By studying the characteristics of ice pellets,it is possible to understand freezing weather more comprehensively.Such research also contributes to freezing-weather warning and forecasting capabilities.However,due to a lack of observational data,among other concerns and reasons,there has been little relevant research on ice particles in China.Accordingly,in the present study,a comprehensive analysis of the characteristics of the spatiotemporal distribution and duration of ice-pellet precipitation,along with associated ground-level meteorological variables,was conducted using data from hourly or half-hourly surface observations and special observations of Chinese civil airports during the years 2011-2013.By matching the airport ground observations from airports with radiosonde data during the study period,the characteristics of the vertical atmospheric environment were analyzed.
It was found that latitude had a very important influence on the distribution of ice pellets.Ice pellets were mostly distributed in midlatitude areas,including the lower reaches of the Yellow River and the middle and lower reaches of the Yangtze River.There was no substantial correlation between the occurrence and duration of ice pellets.Events may last for a relatively long time where ice-pellet precipitation is rare.Ice-pellet precipitation occurred most often from January to February,and regional differences were found to exist.Ice pellets occurred frequently in January in southern and southwestern regions;and in February in midlatitudes.In the northeast,the peak month for ice pellets was April,while it was March in the north.
The duration of a single ice-pellet event was relatively short-mostly less than 2 hours.In the observation data,the number of single ice-pellet events that lasted less than 2 hours was 196,which was 87.50% of the total;Meanwhile,the number that lasted less than 1 hour was 152,which was 67.86% of the total.In addition,the number of single ice-pellet events that lasted more than 2 hours decreased exponentially:the number lasting more than 4 hours was 9 (less than 5% of the total);the number lasting more than 6 hours was only 2 (less than 2% of the total).
Ice pellets mostly occurred within the surface temperature range of -4 ℃ to 4 ℃.Therein,their occurrence was most common at 0-3 ℃ and within that range,most often at 1 ℃.Ice pellets often occurred at the dew-point temperature within the range of -5 ℃ to 2 ℃and most often at the dew-point temperature of 0 ℃.When the range of the depression of dew point dropped to 0-3 ℃,conditions were highly conducive to the occurrence of ice pellets;and when the depression was 1 ℃ they occurred most frequently.
Ice-pellet precipitation in China is often mixed with mist(45% in total),rain(34% in total),and seldom with snow(only 2% in total).However,freezing rain in China is often mixed with snow (17% of the total).The weather phenomenon of ice pellets in China is significantly different from that in the USA and Canada,in which the proportion of snow(37%) is significantly different.In China,ice-pellet precipitation is mostly caused by the melting mechanism and the warm-rain mechanism.Ice pellets can form if there is only partial melting of snow or ice pellets in the lower weak melting layer,allowing precipitation to eventually fall to the ground in the form of ice pellets.Overall,the melting parameter of ice pellets is less than that for freezing rain,while the freezing parameter of ice pellets is more than that for freezing rain.The temperature decreases,while pressure increases,before the occurrence of ice pellets.The wind speed changes were found to be small throughout ice-pellet events,and accompanied by nearly saturated conditions before and after the event.The evolution of precipitation types during ice-pellet events was found to be variable and complex.