Manabu Kanda

Roughness lengths for momentum and heat derived from outdoor urban scale models

Abstract Urban climate experimental results from the Comprehensive Outdoor Scale Model (COSMO) were used to estimate roughness lengths for momentum and heat. Two different physical scale models were used to investigate the scale dependence of the roughness lengths; the large scale model included an aligned array of 1.5-m concrete cubes, and the small scale model had a geometrically similar array of 0.15-m concrete cubes. Only turbulent data from the unstable boundary layers were considered. The roughness length for momentum relative to the obstacle height was dependent on wind direction, but the scale dependence was not evident. Estimated values agreed well with a conventional morphometric relationship. The logarithm of the roughness length for heat relative to the obstacle height depended on the scale but was insensitive to wind direction. COSMO data were used successfully to regress a theoretical relationship between κB−1, the logarithmic ratio of roughness length for momentum to heat, and Re*, the roug...

Organized structures in developing turbulent flow within and above a plant canopy, using a Large Eddy Simulation

A Large Eddy Simulation (LES) model representing the air flow within and above a plant canopy layer has been completed. Using this model, the organized structures of turbulent flow in the early developmental stages of a crop are simulated and discussed in detail.The effect of the drag due to vegetation is expressed by a term added to the three-dimensional Navier-Stokes equation averaged over the grid scale. For the formulation of sub-grid turbulence processes, the equations for the time-dependent SGS (Sub-Grid-Scale) turbulence energy equation is used, which includes the effects of dissipation (both by viscosity and leaf drag), shear production and diffusion.The organized structure of turbulent flow at the air-plant interface, obtained numerically by the model, yields its contribution to momentum transfer. The three-dimensional large eddy structures, which are composed of spanwise vortices (‘rolls’) and streamwise vortices (‘ribs’), are simulated near the air-plant interface. They are induced by the shear instability at inflection points of the velocity profile. The structure clearly has a life cycle. The instantaneous image of the structure is similar to those observed in the field observations of Gaoet al. (1989) and in the laboratory flume experiments of Ikeda and Ota (1992). These organized structures also account for the well known fact that the sweep motion of turbulence dominates momentum transport within and just above a plant canopy, and the motion of ejection prevails in the higher regions.

AREA-AVERAGED SENSIBLE HEAT FLUX AND A NEW METHOD TO DETERMINE ZERO-PLANE DISPLACEMENT LENGTH OVER AN URBAN SURFACE USING SCINTILLOMETRY

Field observations of area-averagedturbulence characteristics were conducted in a densely built-up residential neighbourhood in Tokyo, Japan. In addition to eddy-correlation (EC) sensors a scintillometer was used for the first time in a city. Significant results include: (1) Scintillometer-derived sensible heat fluxes, QH, obtained at a height 3.5 times the building height agree well with those using the EC technique; (2) source areas for the scintillometer fluxes are larger than for the EC sensors, so that at low heights over inhomogeneous terrain scintillometry offers advantages; (3) new similarity relationships for dissipation rates are proposed for urban areas; (4) a new technique that uses simultaneous scintillation measurements at two heights to directly estimate area-averaged zero-plane displacement height, zd, is proposed. zd estimated in this way depends slightly on atmospheric stability (lower zd under more unstable conditions).

Turbulent flow similarity over an array of cubes in near-neutrally stratified atmospheric flow

The main objective of this study is to examine the robustness of the inner-layer scaling similarity of near-wall turbulence. The turbulent boundary layer of interest is over a very rough surface with a very high Reynolds number and significant outer-layer disturbances. This is not consistent with the canonical turbulent flows studied in laboratories, but it is common in urban areas. The investigation was conducted using the comprehensive outdoor scale model (COSMO) facility. COSMO is composed of a regular array of 1.5 m concrete cubes on a 50 x 100 m 2 flat concrete base. This unique facility allows us to obtain the turbulent dataset within the vertical constant stress region under near-neutral stratification at high Reynolds numbers. The turbulent spectra and the standard deviation of velocity fluctuations from COSMO were compared with the values obtained over rural and urban surfaces, and in wind-tunnel experiments. The results confirmed that the inner-layer scaling similarity was robust for the wall-normal fluctuations and the Reynolds stress, independent of the roughness types and the outer-layer conditions. The inner-layer scaling similarity failed for the horizontal velocity fluctuations owing to the influence of the outer-layer disturbance. The relative importance of outer-layer turbulence to inner-layer-scale eddies in the horizontal velocity fluctuations was successfully quantified in terms of the roughness scale normalized by the outer-layer scale.

Evaluation of the Simple Urban Energy Balance Model Using Selected Data from 1-yr Flux Observations at Two Cities

Abstract The authors’ objective was to apply the Simple Urban Energy Balance Model for Mesoscale Simulation (SUMM) to cities. Data were selected from 1-yr flux observations conducted at three sites in two cities: one site in Kugahara, Japan (Ku), and two sites in Basel, Switzerland (U1 and U2). A simple vegetation scheme was implemented in SUMM to apply the model to vegetated cities, and the surface energy balance and radiative temperature TR were evaluated. SUMM generally reproduced seasonal and diurnal trends of surface energy balance and TR at Ku and U2, whereas relatively large errors were obtained for the daytime results of sensible heat flux QH and heat storage ΔQS at U1. Overall, daytime underestimations of QH and overestimations of ΔQS and TR were common. These errors were partly induced by the poor parameterization of the natural logarithm of the ratio of roughness length for momentum to heat (κB−1); that is, the observed κB−1 values at vegetated cities were smaller than the simulated values. The...

Urban Energy Balance Obtained from the Comprehensive Outdoor Scale Model Experiment. Part I: Basic Features of the Surface Energy Balance

Abstract The objective of this study is to examine the basic features of the surface energy balance (SEB) using the data obtained from the Comprehensive Outdoor Scale Model (COSMO). COSMO is an idealized miniature city that has no vegetation, no human activity, and no heterogeneity of the surface geometry. The basic features of the SEB such as energy balance closure, the ensemble mean of the diurnal variation of the energy balance, and the daytime and daily statistics of the energy balance were investigated. The following were the main findings of the study: 1) A surface energy imbalance was observed. The sum of sensible and latent heat fluxes estimated by the eddy correlation method underestimated the available energy by 1% during the daytime and by 44% during the night. 2) Large heat storage in the daytime and small radiative cooling at night sustained positive sensible heat fluxes throughout the night in all seasons and in all sunshine conditions. 3) The daytime ratio of heat storage ΔQS to net radiati...

Urban Surface Energy Balance Models: Model Characteristics and Methodology for a Comparison Study

Many urban surface energy balance models now exist. These vary in complexity from simple schemes that represent the city as a concrete slab, to those which incorporate detailed representations of momentum and energy fluxes distributed within the atmospheric boundary layer. While many of these schemes have been evaluated against observations, with some models even compared with the same data sets, such evaluations have not been undertaken in a controlled manner to enable direct comparison. For other types of climate model, for instance the Project for Intercomparison of Land-Surface Parameterization Schemes (PILPS) experiments (Henderson-Sellers et al., 1993), such controlled comparisons have been shown to provide important insights into both the mechanics of the models and the physics of the real world. This paper describes the progress that has been made to date on a systematic and controlled comparison of urban surface schemes. The models to be considered, and their key attributes, are described, along with the methodology to be used for the evaluation.

Length-Scale Similarity of Turbulent Organized Structures over Surfaces with Different Roughness Types

We examine the similarity of turbulent organized structures over smooth and very rough wall flows. Turbulent flow fields in horizontal cross-sections were measured using particle image velocimetry, and the characteristics of turbulent organized structures over four types of surfaces were investigated. Measurements were conducted at several measurement heights across the internal boundary layer. The length and width of turbulence structures were quantified using a two-point correlation method. We selected two thresholds of two-point correlation coefficients to consider both large-scale and small-scale structures; the validity of these choices was examined through the analyses using proper orthogonal decomposition. For large-scale structures, the length and aspect ratios (streamwise length/spanwise width) of structures were highly correlated with the velocity gradient for each measurement height and boundary-layer thickness. This relationship was also examined in the results of previous studies, and the scaling of the aspect ratio with the non-dimensional velocity gradient again showed the importance of the velocity gradient, with slight differences found between smooth and rough surfaces. In contrast, the small-scale structures exhibited weak dependency on the velocity gradient and boundary-layer thickness. Instantaneous snapshots of turbulent organized structures at the same shear level also displayed differences in small-scale structures, but the structures of the organized motions resembled each other, as in the results of the two-point correlation method.

Numerical simulation of infiltration and solute transport in an S-shaped model basin by a boundary-fitted grid system

Abstract The boundary-fitted grid method, which transforms a complicated computational domain into a rectangular domain, is applied for the moisture and solute transport problems in the unsaturated soil layer of an S-shaped model hillslope. In the transformed domain, the fundamental equations of moisture and solute transport (i.e. the Richards equation and the Fickian equation) can be easily discretized into the finite-difference form. For three types of rainfall pattern (a unit rainfall, two-unit successive rainfalls and two unit ranfalls with a lag between them), hydrographs of total (pre-event and event water) and event water discharges, contour lines of soil moisture, pressure and total hydraulic head distributions, and flow velocity vectors are compared. For a relatively thick soil layer or for a relatively light rainfall, the capillary fringe effect plays an important role, whereas for a relatively thin soil layer or for a relatively heavy rainfall, overland flow from a variable source area is superimposed on the capillary-fringe-type runoff.

Urban Energy Balance Obtained from the Comprehensive Outdoor Scale Model Experiment. Part II: Comparisons with Field Data Using an Improved Energy Partition

Abstract The objective of this study is to examine the differences and similarities in the annual trends of the urban surface energy balance (SEB) among long-term field measurements. Four datasets analyzed for the study were collected in the following experiments or observational sites: Comprehensive Outdoor Scale Model experiments (COSMO), the Kugahara site in Tokyo, Japan (Ku04), and the Sperrstrasse and Spalenring sites in Basel, Switzerland (BuU1 and BuU2). A new variable, the forcing radiation QFR, has been proposed to replace the conventional net radiation Q* for the normalization of the SEB components. Here, QFR is defined as the sum of net shortwave radiation and downward longwave radiation. Because QFR does not include the upward longwave radiation, it is independent of the surface temperature, which is determined by the energy partitioning process. Therefore, QFR is independent of the energy partitioning process itself. With the use of QFR, the characteristics of the daytime normalized component...