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

Analysis of energetic radiation associated with thunderstorms in the Ebro delta region in Spain

The analysis of high-energy background radiation (0.1 – 2 MeV) enhancements during eight winter thunderstorms and ¿ve summer storms in the Ebro delta region in the northeast of Spain is presented. For the ¿rst time, high-energy radiation counts, precipitation, radar re¿ectivity, and very high frequency lightning detections to infer charge regions altitude have been analyzed in order to ¿nd out what produces the measured background radiation increments associated with storms. The good agreement between radar re¿ectivity and precipitation with increases in background radiation counts coupled with the spectrum analysis comparing rain/no rain periods suggests that radon-ion daughters play a major role in the radiation increments reported. No evidence has been found supporting that measured background radiation enhancements can be produced by storm electric ¿elds. Finally, a single case of a high-energy radiation increase was prior to a cloud-to-ground lightning stroke, which reinforces the theory that a lower positive charge layer’s existence is important for the production of Terrestrial Ground Enhancements.Peer ReviewedPreprin

X-rays and microwave RF power from high voltage laboratory sparks

Lightning flashes involve high energy processes that still are not well understood. In the laboratory, high voltage pulses are used to produce long sparks in open air allowing the production of energetic radiation. In this paper X-rays emitted by long sparks in air are simultaneously measured with the RF power radiation at 2.4 GHz. The experiment showed that the measured RF power systematically peaks at the time of the X-rays generation (in the microsecond time scale). All of the triggered sparks present peaks of RF radiation before the breakdown of the gap. The RF peaks are related to the applied voltage to the gap. RF peaks are also detected in discharges without breakdown. Cases where X-rays are detected presented higher RF power. The results indicate that at some stage of the discharge, before the breakdown, electrons are very fast accelerated letting in some cases to produce X-rays. Microwave radiation and X-rays may come from the same process.Preprin

Basic lightning flash properties derived from lightning mapping array data

The size and duration of lightning flashes are examined. Data from the Ebro Valley Laboratory Lightning Mapping Array is used as reference. Additional data from the VLF/LF LINET network is included. In the analysis, each flash is simplified by a confidence ellipse fitting most of the detected sources. The major axis of the ellipse is adopted as the flash length. Flash durations are computed too. The analysis of 778 flashes results in a median flash length of ¿14 km with a median duration of ¿0.3 s. The results presented, besides characterizing the storm activity, they can be useful to define stroke grouping criteria, lightning flash density calculations and lightning warning purposes.Postprint (published version

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

Lightning mapping observations of downward lightning flashes to wind turbines

Negative downward leaders that produced lightning strokes to wind turbines are identified by means of the Lightning Mapping Array data of the Ebro Valley Laboratory (NE Spain). Four cases are analyzed together with weather radar imagery. All flashes hitting wind turbines were originated in small convective cells with moderate development. Notwithstanding the moderate convection, all cases occurred under what can be called “out of season conditions”, where the ‘‘charging zone’’ is located closer to the ground and may favour downward leaders to tall structures. Cloud charge structures showed negative region from 3 km to more than 5 km with a low positive charge below (not always detectable)

Analysis of energetic radiation associated with thunderstorms in the Ebro delta region in Spain

The analysis of high-energy background radiation (0.1 – 2 MeV) enhancements during eight winter thunderstorms and ¿ve summer storms in the Ebro delta region in the northeast of Spain is presented. For the ¿rst time, high-energy radiation counts, precipitation, radar re¿ectivity, and very high frequency lightning detections to infer charge regions altitude have been analyzed in order to ¿nd out what produces the measured background radiation increments associated with storms. The good agreement between radar re¿ectivity and precipitation with increases in background radiation counts coupled with the spectrum analysis comparing rain/no rain periods suggests that radon-ion daughters play a major role in the radiation increments reported. No evidence has been found supporting that measured background radiation enhancements can be produced by storm electric ¿elds. Finally, a single case of a high-energy radiation increase was prior to a cloud-to-ground lightning stroke, which reinforces the theory that a lower positive charge layer’s existence is important for the production of Terrestrial Ground Enhancements.Peer Reviewe

Lightning flash properties derived from lightning mapping array data

in this paper size and duration of lightning flashes are examined. Data from the Ebro Valley Laboratory Lightning Mapping Array is used as reference. Additional data from the VLF/LF LINET network and meteorological radar is included. In order to simplify the complex geometry of a lightning flash, each flash is described by a confidence ellipse fitting most of the detected sources. The flash length is represented by the major axis of the ellipse. The analysis of 778 flashes results in a median flash length of 14 km with a median duration of 0.3 s. The results presented here, besides characterizing the storm activity, provide valuable information in order to improve the stroke grouping criteria in the flash algorithm, lightning flash density calculations and lightning warning procedures.Peer ReviewedPostprint (published version

X-rays and microwave RF power from high voltage laboratory sparks

Lightning flashes involve high energy processes that still are not well understood. In the laboratory, high voltage pulses are used to produce long sparks in open air allowing the production of energetic radiation. In this paper X-rays emitted by long sparks in air are simultaneously measured with the RF power radiation at 2.4 GHz. The experiment showed that the measured RF power systematically peaks at the time of the X-rays generation (in the microsecond time scale). All of the triggered sparks present peaks of RF radiation before the breakdown of the gap. The RF peaks are related to the applied voltage to the gap. RF peaks are also detected in discharges without breakdown. Cases where X-rays are detected presented higher RF power. The results indicate that at some stage of the discharge, before the breakdown, electrons are very fast accelerated letting in some cases to produce X-rays. Microwave radiation and X-rays may come from the same process