The assessment of the influence of ground clutter on incorrect reflectivity appearance in Guilan meteorological radar products

The assessment of Guilan’s meteorological radar data (PPI and SRI) during autumn 2013 and winter 2014 shows the presence of a large number of pixels, mostly located in Guilan mountainous areas, involving incorrect reflectivities. For example, the daily average of incorrect reflectivity on the 26th of December 2014 calculated by PPI (plan position indicator) products of Guilan’s meteorological radar resulted in 47.014 dBZ in Lahijan synoptic station which would be equal to 31.6 mm/hr rain rate through Marshall-Palmer Z-R relationship. The importance of these results could be appreciated when we note that Lahijan synoptic station had a daily gauge rainfall height equal to zero on the same day. Thus, according to the position of incorrect reflectivities of Guilan’s meteorological radar, this study aims to evaluate the ground clutter influence on incorrect reflectivity appearance. The method used is to study the overlapping of the beam blocking map and the daily average of incorrect reflectivity map by changing the elevation angle of the radar beam at a clear air day. The results show that the ground clutter is the main factor responsible for incorrect reflectivities in %75.34 pixels of the image. Using comparison of the mean distance of the blocked pixels and the pixels containing the ground clutter to the radar antenna leads us to conclude that the real amount of the elevation angle is equal to -0.20°. The method adopted by assuming 5 km donut-shape regions around the radar antenna and using it to determine the radar elevation angle is one of the innovations of this study. Furthermore, to ensure the working of the actions undertaken for the clutter contamination mitigation, one requires a numerical threshold to determine the effectiveness of the actions. For this purpose, therefore, a method is developed based on the correlation between the radar’s 24-hr rainfall heights and their respective gauge data. The method provides a maximum threshold in dBZ unit for the clutter contamination while Z = 200 R1.6 is applied. The numerical thresholds resulted from the application of the method for the meteorological stations of Rasht (the Airport), Rasht (the Agriculture Faculty), Anzali, Talesh, Lahijan, Jirandeh, Masouleh, Deylaman and Manjil are equal to -3.748, -1.638, -6.074, 14.952, -3.611, -1.482, 12.466, 4.872 and 6.852 in dBZ, respectively. Also, a value of 9.536 dBZ is obtained for all the stations considered together. The latter value can be extended to all pixels of the radar image. As the conclusion, in spite of the fact that on the 26th of December 2014 as a non-rainy day, the average clutter was considerable, a close correlation between the radar’s rainfall heights and their respective gauge data is observed during the 15 days studied. The Pearson correlation coefficients (Pcc) in the meteorological stations of Rasht (the Airport), Rasht (the Agriculture Faculty), Anzali, Lahijan, Jirandeh, Deylaman and Manjil are significant at 1% statistical significance level, and also Pcc values in the meteorological stations of Talesh and Masouleh are larger than that can be excluded. Therefore, the clutter contamination of Guilan’s meteorological radar data cannot be considered an impediment to its application in practical purposes. However, for further progress, a higher degree of accuracy would be required.