Cirrus Macro-physical Properties And Meteorological Fields At The SACOL Site

Cirrus clouds have an important impact on the energy budget of the earth-atmosphere system.In this study,we use two-year observation data of Ka-Band Zenith Cloud Radar(KAZR)from August 2013 to July 2015 at the Semi-Arid Climate and Environment Observatory of Lanzhou University(SACOL)to analyze the macro-physical properties of cirrus.We use the k-means cluster analysis method to classify the cirrus clouds into four regimes,according to their persistent time,thickness and top height.Our results show that the four grouped cirrus clouds have distinct characteristics.We also analyze the relationship between the physical properties of these cirrus clouds and the large-scale meteorological fields.The main conclusions are as follows:(1)Within the entire observation period,the occurrence frequency of cirrus is 41.6%.The occurrence frequency of cirrus clouds has obvious seasonal variations.It has larger value in cold season than it in warm season.Cirrus occurs most frequently at the height of 7.2km above the ground.Generally,the reflectivity of most cirrus clouds decreases with height,but there is a second mode where the reflectivity increases with height,this mode accounting for 4.7% of the whole data.The top and base height of cirrus clouds are the lowest in January and the highest in August.About 84.3% cirrus clouds are single-layer.(2)The four cirrus clouds have distinct macro-physical properties.The cloud thickness,top height and persistent time in the first regime are 2.22 km,8.31 km and 7.5 hours,respectively;Cirrus clouds in the second group have the highest location,with a mean top height of 9.8 km.The cloud thickness and persistent time are 0.9km and 1.5 hours,respectively.The thickness and persistent time of the third cirrus clouds are much larger than those of the other three cirrus clouds.The thickness and persistent time are 2.83 km and 17.7 hours,respectively,and its mean cloud top height is 8.63 km.Cirrus clouds in the fourth regime are located in the middle troposphere.The average thickness and persistent time are similar to those of the second cirrus cloud,but the mean cloud top height is significantly lower than that of the second cirrus group,with a value of only 6.7 km.(3)The occurrence frequency of all kinds of cirrus clouds has obvious seasonal variations,because they are affected by large-scale meteorological fields.When large-scale meteorological fields change,the type of cirrus clouds changes.When thick cirrus clouds(the first cirrus clouds and the third cirrus clouds)emerge,their environment fields have a large area of high relative humidity,which are dominated by ascending movements.When the thin cirrus clouds(the second cirrus clouds and the forth cirrus clouds)emerge,there are no large area of high relative humidity in their environment fields,and subsidence movements dominate.