Relationship between Types of Precipitation on the Ground and Surface Meteorological Elements

Abstract

Routine surface meteorological data at three weather stations in Japan, i.e., Wajima (sea shore), Matsumoto (inland), and Nikko (inland), were analyzed in order to obtain the relation-ship between the occurrence of the types of precipitation of snow, sleet, and rain on the ground and surface meteorological elements. The results indicate that the types of precipi-tation were dependent on surface relative humidity as well as surface air temperature. Even at the same air temperature, the precipitation types varied depending on humidity, and that critically. Two critical humidities were recognized at a given air temperature and the precipi-tation types were characterized by these critical humidities. Below a lower critical humidity, precipitations were all snow. Within some range above the critical humidity, they could be snow, sleet, or rain. Above a higher critical humidity on the top of the range, only rains were observed. These facts suggest that melting of snowflakes proceeds rapidly at higher relative humidity.
From statistical analyses, two critical humidities for snow and rain (%) were obtained as a function of surface air temperature (°C). The obtained relations at each station were as follows: RH_cri(snow)=-7.5T+93, RH_cri(rain)=46√6.2-T, Wajima RH_cri(snow)=-7.3T+96, RH_cri(rain)=39√7.2-T, Matsumoto RH_cri(snow)=-6.2T+91, RH_cri(rain)=43√6.8-T, Nikko where RHcri (snow) is a lower critical humidity and RHcri (rain) a higher critical one.
The relations for snow are hardly different among the observation stations, but those for rain are different. The difference of the relations for rain suggests that the size and density of snowflakes are different from station to station and that the difference affects the speed of the generation of raindrops due to melting. The non-difference of the relations for snow, on the other hand, suggests that the, difference in physical properties of snowflakes does not exert any influence on the relations for snow.
Sleet was obtained in the transition region surrounded by two relations for snow and rain and its occurrence was associated with high precipitation rate together with air tempera-ture and humidity.
These analytical results are explained well using the calculated results previously obtained by Matsuo and Sasyo (1981,a).