Evolution of Urban Surface Exchange in the UK Met Office’s Unified Model - Meteorological and Air Quality Models for Urban Areas - page 67

Chapter 9
Evolution of Urban Surface Exchange
in the UK Met Office's Unified Model

Peter Clark, Martin Best, and Aurore Porson

Abstract The UK Met Office model the Unifed Model (UM) has undergone a
series of refinement to introduce urban characteristics in the surface scheme over
the last decade. As these have been used operationally the philosophy is to retain
simplicity and computational cheapness while capturing as much as possible the
behaviour of urban areas in modifying surface fluxes. The different steps that have
been taken and their performance are outlined.

9.1 Basic Approach

The resolution of practical numerical weather prediction (NWP) models has
increased substantially in recent years, especially in regional models. As recently as
1998, the Met Office's Unified Model (UM) included no treatment of urban areas in
its 3D operational configurations, beyond an artificial inclusion of enhanced rough-
ness over London in the 12 km resolution version. This was justified because the
true resolution of numerical models is several grid boxes (typically 5), so few, if
any, cities are resolved. However, models with grid lengths of only a few km (or
finer) are becoming available, necessitating the treatment of urban fluxes.
The current UM surface exchange scheme is designed for NWP use, and hence,
to determine the effect of urban areas on the atmosphere (and so on the evolution
of flow), and not vice versa. The implication of this is that the details of the urban
canopy are regarded as unimportant and only the surface-layer fluxes are computed.
In doing so, we assume that a surface layer exists and that standard Monin-Obukhov
Similarity Theory (MOST) applies. This obviously has limitations. At present, the
finest resolution used in the UM is typically ≈ 1 km, though higher resolution has

been used for research purposes.