Use In Large-scale Models Research Articles

The surface layer above a landscape with a rectangular windbreak pattern

Observations of the surface layer over a landscape with regularly spaced windbreaks were analyzed to determine the lower boundary conditions for use in larger scale models. Under near neutral conditions, the wind profile was used to estimate the average roughness length of the landscape. The observed roughness length is used to validate several aggregation rules for average exchange between a heterogeneous landscape and the surface layer. It is concluded that drag of the windbreaks must be included to find an acceptable roughness length of the landscape. The zero-plane displacement of the windbreak-dominated landscape is negligible in unstable and neutral conditions, but increases to 8 m in stable conditions. The increase of zero-plane displacement is accompanied by a decrease of roughness length and is explained by a transition from wake interference to skimming flow. The influence of the windbreak system on surface layer turbulence and temperature is discussed for stable and unstable stratification.

Determination of the Mean Wind Speed and Momentum Diffusivity Profiles above Tall Vegetation and Forest Canopies Using a Mass Conservation Assumption

Abstract A semianalytical method based on a mass conservation principle is presented for describing the transition- layer profiles of mean wind speed and momentum diffusivity and for estimating the aerodynamic characteristics of forest and tall vegetation canopies. This method incorporates density and vertical structure of the canopy and assumes that the transition-layer mean wind speed profile can be expressed in a polynomial form having second-order osculation. It is also suggested that canopy structure has a major influence on the transition-layer mean wind speed and momentum diffusivity profile. The proposed methodology may help in simulating airflow for use in large-scale models of plant-atmosphere exchanges.

A Scheme for Representing Cumulus Convection in Large-Scale Models

Abstract Observations of individual convective clouds reveal an extraordinary degree of inhomogeneity, with much of the vertical transport accomplished by subcloud-scale drafts. In view of these observations, a representation of moist convective transports for use in large-scale models is constructed, in which the fundamental entities are these subcloud-scale drafts rather than the clouds themselves. The transport by these small-scale drafts is idealized as follows. Air from the subcloud layer is lifted to each level i between cloud base and the level of neutral buoyancy for undilute air. A fraction (ϵi) of the condensed water is then converted to precipitation, which falls and partially or completely evaporates in an unsaturated downdraft. The remaining cloudy air is then assumed to form a uniform spectrum of mixtures with environmental air at level i; these mixtures ascend or descend according to their buoyancy. The updraft mass fluxes Mi are represented as vertical velocities determined by the amount o...

Comments and further analysis on effective roughness lengths for use in numerical three-dimensional models

Simple calculations of the apparent roughness length for the areally averaged flow over flat but heterogeneous terrain are presented. These results could be used to specify effective roughness lengths for use in large-scale models. Some of our conclusions differ significantly from those reached recently by Andre and Blondin (1986).

A simple model of the atmospheric boundary layer; sensitivity to surface evaporation

A simple formulation of the boundary layer is developed for use in large-scale models and other situations where simplicity is required. The formulation is suited for use in models where some resolution is possible within the boundary layer, but where the resolution is insufficient for resolving the detailed boundary-layer structure and overlying capping inversion. Surface fluxes are represented in terms of similarity theory while turbulent diffusivities above the surface layer are formulated in terms of bulk similarity considerations and matching conditions at the top of the surface layer. The boundary-layer depth is expressed in terms of a bulk Richardson number which is modified to include the influence of thermals. Attention is devoted to the interrelationship between predicted boundary-layer growth, the turbulent diffusivity profile, ‘countergradient’ heat flux and truncation errors.

On the effective roughness length for use in numerical three-dimensional models

We present analytical and numerical calculations of the effective roughness length (ERL) over a flat surface with varying roughness elements, for use in large-scale models. It is shown that ERL is mostly determined by the roughest elements present inside the averaging domain and that, more surprisingly, the ERL increases as the first level of the numerical model gets closer to the surface and its altitude approaches the value of the largest local roughness length. This effect further increases the drag coefficient, in addition to the well-known increase due to the lowering of the first model level.

A Study on Fatigue Strength of Welded Structure by means of Large Scale Model Test

Large scale model tests have been now increasingly attempted for the study on fatigue strength of welded structures. Main advantages of the use of large scale model are the possibility to confirm directly the fatigue behaviour of total structure.The authors carried out a series of full scale model fatigue tests on a slotted part of ship hull structure. Much efforts are exerted to detect the crack initiation and to pursue the crack propagation during the test. An approximate analysis of crack growth was attempted by a fracture mechanics approach. It is noted that the crack growth behaviours are much influenced by various factors such as defects or distortion of structure induced by the fabrication, redistribution of stress among adjoining structural members, etc. Empirical formulas are obtained to estimate crack initiation life and failure life of total structure.