Procedures for improved weather radar data quality control

[eng] Weather radar data and its downstream products are essential elements in weather surveillance and key parameters in the initialisation and validation of hydrological and meteorological models, among other downstream applications. Following the quality standards established by the European and global weather radar networking referents, the present thesis aims for the improvement of the base data quality control in the regional weather radar network operated by the Meteorological Service of Catalonia, the XRAD. This objective is accomplished through the analysis, development and implementation of new or existing procedures and algorithms for radar data quality assessment and improvement. Attending to the current radar technology and to the already implemented quality control procedures for the XRAD, the work is focused on the continuous evaluation of the radar system calibration status and on the correction of Doppler velocity data. The quality control algorithms and recommendations presented are easily translatable to any other operative weather radar networking environment. A Sun-based, fully automatic procedure for online monitoring the antenna alignment and the receiver chain calibration is adapted and operationally implemented for the XRAD. This Sun-monitoring technique was developed at the Royal Netherlands and Finnish Meteorological Institutes and is included in the quality control flow of numerous weather radar networks around the world. The method is modified for a robust detection and characterisation of solar interferences in raw data at all scan elevations, even when only data at relatively short ranges is available. The modified detection algorithm is also suitable for detecting interferences from wireless devices, which are stored for monitoring their incidence in the XRAD. The solar interferences detected, in turn, are input observations for the inversion of a two-dimensional Gaussian model that yields estimates of the calibration parameters of interest. A complete theoretical derivation of the model establishes its validity limits and provides analytical estimates of the effective solar widths directly from radar parameters. Results of application of this Sun-monitoring methodology to XRAD data reveal its ability to determine the accuracy of the antenna pointing and to detect changes in receiver calibration and radar system operation status. In order to facilitate the usage of the Sun-monitoring technique and the interpretation of its estimates, the methodology is reproduced under controlled conditions based on the distributions of solar observations collected by two of the XRAD radars. The analysis shows that the accuracy of the estimated calibration parameters is conditioned by the precision, number and distribution of the solar observations which constitute key variables that need to be controlled to ensure reliable estimates. In addition, the Sun-monitoring technique is compared under actual operative conditions with two other common techniques for quantifying the antenna azimuth and elevation pointing offsets. Pointing bias estimates gathered in a dedicated short-term campaign are studied in a direct inter- comparison of the methods that reflects the advantages and limitations in each case. The analysis of the bias estimates reported by the methods in the course of a one-year period reveals that the performance of the techniques depends on the antenna position at the time of the measurement. After this study, a reanalysis of the Sun-monitoring method results is proposed, which allows to additionally quantify the antenna pedestal levelling error. Finally, a post-processing, spatial image filtering algorithm for identification and correction of unfolding errors in dual-PRF Doppler velocity data is proposed. The correction of these errors benefits the usage of radar velocity data in downstream applications such as wind- shear and mesocyclone detection algorithms or assimilation in numerical weather prediction models. The main strengths of the proposed algorithm, in comparison with existing correction techniques, are its robustness to the presence of clustered unfolding errors and that it can be employed independently of post-processing dealiasing algorithms. By means of simulated dual-PRF velocity fields, the correction ability of the algorithm is quantitatively analysed and discussed with particular emphasis on the correction of clustered errors. The quality improvement in real dual-PRF data brought out by the new algorithm is illustrated through application to three selected severe weather events registered by the XRAD.[cat] Seguint els estàndards de qualitat establerts per a les xarxes de radars meteorològics de referència a nivell europeu i global, la present tesi té com a objectiu la millora del control de qualitat de les dades de la xarxa regional de radars meteorològics operada pel Servei Meteorològic de Catalunya (la XRAD). Atenent als procediments de control de qualitat ja implementats per a la XRAD, el treball es centra en l'avaluació contínua de l'estat del calibratge del sistema radar i en la correcció de les dades de velocitat Doppler. Es presenta l'adaptació i aplicació d’un procediment totalment automàtic basat en el Sol, que permet la quantificació remota dels errors d'alineació de l'antena i de calibratge en recepció del radar a la XRAD. El mètode ha estat modificat per a la detecció i caracterització robusta d'interferències solars a les dades primàries de radar. Les interferències solars són utilitzades per a la inversió d'un model físic que proporciona estimacions dels paràmetres de calibratge d'interès. L'algoritme de detecció modificat també és adequat per a la identificació d'interferències procedents de dispositius electrònics externs. Aquestes interferències són emmagatzemades per al seguiment de la seva incidència a la XRAD. La metodologia solar esmentada es modelitza en condicions controlades a partir de la distribució de les observacions solars recollides per dos dels radars de la XRAD. L'anàlisi mostra que la precisió, el nombre i la distribució de les observacions solars constitueixen variables clau que necessiten ser controlades per garantir estimacions fiables dels paràmetres de calibrage. A més, la tècnica solar es compara, sota condicions operatives reals, amb altres dues tècniques habitualment emprades per a la quantificació de l'error d'apuntament de l'antena. A partir d'aquest estudi, es proposa un nou mètode d'anàlisi de les interferències solars, el cual permet quantificar l'error d'anivellament del pedestal de l'antena. Finalment, es desenvolupa i valida un algoritme de filtrat d'imatges per a la identificació i correcció dels errors característics que es donen lloc a les dades dual-PRF de velocitat Doppler. Els punts forts de l'algoritme proposat, en comparació amb les tècniques de correcció existents, són la seva robustesa en la correció d'errors agrupats i que pot emprar- se amb independència dels algoritmes de dealiasing. La millora de la qualitat de les dades reals de velocitat s'il·lustra mitjançant l'aplicació de l’algoritme a tres episodis de temps sever enregistrats per la XRAD

Quality control of antenna alignment and receiver calibration using the sun: adaptation to midrange weather radar observations at low elevation angles

A quality control method for combined online monitoring of weather radar antenna pointing biases and receiver calibration using solar signals detected by an operational radar is adapted for application to midrange radar data (80-150 km). As the original method was developed using long-range data, additional criteria based on robust statistical estimators are imposed in the sun signature detection and selection process, allowing to discard observations biased by ground clutter or precipitation and to remove very influential outliers. The validity ranges of the physical model describing the solar interferences detected by the scanning radar antenna are explicitly defined and an equation for estimation of the effective scanning width in reception is provided in a thorough theoretical derivation. The method proposed reveals its sensitivity to changes in the antenna pointing accuracy and receiver calibration when applied to operational data obtained with three C-band radars during one year. A comparative study on the goodness of fit between a three- and a five-parameter model highlights the effect on the stability and accuracy of the antenna and receiver parameters retrieved for each radar system, considering the dissimilar information content of the observations collected by each radar. The performance of the proposed methodology under the effects of the presence of ground clutter and radio local area network interferences is discussed in the results presented

Inter-comparison and potential synergies of three methods for weather radar antenna pointing assessment

Three methods for estimation of the weather radar antenna azimuth and elevation pointing offsets are compared. Two of the methods reviewed use the known location of the sun as a reference. The first of these methods is based on an offline scan of the sun disk. The second method detects and characterizes solar interferences in operative scans. The third method consists of correlating measured ground clutter echoes with echoes simulated using a high-resolution digital elevation model. The main objectives are to review the characteristics in each case, studying their performance in actual operative conditions, and to examine the reasons for the discrepancies between the reported pointing bias estimates, with the aim of laying the groundwork for an optimized individual or combined application and interpretation of the methods. Daily pointing biases estimated through the sun-scanning procedure in a dedicated one-month, short-term campaign are the base for the intercomparison. When applied to the three weather radars operated by the Meteorological Service of Catalonia, the short-term study reveals the advantages and limitations of the methods. A one-year, long-term analysis serves to confirm and clarify the discrepancies inferred from the short-term study and highlights how the antenna position at the time of the measurement may influence the pointing bias estimates. Based on the long-term results, a combination of the two sun-based methods for detection and simultaneous quantification of the pointing bias and the system leveling error is discussed

Inter-comparison and potential synergies of three methods for weather radar antenna pointing assessment

Three methods for estimation of the weather radar antenna azimuth and elevation pointing offsets are compared. Two of the methods reviewed use the known location of the sun as a reference. The first of these methods is based on an offline scan of the sun disk. The second method detects and characterizes solar interferences in operative scans. The third method consists of correlating measured ground clutter echoes with echoes simulated using a high-resolution digital elevation model. The main objectives are to review the characteristics in each case, studying their performance in actual operative conditions, and to examine the reasons for the discrepancies between the reported pointing bias estimates, with the aim of laying the groundwork for an optimized individual or combined application and interpretation of the methods. Daily pointing biases estimated through the sun-scanning procedure in a dedicated one-month, short-term campaign are the base for the intercomparison. When applied to the three weather radars operated by the Meteorological Service of Catalonia, the short-term study reveals the advantages and limitations of the methods. A one-year, long-term analysis serves to confirm and clarify the discrepancies inferred from the short-term study and highlights how the antenna position at the time of the measurement may influence the pointing bias estimates. Based on the long-term results, a combination of the two sun-based methods for detection and simultaneous quantification of the pointing bias and the system leveling error is discussed