Stefan Emeis

Surface-based remote sensing of the mixing-layer height : a review

Presently available methods for the determination of the mixing-layer height from surface-based acoustic, optical and electro-magnetic remote sensing are summarized and compared. Most complete information on the structure of the ABL can be expected from a combined use of acoustic together with optical or electro-magnetic remote sensing.

Boundary-layer anemometry by optical remote sensing for wind energy applications

The effective use of wind energy requires precise wind speed measurements. However, there is a risk that existing in-situ techniques with cup anemometers mounted on masts will exceed mechanical and financial limits at future hub heights. Ground-based optical remote sensing methods that measure the vertical profile of wind speed up to several hundred metres height may be a solution to these problems. This review paper will discuss the basic principles of anemometry by remote sensing and will present some optical methods in more detail.

Influence of mixing layer height upon air pollution in urban and sub-urban areas

The mixing layer height is an important parameter characterising the potential of the atmospheric boundary layer to take up emitted air pollutants. During continuous measurements in Hanover, Germany, from 2001 until 2003 and around Munich, Germany, in summer and winter 2003 mixing layer heights (MLH) were determined by different remote sensing systems mainly from the thermal structure and turbulence of the air (SODAR), for some time from the aerosol layering of the air (ceilometer), and for a short period directly from the temperature profile (RASS). The temporal variations of the concentrations of PM 10 and PM 2.5 as well as of CO and NO x simultaneously measured near the surface were investigated and correlated with the MLH derived from SODAR data. The pollution measurements were performed inside a street canyon and at an urban background station close to Hanover and at three measurement locations inside and outside of Munich complementing the available monitoring networks. The analyses show that the correlations of pollutant concentrations with MLH are smallest inside street canyons. Correlations at the urban background stations are larger in winter than in summer, and they are larger for the urban stations than for the rural stations. It turns out further that the correlation of NO X concentrations with MLH is larger than the correlation of particles concentrations. Explanations for these findings must consider the varying emission source strengths for NO X and particles and the influence of gas-to-particle conversion within air masses especially during daytime in summer.

Multiple atmospheric layering and mixing-layer height in the Inn valley observed by remote sensing

Automatic mixing layer height monitoring was performed by continuous sodar and ceilometer measurements in the Inn valley east of Innsbruck, Austria during a winter measuring campaign on air and noise pollution. The ceilometer, an eye-safe commercial lidar originally designed to detect cloud base heights and vertical visibility for aviation safety purposes, was operated for about two months; the sodar was operated for more than four months. Special software for this ceilometer provides routine retrievals of multiple aerosol layers and mixing layer height from the optical backscatter data. An existing retrieval method for the mixing layer height from sodar data has also been enhanced in order to detect multiple atmospheric layering. Particular emphasis is given to the detection of thermally stable layers and inversions within the lower valley atmosphere and their temporal development. Such layers influence significantly the diurnal variations of air pollution and traffic noise impact. A comparison is performed with parallel mixing layer height retrievals from the sodar and the ceilometer. In clear and cold winter nights sometimes several layers one above the other can be detected with both instruments. These multiple layers form due to an interaction between the mountain wind and the down-slope winds. In the absence of low clouds and precipitation ceilometers can estimate the mixing-layer-height fairly well. Ceilometer and sodar partly complement each other.

Surface-Based Remote Sensing of the Atmospheric Boundary Layer

1. Introduction.- 2. Analytical description and vertical structure of the ABL.- 3. Basic principles of surface-based remote sensing.- RADAR.- SODAR.- RASS.- LIDAR.- Radiometers.- 4. Applications.- Vertical layering of the ABL.- Vertical profiles of atmospheric variables.- Vertical flux profiles.- Regional and local flow systems.- Conclusions on the applicability of ground-based remote sensing for ABL research and monitoring.- 5. Outlook.- Appendix: History of ISARS conferences.- Glossary.- Index.

Reduction of Horizontal Wind Speed in a Boundary Layer with Obstacles

The reduction of horizontal wind speed at hub height in an infinite cluster of wind turbines is computed from a balance between a loss of horizontal momentum due to the drag and replenishment from above by turbulent fluxes. This reduction is derived without assumptions concerning the vertical wind profile above or below hub height, only some basic assumptions on turbulent exchange have been made. Two applications of the result are presented, one considering wind turbines and one pressure drag on orographic obstacles in the atmospheric boundary layer. Both applications are basically governed by the same kind of momentum balance.

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.

Frequency distributions of the mixing height over an urban area from SODAR data

The height of the mixing-layer (MLH) is an important parameter in the assessment of the dilution of primarily emitted or secondarily formed air pollutants in the atmospheric boundary-layer. A continuous measurement of MLH is possible only by remote sensing. Here, 17 months of SODAR data have been analysed automatically to derive the MLH over the city of Hannover in Northern Germany. In contrast to earlier studies the MLH has been determined from vertical profiles of the acoustic backscatter intensity and from the variance of the vertical velocity component. The results are presented in form of monthly frequency distributions and mean daily courses of MLH. These statistical evaluations of MLH show a clear annual course and interannual variability. The study shows the possibility to derive meaningful climatological information from long-term SODAR measurements for air quality issues.

Observation of the structure of the urban boundary layer with different ceilometers and validation by RASS data

U rban air quality assessment requires the knowledge of the temporal and spatial structure of the mixing layer, because this structure controls the vertical dilution of near-surface pollutants. The behaviour of the mixing layer is a consequence of vertical temperature and moisture profiles in the lower atmosphere so that remote sensing can be a suitable tool to monitor it. Three ceilometers, a Vaisala LD40 and two Vaisala CL31, have been operated for many months in the German city of Augsburg in order to observe the vertical aerosol distribution. Wind and temperature profile information have been obtained for a part of the period from radioacoustic sounding system (RASS) observations. This paper investigates the abilities of the instruments and compares the information received by the ceilometers among each other and with temperature profiles from the RASS data. Zusammenfassung

Modification of air flow over an escarpment — Results from the Hjardemål experiment

Mean flow, turbulence, and surface pressure measurements over an escarpment are presented. The speed-up in the mean wind field shows the known dependence on stratification. Cross-sections of the standard deviation of horizontal and vertical wind components and of the friction velocity are derived from the data and compare favorably with the numerical model of Zeman and Jensen (1987). The modification of turbulent power spectra at intermediate frequencies can be explained by rapid distortion theory. At very low frequencies, there is a quasi-stationary response to the disturbance. Except for speed-up and standard deviations of the wind components, which are also shown for downslope wind, all results in this paper refer to upslope winds.An analysis of the vertical momentum flux reveals that upstream of the escarpment, most of the flux is transported in sweeps of fast, sinking motion to the ground. Downstream of the escarpment, ejections of slow, rising motion dominate the turbulent transport.