2ND-ORDER SPECTRAL LOCAL ISOTROPY OF THE HUMIDITY AND TEMPERATURE-FIELDS IN AN URBAN AREA

A theoretical and experimental analysis of the humidity and temperatur e spatial differentials (Delta/Delta x(j)) in an urban area is present ed. The local isotropy form of the spectra of the streamwise and verti cal differentials of a scalar field is derived. The experimental resul ts for the humidity and temperature fields in the lowest part of the u rban boundary layer are in satisfactory agreement (at least in form un der stable conditions) with the local isotropy model indicating that t hese fields can be locally isotropic. A strong anisotropy was observed at small wavenumbers of the vertical differentials spectra and it is probably connected with surface-layer plumes. The analysis of the humi dity and temperature differentials is extended to their co-spectra and the spectra of the vertical differentials are compared to the streamw ise ones. The comparison of the structure parameters of the temperatur e or the humidity field in the streamwise and the vertical direction, estimated using a spectral and a variance (structure function) method, showed no systematic departure from the isotropic prediction of equal ity.