A study of the relationship between precipitable water and observed precipitation over Tehran


Institute of Geophysics, University of Tehran, P.O. Box 14155-6466, Tehran, Iran


To cloud physicists, precipitable water (PW) is one of the most important meteorological quantities. PW is usually measured using radar. However, because of the lack of suitable radar coverage in the Tehran area, the current study is based on radio sound measurements. In this paper a comparison is made between computed PW using thermodynamical diagrams (Skew-T) and precipitation measured by the ground meteorological stations, with a view to constructing a relationship for precipitation forecasting. The PW derived from thermodynamic diagrams for 00 and 12 (GMT) from 1984 to 1998, were analyzed. In addition three sets of relationships for each (GMT) time were found between the measured precipitation and precipitable water for three different precipitable water intervals of (0-15) , (15-30) and (30-45)mm;  all relationships were found to be statistically significant. The results show that the 20-25 mm, precipitable water interval could be selected as a suitable threshold for convective cloud seeding decision in the Tehran area. The results confirmed findings by previous cloud physicists. Furthermore it is concluded that there is a possibility of precipitation occurring in the Tehran area when precipitable water exceeds 7mm. The results also show that if the precipitable water estimated at 00 (GMT), exceeds 40mm, then flood may occur in this area.