Meteorological radars estimate the precipitation using a reflection-precipitation relation as Z=aR^b with a, b coefficients. These coefficients will change from one precipitation to the other one. For the Tehran radars they are considered as a=200 and b=1.6. These values are proper when we have moderate precipitations. This assumption causes an error in the estimation of different precipitation which is done by radars. In order to know how the evaluations of rain are done by Tehran radars, we consider the amount of rain in three chosen rain recording stations which are Qom, Kooshk Nosrat and Pakdasht.
Our aim of these studies is to evaluate the estimation of the Tehran meteorological radars for different amounts of precipitations.  Therefore three intervals, including 11/1/2010-11/4/2010, 1/8/2011-1/11/2011 and 1/15/2011-1/18/2011 have been chosen in which the precipitations have been reported.
The primary results indicate that Tehran meteorological radars estimate the amount of the precipitation less than the amounts which are registered by the rain – gauge of meteorological stations. These differences become more when the precipitation rates are considered. To amend the estimated values of the precipitation which are taken by Tehran radars, these amounts are evaluated by data which are given by rain recovery stations and finally are rescaled by the logarithmic relation Z-R. Using this relation new values for the concerned coefficients are obtained for different dates of precipitation. After depicting the linear regression equation and getting new coefficients to estimate the intensity of rain, we can access to a plot which indicates how the precipitation will change by employing the required corrections. We are also able to plot the precipitation changes in the used rain recording stations. The results indicate that we achieve good progress of estimations if we use new coefficients. Comparison of the gathered rain data by radars with the data of the rain recording stations shows a 40% agreement between them. If we use new coefficients, this agreement will increase to 90% which confirm that these new coefficients are appropriate in the evaluation equation of precipitation.
Results of this study illustrated that the error of the radar lies in two factors, the first one is that the raindrops absorb some of the radar signals and this causes that return signals to be reduced; therefore more rainfall intensity, weakens the wave reflections The second reason is that the reflectivity values are proportion to sixth power of raindrops diameters and this means that the reflection of radar waves is affected by the droplet size and the larger droplet diameter increases the reflection, while the rainfall measured by rain gauge is not affected by droplet size, and value of the recorded rainfall or precipitation volume is not affected. In other words, the radar estimates precipitation more than the actual amount occurring in rainfall with larger drop size while estimates are less than the actual amount of precipitation in rainfall with smaller drop size.