Oleg A. Krasnov

Cloudnet: Continuous Evaluation of Cloud Profiles in Seven Operational Models Using Ground-Based Observations

Cloud fraction, liquid and ice water contents derived from long-term radar, lidar and microwave radiometer data are systematically compared to models to quantify and improve their performance.

Basics and first experiments demonstrating isolation improvements in the agile polarimetric FM-CW radar – PARSAX

The article describes the IRCTR PARSAX radar system, the fully polarimetric FM-CW radar with dualorthogonal sounding signals, which has the possibility to measure all elements of the radar targets polarization scattering matrix simultaneously, in one sweep.

Advances in Continuously Profiling the Thermodynamic State of the Boundary Layer: Integration of Measurements and Methods

This paper describes advances in ground-based thermodynamic profiling of the lower troposphere through sensor synergy. The well-documented integrated profiling technique (IPT), which uses a microwave profiler, a cloud radar, and a ceilometer to simultaneously retrieve vertical profiles of temperature, humidity, and liquid water content (LWC) of nonprecipitating clouds, is further developed toward an enhanced performance in the boundary layer and lower troposphere. For a more accurate temperature profile, this is accomplished by including an elevation scanning measurement modus of the microwave profiler. Heightdependent RMS accuracies of temperature (humidity) ranging from 0.3 to 0.9 K (0.5–0.8 g m 3 )i n the boundary layer are derived from retrieval simulations and confirmed experimentally with measurements at distinct heights taken during the 2005 International Lindenberg Campaign for Assessment of Humidity and Cloud Profiling Systems and its Impact on High-Resolution Modeling (LAUNCH) of the German Weather Service. Temperature inversions, especially of the lower boundary layer, are captured in a very satisfactory way by using the elevation scanning mode. To improve the quality of liquid water content measurements in clouds the authors incorporate a sophisticated target classification scheme developed within the European cloud observing network CloudNet. It allows the detailed discrimination between different types of backscatterers detected by cloud radar and ceilometer. Finally, to allow IPT application also to drizzling cases, an LWC profiling method is integrated. This technique classifies the detected hydrometeors into three different size classes using certain thresholds determined by radar reflectivity and/or ceilometer extinction profiles. By inclusion into IPT, the retrieved profiles are made consistent with the measurements of the microwave profiler and an LWC a priori profile. Results of IPT application to 13 days of the LAUNCH campaign are analyzed, and the importance of integrated profiling for model evaluation is underlined.

Investigation of Droplet Size Distributions and Drizzle Formation Using A New Trajectory Ensemble Model. Part II: Lucky Parcels

Abstract A novel trajectory ensemble model of the cloud-topped boundary layer containing 1340 Lagrangian parcels moving with a turbulent-like flow with the observed statistical properties was applied to investigate the formation of the microphysical structure of stratocumulus clouds (Sc) in a nonmixing limit (when turbulent mixing between the parcels is not taken into account). The Sc observed in two research flights during the Second Dynamics and Chemistry of the Marine Stratocumulus field study (DYCOMS II)—RF01 (no drizzle) and RF07 (weak drizzle)—are simulated. The mechanisms leading to a high variability of droplet size distributions (DSDs) with different spectrum width and dispersion are discussed. Drizzle formation was investigated using the radar reflectivity–LWC and LWC–effective drop radius diagrams simulated by the model in the nondrizzle and drizzle cases. It is shown that in the RF07 case large cloud droplets that trigger drop collisions and drizzle formation form only in a small fraction (abo...

Combined Observational and Model Investigations of the Z–LWC Relationship in Stratocumulus Clouds

In situ measurements indicate the complexity and nonunique character of radar reflectivity–liquid water content (Z–LWC) relationships in stratocumulus and cumulus clouds. Parameters of empirical (statistical) Z–LWC dependences vary within a wide range. Respectively, the accuracy of retrieval algorithms remains low. This situation is partially related to the fact that empirical algorithms and parameters are often derived without a corresponding understanding of physical mechanisms responsible for the formation of the Z–LWC diagrams. In this study, the authors investigate the processes of formation of the Z–LWC relationships using a new trajectory ensemble model of the cloud-topped boundary layer (BL). In the model, the entire volume of the BL is covered by Lagrangian parcels advected by a turbulent-like velocity field. The time-dependent velocity field is generated by a turbulent model and obeys the correlation turbulent laws. Each Lagrangian parcel represents the “cloud parcel model” with an accurate description of processes of diffusion growth–evaporation of aerosols and droplets and droplet collisions. The fact that parcels are adjacent to each other allows one to calculate sedimentation of droplets and precipitation (drizzle) formation. The characteristic parcel size is 50 m; the number of parcels is 1840. The model calculates droplet size distributions (DSDs), as well as their moments (e.g., aerosol and drop concentration, mass content, radar reflectivity) in each parcel. In the course of the model integration, Z–LWC relationships are calculated for each parcel, as well as the scattering diagram including all parcels. The model reproduces in situ observed types of the Z–LWC relationships. It is shown that different regimes represent different stages of cloud evolution: diffusion growth, beginning of drizzle formation, and stage of heavy drizzle, respectively. The large scattering of the Z–LWC relationships is found to be an inherent property of any drizzling cloud. Different zones on the Z–LWC diagram are related to cloud volumes located at different levels within a cloud and having different DSD. This finding allows for improvement of retrieval algorithms.

Investigation of Droplet Size Distributions and Drizzle Formation Using a New Trajectory Ensemble Model. Part I: Model Description and First Results in a Nonmixing Limit

Abstract A novel trajectory ensemble model of a stratocumulus cloud is described. In this model, the boundary layer (BL) is fully covered by a great number of Lagrangian air parcels that during their motion can contain either wet aerosols or aerosols and droplets. The diffusion growth of aerosols and droplets, as well as drop collisions, is accurately described in each parcel. Droplet sedimentation is taken into account, which allows simulation of precipitation formation. The Lagrangian parcels are advected by the velocity field generated by the turbulent-like flow model obeying turbulent correlation laws. The output of the numerical model includes droplet and aerosol size distributions and their moments, such as droplet concentration, droplet spectrum width, cloud water content, drizzle content, radar reflectivity, etc., calculated in each parcel. Horizontally averaged values are calculated as well. Stratocumulus clouds observed during two research flights (RF01 and RF07) in the Second Dynamics and Chemi...

Nearly Orthogonal Waveforms for MIMO FMCW Radar

Herein is proposed a set of nearly orthogonal waveforms based on offset linear frequency modulated (LFM) signals for multiple-input, multiple-output frequency modulated continuous wave (MIMO FMCW) radar operation. The orthogonality of sounding signals is a critical point for many multi-channel radar applications because the interference between signals can significantly limit the radars ability for observation of weak targets in the presence of strong targets and clutter. High orthogonality for the proposed waveforms can be obtained by using de-ramping processing in the MIMO FMCW radar receiver. The experimental results presented demonstrate that a high orthogonality level is achievable for a specific multi-channel FMCW radar.

Interleaved OFDM Radar Signals for Simultaneous Polarimetric Measurements

A novel pair of mutually-orthogonal radar waveforms is proposed for simultaneous polarimetric measurements (SPM). The proposed waveform pair exploits the orthogonality features between different subcarriers within a single orthogonal frequency division multiplexing (OFDM) chip, and is called interleaved OFDM (I-OFDM). With I-OFDM signals the isolation limitation defined by the bandwidth time (BT) product for frequently used waveform pairs like linear frequency modulated (LFM) and phase-code modulated (PCM) signals, can be overcome. The performance features of I-OFDM signals are theoretically analyzed; the application of I-OFDM signals and the corresponding signal processing scheme in SPM are presented. A significant increase of the polarimetric measurements efficiency by utilization of the proposed I-OFDM signals has been verified by experiments using an operational radar system.

A simple direct target localization and deghosting algorithm in active radar network

Recently studies proposed to solve the target localization problem processing directly all raw signals received on all nodes in a centralized one step approach. Even if such approach can be more accurate than classical approach it can also be prone to strong sidelobes that makes the detection step complicated. In the context of active radar network, we propose here a simple detection based on a one step approach that increase the sidelobes rejection compared to a classical approach. The proposed simple test enables both the target localization and deghosting in the context of active radar. The method is tested on synthetic data and on real data measured in a realistic scenario.

Radar micro-Doppler of wind turbines: simulation and analysis using rotating linear wire structures

A simple electromagnetic model of wind-turbines main structural elements as the linear wired structures is developed to simulate the temporal patterns of observed radar return Doppler spectra (micro-Doppler). Using the model, the micro-Doppler for different combinations of the turbines rotation frequency, radar pulse repetition frequency, and duration of the Doppler measurement interval are analyzed. The model is validated using the PARSAX radar experimental data. The model ability to reproduce the observed Doppler spectra main features can be used for development of signal-processing algorithms to suppress the wind-turbines clutter in modern Doppler radars.