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Gerhard Peters

Abstract

High-resolution measurements of the frequency shift of radio acoustic sounding system echoes show small (≤10−3) systematic deviations from values that are expected according to basic theory. It is shown that these deviations can be explained by two successive scattering processes: 1) scattering of acoustic waves from atmospheric turbulent (acoustic) refractive-index structures, and 2) scattering of radar waves from that component of the scattered acoustic field that satisfies the condition for specular reflection. An expression for the frequency shift is derived, which agrees with the theory of Lataitis for the case of “strong” turbulence. Preliminary experimental observations are presented, which support this analysis.

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Stefan Kowalewski and Gerhard Peters

Abstract

The inclusion of polarimetric measurements for the quantitative precipitation estimation (QPE) by weather radars as well as space- and airborne radars is considered most promising now-a-days. Because the melting layer region is usually marked by a distinct peak of the linear depolarization ratio (LDR), a possible correlation between LDR peak values and underlying drop sizes in terms of the ZR relation is investigated, that is, the empirical relation between radar reflectivity factor Z and rain rate R. For this purpose, data taken during the Convective and Orographically Induced Precipitation Study (COPS) campaign in 2007 from two vertically pointing radars—a 24.15-GHz Micro Rain Radar (MRR) and a 35.5-GHz polarimetric cloud radar—were analyzed.

In this analysis a correlation between parameters of the ZR relation and LDR peak values are revealed, implying that the LDR magnitude within the melting layer must be influenced by the size of melting particles. Furthermore, an LDR classification scheme shows an improvement of R retrieval with respect to the global ZR relation optimized for the dataset herein. However, to asses the impact for improved QPE in the above-mentioned applications, future research is necessary.

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Gerhard Peters and Hans Jürgen Kirtzel

Abstract

Ground-based remote measurements of the turbulent vertical flux of horizontal momentum in the atmospheric boundary layer are described. For this experiment a wind profiler was operated in the radio acoustic sounding system mode, which uses the scattering at artificially generated sound waves rather than at natural microturbulent refractive-index variations as in the case of conventional clear-air radar. The method used to derive the momentum flux from sound velocities measured parallel to different rather beams is explained. The accuracy and resolution is apparently high enough to obtain useful measurements at least under conditions with moderate winds. The height range is limited to a few hundred meters in this first experiment due to the advection and absorption of the scattering sound waves.

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Gerhard Peters, Bernd Fischer, and Hans Münster

Abstract

Humidity spectra obtained with a closed-path optical hygrometer (LI-6262) show an apparent low-pass characteristic with a cutoff frequency varying between 0.01 and 0.1 Hz. Laboratory measurements revealed that contamination of the intake filter is responsible for the damping of high frequencies. It was further found that the effect of the intake filter on the response function has a nonlinear component with the consequence that the cutoff frequency is a function of the humidity variance. The uncontaminated “lifetime” of the intake filter depends on the particular air quality and may be less than 24 h. The usefulness of closed-path hygrometers for long-term eddy covariance measurements of humidity is therefore questioned.

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Gerhard Peters, Bernd Fischer, and Marco Clemens

Abstract

The classical rain attenuation correction scheme of Hitschfeld and Bordan (HIBO) and the newer iterative approach by Hildebrand (HL) are reconsidered. Although the motivation for the HL algorithm was an extension into ranges, where HIBO tends to be unstable, it is shown here that the contrary is the case. The finite-range resolution causes an intrinsic instability of HL already at moderate attenuation, where HIBO would still deliver stable results. Therefore, the authors concentrate the further analysis on HIBO, and confirm that the usual implementation of HIBO does not account correctly for finite-range resolution. They suggest a modified scheme that produces exact retrievals in the ideal case of perfect measurements.

For vertically pointing Doppler radars a new element is explored in the attenuation correction—namely, calculating rain attenuation κ and rainfall R from Doppler spectra via the raindrop size distributions (RSDs). Although this spectral scheme (SIBO) avoids the uncertainty of ZR and Zκ relations, the superiority of this approach is not a priori obvious because of its sensitivity to vertical wind. Therefore, radar rain rates, based on a ZR relation and on RSDs, respectively, are compared with in situ measurements. The results indicate better agreement for RSD-based retrievals. Because κ is closely correlated with R, the authors assert the advantage of RSD-based retrievals of κ.

The application of HIBO and SIBO to real data shows that the uncertainty of standard ZR relations is the main source of deviation between the two versions. In addition, the comparison of profiles suggests that the parameters of ZR relations aloft can deviate considerably from near-surface values. Although artifacts cannot be excluded with certainty, there is some evidence that this observation actually reflects microphysical processes.

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Christoph Senff, Jens Bösenberg, and Gerhard Peters

Abstract

A remote-sensing method to retrieve vertical profiles of water vapor flux in the convective boundary layer by using a differential absorption lidar and a radar-radio acoustic sounding system is described. The system's height range presently extends from 400 to 700 m above the surface, and flux data can be sampled with a height resolution of 75 m and a time resolution of 60 s. The results of a first measurement in July 1991 under predominantly convective conditions are presented. The resolution of the remote-sensing system apparently is sufficient to resolve the major contributions to the flux in the convective mixed layer. In addition, the advantages and limitations of this method are discussed.

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Gerhard Peters, Bernd Fischer, and Hans Jürgen Kirtzel

Abstract

A Doppler sodar and an ultrasonic anemometer–thermometer, which are operated continuously at a nuclear research center in Germany, are described. Mean values and second moments including turbulent surface fluxes recorded over 1 yr are analyzed in order to assess their credibility. Particularly, the effects of rain on the data quality and the statistical bias caused by a reduced data availability—typical for remote sensing measurements—are investigated. Mean profiles of the vertical wind variance for different classes of Monin–Obukhov length, the mean vertical wind component versus wind direction, and the annual variation of 24-h averages of the surface heat flux are presented.

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Gerhard Peters, Bernd Fischer, Hans Münster, Marco Clemens, and Andreas Wagner

Abstract

Data of vertically pointing microrain radars (MRRs), located at various sites around the Baltic Sea, were analyzed for a period of several years. From the Doppler spectra profiles of drop size distributions (DSDs) are obtained. A significant height dependence of the shape of the DSDs—and thus of the ZR relations—is observed at high rain rates. This implies, for the considered sites, that ground-based ZR relations lead to underestimation of high rain rates by weather radars.

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Ulrich Görsdorf, Volker Lehmann, Matthias Bauer-Pfundstein, Gerhard Peters, Dmytro Vavriv, Vladimir Vinogradov, and Vadim Volkov

Abstract

A 35-GHz radar has been operating at the Meteorological Observatory Lindenberg (Germany) since 2004, measuring cloud parameters continuously. The radar is equipped with a powerful magnetron transmitter and a high-gain antenna resulting in a high sensitivity of −55 dBZ at 5-km height for a 10-s averaging time. The main purpose of the radar is to provide long-term datasets of cloud parameters for model evaluation, satellite validation, and climatological studies. Therefore, the system operates with largely unchanged parameter settings and a vertically pointing antenna. The accuracy of the internal calibration (budget calibration) has been appraised to be 1.3 dB. Cloud parameters are derived by two different approaches: macrophysical parameters have been deduced for the complete period of operation through combination with ceilometer measurements; a more enhanced target classification and the calculation of liquid and ice water contents are realized by algorithms developed in the framework of the European CloudNet project.

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Bjorn Stevens, Felix Ament, Sandrine Bony, Susanne Crewell, Florian Ewald, Silke Gross, Akio Hansen, Lutz Hirsch, Marek Jacob, Tobias Kölling, Heike Konow, Bernhard Mayer, Manfred Wendisch, Martin Wirth, Kevin Wolf, Stephan Bakan, Matthias Bauer-Pfundstein, Matthias Brueck, Julien Delanoë, André Ehrlich, David Farrell, Marvin Forde, Felix Gödde, Hans Grob, Martin Hagen, Evelyn Jäkel, Friedhelm Jansen, Christian Klepp, Marcus Klingebiel, Mario Mech, Gerhard Peters, Markus Rapp, Allison A. Wing, and Tobias Zinner

Abstract

A configuration of the High-Altitude Long-Range Research Aircraft (HALO) as a remote sensing cloud observatory is described, and its use is illustrated with results from the first and second Next-Generation Aircraft Remote Sensing for Validation (NARVAL) field studies. Measurements from the second NARVAL (NARVAL2) are used to highlight the ability of HALO, when configured in this fashion, to characterize not only the distribution of water condensate in the atmosphere, but also its impact on radiant energy transfer and the covarying large-scale meteorological conditions—including the large-scale velocity field and its vertical component. The NARVAL campaigns with HALO demonstrate the potential of airborne cloud observatories to address long-standing riddles in studies of the coupling between clouds and circulation and are helping to motivate a new generation of field studies.

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