واسنجی بارش رادار بر پایه اقلیم کوهستانی غرب ایران (مطالعه موردی: کرمانشاه)

Validation is the relationship of converting reflectance to precipitation rate compared to station precipitation. Based on this, in this study, the radar data of Kermanshah synoptic station and the heavy rains of Kermanshah, Sarpol Zahab, Qasr Shirin, Islamabad, Kangavar, Ravansar, Sanghar, Gilan gharb, Javanrood, Harsin, Somar and Taz-Abad stations, which are within the range 30 to 100 km from the Kermanshah radar, it was checked. In the studied precipitations, for each of the studied points, the optimal height angle of the selection beam and the relationship related to that extraction point and correction coefficients were obtained. By using this relationship, the radar's estimated rainfall increased from 33% to 93% and the total average of the radar's estimated rainfall reached from 9.4 to 31.1 mm, which is only 5 mm less than the actual average. Then, in order to calibrate Kermanshah radar rainfall data based on climatic conditions, Kermanshah province was divided into 3 climatic regions and an equation was calculated for each region, which slightly reduced the accuracy of the estimated rainfall data. The results showed that the radar rainfall can be reliably estimated with the mentioned method, and the total average radar rainfall estimate increased from 9.8 to 24.5 mm. In sum, the findings of the research showed that if the radar coefficients for different regions are corrected correctly, heavy rains can be predicted and thus it is possible to prevent the occurrence of unexpected events.
 
Extended Abstract
 
Introduction
The estimation of precipitation patterns (modeling) is considered one of the fundamental aspects of spatial climate research. Forecasting torrential rains in terms of intensity, amount, and duration is often not accurately possible with conventional methods. It requires highly skilled forecasters who are familiar with local conditions, especially in western Iran, where the weather is mainly orographic and barrage-like, similar to other mountainous areas. This often leads to severe, destructive, and sometimes catastrophic floods in these regions.Today, weather radars are a valuable tool for forecasters in predicting torrential rains, provided they are checked according to local conditions, recalibrated over time, and adjusted in response to climate changes. Rainfall forecasting is carried out using different methods, the most important of which are synoptic forecasting, thermodynamic forecasting, satellite forecasting, and radar forecasting. In the first three methods, it is not possible to accurately determine the exact amount, time, and location of precipitation. Additionally, the observations have time intervals of at least 6 to 12 hours. The main reason for the low accuracy in predicting the amount and location of precipitation is related to the arrangement and large distances between ground stations and their low resolution, resulting in many areas being overlooked and ignored.In radar forecasting, the exact location and amount of precipitation in small areas can be determined with a high degree of accuracy. This is because radar has high resolution and can penetrate clouds to measure the amount of moisture within their layers. The radar can quickly and accurately estimate and calculate the location and amount of precipitation caused by different clouds. However, the amount of precipitation measured by radar can differ from the amount received on the ground. This discrepancy has several causes, some of which are related to the nature of radar technology itself, while others are linked to the weather conditions of each region and the characteristics of the earth's surface.The current research aims to investigate and analyze the relationship between the rainfall measured by radar and the rainfall measured by ground stations. This will enable quick precipitation estimates by applying corrected relationships to the data recorded from the Kermanshah weather radar. In line with the objectives of the current research, validating the Kermanshah meteorological radar against the province's conditions will assist forecasters in predicting rainfall characteristics before they occur, thereby enabling them to issue necessary warnings to the public and authorities. Finally, these estimates will help identify high-risk areas.
 
Methodology
In this research, data from rain gauges located within a horizontal distance of 30 to 100 km from the Kermanshah radar were used to correct the coefficients. After qualitatively and quantitatively controlling the rainfall data, ten stations (Kermanshah, Sarpol-Zahab, Qasr-Shirin, Islamabad, Kangavar, Ravansar, Sanghar, Gilan-Gharb, Harsin, and Tesh-Abad) were selected. Other stations were deemed unsuitable for this research due to a lack of statistical continuity, inaccuracies in the rain gauges, and obstruction of the radar beam.To facilitate the analysis, a geographic coordinate grid was created in MATLAB software, covering the range of the Kermanshah radar, with each pixel representing one square kilometer. Consequently, an 800 x 800 matrix was created for the radar data. Two rainfall events, occurring between November 4th and March 21st, 2015, which had a greater horizontal extent as well as significant intensity and amount, were selected for this study. Subsequently, Kermanshah province was divided into three climatic regions, and a general formula was derived for each region.
 
Results and Discussion
The findings of the research showed that the radar precipitation can be reliably estimated with the mentioned method, and the average total radar precipitation estimate increased from 9.8 to 24.5 mm. In sum, the findings of the research showed that if the radar coefficients for different regions are corrected correctly, heavy rains can be predicted and in this way it is possible to prevent the occurrence of unexpected incidents.
 
Conclusion
In this research, data collected from rain gauges located within a horizontal distance of 30 to 100 km from the Kermanshah radar were used to correct the coefficients. The precipitation dates were selected to effectively demonstrate the distribution and characteristics of precipitation throughout the year, thereby showcasing the performance of the meteorological radar. Based on climatic conditions, the study area was divided into three regions, and a final equation was obtained for each.The research results showed that the average value of radar parameter a ranged from 15 to 42, with an average value of 27. The range of parameter b was between 1.05 and 1.6, with an average value of 1.24. Although the accuracy of rainfall estimation decreased, the overall accuracy remained at 81%, a 12% reduction. This method, with an error of less than 19%, provided estimates for the amount, intensity, and distribution of precipitation. Given the radar's coverage range, the dispersion of rain gauge stations, and the radar's resolution, which provides numerical values for each square kilometer, this method proved very beneficial.Using this method, the amount and intensity of precipitation can be directly obtained as the rain begins, utilizing new parameters. The study concluded that radar-estimated precipitation using default radar gauges, which averaged 36.9% of the actual precipitation, is not particularly useful. However, radar-estimated rainfall using the average of the parameters, which averaged 83.1% of the actual rainfall, can be valuable for determining runoff and issuing immediate flood warnings. Furthermore, radar-estimated rainfall using specific parameters for particular rainfall events and locations, which showed an average of 2.89% of the actual precipitation, can be highly useful.The research also indicated that by adjusting the radar coefficients for each region and season, the radar precipitation estimate becomes closer to the actual value. If Kermanshah radar experts and officials use the formulas obtained from this research, they can make accurate forecasts before rainfall occurs and issue necessary warnings to relevant departments and organizations to prevent accidents caused by heavy rains.Finally, it is recommended that the coefficients of the radar equation be calculated for each area covered by the meteorological radar to accurately predict rainfall and provide necessary warnings to different centers.
 
Financial sponsor
According to the responsible author, this article has no financial sponsor.
 
Contribution of the authors to the research
First author: supervisor
Second author: data analysis, compilation of findings and conclusions.
 Third author: writing the introduction and literature review and research records and final review
 
Conflict of interest
The authors declare that they have no conflict of interest in writing or publishing this article.
 
Appreciation and thanks
The authors sincerely thank and appreciate the cooperation of the General Department of Meteorology of Kermanshah province for their cooperation in collecting the required data.