Differential Emissivity Imaging for Measurement of Hydrometeor Particle Mass and Type

An evaporation-based optical and thermal instrument designed to measure the mass, size, density, type, and fall velocity of hydrometeors (i.e., snow, rain, and mixtures) is presented. This Differential Emissivity Imaging Disdrometer (DEID) consists of a hot plate with a low emissivity top surface, a thermal camera, a laser sheet, and an SLR camera. The DEID measures hydrometeors at sampling frequencies up to 0.5-1 Hz with masses and sizes greater than 0.01 mg and 50 µm respectively with a resolution limited by the size of the hot plate and the thermal camera resolution. When a hydrometeor falls on the hot plate, the droplet evaporates and its mass is directly related to loss of heat from the hot plate assuming energy conservation and that heat loss from the plate is only conductive and one dimensional. Physical dimensions of the droplets are measured on the hot plate using differential emissivity imaging where water/snow has high emissivity and the metal plate has low emissivity (figure 1). DEID-mass measurements have been carefully validated by comparing against direct mass measurements using micropipettes droplets of known mass in a wind tunnel (coefficient of determination of 0.99 with a root mean square error of 1 mg). The method has been extended to snow and rain-snow mixtures. Preliminary field experiment measurements of snow and rain from winter 2019 show size distributions and precipitation rates that agree well with the canonical results obtained by Marshall-Palmer and Marshall-Gunn. Summarizing, the DEID provides a new technique accurately measuring: fall-velocity, precipitation rate, snow density, particle size distribution, and particle mass distribution.