Multi-beam raindrop size distribution retrievals on the Doppler spectra
Acquiring the raindrop size distribution from radar data is still a challenge. Generally this distribution is retrieved using the reflectivity, Z, and the differential reflectivity, Zdr, at S-band. The specific differential phase, Kdp, provides a third radar observable in the case of heavy precipitation to strengthen the retrieval technique. For S-band profiling radars, Kdp cannot be used, mainly because of the limited number of range bins containing rain. The Zdr measurement cannot be employed when the elevation is near the vertical, or in the case of drizzle and light rain. Therefore retrievals have to be performed on the measured Doppler spectra. For this purpose, the Doppler spectra of rain are modeled using Rayleigh scattering, the gamma distribution for the raindrop size, a size-shape relationship and a Gaussian kernel for the turbulence. Comparing the measured Doppler spectrum with the modeled one, a non-linear least-square fit technique is employed to obtain the parameters of the raindrop size distribution (D0, ?) and the turbulence broadening factor of the raindrop size spectrum (?0). The intercept parameter (Nw) and the radial wind component (v0) are estimated by scaling the broadened drop size distribution (DSD) along the spectral reflectivity axis and the Doppler velocity axis respectively. This approach has been proposed in (Moisseev et al, 2006). So far, this technique has not been further developed, exploited and validated. It is our intention to investigate this methodology using the Doppler-polarimetric TARA radar to retrieve raindrop size distribution from drizzle to heavy precipitation with high spatial and time resolution. Since TARA can profile in three directions, three raindrop size distribution profiles are estimated, which can give insight in the microphysical homogeneity of the precipitation
Laboratory Measurements of Axis Ratios for Large Raindrops
Abstract The oscillations of moderate to large raindrops are investigated using a seven-story fall column with shape data obtained from multiple-strobe photographs. Measurements are made at a fall distance of 25 m for drops of D = 2.5-, 2.9-, 3.6-, and 4.0-mm diameter, with additional measurements at intermediate distances to assess the role of aerodynamic feedback as the source of drop oscillations. Oscillations, initiated by the drop generator, are found to decay during the first few meters of fall and then increase to where the drops attained terminal speed near 10 m. Throughout the lower half of the fall column, the oscillation amplitudes are essentially constant. These apparently steady-state oscillations are attributed to resonance with vortex shedding. For D = 2.5 and 3.6 mm, the mean axis ratio is near the theoretical equilibrium value, a result consistent with axisymmetric (oblate/prolate mode) oscillations at the fundamental frequency. For D = 2.9 and 4.0 mm, however, the mean axis ratio is larg...
Designing for TARA: Data processing, visualization and storage
The TARA scans through a column of atmosphere in order to observe and study cloud system behavior. After 10 years of operation new knowledge of processing has been acquired which allows for an update that will improve measurement quality and processing speed. The main question of this thesis is: How to implement data processing, visualization and storage for TARA? The system, from the radar control to the visualization and storage, needs to be built from scratch. In order to most effectively achieve this, the entire system has been split up into two parts that will be designed separately. To be able to combine the parts, the groups will work together to build an overarching structure in which the separate blocks can be easily implemented (chapter 2). In chapter 3 the program of requirements is given, in which all the requirements for the system are mentioned. Different topics have been researched, in order to be able to do this project, this can be read in chapter 4. Over the years a variety of radar types have been developed, each having its own advantages and disadvantages. Four of the different properties are important in TARA. That are: monostatic vs. bistatic, FM-CW radar vs. pulsed radar, Doppler radar and polarimetric radar. Frequency modulation is used to be able to determine the range of the measured objects. By comparing the received frequency with the current transmitted frequency the time that was needed for the signal to travel can be determined, also the Doppler speed of the particles can be calculated. For the implementation of this project we will work with LabVIEW and Matlab. An overall block diagram has been made, after which the more detailed block diagrams and state diagrams of the three blocks that group 2 has to design, could be made. The overall block diagram shows the signals that every block in the systems sends out or receives (chapter 5). The final block diagrams are shown in Appendix A and the state diagrams in Appendix B. The state diagrams are used to describe how the sub-systems accomplish their tasks. Incoming data is processed by Matlab code which is provided. This is one of the main reasons for the choice of LabVIEW, because Matlab code can be implemented in the program via Matlab nodes. Variables that should not be changed during measurement and should be used by multiple different systems are put into global functional variables. Variables that can be changed are sent via notifiers. After everything was designed and some choices for implementation were made, the implementation could begin (chapter 6). First an overarching system was made, but during the development it became apparent that the actions of this system could be interpreted as those of the Radar Control block, a block that was originally designed by group 1. After the choices were made on how the Radar Control operates, the design process of the GUI started and the data processing and visualization block were implemented. Implementation of the storage block has however not been realized, because of time constraints and task re-allocation between the two sub-groups. Finally the system has been implemented in the actual TARA.First real-time measurements have been succesfully performed. An evaluation about what has been achieved compared to the requirements discussed in chapter 3, can be read in chapter 7. As already stated before, the storage has not been implemented, so requirements considering storage have not been met. Besides that, most requirements are met, although improvements can still be made. In chapter 8 a discussion is given about the points on which the project could be improved. In chapter 9 the conclusion is drawn that most of the requirements are met and recommendations are given on how to improve the system further.