| Doppler weather observing data is not a forecasting model explicit variable, in order to add Doppler weather radar data into mesoscale numerical models, we need to transform radar observing data into model variables. In this paper the relationship between radar reflectivity of rainfall, also called radar echo intensity, and relative humidity of atmosphere was studied, so that radar echo intensity data can be used in numerical models by transforming it into relative humidity, and increase the accuracy of mesoscale forecast.Firstly, some features of relative humidity of atmasphere was analyzed in this paper. Saturation vapor pressure in the pure phase with respect to water or to ice is a function of temperature, both increase with the 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 between saturation vapour pressure of ice and water △ E= E _w - E_i. is also a function of temperature, and thesupersaturation △S = △E/E ; increase when temperature decrease. When the vapor of airis saturated with respect to ice, the relative humidity will drop from 100% to 60% as temperature decrease 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 soliddroplets 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 with cold cloud in the part with temperature below 0℃.In this paper, the observing data of Datian, Youxi, Jianning, Ninghua, Jiangle, Yong'an meteorological stations in Fujian province in 2003 was used. Rainfall information at the time of humidity observing was obtained from weather phenomenon data and rainfall record. First rainfall intensity was calculated by using hour-to-hour amount of precipitation data from automatic weather station. Next, rainfall intensity was converted into the radar reflectivity through Z-I relationship, and then get a data array of radar reflectivity and relative humidity. Analyzing these data arrays we can see that: 1. the relative humidity observed by surface synoptic stations is not 100 % when it's raining. 2. the relationship between radar reflectivity of rainfall 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 of each station was obtained as follow:Datian: f= 81.661 +0.2843 · dbz Jianning: f= 81.587 +0.3687·dbzYouxi: f= 81.756+ 0.2763·dbz Yong'an: f= 86 + 0.2588 · dbzJiangle: f = 81.26 + 0.2843 · dbz Ninghua: f = 83.066 + 0.3237 · dbzThe fitting linear relationship between radar reflectivity and relative humidity of all the stations is:f = 83.7482681+ 0.2777882· dbzThe linear relationship varies with the temperature. Classify the relationship by temperature, we get the following relationships:5.0-19.8 ℃ : f= 91.802069+ 0.1181334·dbz19.9-22.9 ℃: f= 89.1717078 + 0.1355859 · dbz 23.0~25.9 ℃: f= 84.8248638 + 0.1670182· dbz 26.0-26.9 ℃: f= 79.1646627+ 0.2244879·dbz 27.0-28.2 ℃: f = 74.4652529 + 0.2743671 · dbz 28.2-29.9 ℃: f= 69.18536+ 0.28124955·dbz Based on the discussion above, we get some conclusions: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 on the grid points of each height of the model and decide the values of the echo intensity...
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