Observing crosswind over urban terrain using scintillometer and Doppler lidar

In this study, the crosswind (wind component perpendicular to a path, U⊥) is measured by a scintillometer and Doppler lidar above the urban environment of Helsinki, Finland, for 3 weeks. The scintillometer allows acquisition of a path-averaged value of U⊥ (U⊥), while the Doppler lidar allows acquisition of path-resolved U⊥ (U⊥ (x), where x is the position along the path). The goal of this study is to evaluate the applicability of scintillometer U⊥-measurements for conditions where U⊥ (x) is variable. If the scintillometer is applicable in such variable-wind conditions, it can also be used in the urban environment. Two methods were applied to obtain U⊥ from the scintillometer signal; the cumulative spectrum method (relies on scintillation spectra), and the lookup table method (relies on time-lagged correlation functions). Both methods compared reasonably well with the Doppler lidar measurements, especially considering the challenging urban environment in which they were measuring; with RMSE of 0.71 and 0.73 m s−1. This indicates that both measurement technologies are able to obtain U⊥ in the complex urban environment. The in detail investigation of four cases indicate that the cumulative spectrum method is less susceptible to a variable U⊥ (x) than the lookup table method. However, the lookup table method can be adjusted to improve its capabilities to obtain U⊥ for conditions where U⊥ (x) is variable.