How to Use Computational Fluid Dynamics Models for Urban Canopy Parameterizations - Meteorological and Air Quality Models for Urban Areas - page 24

Chapter 4
How to Use Computational Fluid Dynamics
Models for Urban Canopy Parameterizations

Alberto Martilli and Jose Luis Santiago

Abstract The highest spatial resolution of today's mesoscale models is few hun-
dreds of meters. However, in urban areas important atmospheric features occur at
the scale of the hetereogenity (few tenths of meters). Mesoscale models can not
resolve these features, and their effect must be parameterized. In this contribution,
starting from the basic averaging schemes used in mesoscale models, it is explained
why Computational Fluid Dynamics models can be used to test and derive such
parameterizations. An example of the technique is presented based on simulations
over an array of cubes.

4.1 Introduction

The spatial resolution of mesoscale atmospheric models is the result of a balance
between three factors:

• the size of the mesoscale circulations object of investigation, which should be

contained within the model domain;

• the scale of the surface heterogeneities, which should be resolved at best by model

resolution;

• the computational time needed, that should be kept reasonable (few hours).

In general these factors fix, for today's computer, the spatial resolution for
mesoscale models at few kilometres (or several hundreds of meters at best). This
means that only atmospheric structures larger than the model grid cell can be
resolved (a more rigorous estimate may fix the smallest size of resolvable struc-
tures to, at least, twice, or 2

x, the size of the grid cell, but for simplicity thereafter
we will always refer to the grid cell size as lower limit).