Low Clouds Physical Properties And Their Links To Meteorology At The SACOL Site

The low-level cloud response and feedback are important sources of uncertainties in climate prediction.Arid and semi-arid regions are susceptible to climatic changes.Understanding the distribution of low clouds and how they respond to changes in the meteorological field in semi-arid regions through observational data will help narrow the uncertainty of low cloud feedback.Based on six-year Ka-band Zeinith radar observations at the Semi-Arid Climate and Environment Observatory of Lanzhou University(SACOL)site,we first analyze the distribution of the physical properties of low clouds and their annual and diurnal cycle of low clouds and then examine the relationship between low clouds and meteorology.The main conclusions are as follows:(1)The frequency of low clouds over the site is 15.07% during the observation.The median cloud base height was 0.96 km,and the median cloud top height was2.58 km.The maximum reflectivity factor of low clouds increases with the cloud thickness,consistent with the logarithmic regression relationship.Based on the cloud base height,continuous time,and cloud thickness,this paper uses cluster analysis to partition the low clouds into three clusters.The first type of low clouds(C1)has the lowest height above the ground,with a median of 0.32 km.The second type of low clouds(C2)has a slightly higher cloud base height of 1.42 km.The third type of low clouds(C3)has the highest frequency of 7.32%,the most extensive distribution of persistence,with a median of 15.08 hours,and the thickest cloud layer,with a median of 2.71 km.(2)Low clouds over the SACOL site have a significant diurnal cycle and annual cycle.The greatest vertical development of low clouds occurs around 9 am.The most significant thin cloud(C2)frequency occurs around 8 am,and the greatest frequency of thicker cloud(C3)occurs around 16 am.The occurrence frequency of low clouds is the highest in March at 21.9%,and is the lowest in July at 5.2%.The cloud base height is the lowest in January,with a mean value of 0.67 km,and is the highest in June,with a mean value of 1.60 km.The thickness of the cloud layer is the lowest in December and the highest in April,with values of 1.46 km and 4.88 km,respectively.The low clouds with a higher distance from the ground(C2 and C3)mainly occur in spring and autumn,and the low clouds with a lower distance from the ground(C1)mainly occur in winter.(3)When low clouds occur over the SACOL site,there is a decrease in temperature decrement rate,an increase in specific humidity gradient,and an increase in relative humidity at the corresponding positions of cloud occurrence.When the low clouds with strong vertical development(C3)occur,the upward movement is enhanced at all heights;when the low clouds with weak vertical development(C1,C2)occur,the sinking is enhanced above 3 km.(4)In this paper,we investigate the correlation between low clouds and meteorological fields at hourly resolution by the mean values of low cloud characteristics in different intervals of three meteorological variables and find that:ground temperature,estimated inversion strength(EIS),and low-level wind shear have good correlations with the frequency of low cloud occurrence.The correlation coefficients are 0.94,-0.96,and 0.91,respectively.Among the three clusters of low clouds,the occurrence frequency of the thin and low clouds(C1)has the highest correlation with these three variables.The correlation coefficients are 0.86,-0.84,and 0.94,respectively.While the frequency of thick low clouds with a high cloud base(C3)decreases when the EIS is greater than 16 K,the surface temperature is less than 275 K,or the wind shear is greater than 10 m/s.Correspondingly,When the EIS increases,the wind shear increases,or the surface temperature decreases,the cloud base height and the thickness of the cloud layer of low clouds will decrease.