Subgrid Scheme Research Articles

Large-Eddy Simulation Of A Nocturnal Stratocumulus-Topped Marine Atmospheric Boundary Layer: An Uncertainty Analysis

A large-eddy simulation (LES) model has been used to study a nocturnalstratocumulus-topped marine atmospheric boundary layer. The main objectivesof our study have been first to investigate the statistical significance of LES-derived data products. Second, to test the sensitivity of our LES results with respect to the representation of subgrid-scale mixing and microphysical processes, and third to evaluate and to quantify the parametric uncertainty arising from the incomplete knowledge of the environmental parameters that are required to specify the initial and boundary conditions of a particular case study. Model simulations were compared with observations obtained in solid stratocumulus during the third flight of the first 'Lagrangian' experiment of the Atlantic Stratocumulus Transition Experiment (ASTEX). Based on these simulations the following conclusions could be drawn. Resolution(50 × 50 × 25 m3) and domain size (3.2 × 3.2 × 1.5 km3) of the LES calculations were adequate from a numerical point of view to represent the essential features of the stratocumulus-topped boundary layer. However, the ensemble runs performed in our study to investigate the statistical significance of LES-derived data products demonstrate that the area-time averaging procedure for the second-order moments produces only a low degree of statistical reliability in the model results. This illustratesthe necessity of having LES model results that are not only of adequate resolution but also of sufficiently large domain. The impact of different subgrid schemes was small, but the primary effects of drizzle were found to influence the boundary-layer structure in a climatologically significant way. The parametric uncertainty analysis revealed that the largest contribution to the variance of the LES-derived data products is due to theuncertainties in the cloud-top jump of total water mixing ratio and the net radiative forcing. The differences between the model and measurements for most of the simulated quantities were within the modelling uncertainties, but the calculated precipitation rate was found to differ significantly from that derived in the observations.

On the sensitivity of projected oceanic thermal expansion to the parameterisation of sub‐grid scale ocean mixing

A coupled model of intermediate complexity is used to examine the importance of the parameterisation of sub‐grid scale ocean mixing on the global mean steric sea level rise in global warming simulations. It is shown that when mixing associated with mesoscale eddies is treated in a more physically realistic way than the commonly used horizontal/vertical scheme, quasi‐equilibrium projected steric sea level rise is more than two times lower in both 2 × CO2 and 4 × CO2 climates. This occurs despite the invariance of the coupled model climate sensitivity to the particular sub‐grid scale mixing scheme employed. During the early phase of the transient integrations thermal expansion differences are smaller, although experiments using the Gent and McWilliams parameterisation for mixing associated with mesoscale eddies approach equilibrium more rapidly once the radiative forcing is held fixed. This reduced expansion commitment reflects a greater decoupling of the surface ocean from the deep ocean, due to a reduction in spurious high latitude convection that occurs when a horizontal/vertical mixing scheme is used.

Parameterizing Subgrid Orographic Precipitation and Surface Cover in Climate Models

Abstract Previous development of the Pacific Northwest National Laboratory’s regional climate model has focused on representing orographic precipitation using a subgrid parameterization where subgrid variations of surface elevation are aggregated to a limited number of elevation classes. An airflow model and a thermodynamic model are used to parameterize the orographic uplift/descent as air parcels cross over mountain barriers or valleys. This paper describes further testing and evaluation of this subgrid parameterization. Building upon this modeling framework, a subgrid vegetation scheme has been developed based on statistical relationships between surface elevation and vegetation. By analyzing high-resolution elevation and vegetation data, a dominant land cover is defined for each elevation band of each model grid cell to account for the subgrid heterogeneity in vegetation. When larger lakes are present, they are distinguished from land within elevation bands and a lake model is used to simulate the the...

Stable Nocturnal Boundary Layers: A Comparison of One-Dimensional and Large-Eddy Simulation Models

A one-dimensional model and a large-eddy simulation (LES) model are used to simulate the turbulence structure of a stably stratified atmospheric boundary layer. The results are compared with those obtained by Andren (1995) using a similar three-dimensional model and with experimental data. The one-dimensional model solves the fundamental equations of momentum, entropy and conservation of two scalars, and a diagnostic second-order closure model is used to determine the turbulent fluxes of the variables as well as the variances of the velocity and scalars. The two LES models differ in the definition of the sub-grid scale scheme. The comparison of the model results outlines the differences and the similarities between the three- and the one-dimensional simulations. Although the results of the two LES models differ in the description of some of the stable boundary layer turbulence features, it is shown that good agreement is obtained with local-similarity theory and experimental data. The differences between the results of three models relate to the stratification within the layer, the asymptotic value of the various turbulent variables for large stability regimes and their values close to the surface.

Eddy parametrization and the oceanic response to idealized global warming

A coarse-grid global ocean general circulation model (OGCM) is used to determine the role of sub-grid scale eddy parametrization schemes in the response to idealized changes in the surface heat flux, of the same order as expected under increased atmospheric CO2 concentrations. Two schemes are employed. The first (H) incorporates standard horizontal mixing, whereas the second (G) combines both enhanced isopycnal mixing and eddy-induced transport. Uniform surface heating anomalies of +2 W m-2 and −2 W m-2 are applied for 50 years, and the results are compared with a control experiment in which no anomalous heating is imposed. A passive “heat” tracer is applied uniformly (at a rate of 2 W m-2 for 50 years) in a separate experiment. The sea-surface temperature response to global surface heating is generally larger in G, especially in the northern subtropical gyres, along the southern coast of Australia and off the Antarctic coast. A pronounced interhemispheric asymmetry (primarily arising from an anomalous response south of 35 °S) is evident in both H and G. The surface trapping of passive tracers in the Southern Hemisphere is generally greater in G than it is in H, and is particularly pronounced along the prime meridian (0 °E). Dynamical changes (i.e., changes in horizontal and vertical currents, convection, and preferred mixing and eddy transport pathways) enhance surface warming in the tropics and subtropics in both G and H. They are dominated by an anomalous meridional overturning centred on the equator, which may also operate in greenhouse warming experiments using coupled atmosphere-ocean GCMs. Over the Southern Ocean the passive tracer experiments and associated ventilation rates suggest that surface warming will be greater in G than in H. In fact, the contrast between the dynamical responses evident in G and H in the actual heating experiments leads to a situation in which the reverse is often true. Overall, dynamical changes enhance the interhemispheric assymetry, more so in G than in H.

Simulation of summertime ozone over North America

The concentrations of O3 and its precursors over North America are simulated for three summer months with a three‐dimensional, continental‐scale photochemical model using meteorological input from the Goddard Institute for Space Studies (GISS) general circulation model (GCM). The model has 4°×5° grid resolution and represents non linear chemistry in urban and industrial plumes with a subgrid nested scheme. Simulated median afternoon O3 concentrations at rural U.S. sites are within 5 ppb of observations in most cases, except in the south central United States where concentrations are overpredicted by 15–20 ppb. The model captures successfully the development of regional high‐O3 episodes over the northeastern United States on the back side of weak, warm, stagnant anticyclones. Simulated concentrations of CO and nonmethane hydrocarbons are generally in good agreement with observations, concentrations of NOx are underpredicted by 10–30%, and concentrations of peroxyacylnitrates (PANs) are overpredicted by a factor of 2 to 3. The overprediction of PANs is attributed to flaws in the photochemical mechanism, including excessive production from oxidation of isoprene, and may also reflect an underestimate of PANs deposition. Subgrid nonlinear chemistry as captured by the nested plumes scheme decreases the net O3 production computed in the United States boundary layer by 8% on average.

Reconstruction of oceanic flow characteristics from quasi‐Lagrangian data: 1. Approach and mathematical methods

We describe a nontraditional method for processing data derived from quasi‐Lagrangian tracers. The approach relies on a representation of the thermohydrodynamic field as a set of a finite number of functions (modes) with the subsequent computation of mode amplitudes from quasi‐Lagrangian data. This requires special methods for calculating mode components of the thermohydrodynamic field from Lagrangian observations. Also, because of the quasi‐Lagrangian nature of the data there are special problems caused by the need to filter in both space and time and to account for motions unresolved by such observations. The former problem is addressed by a filtering procedure based on a variational criteria and Pontryagin's principle. To handle the latter problem, a subgrid parameterization scheme appropriate for Lagrangian data is proposed.

The effects of land-use and anthropogenic heating on the surface temperature in the Tokyo Metropolitan area: A numerical experiment

In order to investigate the effects of land-use and anthropogenically produced heat on the surface air temperature in the Tokyo Metropolitan area, numerical experiments were carried out under typical summer synoptic conditions. The model is based on the three-dimensional Boussinesq equations, taking into account the hydrostatic assumption. Since land-use differs over every subdivision in Tokyo, the model includes a sub-grid parametrization scheme which can calculate the total heat flux a grid surface composed of different surfaces. The diurnal variation of the simulated surface air temperature agrees well with the observed value; an average over 36 days which represent typical summer days; i.e. negligible gradient winds and almost clear skies. The model shows that the contribution of anthropogenic heat is much larger at night, in spite of the lower energy consumption as compared to daytime use. Due to the scarcity of green vegetated areas in the central part of the city, the surface air temperature is enhanced in this region during daytime, however this enhancement is small after midnight.

Climatic effects of land reclamation in Tokyo bay — numerical experiment

To investigate the climatic effects of land reclamation in Tokyo Bay, numerical experiments were carried out under typical summer and winter synoptic conditions. The model is based upon the three-dimensional Boussinesq equations with the hydrostatic assumption. Since land-use is different for every subdivision in Tokyo, the model includes a sub-grid parameterization scheme which can calculate heat budget on the mixture surface of various surface conditions.Diurnal variations of the simulated surface temperature in typical summer conditions without the land reclamation agree well with observed ones averaged over about 30 days each which have been chosen from six years for typical summer and winter days.The model predicts that the surface temperature at midnight on a typical winter day in the central part of Tokyo will increase by about 1.5 K after reclamation of the northern half of the Bay.

Numerical simulations of stratiform boundary-layer clouds on the meso-γ-scale. Part II: The influence of a step change in surface roughness and surface temperature

A higher order closure mesoscale model is used to study the influence of different surface properties on stratiform boundary-layer clouds. The model is hydrostatic, has a terrain-following coordinate system and a sub-grid scale condensation scheme. It also has a radiation parameterisation for shortwave and longwave radiation in order to calculate radiative cooling/heating. The simulations show the effects on a cloud field when cool or cold air is advected over warm water, the possible influence of local circulation systems on cloud fields in situations with weak synoptic forcing and the influence on a cloud field by growing internal boundary layers. Some of the results are compared with simpler physical models, and limitations in those are demonstrated.

Numerical simulations of stratiform boundary-layer clouds on the meso-γ-scale. Part I: The influence of terrain height differences

A higher order closure mesoscale model is used to study the influence of terrain height differences on the meso-γ-scale on stratiform boundary-layer clouds. The model is hydrostatic, has a terrain-following coordinate system and a sub-grid scale condensation scheme. It also has a radiation parameterisation for shortwave and longwave radiation in order to calculate radiative cooling/heating. The simulations show that the cloud base height variations induced by the terrain can be much larger than motivated by terrain height variations alone. It is also shown how this behavior is dependent on upstream boundary-layer conditions and/or changes in the turbulence field. Other features studied include the wave in the lee of a ridge/hill and the associated lifting of the cloud base. The results are compared with some simpler physical models, and limitations in those models are demonstrated.