a theoretical analysis of the heat transfer from the vegetation towards the air, it is shown that the ratio of the roughness length for heat and momentum is a function of the friction velocity, the leaf-area index (LAI), and the Prandtl number. It is shown that the observational results compare reasonably well with the theoretical prediction. In addition to the ratio of the roughness length for heat and momentum, the roughn...

Flux Parameterization over Land Surfaces for Atmospheric Models

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.