The OceanRAIN Optical Disdrometer ODM470

The backbone of the OceanRAIN project is the Eigenbrodt ODM470 optical disdrometer (left sketch and A in the right image). An infrared light diode homogeneously illuminates the measuring volume. Hydrometeors passing this volume cause light extinction proportional to their cross sectional area. The detected light reduction is stored as an activation voltage. During the integration time of 60 s all hydrometeors are counted and sorted into size bins ranging from 0.04 to 22 mm to obtain the particle size distribution. The OceanRAIN instrumentation is complemented by a cup anemometer (B) to measure relative wind speed and a IRSS88 precipitation detector (C) to activate the disdrometer when precipitation occurs. The precipitation phase and consequently the rain, snow and mixed-phase precipitation occurrence, intensity and accumulation are derived from particle size distributions. Additionally, microphysical parameters and radar-related parameters are provided.

The ODM470 is specifically designed for automated all-weather shipboard operation. The main advantages of the ODM470 system over other existing disdrometers can be specified in five points:

1) the measurement volume of the disdrometer has a cylindrical shape causing precipitation particles to be independent of their incidence angle,

2) the measurement volume is always kept perpendicular to the local wind direction. This is achieved by a wind vane that pivots the instrument around a vertical axis. Consequently, local up- and downdrafts as well as turbulence induced by the ships superstructure cause a minimal impact on the measurement of the particle size distribution,

3) the high-resolution optical unit allows discriminating hydrometeors into 128 size bins with an logarithmically increased resolution towards smaller size bins,

4) an automated precipitation phase detection algorithm allows to process particle size distributions using either the rainfall or snowfall algorithm, and

5) the instrument is fully automated, robust and requires minimal maintenance during operation. Thus, it is ideally suited for long-term monitoring of precipitation over the global oceans in all-weather conditions.

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