Investigating airflow effects on the accuracy of cloud particle measurement

Accurate cloud droplet measurements are important for an increased understanding of cloud microphysical processes, especially the nucleation and growth of cloud particles through condensation and coalescence. In addition, cloud droplets play a significant role in the formation of precipitation; therefore, accurate measurements are important for precipitation forecasting. A Forward Scattering Spectrometer Probe (FSSP) is a cloud droplet measuring instrument that uses light scattering intensity in determining size distribution and concentration. A German version of the FSSP was developed, with minor structural changes, in order to reduce inaccuracies associated with cloud droplet measurements. The goal of the research was to investigate the effects of changes in droplet velocities and trajectories during measurement using the standard and modified FSSP probes. This investigation will help determine a probe configuration with improved measurement capabilities. Two modeling and simulation packages were used; Gambit and Fluent. Gambit was used to generate probe prototype models, 3d computational grids and the computational flow domain. Fluent was used for flow simulations under specified boundary and operating conditions. The analysis examined flow fields at various airspeeds (focusing on 145m/s as a case study), angles of attack and trajectories of 20-50 microns sized particles passing through the measurement volume. The results obtained showed change in particle velocities within the sampling volume for both probes. In addition, both probe simulations showed evidence of particle trajectories approaching the sampling area. This plays an important role in determining the amount of particle passing through the laser beam. Further investigations have to be carried out in order to determine the more suitable probe configuration.