Radar Technology

1. Bistatic Radar

The knowledge of wind vector fields throughout the atmosphere is extremely desirable for both meteorological research and operational applications. While with conventional methods for deriving wind vector fields within precipitation, e.g., combining two Doppler weather radars (Fig. 1, monostatic dual-Doppler) at a distance between 40-80 km are quite expensive. An alternative could be a bistatic multiple Doppler technique where several inexpensive receivers are grouped around a transmitting/receiving radar system (Figs. 1 and 2).

Doppler Comparisons

Large parts of my dissertation focuses on optimal setup, algorithm development, and antenna configurations, and wind synthesis. Followed up publications show results and potential applications. This system is currently operating at the German Aerospace Center (Deutsches Zentrum fuer Luft- und Raumfahrt, DLR) in Oberpfaffenhofen near Munich. Figure 3 shows an example of wind measurements during a thunderstorm event during the VERTIKATOR field campaign. About one hour later the convective cell south-southeast of the radar developed a severe downburst.

Figure 3: Radar reflectivity (color coded) and horizontal wind fields (arrows) measured by a bistatic radar system during a supercell thunderstorms.

2. Advanced dual-polarization/dual-Doppler capabilities for Doppler on Wheels radars

The Doppler on Wheels (DOW) radars, operated by the Center for Severe Weather Research, were upgraded to a new dual-polarization design, permitting fast, full dual-polarization (including LDR), or very fast 45-degree polarization modes, using two transmitters. The two transmitters, operating at slightly different frequencies, also permit doubled independent sampling for both velocity and dual-polarization products, permitting doubled scan rates. Current mobile dual-polarization technologies do not permit both fast scanning and dual-polarization scanning, e.g., dual-polarization X-band radars in VORTEX2 will be slowed to 3 min update rates when in dual-polarization modes. Some features in supercells evolve too rapidly to be resolved with this update rate. Additionally, microphysical processes relevant for storm development, intensification, severe weather outbreaks or precipitation enhancement occur on the order of minutes within the entire 3-dimensional volume and are often coupled to processes occurring above the 0º isotherm. Therefore, fast and dense volume scans up to 12-15 km height are required. With the novel technical approach described in section 4, the proposed DOW upgrades would permit scanning up to 30-50 deg per second accomplishing volume scans with LDR within 2 minutes and volume scans with ZDR/Phi-DP only within 60-90 seconds. In addition to fast scanning, the proposed DOW upgrades would include the measurement of the linear depolarization ratio (LDR), which is absent from current mobile dual-polarization X-band radars which only measure ZH, ZV, ZDR, Rho-HV, Phi-DP.

The dual-polarization DOW6 and DOW7 will be first tested during the VORTEX2 field campaign conducted between 1 May - 17 June 2010 in the U.S. Great Plains.

3. How do mountain affect the quality of dual-polarization measurements?

Usually the orographic environment around the radar is characterized in terms of calculating the area that is illuminated by the radar beam for a given scanning strategy and radar characteristics (Fig. 4).While numerous studies have focused on the influence of ground clutter on the quality of radar reflectivity only few investigated the effects of mountains on the quality of polarimetric measurements.

Radar reflectivity (Zh), differential reflectivity (Zdr), and specific differential phase (Kdp) measured from the operational, polarimetric weather radar located in Trappes, France, were used to examine the effects of radar beam shielding on rainfall estimation. The objective of this study is to investigate the degree of immunity of Kdp-based rainfall estimates to beam shielding for C-band radar data during four typical rain events encountered in Europe.  The large effects of beam shielding on rainfall accumulation were observed for algorithms using Zh and Zdr with differences of up to ~2 dB (40%) compared to a Kdp-based algorithm over a power loss range of 0–8 dB (Fig. 5). This analysis reveals that Zdr and Kdp are not affected by partial beam shielding. Standard reflectivity corrections based on the degree of beam shielding would have overestimated rainfall rates by up to 1.5 dB for less than 40% beam shielding and up to 3 dB for beam shielding less than 75%. The investigation also examined the sensitivity of beam shielding effects on rainfall rate estimation to (i) axis–ratio parameterization and drop size distribution, (ii) methods used to smooth profiles of differential propagation phase (phi_dp) and estimate Kdp, and (iii) event-to-event variability. Although rainfall estimates were sensitive to drop size distribution and axis–ratio parameterization, differences between Zh- and Kdp-based rainfall rates increased independently from those parameters with amount of shielding. Different approaches to smoothing phi_dp profiles and estimating Kdp were examined and showed little impact on results.

Figure 5: Schematic diagram summarizing the results of this study. The thick black lines illustrate the behavior of R(Zh), R(Zdr), and R(Kdp, Zdr) with respect to R(Kdp). The results are based on four rainfall events with different rainfall characteristics. The gray lines indicate the theoretical loss of reflectivity-based rainfall rate according to the power loss from the beam shielding map. (Friedrich et al. 2009)

___________________________________________________________________________________________________________________________________

Friedrich, K., U. Germann, and P. Tabary, 2009: Influence of ground clutter contamination on the accuracy of polarimetric quantities and rainfall rate. J. Atmos. Oceanic Technol., 26, 251-269.

Friedrich, K., U. Germann, J. J. Gourley, and P. Tabary, 2007: Effects of radar beam shielding on rainfall rate estimation for polarimetric C-band radar. J. Atmos. Ocean Technol., 24, 1839-1859.

Friedrich, K., M. Hagen, and T. Einfalt, 2006: A quality control concept for radar reflectivity, polarimetric parameters, and Doppler velocity. J. Atmos. Ocean Technol., 23, 865-887.

Friedrich, K., and M. Hagen, 2004: Evaluation of wind vectors measured by a bistatic Doppler radar network. J. Atmos. Oceanic Technol., 21, 1840-1854.

Friedrich, K., and M. Hagen, 2004: On the use of advanced Doppler radar techniques to determine horizontal wind-fields for operational weather surveillance. Meteor. Appl., 11, 155-171.

Friedrich, K., and O. Caumont, 2004: Dealiasing Doppler velocities measured by a bistatic radar network during a downburst-producing thunderstorm. J. Atmos. Oceanic Technol., 21, 717-729.

Friedrich, K., and M. Hagen, 2004: Wind synthesis and quality control of dual-Doppler derived horizontal wind-fields. J. Appl. Meteor., 43, 38-57.

Friedrich, K., 2002: Determination of three-dimensional wind-vector fields using a bistatic Doppler radar network. Ph.D. thesis, Dep. of Physics, Ludwig-Maximilians University Munich; 135 pp., DLR-FB2002-05.