The capability to accurately measure the raindrop size distribution (DSD) is crucial for a variety of applications: quantitative interpretation of weather radar measurements, erosion investigation (the kinetic energy is related to the size and fall speed of the drops), validation of simulated microphysics in numerical weather models, to name a few. In addition, the shape of the drops is also a key information for dual-polarization weather radar systems.
To collect data about the shape and size of raindrops (and other hydrometeors like snowflakes and hail stones), the reference instrument is the 2D video-disdrometer, developed and sold by Joanneum Research, Austria. This sensor has however the drawbacks of being large, heavy, and and requires a significant power supply (500 W). Optical disdrometers (e.g., from OTT or Thies Clima) are relatively cheap and robust, but they only provide equivolumetric size and still have significant weight and power consumption.
The objective of the present work is to develop a prototype disdrometer which will provide size as well as (qualitative) shape, be light and require limited power. This instrument will take advantage of the recent progress in the design and implementation of Single Photon Avalanche Diodes (SPAD). Such a light disdrometer will have many applications, in particular for installation on autonomous and transportable weather stations, like Sensorscope.
DSD
The Drop Size Distribution (DSD) represents a statistical way of summarizing information on the raindrop size within a unit volume.
The raindrop
Hydrometeors range in size from a few micrometers to a few centimeters. The largest are snowflakes and the smallest are raindrops.
Our project targets raindrops comprised between 0.1mm and 10mm, falling at a speed of up to 10 m/s.
The spatial resolution must be sufficient to determine the vertical and horizontal drop shapes.
The SPAD
The SPAD (Single Photon Avalanche Diode) sensor is at the heart of the single photon sensitive cameras we employ, as it basically allows to count individual photons. The actual camera is an array of SPADs, delivering a native fully digital image and capable of a high dynamic range.
First experimentations
First tests were carried out with the Radhard2 chip, a SPAD array of 32×32 pixels that can operate up to 375‘000 1 bit images per second. Several frames have to be added to assemble an image with multiple grey levels (e.g., 256 frames for 256 gray levels).
Background light is generated with a LED backlight from an LCD screen.
In order to simulate rain drops, we use glass beads. A mechanical setup moves the target object, simulating a falling drop.
Experimental setup
A glass beads, with 4 grey levels