# Dual-Polarization Observations of Slowly Varying Solar Emissions from a Mobile X-Band Radar  
[PERSON]  
1 MeteoSwiss, via ai Monti 146, Locarno-Monti CH-6605, Switzerland; [EMAIL_ADDRESS] MeteoSwiss, via ai Monti 146, Locarno-Monti CH-6605, Switzerland; [EMAIL_ADDRESS] MeteoSwiss.ch1 MeteoSwiss. phase in approximately 15 min. Later, [PERSON], [PERSON] and [PERSON] further extended the methodology, but, probably because of the long off-line time (three to five sun-tracking scan programs were generally run and the median value was kept as the observation of the specific day), it only became popular a few decades later, when the data acquisition time became considerably shorter. The method was eventually transferred to the United States weather radar network by [PERSON] and colleagues [7] for use with the S-band, polarimetric upgraded WSR-88D radar. A detailed quantitative comparison of C-band dual-polarization radar-derived solar flux values and accurate reference measurements provided by DRAO was also published in 2015 [8]: the standard deviation of the difference between 7 radar observations, measured during the Site Acceptance Tests of the MeteoSwiss Weissfluhgipfel radar at an altitude of 2850 m, and the (frequency transformed) DRAO reference was as small as 0.09 (0.13) dB for the vertical (horizontal) polarization. These preliminary results were so encouraging and promising that MeteoSwiss decided to implement a fast, semi-automated sun-tracking technique, which has also been applied to the other four polarimetric operational weather C-band radars [9]. With such a semi-automated sun-tracking technique, data acquisition is performed in just a few minutes. Remarkably, this methodology tackles both the relative (standard deviation of the error around the mean) and absolute calibration of the receiver for the first time [8; 9]. The major limitation of the sun-tracking technique is that it requires the weather radar to be off-line for a few minutes. In order to overcome this limitation, a complementary on-line technique that automatically detects and analyzes signals from the sun which are stored in the polar volume radar reflectivity data acquired during the operational weather scan program was conceived [10; 11; 12]. This operational monitoring method has been implemented successfully to determine the electromagnetic antenna pointing [10], and assess receiver stability [11] and the differential reflectivity offset [12]. The results of such a method were first applied successfully during a period of quiet solar flux activity in 2008 [11; 12] and then to an active solar period in 2014 [13]. All the Swiss radars were able to capture and describe the \(\sim\)27-day period of the slowly varying solar component. The technique has allowed a relative calibration and a mutual inter-comparison to be made between the vertical and horizontal channels and has the great advantage of requiring no interruption of the weather surveillance. A recent work by [PERSON] et al. [14] presents further developments of the method: for the daily monitoring of the differential reflectivity bias of polarimetric weather radars, an improved quality control is applied to the solar signals detected during the operational scanning, which efficiently removes rain and clutter contaminated gates occurring in the solar hits. In short, the focus of most of the sun related papers [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14] presented in this introduction has been on relative calibration: horizontal and vertical polarization signals have been (mutually) compared and evaluated versus the DRAO reference signal, mainly in terms of standard deviation of the difference between radar-retrieved and reference flux values. Nevertheless, some authors [8; 9] have presented a novel method aimed at the absolute calibration of the dual-polarization weather radar receivers.