Wind Tunnel Tests of the Airborne PVM-100A Response to Large Droplets

The Gerber Scientific, Inc. Particle Volume Monitor (PVM) is widely used to measure the liquid water content (LWC) and droplet effective radius (r^sub eff^) of water clouds. The LWC response of the airborne version of this instrument, the PVM-100A, was evaluated in two independent wet wind tunnel experiments under well-controlled conditions. Earlier studies predict that the PVM-100 (the ground-based version of the PVM) theoretical response to monodisperse droplets diminishes for droplet diameters larger than about 40 (mu)m. However, results from the wind tunnel experiments presented in this paper show that the response of the PVM-100A to monodisperse droplets begins to decrease when the droplet diameter is between 20- and 30-(mu)m diameter. For polydisperse droplet populations (such as those found in natural clouds) the efficiency of the airborne PVM-100A for sensing LWC begins to decrease when the median volume diameter (MVD) of the droplet size distribution is above 20 (mu)m, falling to 50% efficiency for an MVD of 50 (mu)m. Therefore, measurements of LWC obtained from the PVM-100A in natural clouds with broad droplet size distributions (and large values of MVD) should be treated with caution.