Volker Lehmann

Enhanced Accuracy of RASS-Measured Temperatures Due to an Improved Range Correction

Abstract The accuracy of virtual temperature measurements taken by the Radio Acoustic Sounding System (RASS) has been improved essentially by the application of a more precise range correction that takes into account the profile of reflectivity and the range weighting function of the receiver. Especially, the height dependence of the RASS temperature error compared to radiosondes was significantly reduced. The improved range correction was verified with a 1-yr dataset consisting of more than 1000 comparisons of 482- and 1290-MHz wind profiler/RASS with radiosondes observations. Other corrections discussed in the literature accounting for the displacement of the acoustic source, horizontal wind, and turbulence have been neglected because they are usually not significant. After the application of the improved range correction together with the correction of vertical wind as well as more accurate constants in the temperature retrieval, the bias with respect to radiosondes is smaller than 0.3 K and the standa...

A 35-GHz Polarimetric Doppler Radar for Long-Term Observations of Cloud Parameters—Description of System and Data Processing

AbstractA 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 Eur...

Advanced radar wind profiling

During the last three decades, radar wind profiling (RWP) has evolved into a key technology for atmospheric science and operational meteorology. In this tutorial stat us report, RWP is divided into three distinct areas: single-signal RWP, two-signal RWP, and multi-signal RWP. While single-signal RWP, or standard RWP, is a mature technology in many respects, there is still much room for improvement, particularly in the interpretation of signals that are severely contaminated by radi o interference or by clutter from aircraft, birds, hydrometeors, etc. Two-signal RWP, the best known examples of which are the spaced-antenna (SA) and frequency-domain interferometry (FDI) techniques, have been used to overcome some of the limitations inherent in standard RWP. Multi-signal RWP is, to a large extent, still unexplored territory. This paper attempts to provide a coherent conceptual framework of advanced RWP and to identify areas of future research and development. Zusammenfassung Im Laufe der letzten drei Jahrzehnte hat sich Radar-Windprofiling (RWP) zu einer Schlusseltechnologie in der Atmospharenforschung und der operationellen Meteorologie entwickelt. Im Rahmen einer einfuhrenden Bestandsaufnahme wird RWP in drei Kategorien eingeteilt: Einzel-Signal-RWP, Zwei-Signal-RWP und Multi-Signal-RWP. Obwohl Einzel-Signal-RWP, d.h. Standard-RWP, in vielerlei Hinsicht eine ausgereifte Technologie ist, gibt es dennoch Verbesserungsmoglichkeiten, insbesondere hinsichtlich der Auswertung von Messungen, die durch Radio-Einstreuung oder Storechos von Flugzeugen, Vogeln, Hydrometeoren usw. stark beeintrachtigt sind. Zwei-Signal-RWP, als deren Hauptvertreter die Technik der versetzten Antennen und die Frequenzbereich-Interferometrie gelten konnen, haben sich als hilfreich zur Uberwindung einiger Limitierungen der Standard-RWP erwiesen. Multi-Signal-RWP hingegen ist im wesentlichen noch unbekanntes Territorium. Dieser Beitrag versucht, einen einheitl ichen begrifflichen Rahmen der fortgeschrittenen RWP-Technologie zu liefern. Zudem werden mogliche Bereiche zukunftiger Forschung und Entwicklung aufgezeigt.

Optimal Gabor-Frame-Expansion-Based Intermittent-Clutter-Filtering Method for Radar Wind Profiler

AbstractIntermittent clutter signals are frequently observed by radar wind profilers during the seasonal bird migration. A novel statistical filtering algorithm based on a simultaneous time–frequency analysis of the profiler’s raw data was recently proposed to address shortcomings of existing methods. The foundation of this method is a Gabor frame expansion of the complex time series of the demodulated receiver voltage. In this paper, two objective criteria are suggested to obtain an optimal setup for the discrete Gabor frame expansion from the multitude of possibilities: first, the choice of almost-tight frames for a predefined maximum redundancy and second, the requirement that the analyzing bandwidth of the used Gaussian window function should provide a simultaneously sparse representation of both atmospheric signal components and intermittent clutter. The question of optimal sampling settings, especially dwell time, for a maximum reduction of bird interference is also discussed. Using data obtained du...

Wavelet-based methods for clutter removal from radar wind profiler data

The common way to process radar wind profiler (RWP) data by moments estimation of the Fourier power spectrum fails in presence of transient intermittent clutter contributions. Wavelets are especially suitable for detecting and removing transient components because of their high localization in time and frequency domain. We give an overview on the wavelet filtering of contaminated discrete RWP signals and introduce a new technique involving the wavelet packet decomposition and a splitting in progressive and regressive signal components. This technique has been successfully tested on severely real-data sets where classical wavelet routines fail.

Combining cloud radar and radar wind profiler for a value added estimate of vertical air motion and particle terminal velocity within clouds

Vertical-stare observations from a 482MHz radar wind profiler and a 35GHz cloud radar are combined on the level of individual Doppler spectra to measure vertical air motions in clear air, clouds and precipitation. For this purpose, a separation algorithm is proposed to remove the influence of falling particles from the wind profiler Doppler spectra and to calculate the terminal fall velocity of hydrometeors. The remaining error of both vertical air motion and terminal fall velocity is estimated to be better than 0.1ms−1 using numerical simulations. This combination of instruments allows direct measurements of in-cloud vertical air velocity and particle terminal fall velocity by means of ground-based remote sensing. The possibility of providing a profile every 10s with a height resolution of < 100m allows further insight into the process scale of in-cloud dynamics. The results of the separation algorithm are illustrated by two case studies, the first covering a deep frontal cloud and the second featuring a shallow mixed-phase cloud.

Synergy between ground-based remote sensing systems in microphysical analysis of cirrus clouds

A broad suite of ground-based remote sensing instruments of the Meteorological Observatory Lindenberg, Germany, is combined for the first time to synergistically analyze cirrus cloud microphysics, including a Raman lidar, a Ka band cloud radar and a 5ff tilted ceilometer. 84 days of cirrus cloud measurements have been selected to study the correlation between, and the dependences of, the different measured variables. The presented study investigates the effect of the spatial orientation and the shape of solid cloud particles on particle optical properties and their relation to wind and turbulence parameters. A sensitive indicator of particle spatial orientation is the particle depolarization ratio (PDR). When ice crystals are horizontally aligned, mirror reflections can occur, which is evidenced by low PDR if observed with a vertically pointing Raman lidar. Observations are grouped according to the prevailing weather condition. It is found that on some days PDR is constant for long time periods. Interestingly, during warm fronts the PDR is generally small (<0.2), while during cold fronts it is high (> 0.4). Moreover, the mean lidar ratio of cirrus with high PDR is about 20 sr, two times larger than of cirrus with low PDR. Similar dependences on PDR have been found for the particle extinction coefficient, and for the backscatter coefficient from the tilted ceilometer, but for the Raman lidar backscatter coefficient in perpendicular polarization the opposite behavior is observed.