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 estimated with Doppler lidar above the urban environment of Helsinki, Finland, for 15 days. The scintillometer allows acquisition of a path-averaged value of U¿ (U¿), while the 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 performance of scintillometer U¿ estimates for conditions under which U¿ (x) is variable. Two methods are applied to estimate U¿ from the scintillometer signal: the cumulative-spectrum method (relies on scintillation spectra) and the look-up-table method (relies on time-lagged correlation functions). The values of U¿ of both methods compare well with the lidar estimates, with root-mean-square deviations of 0.71 and 0.73 m s-1. This indicates that, given the data treatment applied in this study, both measurement technologies are able to obtain estimates of U¿ in the complex urban environment. The detailed investigation of four cases indicates that the cumulative-spectrum method is less susceptible to a variable U¿ (x) than the look-up-table method. However, the look-up-table method can be adjusted to improve its capabilities for estimating U¿ under conditions under for which U¿ (x) is variable