Precipitable water (PW) is an important meteorological quantity that cloud physics scientists have paid special attention to it. In fact, PW can be an estimation of the total column water vapor in the atmosphere which is the result of convergence of water vapor in the lower atmosphere. Awareness of the relationship between this quantity and rainfall amount and intensity is important in predicting the atmospheric conditions. In most part of the world, PW is measured with weather radars and satellites.
    The first purpose of this research is to find a relationship between precipitation and cloud precipitable water by utilizing Tehran weather radar data. The second purpose is checking the accuracy and skill of WRF model in forecasting and simulating the PW value and its changes. For fullfilling this purpose, particular rain gauges with the ability to record rainfall in short intervals (e.g. 1 hour intervals) are needed. Thus, three automatic rain gauges of Meteorological Organization (Pakdasht, Kooshk-e-Nosrat and Qom) were chosen. The Rainbow software was used to illustrate the Pakdasht, Kooshk-e-Nosrat and Qom Radar data. Desired products of radar data for this research were Surface Precipitation Intensity (SRI) and Vertical Integrated Liquid (VIL) (Actually VIL product is cloud precipitable water). For running the WRF model, three domains were considered including a 36 km horizontal range of parent domain and two nest domains with ranges of 12 and 4 km.  It is worth noting that the relation between cloud precipitable water (CPW) and precipitation of radar measurements was investigated in the time period of 11/1/2010-11/4/2010.
    The following results were obtained by comparing the quantity of the cloud precipitable water (CPW) and radar surface rainfall intensity (SRI) in three considered stations. It was found that before the beginning of the rainfall, the amount of CPW of cloud always increases, and then with passing the amount of CPW through the 0.1 mm at that station, the radar shows SRI product on its display screen; this means that the rainfall is recorded by radar. After starting of the precipitation, the general trends of rainfall amounts would follow the trends in precipitable water values. However, the radar recorded a less SRI product at Kooshk-e-Nosrat and Qom stations compared with Pakdasht station for similar amounts of CPW. Meanwhile, the WRF model predictions for PW were compared with observations of this quantity by the weather Radar. Finally, it should be noted that the outcomes of model predictions for PW and Radar observations for CPW in the region under the cover of Tehran Weather Radar showed a very high accuracy (significant correlation at 5% significance level) in temporal-spatial forecasting and also in model simulation outputs for the changes of PW quantity. The results also revealed that whenever predicted PW by model having values more than 20 mm, radar observations displayed values more than 0.1 mm for the CPW.