Veronica Santalla del Rio

3-Pol Polarimetric Weather Measurements With Agile-Beam Phased-Array Radars

Most actual polarimetric weather radars consider either alternate transmission of horizontal and vertical polarizations or simultaneous transmission of both polarizations. Simultaneous reception of vertical and horizontal components is always performed. The way these operating modes have to be modified, to cope with the loss of polarization purity and the increase of polarization channels coupling at some pointing directions when planar phased-array radars (PARs) are used, has already been discussed. Complete correction of these effects showed to be difficult, requiring precise control of the relative amplitudes and phases of the dipoles feeding signals on transmission in a pulse-to-pulse basis. If simultaneous transmission of vertical and horizontal polarizations is considered, complete correction has not been achieved. In this paper, the implementation of a third polarimetric measurement mode based on alternate transmission of three different polarizations with PARs is discussed. It is found that implementation of this method, which provides minimum-variance unbiased linear polarimetric covariance matrix estimates, with phased-array systems only requires simple adjustments at reception.

High Temporal Resolution Refractivity Retrieval from Radar Phase Measurements

Knowledge of the spatial and temporal variability of near-surface water vapor is of great importance to successfully model reliable radio communications systems and forecast atmospheric phenomena such as convective initiation and boundary layer processes. However, most current methods to measure atmospheric moisture variations hardly provide the temporal and spatial resolutions required for detection of such atmospheric processes. Recently, considering the high correlation between refractivity variations and water vapor pressure variations at warm temperatures, and the good temporal and spatial resolution that weather radars provide, the measurement of the refractivity with radar became of interest. Firstly, it was proposed to estimate refractivity variations from radar phase measurements of ground-based stationary targets returns. For that, it was considered that the backscattering from ground targets is stationary and the vertical gradient of the refractivity could be neglected. Initial experiments showed good results over flat terrain when the radar and target heights are similar. However, the need to consider the non-zero vertical gradient of the refractivity over hilly terrain is clear. Up to date, the methods proposed consider previous estimation of the refractivity gradient in order to correct the measured phases before the refractivity estimation. In this paper, joint estimation of the refractivity variations at the radar height and the refractivity vertical gradient variations using scan-to-scan phase measurement variations is proposed. To reduce the noisiness of the estimates, a least squares method is used. Importantly, to apply this algorithm, it is not necessary to modify the radar scanning mode. For the purpose of this study, radar data obtained during the Refractivity Experiment for H 2 O Research and Collaborative Operational Technology Transfer (REFRACTT_2006), held in northeastern Colorado (USA), are used. The refractivity estimates obtained show a good performance of the algorithm proposed compared to the refractivity derived from two automatic weather stations located close to the radar, demonstrating the possibility of radar based refractivity estimation in hilly terrain and non-homogeneous atmosphere with high spatial resolution.

Horizontal and vertical gradient refractivity joint estimation from weather radar phase measurements

Refractivity variations can be easily obtained from radar phase measurements using based-ground clutter returns under certain basic assumptions such as flat terrain and zero vertical gradient of the refractivity. However, in more realistic conditions, phase measurements can be significantly affected by the vertical gradient variations of the refractivity and the height variations between the different based-ground targets. This paper focuses on retrieving jointly the average horizontal atmospheric refractivity and its vertical gradient variations over hilly terrain using triple targets located at different heights in order to obtain a higher spatial resolution. A linear decrease of the refractivity with the height is assumed. Refractivity measurements were obtained with an S-Band coherent radar during the International H2O Project (IHOP_2002) held in the Southern Great Plains of Oklahoma (United States). Radar results have been compared with in situ measurements obtained from different weather based-ground stations located inside the influence area of the radar. A good correlation between both observations has been found.

Antenna Cross-Polar Requirements for 3-PolD Weather Radar Measurements

The analysis and correction of the bias occurring in weather radar polarimetric measurements is a challenging problem. Polarization coupling due to the cross-polar radiation pattern of the radar’s antenna is known to be responsible for errors in the estimation of the polarimetric covariance matrix, and consequently in the hydrometeor classification and quantification, either when using the ATSR or the SHV method. An alternative method for Doppler and polarimetric measurements based on transmitting three different polarizations (3-PolD) has been proven to provide accurate polarimetric covariance matrix estimates without making any hypothesis about the target polarimetric response or its Doppler spectrum. This method does not reduce the Doppler capabilities or the unambiguous range of the radar despite alternately transmitting 3-PolD. These characteristics have encouraged evaluating the polarimetric parameter biases due to cross-polar radiation when this method is used. Biases are calculated considering reflector antenna systems as well as phased-array antenna systems. The results show that this method may guarantee a tolerable bias level even with a poor co- to cross-polar antenna pattern ratio.

Estimation of the vertical gradient of the atmospheric refractivity from weather radar data using square trihedral corner reflector returns

The aim of this paper is to characterize the atmospheric refractivity variations from radar phase measurements corresponding to responses from different stationary ground-based targets. Phase variations due to the different heights of the targets and the vertical gradient variation of the refractivity are considered. Square trihedral corner reflectors have been used as test targets to remove the uncertainty about the specific position of the targets and enhance the level of the backscattered signal.

Statistical characterization of the ground clutter variability from Dual-Polarization Radar measurements

In this paper, the potential of dual polarization measurements to assess the temporal variability of the backscattered signals from ground clutter is presented. The stationarity of ground clutter is a fundamental requirement to estimate the atmospheric refractivity from measurements of phase variation between responses from different targets at different time instants. It is investigated if the phase difference between the two co-polarized components of the polarimetric radar returns helps to remove the contributions that are not due specifically to the signal variability, such as the variation of the atmospheric refractivity. Polarimetric data are obtained using a C-Band Dual Polarization Weather Radar. The proposed method is compared with three different single polarization methods.