The Variation Of Cirrus Cloud And Its Linkages To Meteorology At The SACOL And SGP Sites

Clouds play critical roles in the energy budget of the earth-atmosphere system,the hydrological cycle and the atmospheric circulation.Due to the complexity and variability of cloud physical properties and the uncertainty of cloud feedback process,it is difficult to represent and simulate cloud accurately.Therefore,understanding the microphysical processes and dynamic mechanism of the cloud is important to improve the cloud parameterization scheme in the global climate model.In this paper,based on the Ka-band zenith radar(KAZR)observation datasets from two mid-latitude sites,Semi-Arid Climate and Environment Observatory of Lanzhou University(SACOL)and the Atmospheric Radiation Measurement(ARM)Program Southern Great Plains(SGP)from August 2016 to July 2019,the frequency and physical properties of cirrus clouds are analyzed statistically,and their relationships to the large-scale meteorology are examined.The main conclusions are as follows:(1)The occurrence frequency and vertical distributions of cirrus have obvious seasonal variation.Cirrus clouds tend to be less frequency but occur at higher altitudes in the warm months than cold months,and the seasonal variation of altitude of cirrus is consistent with that of the tropopause.The average top and base of cirrus cloud at SACOL and SGP have the highest altitude in summer but the lowest in winter.On average,the frequency of cirrus occurrence observed at SACOL and SGP sites are41.72%and 27.23%,respectively,and the thicknesses of cirrus are 2.06km and1.69km,respectively.The frequency and layer thickness of cirrus cloud observed by KAZR at SACOL is larger than that of SGP.On the diurnal timescale,cirrus appears to be a less occurrence frequency and thinner cloud thickness during daytime than the nighttime,whereas the height of cloud top and base have no significant diurnal cycle.The cirrus observed at SGP shows a remarkable diurnal variation in summer,while the diurnal cycle of cirrus at SACOL has no obvious difference between four seasons.(2)The relationships between the frequency and physical properties of cirrus with the variation of upper tropospheric meteorology are almost similar at SACOL and SGP.The greater frequency of cirrus occurrence is concentrated in the regions of larger relative humidity,stronger vertical upward motion,lower stability,and colder temperature between 7-11km ASL(above sea level),indicating that these meteorological conditions are key to support the occurrence of cirrus clouds.The correlation analysis of the physical properties of cirrus clouds with four meteorological variables,including relative humidity at 300h Pa(RH₃₀₀),vertical velocity at 300h Pa(?₃₀₀),upper tropospheric stability(UTS),and convective tropopause temperature(T_UTS),shows that the thickness,top and base,infrared emissivity,albedo,short-wave radiative forcing and long-wave radiative forcing of cirrus clouds are correlated with the four meteorological parameters in different extent,and their sensitivities vary with different meteorological variables.(3)The principal component analysis(PCA)is applied to extract common patterns of meteorology variability by combining RH₃₀₀,?₃₀₀,UTS and T_UTS,and the explained variance of the first principal component(PC1)and the second principal component(PC2)are approximately 40%and 20%,respectively.The relationships of the vertical distribution of the cirrus clouds with PC1 and PC2 show that the occurrence frequency of cirrus is significantly positively related to PC1,while little related to PC2.The cirrus mainly occurs in the region of large PC1 at 6?11km ASL.The relationships between the frequency and properties of cirrus with PC1 are stronger than that of a single meteorological variable.The positive PC1 is associated with the larger relative humidity,stronger upward motion,lower stability,colder temperature,warm advection,and positive vorticity advection.Furthermore,PC1 and the frequency of cirrus have a similar diurnal cycle.In a word,PC1 represents common patterns of variability of these meteorological variables,and is a better measure of large-scale atmospheric states that indicate cirrus clouds occurrence.