AN OPTICAL DISDROMETER FOR USE IN HIGH WIND SPEEDS

A new optical disdrometer has been developed that is optimized for use in high wind speeds, for example, on board ships. The minimal detecta ble size of dt droplets is 0.35 mm. Each drop is measured separately w ith regard to its size and residence time within the sensitive volume. From the available information, the drop size distribution can be cal culated with a resolution of 0.05 mm in diameter either by evaluation of the residence time of drops or by drop counting knowing the local w ind. Experience shows that using the residence time leads to better re sults. Rain rates can be determined from the droplet spectra by assumi ng terminal fall velocity of the drops according to their size. Numeri cal modeling of disdrometer measurements has been performed, allowing the study of the effect of multiple occupancy of the sensitive volume and grazing incidences on disdrometer measurements. Based on these stu dies an iterative procedure has been developed to eliminate the impact of these effects on the calculated drop size distributions. This tech nique may also be applied to any other kind of disdrometer. Long-term simultaneous measurements of the disdrometer and a conventional rain g auge have been used to validate this procedure.