Derivation of Vertical Wind and Turbulence Profiles, the Mixing-Layer Height, and the Vertical Turbulent Exchange Coefficient from Sodar and Ceilometer Soundings in Urban Measurement Campaigns - Meteorological and Air Quality Models for Urban Areas - page 120

Remote measurements of turbulent viscosity are only available for complex rural
terrain (Campistron et al., 1991; Emeis, 2004b; Kouznetsov et al., 2007) and not for
urban areas. However the ability of remote sensing of turbulent viscosity is closely
related to the ability of remote sensing of vertical turbulent fluxes (see e.g. Engelbart
et al., 2007).
Here typical results for profiles, MLH, and turbulent viscosity from optical and
acoustic ground-based remote sensing from some of the above mentioned stud-
ies and further measurements made in Budapest (Hungary) and Paris (France) are
presented.
13.2 Results
13.2.1 Profiles and Diurnal Variation of Mean Wind Speed
Monthly mean wind speed profiles have been measured by a sodar over Hanover
(Germany) (Emeis, 2004a), Budapest (Hungary) (Fig. 13.1), and at Charles de
Gaulle airport near Paris (France) (Fig. 13.2). A typical feature of all these pro-
files is the strong vertical wind speed shear in the first hundreds of metres above
ground due to the large surface roughness of cities.
The diurnal variation of wind speed at several heights above ground (Emeis,
2004a; Fig. 13.1a) is also derived from this dataset. The usual nocturnal decrease of
wind speed near the ground (at about from 50 to 100 m height) is missing or, at least,
considerably reduced over cities because a higher nocturnal level of turbulence. This
turbulence is due to the larger surface roughness, and the reduced thermal stability
is due to the larger heat storage of cities.
13.2.2 Profiles and Diurnal Variation of the Variance
of the Vertical Wind Component
Mean vertical profiles of the variance of the vertical wind component over Hanover,
Budapest (Fig. 13.3), and Paris (Fig. 13.4) have been derived from sodar measure-
ments. The square of this variance describes one part of the turbulent kinetic energy
(TKE) contained in the air. If the turbulence is isotropic it describes just one third
of TKE. A typical feature in the urban boundary layer (UBL) is the increase of
this variance with height even at night time in a layer which is several hundred
metres deep. For the lower 100 m this indicates that the UBL often has a very unsta-
ble thermal stratification. Above this near-surface layer the increase of the variance
continues, especially in spring and summer, due to the frequent occurrence of low-
level jets which produce mechanical turbulence through the large wind speed shear
underneath of them. The nocturnal production of mechanical turbulence had been