The development of a laboratory drip rig for performing reproducible comparisons on different precipitation sensors by correcting for environmental conditions and rig set up

Assessing the accuracy of precipitation sensors can prove very challenging due to the lack of a universal test standard, stemming from difficulties in creating a controlled test scenario. We propose a refined method of testing that is highly reproducible and can be applied to any precipitation sensor regardless of sensing technology.

It is widely understood that two identical disdrometers mounted close together in a real rain event are not likely to report the same precipitation measurements due to the small scale spatial variation of rain. This makes it difficult to draw comparisons between sensors of the same type and even more difficult to compare rain sensors that have different sensing areas and use different sensing technologies. It is therefore desirable to simulate rainfall in the laboratory that is representative of real world conditions but this presents its own set of challenges, primarily in creating rain drops that travel at terminal velocity. This test method significantly reduces the impact of this issue.

This is particularly important for sensors such as optical, acoustic, radar or impact, where the calculations used to obtain rainfall accumulation and drop size distribution assume that the droplets are at terminal velocity. Even for sensors such as capacitive rain gauges and tipping buckets, where the velocity of fall is not directly related to the measurements, more valid conclusions can be drawn about the sensor’s ability to measure precipitation when the droplets imitate real rainfall as closely as possible.

Here, the development of a drip rig capable of creating raindrops of a controlled size is documented. The drip rig can be mounted at a known height and used to test a variety of different precipitation sensors. However, due to height restrictions in the laboratory, it is not possible to get larger raindrops to terminal velocity. Mounted at a height of 7.4m, drops above 2 mm in diameter do not reach 99% terminal velocity, and drops above 3 mm do not reach 95%. For this reason, corrections must be applied to the calculations. It is therefore essential to have an understanding of the change in fall velocity of a water droplet with fall distance.

This work documents the equations used to calculate drop velocity with fall distance for different drop masses. Temperature, humidity and air pressure define air density, which has a significant impact on the velocity of a falling water droplet. The effect of each of these environmental factors has been investigated in order to allow for further corrections. Performing these corrections greatly improves the validity and repeatability of the tests carried out on precipitation sensors.