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An Optical Disdrometer for the Measurement of Raindrop Size Spectra in Windy Conditions

A. J. IllingworthDepartment of Physics UMIST, Manchester M60 1QD, England

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C. J. StevensDepartment of Physics UMIST, Manchester M60 1QD, England

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Abstract

An instrument for measuring the size and concentration of raindrops is described which has the ability to function equally well in calm conditions and strong winds. Raindrops are detected optically in a shadowgraph-type imaging system. A unique feature of the device is the sensing of drop size as drops enter and leave a cylindrical sample volume. Each drop produces two equally sized pulses, the heights of which are proportional to the drop's diameter and their separation yields the drop's transit time. The drop concentration can be derived from this data without assuming or measuring drop velocities. The instrument has a large sample volume, but a negligible loss of data resulting from two drops being sampled simultaneously. Drops above 300 μm can be detected, but beam divergence limits accurate sizing to drops larger than 400 μm. Laboratory calibration shows that drops greater than 1 mm in diameter may be sized to better than 5%. Comparison tests with a tipping bucket raingage reveal that time integrations of the raindrop size spectra yield rainfall totals generally within 10% of the bulk-measured values. These tests also show that disdrometers which sense raindrop flux win seriously overestimate drop concentrations in windy conditions but that this new device performs satisfactorily for winds of up to 20 m s−1.

Abstract

An instrument for measuring the size and concentration of raindrops is described which has the ability to function equally well in calm conditions and strong winds. Raindrops are detected optically in a shadowgraph-type imaging system. A unique feature of the device is the sensing of drop size as drops enter and leave a cylindrical sample volume. Each drop produces two equally sized pulses, the heights of which are proportional to the drop's diameter and their separation yields the drop's transit time. The drop concentration can be derived from this data without assuming or measuring drop velocities. The instrument has a large sample volume, but a negligible loss of data resulting from two drops being sampled simultaneously. Drops above 300 μm can be detected, but beam divergence limits accurate sizing to drops larger than 400 μm. Laboratory calibration shows that drops greater than 1 mm in diameter may be sized to better than 5%. Comparison tests with a tipping bucket raingage reveal that time integrations of the raindrop size spectra yield rainfall totals generally within 10% of the bulk-measured values. These tests also show that disdrometers which sense raindrop flux win seriously overestimate drop concentrations in windy conditions but that this new device performs satisfactorily for winds of up to 20 m s−1.

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