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NCAS Thies Laser Precipitation Monitor (LPM) number 7

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The UK's National Centre for Atmospheric Sciences (NCAS) operate a suite of disdrometers, each given the designation 'ncas-disdromer' followed by a number. This record links to datasets produced using the 'ncas-disdrometer-7' instrument and any specific details for this instrument that have been recorded. Each instrument is a Thies™ Laser Precipitation Monitor (LPM), model number 5.4110.00.200, and is described in detail by Pickering et al. (2019, see linked documentation). The full instrument operational manual is also available via the linked documentation. The suite of instruments was initially deployed as part of the Disdrometer Verification Network (DiVeN) project.

The Thies LPM instrument utilises an infrared (785 nm) beam with dimensions 228 mm x 20 mm x 0.75mm, a total horizontal area of 45.6cm^2. The infrared beam is emitted from one end of the instrument and is directed to the other. A photo-diode and signal processor determine the optical characteristics including optical intensity which is reduced as a particle falls through the beam. The diameter of the hydrometeor is inferred by the maximum amplitude of the signal reduction and the speed of the hydrometeor is estimated by the duration of the signal reduction.

Figure 1 in Löffler-Mang and Joss (2000) describes a similar instrument (Parsivel-1) with the same observing principle and is an excellent visualisation of the technique which is employed by the Thies LPM. The signal processing claims to detect and remove particles that fall on the edge of the beam: "The measured values are processed by a signal processor (DSP), and checked for plausibility (e.g. edge hits)." No further details are given by the manufacturer. The instrument is able to allocate individual hydrometeors into 20 diameter bins from 0.125 mm to > 8 mm, and 22 speed bins from > 0.0 m s^-1 to > 20 m s^-1.

The Thies disdrometer performs additional calculations on the incoming data which it attaches to the Telegram 4 serial output. The quantity, intensity, and type of precipitation (drizzle, rain, snow, ice, grains, soft hail, hail as well as combinations of multiple types) are calculated. Hydrometeor type is recorded as a present weather code. The present weather code is encoded as a number between 1-99 which has a corresponding description of the weather using the standardised codes from the World Meteorological Organization Table 4860 (WMO, 1988). The present weather descriptors cover most hydrometeor types but not all; graupel is not explicitly mentioned, for example. Hydrometeor type is inferred by the instrument, using empirical relationships between hydrometeor size and fall speed.

Abbreviation: ncas-disdrometer-7
Keywords: disdrometer, rainfall, hydrometeors