Observing crosswind over urban terrain using scintillometer and Doppler lidar

In this study, the crosswind (wind component perpendicular to a path, U-perpendicular to) 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-perpendicular to ((U-perpendicular to) over bar), while the lidar allows acquisition of path-resolved U-perpendicular to (U-perpendicular to(x), where x is the position along the path). The goal of this study is to evaluate the performance of scintillometer estimates for conditions under which U-perpendicular to(x) is variable. Two methods are applied to estimate (U-perpendicular to) over bar 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-perpendicular to) over bar of both methods compare well with the lidar estimates, with root-mean-square deviations of 0.71 and 0.73 ms(-1). This indicates that, given the data treatment applied in this study, both measurement technologies are able to obtain estimates of (U-perpendicular to) over bar in the complex urban environment. The detailed investigation of four cases indicates that the cumulative-spectrum method is less susceptible to a variable U-perpendicular to (x) /than the look-up-table method. However, the look-up-table method can be adjusted to improve its capabilities for estimating (U-perpendicular to) over bar under conditions under for which U-perpendicular to (x) is variable.Peer reviewe

Scintillometry in urban and complex environments: a review

Knowledge of turbulent exchange in complex environments is relevant to a wide range of hydro-meteorological applications. Observations are required to improve understanding and inform model parameterisations but the very nature of complex environments presents challenges for measurements. Scintillometry offers several advantages as a technique for providing spatially-integrated turbulence data (structure parameters and fluxes), particularly in areas that would be impracticable to monitor using eddy covariance, such as across a valley, above a city or over heterogeneous landscapes. Despite much of scintillometry theory assuming flat, homogeneous surfaces and ideal conditions, over the last 20 years scintillometers have been deployed in increasingly complex locations, including urban and mountainous areas. This review draws together fundamental and applied research in complex environments, to assess what has been learnt, summarise the state-of-the-art and identify key areas for future research. Particular attention is given to evidence, or relative lack thereof, of the impact of complex environments on scintillometer data. Practical and theoretical considerations to account for the effects of complexity are discussed, with the aim of developing measurement capability towards more reliable and accurate observations in future. The usefulness of structure parameter measurements (in addition to fluxes, which must be derived using similarity theory) should not be overlooked, particularly when comparing or combining scintillometry with other measurement techniques and model simulations

Functional derivatives applied to error propagation of uncertainties in topography to large-aperture scintillometer-derived heat fluxes

Scintillometer measurements allow for estimations of the refractive index structure parameter <i>C_n²</i> over large areas in the atmospheric surface layer. Turbulent fluxes of heat and momentum are inferred through coupled sets of equations derived from the Monin–Obukhov similarity hypothesis. One-dimensional sensitivity functions have been produced that relate the sensitivity of heat fluxes to uncertainties in single values of beam height over flat terrain. However, real field sites include variable topography. We develop here, using functional derivatives, the first analysis of the sensitivity of scintillometer-derived sensible heat fluxes to uncertainties in spatially distributed topographic measurements. Sensitivity is shown to be concentrated in areas near the center of the beam path and where the underlying topography is closest to the beam height. Relative uncertainty contributions to the sensible heat flux from uncertainties in topography can reach 20% of the heat flux in some cases. Uncertainty may be greatly reduced by focusing accurate topographic measurements in these specific areas. A new two-dimensional variable terrain sensitivity function is developed for quantitative error analysis. This function is compared with the previous one-dimensional sensitivity function for the same measurement strategy over flat terrain. Additionally, a new method of solution to the set of coupled equations is produced that eliminates computational error