V. P. Yushkov

Wind and turbulence in the urban boundary layer - analysis from acoustic remote sensing data and fit to analytical relations

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.

Experience in measuring the wind-velocity profile in an urban environment with a Doppler sodar

The experience of long-term acoustic remote measurements of vertical wind-velocity profiles at two sites in Moscow is reported. Equipment performances and measurement conditions are described. Acoustic measurement features characteristic of a large city with high traffic noise and spurious reflections from buildings are discussed. Criteria and techniques of rejecting noisy and false signals are described as well as the methods of statistical data processing suitable in the case of a signal-to-noise ratio rapidly varying in time and a significant number of rejected signals. Preliminary results of measurements are given.

Mean wind speed profiles in the air basin of Moscow

Results of measurements of mean wind speed profiles and direction during two years in the center of the city of Moscow are presented. These measurements were performed with a Doppler acoustic profiler (sodar), which was used for continuous monitoring of the wind speed within the height range from 40 to 250–500 m. The averaged seasonal mean profiles are presented along with other observational data, including diurnal cycle parameters, wind speed and direction frequencies at chosen levels, and frequencies and duration of strong winds and calm weather.

A probabilistic description of atmospheric turbulence

A way from the statistical description of homogeneous and isotropic turbulence to the anisotropic probability distribution is proposed. The role of adiabatic motions in the transfer of energy of turbulent fluctuations by spectrum is discussed. We justify the proposition that for large Reynolds numbers the distribution of the energy of adiabatic fluctuations at scales of homogeneous and isotropic turbulence has the form of a Planck curve.

What can be measured by the temperature profiler

The ability of the ground-based microwave temperature profiler to retrieve the lapse rates in the lower part of the atmospheric boundary layer is verified by analyzing the long-term observation series. It is demonstrated that if the stratification of the whole boundary layer is either unstable or sufficiently homogeneous, measured and retrieved with a mast lapse rates are quite similar. If the stratification is stable, the retrieval algorithms require correction. Demonstrated is the potential for the more accurate retrieving of the temperature gradient over the surface layer by means of the angular scanning. This enables using the data of temperature profilers for estimating the boundary layer stability and the Monin-Obukhov scale.

The structural function of entropy and turbulence scales

Structure functions of sound speed and local entropy in the turbulent atmosphere are analyzed. By long-term measurements in the atmospheric boundary layer the power dependence of these characteristics in the inertial sub-range is significantly diverged from the “2/3 law”. Behavior of structure functions indicates deviation from the asymptotic Kolmogorov-Obukhov law on scales, which significantly exceed Taylor microscale. The exponent of 2/3 is regarded as the limit value, which can be reached under different synoptic conditions. Analytical expressions for the internal and “energy” scales of entropy fluctuations, as well as for relation of these scales with entropy dissipation rate and energy of adiabatic fluctuations are proposed.

Evaluation of atmospheric boundary layer characteristics simulated by the regional model

Carried out is the comparison of the temporal courses of temperature and wind speed at different levels as well as of the wind and temperature profiles in the atmospheric boundary layer obtained from the WRF regional model forecasts and using the upper-air in situ and remote measurements in Moscow region. The errors in temperature and wind speed forecasts at different levels are computed as well as the statistical estimates of the forecast of temperature inversions, atmospheric stratification types, and monthly mean wind speed profiles on the basis of model forecasts and acoustic sounding.

The wind speed shear in the case of stable stratification and the scales of the similarity theory

The wind speed shear in the case of stable stratification in the linear part of the profile spreading high above the surface layer of constant flows is studied using the data of long-term sodar measurements in the atmospheric boundary layer. The wind speed shear in this part remains almost invariable during several hours at the significant change in parameters of the Monin-Obukhov theory. The length of this linear part can be associated with the layer of the critical Richardson number. In the case of the pronounced temperature inversion (with the positive gradient of more than 1°C per 100 m), the wind speed profile is close to the linear function in the most part of the nocturnal mixing layer. Proposed is a scale characterizing the height of the surface layer of constant flows.

The energy and dissipation of turbulent fluctuations of wind velocity and temperature in the boundary layer of the atmosphere

The relationships between the energy of small-scale turbulence and its dissipation rate are studied based on the data of long-term high-frequency measurements of temperature and wind velocity fluctuations in urban area. It is shown that the energy of wind velocity turbulent fluctuations is linearly related to the dissipation rate ɛ. The proportionality coefficient between turbulent kinetic energy (TKE) and ɛ is dimensional and does not depend on the stratification of the atmosphere, the Richardson number, or the Monin-Obukhov scale. Measurements in different seasons show that this coefficient can be related to the mean velocity of adiabatic motions (sound speed or air temperature), which enables one to select a more universal constant, γ. A linear relationship between the temperature fluctuations variance (the characteristic of the inner energy of turbulence) and their dissipation rate is also shown. The revealed proportionality is confirmed by measurements in urban and forest conditions, as well as in the surface layer over a flat desert terrain.

Estimation of spatial inhomogeneities of thermal stratification in the boundary layer of the Moscow megalopolis from remote sensing

The results of a joint study of spatial inhomogeneity and accuracy of retrieving temperature profiles in the atmospheric boundary layer over Moscow are analyzed. Measurements at three sites allowed estimating the spatial variability of thermal stratification in the megalopolis. The effect of local features of the observation sites on the retrieval error at all altitudes is shown. To reduce this effect, the retrieval of temperature lapse rate is suggested. Comparison of temperature gradients showed a gradual decrease in the effect of the local features in the range from 100 to 300 m. Based on the statistical variability analysis of the vertical temperature gradients, a reference mesh of temperature profilers is suggested.