Graduate Exam Abstract
Yuxiang Liu
Ph.D. Final
December 5, 2007, 3:00PM
Eng 120
he Study And Real-Time Implementation Of Attenuation Correction For X-Band Dual-Polarization Weather Radars
Abstract: Attenuation of electromagnetic energy due to rain or other wet hydrometeors along the propagation path is a well-recognized phenomenon which has been studied extensively in both the radar meteorology community as well as the satellite/terrestrial radio wave propagation community. Recently, there has been a tremendous interest in the use of short range dual-polarimetric X-band radar systems for hydrology due to lower system cost compared with the much more expensive S-band (~ 3 GHz) systems (WSR-88D) operated by the US National Weather Service. This interest has been due to advances in dual-polarimetric radar research which show that the specific attenuation (Ah) and differential attenuation (Adp) between horizontal (h) and vertical polarized waves (v) caused by oblate, highly oriented raindrops can be estimated using the specific differential phase measurement (Kdp). This advance leads to correction of the radar measured reflectivity (Zh) and the differential reflectivity (Zdr) due to path attenuation. This thesis addresses via theory, simulations and data analyses the accuracy and optimal estimation of attenuation-correction procedures at X-band (~ 10 GHz) frequency. A primary driving force has been the real-time implementation of the procedures developed herein to the first generation of X-band dual-polarized Doppler radar network (IP1) operated by the NSF Center for Collaborate Adaptive Sensing of the Atmosphere (CASA).<br><br>
The attenuation problem can be formulated within an estimation framework. For the rain attenuation estimation, we first formulate a parametric model based on the Ah-Kdp and Ah-Zh relationships. The parameters in the two relationships are not set a priori but rather estimated based on the consistency between the Kdp and Zh in rain medium. This consistency is achieved by minimizing the model output and the measured differential propagation phase (¦dp) in a least-squares sense. By estimating the parameters based on the consistency, one can reduce the uncertainties inherent in the Ah-Kdp and Ah-Zh relationships due to changes in temperature, rain drop shapes, and to a lesser extent, large variations in the drop size distribution (DSD) along the propagation path. Secondly, we extend the consistency principle to vertically polarized radar variables and an estimation model of the path-integrated differential attenuation is subsequently derived. With the estimated parameters, the path-integrated attenuation and the path-integrated differential attenuation can be used to correct the Zh and Zdr, respectively. Finally, we provide a preliminary method to address the mixed-phase (rain co-existing with wet ice) attenuation problem by which one can separately estimate the attenuation caused by rain and wet ice particles along the propagation path.<br><br>
We evaluate the improved method for correcting the Zh and the Zdr for rain attenuation using simulations and X-band radar data. In the simulations, we apply the method to radar variables generated from constant DSD profiles and variable DSD profiles. We also evaluate the performance under ideal and noisy situations. It is shown that our method is able to adjust the parameters according to the changes in temperature, drop shapes, and a certain class of DSD with very fast convergence. Both Zh and Zdr are corrected to a very good degree of accuracy by comparing the corrected values with the simulation input values. The X-band radar data are obtained from the National Institute of Earth Science and Disaster Prevention (NIED), Japan and from CASA IP1. The improved method accurately corrects NIEDs data when compared with ground truth calculated from in situ disdrometer-based DSD measurements for a Typhoon event. We have implemented, in real-time, the improved method in all the CASA IP1 radar nodes. The corrected Zh PPI scan from the CASA IP1 network radars shows good agreement with the nearly-coincident Zh PPI scan measured by a nearby un-attenuated WSR-88D S-band radar system. One additional advantage is that the estimation of the specific attenuation at the horizontal polarization (Ah) and the specific differential attenuation (Adp) is independent of any systematic offsets in the h- or v-channels of the radar.<br><br>
We also evaluate our preliminary method that separately estimates rain and wet ice attenuation using microphysical outputs from a previous supercell simulation using the CSU-RAMS (Regional Atmospheric Modeling System). The retrieved rain and wet ice specific attenuation fields were found to be in close correspondence to the true fields calculated from the simulation. The wet ice attenuation field is useful in studying the A-Z relationship for wet ice, which can help improve the profiling algorithms used in Tropical Rainfall Measuring Mission (TRMM) or being proposed for the Global Precipitation Measurement (GPM) mission. The concept to correct rain and wet ice attenuation separately can be also applied to the CASA IP1 network with additional constraint information possibly provided by the WSR-88D network.
Adviser: V.N. Bringi
Co-Adviser: V. Chandrasekar
Non-ECE Member: Graeme Stephens (Atmos. Science)
Member 3: N/A
Addional Members: N/A
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