Investigation on Cirrus Clouds Ice Nuclei Characteristics and their effect on Optical Properties

Background: The cirrus ice nuclei characteristics and its effect on the optical properties were investigated from the National Atmospheric Research Laboratory (NARL; 13.5 0 N, 79.2 0 E), Gadanki, Andhra Pradesh, India, an inland tropical station during the period of observation January 2009 to December 2010. From the observations the cirrus ice nuclei characteristics such as effective radius (Re), ice water content (IWC), ice water path (IWP), ice nuclei concentration (INC), ice crystal fall velocity (Vm) were studied. The variation of these tropical ice nuclei properties with mid cloud temperature were also studied. The present study may be useful in predicting the effect of ice nuclei on the cloud dynamics, precipitation and climatology. Materials and Methods: The ground based lidar system installed at Gadanki was operated during clear sky conditions and free from low level clouds. Typically, about 4–6 hours of observations were made during each observation day at night from 22:00 to 05:00 Indian Standard Time (IST; corresponding to 82.5°E). Also, it was aligned to an overlap altitude of greater than 4km in order to avoid the intense backscattered signals from the low altitude clouds and aerosols. For the present analysis, all the temperature observations were taken from radiosonde experiments conducted at the station during the same period. MST Radar observations were taken for the vertical wind transport analysis. The complex time series of the decoded and integrated signal samples are subjected to the process of FFT for on-line computation of the Doppler power spectra for each range. The data processing for parameterization of the Doppler spectrum involves the removal of dc, estimation of average noise level, incoherent integration and computation of the three low order moments. Results: The cloud mean IWC ranged from 2.08x10 -5 to 1.22x10 -3 g/m 3 for the period 2009 and from 3x10 -5 to 6x10 -3 g/m 3 for 2010. The IWP for the SVC was low (<0.2 g/m 2 ), for TC, IWP lie in the range 0.2<IWP<4 g/m 2 and for DC, IWP>4 g/m 2 . The IWC was strongly dependent with the INC and effective size of the ice particle. Observed cirrus with slow updrafts were very thin with low IWC and INC; cirrus with fast updrafts were thick with high IWC and INC. Conclusion: The effective radius of ice crystal was dependent on the ambient temperature and it showed a maximum at the cloud bottom and minimum in the cloud top. Ice crystals formed on the cloud top layer were small in size and less ordered. The ice nuclei concentration and ice water content showed strong correlation. The higher ice water content was most frequently observed at the lower altitude where the temperature was warm and the ice crystals formed were aggregates. Cirrus formed by slow updrafts were thin with low ice water content and ice nuclei concentration while cirrus with fast updrafts were thick with high ice water content and ice nuclei concentration. Key Word: Lidar; Radar; Ice nuclei; Fall velocity; Cirrus. -------------------------------------------------------------------------------------------------------------------------------------Date of Submission: 13-04-2020 Date of Acceptance: 28-04-2020 -------------------------------------------------------------------------------------------------------------------------------------