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《Chinese Journal of Atmospheric Sciences》 2006-02
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A Study of the Relationship Between Radar Reflectivity of Rain and Relative Humidity of Atmosphere

QIN Yan-Yan~(1,2),LI Bai~(1),and ZHANG Pei-Yuan~(1)1 Chinese Academy of Meteorological Sciences,Beijing 1000812 Nanjing Institute of Meteorology,Nanjing 210044  
Doppler weather observational data is not a forecasting model explicit variable,in order to add Doppler weather radar data into mesoscale numerical models,the radar observational data are transformed into model variables.The relationship between radar reflectivity of rain,also called radar echo intensity,and relative humidity of atmosphere is studied,so that radar echo intensity data can be used in numerical models by transforming it into relative humidity,and the accuracy of mesoscale forecast is enhanced.Firstly,some features of relative humidity of atmosphere is analyzed.Saturation vapor pressure in the pure phase with respect to water or ice is a function of temperature,and both increase with temperature.And saturation vapor pressure with respect to water is higher than that with respect to ice at the same temperature,that is when the vapor of atmosphere is saturated with respect to ice,it is not saturated with respect to water.Difference of saturation vapour pressure between ice and water ΔE is also a function of temperature,and the supersaturation ΔS increases when temperature decreases.When the vapor of air is saturated with respect to ice,the relative humidity will drop from 100% to 60% as temperature decreases from 0 ℃ to-50 ℃.In the warm cloud,the relative humidity is 100%.In the cold cloud,when the droplets are all solid water,the vapor of air is saturated with respect to ice,and the relative humidity is a function of temperature;when the droplets are all supercooled water,the vapor of air is saturated with respect to water,and the relative humidity is 100%;when solid droplets and liquid droplets coexist in the same place of the cloud,the relative humidity is lower than 100%.In the mixed cloud,the relative humidity is 100% in the part with temperature above 0℃,and has the same situation as cold cloud in the part with temperature below 0℃.The observational data at Datian,Youxi,Jianning,Ninghua,Jiangle,Yong'an meteorological stations in Fujian Pro-vince in 2003 are used.Rainfall information at the time of humidity observation is obtained from weather phenomenon data and rainfall record.First rain intensity is calculated by using hour-to-hour amount of precipitation data from automatic weather station.Next,rain intensity is converted into the radar reflectivity through ZI relationship,and then a data array of radar reflectivity and relative humidity is got.Analyzing these data arrays it shows that:(1) the relative humidity observed by surface synoptic stations is not 100% when it is raining.(2) The relationship between radar reflectivity of rain and the simultaneous relative humidity of atmosphere can be fitted approximately by a line.By using univariate linear regression,the fitting linear equation of radar reflectivity and relative humidity is obtained.The linear relationship varies with temperature.The relationships classified by temperature are also got.Based on the above discussion,some conclusions are obtained:(1) When induct radar echo intensity into numerical models by transforming it into relative humidity,the relationship of radar echo intensity and relative humidity can be taken as follow: determine whether there are any radar echoes at the grid points of each height of the model and decide the values of the echo intensity by using radar volume scanning data.Then decide height of cloud base,height of zero-temperature level and temperature of each level by radiosonde data. By using radar echo intensity data,relative humidity under the cloud base of each grid points can be decided according to the relationship established in this paper.Relative humidity at the grid points between cloud base and zero-temperature level can be taken as 100%.And those above the zero-temperature level can be decided by temperature at the corresponding grid points.(2) Under the most circumstances relative humidity is not 100% in the corresponding precipitation region when rainfall is observed by ground station.(3) Statistical analysis of observational data at Datian,Youxi,Jianning,Ninghua,Jiangle,Yong'an meteorological stations in Fujian Province in 2003 shows:(1) the relationship between radar reflectivity of rainfall and the simultaneous relative humidity of atmosphere can be fitted approximately by a line.The heavier rain is,the higher the value of relative humidity will be.(2) The corresponding relationship varies with temperature.
【Fund】: 国家自然科学基金资助项目40375006
【CateGory Index】: P457.6
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