The effects of wind-shear on cirrus: a large eddy model (LEM) and radar case study

1. e A Tp Ice Supersaturation Water Saturation Bp Ice Saturation T 0o Tb Tt C The effects of wind-shear on cirrus: a large eddy model (LEM) and radar case study Gourihar Kulkarni* and Steven Dobbie Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK *Now at Atmospheric Science & Global Change Division, Pacific Northwest National Laboratory (PNNL), Richland, WA 99352, USA Summary A new Thermal Gradient ice nucleation Diffusion Chamber (TGDC) has been designed and constructed to investigate ice nucleation characteristics of atmospherically important Ice Nuclei (IN) aerosols. This chamber has been specifically designed so that ice nucleation events can be observed optically whilst the aerosol sample is progressed through various conditions within the static diffusion chamber. The TGDC can establish a range of super-saturations with respect to ice (SSi) over the temperature range of -10 to -34°C and for sufficient time to allow for ice nucleation events of the aerosol particles. The ice nucleation event is determined through observation when an ice embryo forms on an aerosol, which is supported on the Teflon substrate; These events were observed and recorded using digital photography. The design of the TGDC allows for understanding time variations in ice nucleation events for a wide range of SSi conditions and different IN without the need to alter the TGDC conditions makes the TGDC a unique nucleation chamber. Calibration of the system is performed by observing (NH4)2SO4 deliquescence and the results are in good agreement with the literature data. Results are presented for mineral dust IN, taken from the AMMA project, properties such as critical SSi, onset dependence, and active fraction. • Motivation: • Very limited data exists for IN nucleation and the nucleation process is poorly understood • Cloud modelling studies need accurate information about the characteristics of IN, nucleation properties, variations with source regions, transport, and aging • Development of a process chamber that can be adapted not only to determine nucleation critical super-saturations/rates and active fractions but also isolate the effects of specific processes, such as the hysteresis effects • Motivation: • Very limited data exists for IN nucleation and the nucleation process is poorly understood • Cloud modelling studies need accurate information about the characteristics of IN, nucleation properties, variations with source regions, transport, and aging • Development of a process chamber that can be adapted not only to determine nucleation critical super-saturations/rates and active fractions but also isolate the effects of specific processes, such as the hysteresis effects New TGDC design: This TGDC is designed to focus on the nucleation processes of individual aerosol particles. Consequently, we place aerosol particles on a sample holder and insert the holder into the parallel plate system. The nucleation process is observed with an optical system observing through a port in the upper plate. The design is illustrated in Figure 2. Basic Theory: The basic design of a TGDC consists of two parallel plates with the inner walls coated with ice. With both plates at sub-zero conditions and one plate at a colder temperature, the result is that linear profiles establish in water vapour and temperature between the plates. Ice super-saturation profiles establish between the plates, as indicated in Figure 1. Calibration: Figure 1: Shows the diagram of saturation vapour pressure (e) against the temperature (T in °C). Tp and Bp represent the conditions at the top and bottom plates, respectively. Ice supersaturation is the region enclosed between the straight line TpBp and ice saturation. (Not to scale). Acknowledgments: The authors would like to thank Dr Jim McQuaid, Prof Mike Smith, Dr Justin Lingard, Dr Sarah Walker and Dr Barbara Brooks for support during experiments in the aerosol laboratory at Leeds. References: ICAS For more information about this poster please contact Dr Steven Dobbie, Environment, School of Earth and Environment, The University of Leeds, Leeds, LS2 9JT Email: dobbie@env.leeds.ac.uk Tel:+44 (0) 113 343 6725 Insitute for Climate and Atmospheric Science