Ground-Based FSSP and PVM Measurements of Liquid Water Content

Recently published ground-based measurements of liquid water content (LWC) measured in fogs by two microphysical instruments, the FSSP-100 and PVM-100, are evaluated. These publications had suggested that the PVM-100 underestimated LWC significantly in comparison to the FSSP-100 when the fog droplets were large. The present evaluation suggests just the opposite: The FSSP-100 overestimates LWC for large droplets because these droplets are unable to follow the curved streamlines of the flow generated by drawing air into the FSSP-100’s sensitive volume at 25 m s−1. This inertial effect causes droplets to accumulate near the active volume of the instrument’s laser beam and to produce large and spurious droplet concentration and LWC values for the largest droplets. Model calculations estimate the magnitude of this error for the FSSP-100.