عنوان مقاله [English]
Kermanshah Province is one of the western mountainous provinces located in the middle Zagros Mountains. The rainfall in this province is similar to other mountainous regions in orographic and rugby.
Forecasting rainfall in terms of severity, amount and continuity in the usual way is often not precisely possible and requires very expert forecaster and familiarity with local conditions. This leads to severe, destructive and sometimes catastrophic floods in the province.
Today, weather radars can be a valuable tool for experts to forecast rainfall, provided that they are calibrated in accordance with local conditions and calibrated over time with climate change.
If the calibration of weather radar is accurately conducted, the radar can estimate the amount of rain over the vast areas with good accuracy. Relationship between rain and reflectivity radar exponential show Z=aRb, where a and b are coefficients of radar. The amount of R depends on the factors such as the type of rain, rain season, latitude and topography. Variation is ranged of a in few tens to few hundred, and the range of variation b is 1 to 3. Drop size and distribution of rainfall, the coefficient of this.
Materials and methods
The purpose of this study is to calibrate the Kermanshah weather radar with the conditions of the province, so that the specialists be able to predict data using this radar before any hazard. By the way they can provide warnings to the people and authorities and reduce the damage to the dwellings, historic buildings, administrative centers, urban and rural facilities, farms, humans and animals.
In this study, we have used rain data from November 17 to 18, 2015, and November 30 to December 2, 2016. These rain data have been obtained from the stations including Kermanshah, Eslamabad, Sarpol, Ghasre Shirin, Harsin, Javanroud, Tazabad, Songhor, Ravansar, Ghilan Gharb and Soumar. The stations are located in distance of 30 to 100 kilometers from Kermanshah’s radar.
Results and discussion
For rainfall of November 17 to 18 2015 for all stations, we have different angles of radar beam and the reflectance. The radar beam elevation angle is optimized for each station; a separate line equation was obtained. Using the coefficients of the equation and radar, rain intensity level was determined by Surface Rainfall Intensity (SRI) and the total rainfall was an hour and finally total rain was estimated for each station. Radar estimated rainfall for the first part was more than that measured by rain gauges.
Because the error was considerable in all stations, it is assumed that the size and distribution of rainfall in the first part of rainfall vary with the next step. To solve this problem, rain at all stations was divided into two parts. For the first part of the conflict between radar rainfall and gauge rainfall, we have used common linear equations and coefficients of a and b obtained again for all stations. The results showed that the amount of rainfall was estimated in this way better than before.
Using this relationship, rain is estimated 31 percent to 96 percent and the average total rainfall by radar is estimated about 8.9 to 32.4 millimeters. The value is increased greater than that before calibration. Acquiring the optimum beam angle of the radar for any location is time-consuming and difficult. Therefore, when the time is not enough, it is better for each rainfall radar to obtain a relationship.
The results have indicated that the coefficients of radar for any location and at any time are different from those of gauge. To increase the accuracy of radar rainfall, it is better to obtain each separate equation. However, to obtain a separate equation for each location is the best. Finally, it is suggested that the radar equation coefficients for each region of Iran, which is covered by the weather radar, are calculated, so that they can accurately predict the precipitation and give warnings to the different centers.