Detection of Inversions and Mixing Height by REMTECH PA2 Sodar in Comparison with Collocated Radiosonde Measurements

Summary A REMTECH PA2 Doppler Sodar is operated regularly at the Czech Hydrometeorological Institute (CHMI) observatory in Prague, collocated with a routine rawinsonde sounding system. The Air Pollution Control Division of CHMI utilises the sodar data in air pollution studies and as an information support for the smog warning system operated in Prague. Besides of the basic software for echo strength and wind profile evaluation, optional routines for deriving parameters such as inversion and mixing height, stability class etc. were delivered by the sodar manufacturer. Based on a sufficiently large data set (more than one year) of synchronous sodar and rawinsonde measurements, an analysis and comparison of inversion and mixing heights provided by both sounding systems have been accomplished in order to evaluate the correctness and accuracy of sodar estimates of these parameters. In contrast to the wind speed and wind direction data, for which a satisfactory agreement with other kind of measurements has been reported by many studies, the results for inversion and mixing height detection were totally disappointing. A direct applicability of inversion height and mixing height data provided by the REMTECH’s automatic routines in air pollution studies or smog warning systems is quite problematical with the present “state of the art”. Received November 3, 1998 Revised April 20, 1999     

Composing the diurnal cycle of mixing height from simultaneous sodar and wind profiler measurements

Mixing heights have been determined independently from sodar and boundary-layer wind profiler measurements carried out at the Meteorological Observatory Lindenberg of the German Weather Service between 1 June 1994 and 6 July 1994. Good agreement between both systems was found for mixing height values between about 250 m and 700 m, i.e., in an evolving convective boundary layer. Considering the typical sounding ranges of the sodar (50 m up to 800 m) and of the wind profiler (200 m up to higher than 3000 m), simultaneous operation of the two systems is demonstrated to be a promising tool for continously monitoring the mixing height throughout its complete diurnal cycle.     

Sodar Detected Top-Down Convection in a Nocturnal Cloud-Topped Boundary Layer: A Case Study

Nocturnal convection, originating in a well-mixed marine cloud-topped boundary layer, advected onshore, was observed using a Doppler sodar on the Tyrrhenian coast in Italy. The horizontal and vertical dimensions of the downdrafts were evaluated. The oscillation frequency triggered by the downdrafts at the inversion layer, derived from the harmonic analysis of the sodar measured vertical velocity (w), is compared with the Brunt-Vaisala frequency, obtained from the rawinsonde temperature profile. A similarity function for the ²_w vertical profile was used to fit the sodar experimental data and to retrieve the depth of the mixing layer and the sensible heat flux at the top of the cloud layer. The results are in agreement with the convection layer depth observed in the sodar echoes facsimile record, and with the energy budget evaluated at the top of the cloud layer using the rawinsonde profiles.     

北京一次罕见夜间突发性强增温事件成因分析

夜间降温是正常的地表气温日变化,但统计表明北京及周边地区冬半年时常出现入夜后气温不降反升的现象,甚至出现了小时升温超过10℃的剧烈增温事件。这种增温具有明显的突发性,且很快转为降温,常对业务预报造成困扰。2010年11月26日夜间冷锋过境该区域造成了最强达12℃·h^-1的夜间急剧增温事件,与气候统计结果相比,该次增温幅度和影响区域范围非常罕见且极端。文章对其进行了详细诊断分析,使用的资料包括自动气象站、常规地面和探空、铁塔、卫星、风廓线雷达等观测资料和美国环境预报中心(NCEP)最终分析资料。分析结果表明:该次过程对流层低层有非常显著的冷平流;垂直速度诊断、卫星和风廓线雷达观测都表明,对流层中低层都存在显著的强下沉运动;铁塔观测和北京探空观测演变都表明增温过程中近地面大气有显著的湍流运动。分析此次罕见过程的机理包括三个方面:高原地区地面位温显著高于平原地区(二者最大可差10 K),是该次罕见增温事件形成的首要条件;强下沉运动使得低密度高位温空气强迫下沉到边界层,是增温必要条件;强湍流混合作用则是地面空气增温的必要机制。估算结果表明,边界层急流伴随的强湍流混合可引起约8℃的罕见地面空气增温。最后,给出了该次事件的机理概念模型。     

The UHF wind profiler at Vienna airport – data quality control and comparisons to rawinsonde data

Summary A 1290MHz wind profiler (Radian Lap-3000), at present one of three operational wind profilers in Austria, is operated at Vienna airport. In spite of quality assurance procedures as consensus averaging included in the data evaluation process from profiler raw data, some spurious peaks of wind speed and unrealistic changes of the wind vector in time or height occur in the wind measurements. This is especially true for sampling intervals of only 5 minutes which are used to resolve the temporal evolution of summer thunderstorms and frontal passages. Averaging periods of only a few minutes are rather the lower limit apt for wind profiler observations and result in a low data availability of 28%, whereas about 55% of data (relative to the maximum height range according to the parameter setting) are available for 10 to 30 minutes profiles.Approaches to a posteriori quality control using checks for automatic error detection are proposed and tested on a one and a half year data-set: Flagging data when the three-dimensional wind divergence exceeds a predefined limit (0.5s^–1) is in most cases successful in combination with thresholds for wind speed (2 times the median of the daily data-set) or wind shear (0.2s^–1).The wind profiler data is compared to wind profiles from the next radiosonde station where soundings are launched 4 times a day at Hohe Warte, approx. 20km northwest, at the hill-side of the Viennese Woods. Deviations of about 1m s^–1 in wind speed are found between the observations of the two systems. Differences between the wind profiles within the boundary layer can be explained by local differences in the wind regime observed at the airport and the radiosounding – blocking effects of the Viennese Woods during south-easterly flow. Comparing the profiler data to radiosoundings on a monthly basis gives a tool to monitor the profiler performance.     

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 one dimensional wind profile model with stability and eddy viscosity estimations from sodar data

In a one-dimensional wind profile model, methods for eddy viscosity and stability estimations from sodar data have been evaluated with soundings. For eddy viscosity parameterization the ageostrophic method and mixing-length theory have been investigated. Three methods for estimating the static stability have been evaluated; a wind profile adjustment method, gravity wave analysis of sodar backscatter and flux profile functions for windspeed and standard deviation of vertical wind-speed. The wind profile model with variable momentum flux (VF) with height shows better results than an earlier constant flux model (Bergström, 1986). The VF model can be used for extending the sodar profile up to 1500 m.     

Acoustic sounding of land and sea breezes

Sodar measurements have been made at La Spezia, Italy during land- and sea-breeze conditions. The backscatter returns are discussed qualitatively, including their relation to the vertical structure of the boundary layer as revealed by vertical soundings of wind and temperature. During inversion conditions, the sodar signals may be difficult to interpret especially when there is a land breeze flowing over irregular terrain.     

Composite wind and temperature profiles obtained from a complex of in-situ and remote sensing measurement systems for the forcing of a boundary layer model

Summary ?This paper describes the configuration of measurement systems operated continuously at the Meteorological Observatory Lindenberg with the aim of constructing combined profiles of wind and temperature – so-called composite profiles – covering the boundary layer with high temporal and vertical resolution. This is required for the forcing of a micro-α-scale model in order to simulate the atmospheric boundary layer structure over a heterogeneous landscape during the LITFASS-98 experiment. The problems of combining measurements of different remote sensing and in-situ systems are briefly discussed. Although the measuring range of individual remote sensing systems is variable, the height coverage of wind and temperature profile measurements by sodar/RASS and two wind profiler radar/RASS complement each other very well. Using a simple merging procedure composite wind and temperature profiles have been synthesized based on radiosonde, windprofiler/RASS, sodar/RASS and tower measurements. Time-height cross sections of hourly composite profiles show considerably more details of the boundary layer structure than simple radiosonde interpolation due to the higher sampling frequency, higher vertical resolution and increased accuracy at the lower levels. Finally some qualifications of the formulated algorithm are suggested for future application. Received June 18, 2001; revised May 30, 2002; accepted June 6, 2002     

Elevated stratified layers observed with sodar during VTMX

Summary A combination of low frequency sodar, radar wind profiler and in-situ balloon-borne measurements of temperature and water vapor have been used to investigate the structure of elevated stratified layers within the transition layer above the nocturnal boundary layer during the Vertical Transport and Mixing Field Campaign in Salt Lake City Utah, during October, 2000. Elevated layers determined from sodar and radar vertical time sections were penetrated with a balloon-born instrument package to determine the fine scale temperature and moisture structure of the layers. As expected a potential temperature increase was found in the upper half of the layers; however the magnitude was considerably smaller than found above the daytime well-mixed layer and the vertical distance of the increase was quite variable. Mixing ratio, in the mean was found to have a relative maximum in the lower portion of the layers. It was found that the potential temperature within the layers decreased with time relative to background values, regardless of whether the layer descended or ascended.     

Wavy Vertical Motions in the ABL Observed by Sodar

Some characteristics of wavelike motions in the atmospheric boundary layer observed by sodar are considered. In an experiment carried out in February 1993 in Milan, Italy, Doppler sodar measurements were accompanied by in situ measurements of temperature and wind velocity vertical profiles using a tethered balloon up to 600 m. The oscillations of elevated wavy layers containing intense thermal turbulence, usually associated with temperature-inversion zones, were studied by using correlation and spectral analysis methods. The statistics of the occurrence of wavelike and temperature-inversion events are presented. The height distributions of Brunt–Vaisala frequency and wind shear and their correlation within elevated inversion layers were determined, with a strong correlation observed between the drift rate of the wavy layers and the vertical velocity measured by Doppler sodar inside these layers. Spectral analysis showed similarities regarding their frequency characteristics. The phase speed and propagation direction of waves were estimated from the time delay of the signals at three antennae to provide estimates of wavelength. Moreover, wavelengths were estimated from the intrinsic frequency obtained from sodar measurements of the Doppler vertical velocity and oscillations of wavy turbulent layers. The two wavelength estimates are in good agreement.     

Observations of Atmospheric Solitary Waves in the Urban Boundary Layer

The simultaneous operation of a three-axis Doppler sodar system in the centralurban area of Rome and two similar systems in the suburban area, forming atriangle about 20 km on each side, provided evidence of solitary-type wavesin the urban boundary layer. Three events, each lasting from a few minutes toabout 30 min, and ranging in depth from the minimum range of the sodar (39 m) to over 500 m, are reported here. Two events were recognizable onall three sodar records while the third event could be observed at the urbanlocation only. Time-height acoustic echo intensity records showed no-echoregions within the wave indicating transport of trapped recirculating air.This is typical of large amplitude solitary waves. The time series plots ofsodar-derived vertical wind velocity revealed a maximum peak-to-peakvariation of about 5 m s^-1 during periods of wave-associated disturbance.The vertical velocity is found to increase with height up to the top of the closedcirculation within the wave and decreases further above. The normalisedamplitude-wavelength relationship for the two events indicates that theobserved waves are close to a strongly nonlinear regime.     

Atmospheric thermic structure studied by acoustic echo sounder, boundary layer model, and direct measurements

In this work, the thermic structure of the atmospheric boundary layer is analyzed by means of direct measurements with radiosonde equipment, remote exploration with a three-monostatic Doppler sodar, and a boundary layer model of order one-and-a-half. Intercomparisons of radiosonde data, sodar data, and model results are made through the study of radiative nocturnal inversion, subsidence inversion, development and height of the mixing layer, and calculus of the temperature structure parameter. The ability of sodar to find the mixing layer height and to detect stable layers is enhanced when these layers are low enough.     

Some aspects of determining the stable boundary layer depth from sodar data

The question of estimating the height of the stable boundary layer (SBL) based on digitalized vertical profiles of sodar signal intensity has been re-examined. A simple one-dimensional numerical boundary-layer model is used to compute vertical profiles of the temperature structure parameterC _T ² . It is shown that especially at the beginning of the night (when stratification is weak) one can not expect any significant profile structure in the upper part of the SBL if its depth is determined in terms of common turbulent height scales. From this it is concluded that the SBL-height will be underestimated early in the night when derived from the maximum gradient in the signal intensity profiles. Later in the night in contrast, the computations often show elevated maxima or even zones with reduced, and above them enhanced, vertical gradients ofC _T ² , from which a SBL-height can be deduced that compares well with other common height scales. The computed profiles ofC _T ² are shown to be in qualitative agreement with observed profiles of sodar signal intensity for several analysed cases from the HAPEX-MOBILHY experiment.Comparing different SBL-depth scales with sodar observations, it is demonstrated that most of them are often closely related to a sodar-derived SBL-height only during certain phases of the night. Thus the sodar-SBL-height can, after a transition period, be perhaps associated with the lower turbulent layer of the growing surface inversion during the first part and with the height of the low-level wind maximum during the second part of the night.     

2018年北京城区和远郊区低层大气风场特征分析

利用2018年1月1日至12月31日在北京国家综合气象观测实验基地获得的风廓线雷达资料和同时期在河北香河的华北香河全大气层野外科学观测研究站获得的多普勒声雷达资料,比较分析北京城区和远郊区的低层(0～600 m)大气风场特征.结果 表明:水平风速随高度增加而增大,同一高度层,远郊区的平均水平风速大于城区,且受湍流活动影...     

Use of a High-Resolution Sodar to Study Surface-layer Turbulence at Night

Measurements in the atmospheric surface layer are generally made with point sensors located in the first few tens of metres. In most cases, however, these measurements are not representative of the whole surface layer. Standard Doppler sodars allow a continuous display of the turbulent thermal structure and wind profiles in the boundary layer up to 1000 m, with a few points, if any, in the surface layer. To overcome these limitations a new sodar configuration is proposed that allows for a higher resolution in the surface layer. Because of its capabilities (echo recording starting at 2 m, echo intensity vertical resolution of approximately 2 m, temporal resolution of 1 s) this sodar is called the surface-layer mini-sodar (SLM-sodar). Features and capabilities of the SLM-sodar are described and compared with the sodar. The comparison of the thermal vertical structure given by the SLM-sodar and the sodar provides evidence that, in most cases, the surface layer presents a level of complexity comparable to that of the entire boundary layer. Considering its high vertical resolution, the SLM-sodar is a promising system for the study of the nocturnal surface layer. The nocturnal SLM-sodar measurements have shown that, depending on wind speed, the structure of the surface layer may change substantially within a short time period. At night, when the wind speed is greater than 3 m s⁻¹, mechanical mixing destroys the wavy structure present in the nocturnal layer. Sonic anemometer measurements have shown that, in such cases, also the sensible heat flux varies with height, reaching a peak in correspondence with the wind speed peak. Under these conditions the assumption of horizontal homogeneity of the surface layer and the choice of the averaging time need to be carefully treated.     

Evidence of Organized Large Eddies by Ground-Based Doppler Lidar, Sonic Anemometer and Sodar

In an experiment investigating the planetary boundary layer (PBL) wind and temperature fields, and PBL inversion height recorded by various instruments, the results reveal the presence of organized large eddies (OLE) or rolls. The measurements by lidars, anemometers, soundings and sodar gave an overview of the characteristics of the rolls and sources of energy production that maintain them. The experimental results obtained on two consecutive days are compared to model outputs. The agreement is excellent, showing that thermal stratification and wind shear are important factors in the structure and dynamics of OLE. A heterodyne Doppler lidar (HDL) is shown to be a useful tool in the study of OLE.     

Case Studies of the Wintertime Convective Boundary-Layer Structure in the Urban Area of Milan, Italy

The paper describes some aspects of the convective boundary-layer structure based on simultaneous sodar and tethersonde measurements during a field experiment in the urban area of Milan in the period 8 to 20 February, 1993. During this period, fog episodes and strong low-level elevated inversions (with lower boundaries < 400 m) were observed most of the time. A close agreement in the mixing height values, derived from the sodar and tethersonde profiles, has been achieved under these conditions. The validity of the similarity relationships, which have been originally derived to describe the vertical velocity variance and heat flux profiles over horizontally homogeneous terrain under quasi-stationary conditions, was evaluated when applied to the urban boundary layer.     

A COMPARISON BETWEEN THE DOPPLER SODAR AND IN-SITU WIND MEASUREMENTS

In this paper,a comparison is made of the Doppler sodar wind measurement with the in-situ measurements on a 325 m meteorological tower.The results show that when the ratio of signal to noise is larger than 1,the mean wind speeds and directions obtained from the two techniques are well correlated,thus their profiles are close to each other.The mean value and root mean square of vertical velocity measured by the Doppler sodar are a little larger than those measured by a sonic anemometer at a height of 85 m,which is in agreement with the analysis by Gaynor et al.(1983).We notice that there are some differences in the observational results between the Doppler sodar and insitu measurements,especially when comparing the values measured at each time.Therefore,it is important to analyze the reason for the differences and to make corrections to the Doppler sodar measurements.     

基于径向功率谱的风廓线雷达错误风数据处理

选取上海市世博园区站、金山站和嘉定F1赛车场站3个风廓线雷达站2012年3月7日凌晨一次降水过程生成的错误水平风场数据,通过对比该时段高、低模式扫描实时径向功率谱数据,指出在降水初期风廓线雷达软件质量控制出现错误的原因,并重新识别错误时段的功率谱,反演水平风场数据。分析表明:在降水初期由于风廓线雷达各波束探测的数据在空间上不一致,易导致雷达软件采用的质量控制算法并不能全部识别和消除降水对数据的干扰,从而出现偏差。基于风廓线雷达高时空分辨率径向功率谱数据的分析处理方法可有效验证雷达软件质量控制算法,且经过该方法反演后的水平风场更为合理。