Near-surface Turbulent Fluxes Research Articles

Improving the Near-Surface Wind and Turbulence at the Edge of the Orographic Drag Gray Zone by Tuning the Roughness Length

Abstract In this paper, we present the implementation and evaluate the impact of the new roughness length configuration in the ALARO canonical model configuration of the ALADIN system at the edge of the orographic gravity wave drag gray zone. As an essential input for turbulence parameterization, the roughness length affects the near-surface turbulent fluxes and the screen-level interpolation of meteorological parameters. We utilize GMTED2010 and ECOCLIMAP-II databases to derive orographic and vegetation components of the effective roughness length and introduce tuning parameters enabling us to optimize predicted near-surface turbulent momentum fluxes and 10-m wind speed. Based on sensitivity tests, we (i) prove the necessity of tuning the roughness length fields, (ii) considerably reduce the RMSE of near-surface turbulent momentum fluxes (6%–7%) and 10-m wind speed for different groups of stations (3%–10%), and (iii) identify the tree height as the most influential input parameter in our computational domain. The RMSE decomposition indicates that the improvement of 10-m wind speed mostly comes from a decrease in the random error and bias of the mean. The variability is slightly underestimated, thus reducing the model accuracy for wind speeds above the 95th percentile but at an acceptable level. We explain that roughness length tuning also compensates for the missing roughness sublayer correction in our system. Finally, we show that, although the impact of the orographic gravity wave drag scheme at a horizontal mesh size of 1.8 km is generally small, it is still beneficial for capturing some finer features observed in atmospheric soundings. Significance Statement Aiming to improve the 10-m wind speed forecast without sacrificing the accuracy of turbulent momentum fluxes in the kilometric resolution numerical weather prediction model, we derived new roughness length fields from high-resolution physiography databases. Therein, we proved the importance of tuning the input orography and vegetation fields and, depending on the time of day and year, reduced the root-mean-square error of 10-m wind speed by 3%–10%. Further, we demonstrated that orographic gravity wave drag parameterization is still needed to predict finer details seen in wind profiles from atmospheric soundings. Finally, we discussed the related simplifications in our model and their implications and proposed steps toward a more consistent and complete treatment of the near-surface turbulence.

Near-Ground Effects of Wind Turbines: Observations and Physical Mechanisms

AbstractWind turbines generate wakes, which can potentially influence the local microclimate near the ground. To verify and quantify such effects, the Vertical Enhanced Mixing (VERTEX) field campaign was conducted in late summer 2016 to measure near-surface turbulent fluxes, wind speed, temperature, and moisture under and outside of the wake of an operational wind turbine in Lewes, Delaware. We found that, in the presence of turbine wakes from a single wind turbine, friction velocity, turbulent kinetic energy, and wind speed were reduced near the ground under the wake, while turbulent heat flux was not significantly affected by the wake. The observed near-ground temperature changes were <0.4°C in magnitude. Near-ground temperature changes due to the wake correlated well with the temperature lapse rate between hub height and the ground, with warming observed during stable and neutral conditions and cooling during unstable conditions. Of the two properties that define a wake (i.e., wind speed deficit and turbulence), the wind speed deficit dominates the surface response, while the wake turbulence remains aloft and hardly ever reaches the ground. We propose that the mechanism that drives changes in near-ground temperature in the presence of turbine wakes is the vertical convergence of turbulent heat flux below hub height. Above hub height, turbulence and turbulent heat flux are enhanced; near the ground, turbulence is reduced and turbulent heat flux is unchanged. These conditions cause an increase (during stable/neutral stability) or decrease (during unstable stability) in heat flux convergence, ultimately resulting in warming or cooling near the ground, respectively.

Direct numerical simulation of stratified Ekman layers over a periodic rough surface

Abstract

New Modified and Extended Stability Functions for the Stable Boundary Layer based on SHEBA and Parametrizations of Bulk Transfer Coefficients for Climate Models

Abstract Climate models still have deficits in reproducing the surface energy and momentum budgets in Arctic regions. One of the reasons is that currently used transfer coefficients occurring in parameterizations of the turbulent fluxes are based on stability functions derived from measurements over land and not over sea ice. An improved parameterization is developed using the Monin–Obukhov similarity theory (MOST) and corresponding stability functions that reproduce measurements over sea ice obtained during the Surface Heat Budget of the Arctic Ocean (SHEBA) campaign. The new stability functions for the stable surface layer represent a modification of earlier ones also based on SHEBA measurements. It is shown that the new functions are superior to the former ones with respect to the representation of the measured relationship between the MOST stability parameter and the bulk Richardson number. Nevertheless, the functions fulfill the same criteria of applicability as the earlier functions and contain, as an extension, a dependence on the neutral-limit turbulent Prandtl number. Applying the new functions we develop an efficient noniterative parameterization of the near-surface turbulent fluxes of momentum and heat with transfer coefficients as a function of the bulk Richardson number (Rib) and roughness parameters. A hierarchy of transfer coefficients is recommended for weather and climate models. They agree better with SHEBA data for strong stability (Rib > 0.1) than previous parameterizations and they agree well with those based on the Businger–Dyer functions in the range Rib ≤ 0.1.

Evaluation of different methods to model near-surface turbulent fluxes for a mountain glacier in the Cariboo Mountains, BC, Canada

Abstract. As part of surface energy balance models used to simulate glacier melting, choosing parameterizations to adequately estimate turbulent heat fluxes is extremely challenging. This study aims to evaluate a set of four aerodynamic bulk methods (labeled as C methods), commonly used to estimate turbulent heat fluxes for a sloped glacier surface, and two less commonly used bulk methods developed from katabatic flow models. The C methods differ in their parameterizations of the bulk exchange coefficient that relates the fluxes to the near-surface measurements of mean wind speed, air temperature, and humidity. The methods' performance in simulating 30 min sensible- and latent-heat fluxes is evaluated against the measured fluxes from an open-path eddy-covariance (OPEC) method. The evaluation is performed at a point scale of a mountain glacier, using one-level meteorological and OPEC observations from multi-day periods in the 2010 and 2012 summer seasons. The analysis of the two independent seasons yielded the same key findings, which include the following: first, the bulk method, with or without the commonly used Monin–Obukhov (M–O) stability functions, overestimates the turbulent heat fluxes over the observational period, mainly due to a substantial overestimation of the friction velocity. This overestimation is most pronounced during the katabatic flow conditions, corroborating the previous findings that the M–O theory works poorly in the presence of a low wind speed maximum. Second, the method based on a katabatic flow model (labeled as the KInt method) outperforms any C method in simulating the friction velocity; however, the C methods outperform the KInt method in simulating the sensible-heat fluxes. Third, the best overall performance is given by a hybrid method, which combines the KInt approach with the C method; i.e., it parameterizes eddy viscosity differently than eddy diffusivity. An error analysis reveals that the uncertainties in the measured meteorological variables and the roughness lengths produce errors in the modeled fluxes that are smaller than the differences between the modeled and observed fluxes. This implies that further advances will require improvement to model theory rather than better measurements of input variables. Further data from different glaciers are needed to investigate any universality of these findings.

An Efficient Non-iterative Bulk Parametrization of Surface Fluxes for Stable Atmospheric Conditions Over Polar Sea-Ice

In climate and weather prediction models the near-surface turbulent fluxes of heat and momentum and related transfer coefficients are usually parametrized on the basis of Monin–Obukhov similarity theory (MOST). To avoid iteration, required for the numerical solution of the MOST equations, many models apply parametrizations of the transfer coefficients based on an approach relating these coefficients to the bulk Richardson number $$Ri_{b}$$ . However, the parametrizations that are presently used in most climate models are valid only for weaker stability and larger surface roughnesses than those documented during the Surface Heat Budget of the Arctic Ocean campaign (SHEBA). The latter delivered a well-accepted set of turbulence data in the stable surface layer over polar sea-ice. Using stability functions based on the SHEBA data, we solve the MOST equations applying a new semi-analytic approach that results in transfer coefficients as a function of $$Ri_{b}$$ and roughness lengths for momentum and heat. It is shown that the new coefficients reproduce the coefficients obtained by the numerical iterative method with a good accuracy in the most relevant range of stability and roughness lengths. For small $$Ri_{b}$$ , the new bulk transfer coefficients are similar to the traditional coefficients, but for large $$Ri_{b}$$ they are much smaller than currently used coefficients. Finally, a possible adjustment of the latter and the implementation of the new proposed parametrizations in models are discussed.

Near-surface turbulent fluxes in stable stratification: Calculation techniques for use in general-circulation models

Practically oriented flux-calculation techniques based on correction functions to the neutral drag and heat/mass transfer coefficients are further developed. In the traditional formulation, the correction functions depend only on the bulk Richardson number. However, data from measurements of turbulent fluxes and mean profiles in stable stratification over different sites exhibit too strong variability in this type of dependencies. Indirect evidence from climate and weather prediction modelling also shows that the traditional flux-calculation technique is not sufficiently advanced. It is conceivable that other mechanisms besides the surface-layer stratification and, therefore, other arguments besides the bulk Richardson number must be considered. The proposed technique includes a newly discovered effect of the static stability in the free atmosphere on the surface-layer scaling and accounts for the general essential difference between the roughness lengths for momentum and scalars. Besides bulk Richardson number, recommended correction functions depend on one more stability parameter, involving the Brunt–Väisälä frequency in the free atmosphere, and on the roughness lengths. Copyright © 2002 Royal Meteorological Society.

Calculation of Surface Fluxes under Convective Conditions by Turbulence Closure Models

Abstract A method of deriving new relationships between near-surface turbulent fluxes and vertical differences of wind speed and potential temperature between two levels in the atmospheric surface layer from simplified second-order turbulence closure models is presented. The paper aims at a coherent treatment of fair-weather conditions and the asymptotic free-convection limit. The traditionally used representation of wind and temperature profiles is modified to reflect the three-sublayer structure of the surface layer in accordance with recent proposals; this leads to a generalization of the profile method. The modification suggests a new framework for the analysis of empirical data gathered in statically unstable fair-weather conditions. A general formulation of the problem involves a set of integral equations, which is solved numerically; with this aim, a new “convective” vertical coordinate transformation suitable for the computation of the upper part of the profile is proposed. Two popular turbulence ...

Study of roughness lengths and drag coefficients over Nansha sea region, Gobi, desert, Oasis and Tibetan plateau

Near-surface turbulent fluxes observations from three large-scale surface-atmosphere interaction field experiments are studied. In the context of the extended Monin-Obukhov similarity theory, investigation was performed of aerodynamic roughness (Z0), scalar roughness ( ZT, ZQ), neutral drag coefficient (CDN), bulk transfer coefficient (sensible heat exchange coefficient CHN and vapor analog CEN) from a calm and a rough sea of the Nansha region, separately, and of aerodynamic roughness and drag coefficients from Gobi, desert, oasis and the Tibetan plateau, together with the related distribution of the wind u . Some meaningful results have been derived.

Simulation of Atmospheric Boundary Layer Processes Using Local- and Nonlocal-Closure Schemes

A soil‐vegetation‐atmospheric boundary layer model was developed to study the performance of two localclosure and two nonlocal-closure boundary layer mixing schemes for use in meteorological and air quality simulation models. Full interaction between the surface and atmosphere is achieved by representing surface characteristics and associated processes using a prognostic soil‐vegetation scheme and atmospheric boundary layer schemes. There are 30 layers in the lowest 3 km of the model with a high resolution near the surface. The four boundary layer schemes are tested by simulating atmospheric boundary layer structures over densely and sparsely vegetated regions using the observational data from the First ISLSCP (International Satellite Land Surface Climatology Project) Field Experiment (FIFE) and from Wangara. Simulation results indicate that the near-surface turbulent fluxes predicted by the four boundary layer schemes differ from each other, even though the formulation used to represent the surface-layer processes is the same. These differences arise from the differing ways of representing subgrid-scale vertical mixing processes. Results also indicate that the vertical profiles of predicted parameters (i.e., temperature, mixing ratio, and horizontal winds) from the four mixed-layer schemes differ from each other, particularly during the daytime growth of the mixed layer. During the evening hours, after the mixed layer has reached its maximum depth, the differences among these respective predicted variables are found to be insignificant. There were some general features that were associated with each of the schemes in all of the simulations. Compared with observations, in all of the cases the simulated maximum depths of the boundary layer for each scheme were consistently either lower or higher, superadiabatic lapse rates were consistently either stronger or weaker, and the intensity of the vertical mixing was either stronger or weaker. Also, throughout the simulation period in all case studies, most of the differences in the predicted parameters are present in the surface layer and near the top of the mixed layer.