No abstract

No abstract

We evaluate the statistical properties of low-level jets (LLJs) observed by means of a network of Doppler sodars in the Moscow region, Russia. Continuous long-term measurements of the echo-signal intensity and wind-velocity profiles were carried out in July 2005 and in 2008-2010 synchronously in the centre of Moscow and at a rural site. The summertime nocturnal LLJs have a very clear diurnal cycle and exhibit features predicted by the Blackadar mechanism. In contrast, the long-lasting wintertime jets do not have any clear diurnal variability. The urban environment strongly influences LLJs in both seasons: above the city LLJs are higher, weaker and observed more rarely than at the rural site. In very cold periods (air temperature below −8 • C) no LLJs were observed over the city, instead convection emerged in the urban boundary layer. The results are based on observations made in

Two commercial large-aperture scintillometers, Scintec BLS900, were tested on pathlengths of 1840 and 4200 m at about 45–65 m above ground in Helsinki, Finland. From July 2011 through June 2012, large variability in diurnal and annual cycles of both the temperature structure parameter and sensible heat flux were observed. Scintillometer data were compared with data from two eddy-covariance stations. A robust method was developed for the calculation of from raw sonic-anemometer data. In contrast to many earlier studies that solely present the values of , the main focus here is on comparisons of itself. This has advantages, because optical-wavelength scintillometers measure with few assumptions, while the determination of implies the applicability of the Monin–Obukhov similarity theory, which has several inherent limitations. The histograms of compare well between sonic and scintillometer. In-depth analysis is focused on one of the scintillometer paths: both and comparisons gave similar and surprisingly high correlation coefficients (0.85 for and 0.84–0.95 for in unstable conditions), given the differences between the two measurement techniques, substantial sensor separation, and different source areas.

Mean seasonal profiles of wind speed, standard deviation of the vertical velocity and turbulence intensity from SODAR measurements in three cities of different size, Moscow, Hanover and Linz, are compared to analytical approximations for the Prandtl and Ekman layer. Typical urban features in the profiles and differences to measurements at rural sites are discussed. Typical urban features are a greater slope in the wind profiles, enhanced turbulence intensities, and a vertical increase in magnitude of the turbulence. The analytical approach proposed by ETLING ( 2002) for the description of the vertical wind profile in the whole boundary layer is amended in the Prandtl-layer part by a correction function for atmospheric stability. The amended profile description turns out to render the best results for the approximation of urban wind profiles within the lowest 500 m above ground. ZusammenfassungMittlere saisonale Profile der Windgeschwindigkeit, Standardabweichung der Vertikalgeschwindigkeit und der Turbulenzintensität aus SODAR-Messungen in drei unterschiedlich großen Städten, Moskau, Hannover und Linz, werden mit analytischen Annäherungen für die Prandtl-und die Ekman Schicht verglichen. Typische Eigenschaften für diese Profile in Städten sind eine stärkere Zunahme des Windgeschwindigkeitsprofils mit der Höhe, höhere Turbulenzintensitäten und die Zunahme der Turbulenz mit der Höhe. Der analytische Ansatz, welcher von ETLING (2002) für die Beschreibung des Windprofils in der gesamten Grenzschicht vorgeschlagen wird, wird in der Prandtl Schicht durch eine Korrekturfunktion zur Berücksichtigung der atmosphärischen Stabilität erweitert. Die erweiterte Profilbeschreibung ermöglicht die beste Annäherung an städtische Windprofile in den untersten 500 m über Grund.