عنوان مقاله [English]
The aim of this study was to investigate cloud’s characteristics and some effective physical indices on rainfall in Mashhad. We initially studied cloudy sky in Mashhad by consulting meteorological yearbook of 1986-2010. Density and prevalence for medium and low clouds as well as density, prevalence and average for vertical clouds (type 2 cumulus and cumulonimbus) were calculated for every month. This study shows that in Mashhad the highest frequency of low and medium clouds occurs in cold and rainy months during winter and spring, while most of the clouds with vertical development occur in spring. We also studied freezing level in clouds in both rainfall and no-rainfall cases, using skew-T diagrams and radiosonde data during 1992 to 2011. The condition considered here was overcast sky. We obtained a regression equation in order to specify role of physical indices (CAPE، SI, K، PWT) in precipitation. At first, rainfall days of Mashhad were extracted from Khorasan Razavi meteorological department archive (1992 to 2011). On these days, six hour rainfall (00 to 06 Z) were determined. Then, instability and physical indices were obtained from radiosonde data of Mashhad weather station. A regression equation was estimated by analyzing the relationship between natural logarithm of six-hour rainfall as dependent variable and instability indices as independent variable. This research determines which relationship between some indices with natural logarithm of six-hour rainfall is nonlinear. Analysis of regression variance shows that regression was significant at 99% level. This means that at least one of the indices has a linear relationship with logarithm of six-hour rainfall. In this equation, due to significant level, CAPE and PWT were removed. Most relationships were found between SI and natural logarithm of six-hour rainfall.