Performance Limitations of Optical Present Weather and Visibility Sensors under Special Consideration of the Utilized Technology and the Related Measurement Uncertainties

The various performance limitations of optical visibility and present weather sensors need to be recognized and understood in the context of the utilized measurement technology. Conventional optical disdrometers, as well as forward scatter measurement based present weather sensors, incorporate a number of technology dependent weaknesses. The common measurement uncertainty sources and their impact on the precipitation identification, quantification and extinction coefficient determination capabilities will be identified and disclosed in the frame of this paper.

Optical disdrometers (i.e., laser disdrometers) utilize the attenuation behavior of precipitation particles and determine the precipitation type by evaluating the particle width and fall speed and a correlation to the respective scientific models. The visibility determination capabilities of this technology are very limited since only the precipitation particle related portion of the extinction coefficient can be ascertained.

Forward scatter sensors in contrast are optimized to measure the total extinction coefficient under a typical angle in the range from 30° to 50°. The possibility to also estimate size and residence time of precipitation particles that pass the measurement volume enable the design of the conventional present weather sensors in forward scatter geometry. However, the conical transmitter light beam typically incorporates an uneven intensity distribution. Depending upon where a precipitation particle passes the measurement volume, the particle residence time and the detection sensitivity may vary significantly, keeping the size and fall speed analysis capabilities on a rudimentary level. In order to reduce the incorporated measurement and detection uncertainties to an acceptable level, the utilization of additional information is necessary and unavoidable.

The detailed discussion will illustrate that a reliable detection and classification method, especially for small, mixed and frozen precipitation particles, is hard to achieve with the conventional technologies.

In addition to the generic technology dependent limitations, the impact of various common influences on the measurement performance need to be considered.

Conventional optical disdrometers suffer from the fact that spider webs can easily be placed within their measurement area. The opposite enclosure parts and the connecting structure allow the spiders to find a sufficient number of supporting points for their web construction. Spider webs that are partly or entirely placed in their measurement volume may also impact a number of conventional present weather sensors that do not utilize the look down geometry. These sensors react with low meteorological optical range (MOR) indications and false particle detections.

Significant reductions in MOR reported by conventional forward scatter sensors have been observed by various Meteorological Institutes and Weather Services. These reductions where clearly related to the presence of insects in the measurement volume. The transmitter light, in combination with the heat radiation from the weather protection hoods, seem to attract mosquitos and other small flying insects during specific seasons and climate conditions. Especially during sunrise and sunset in humid air, insect swarms may reside close to and/or inside the measurement volume for periods that are sufficiently long to disturb the measured atmospheric scatter signal and reduce the reported MOR. Unnatural MOR “drops” even below 1000 m over time spans of several minutes can be the consequence.

Sun radiation on the photoelectric receiver needs to be avoided for conventional optical disdrometers, as well as for optical forward scatter sensors. In both cases, the strength of the electrical noise will increase with increasing illumination of the photoelectric sensor element, and the risk of false particle detections increases significantly. Conventional optical disdrometers cannot utilize a look down geometry that avoids direct sun radiation entering the sensor window and optical system when the sun is at low elevation angles (i.e. sunrise and sunset).

Conventional optical disdrometers are based on the transmittance measurement concept. Therefore, it is necessary that they utilize a measurement area, which covers the entire path length between light transmitter and light receiver protection windows. However, since the sampling surface starts and ends directly at the enclosure structure, it is unavoidable that the effective size of the measurement area, and therefore the number of “collected” particle events, will heavily be influenced by shadowing effects since the enclosures generate a significant wind direction dependent “shadowing” of the measurement area. Naturally this results in a so-called “undercatch” - not a negligible uncertainty for the precipitation intensity measurements.

Many conventional present weather sensors provide little or no window dirt contamination measurement and correction capability. In some cases, only contamination detection is available (i.e. no correction). Consequently, the windows need to be cleaned more often; always when the window dirt contamination has reached a critical level that might be performance relevant. Signals from single precipitation particles that represent the particle size are proportionally damped by a reduced optical window transparency. The same applies to the scatter signal from the conglomerate of very small hydro- and/or lithometeors that generate the typical visibility reducing phenomena like fog, mist and haze. Growing measurement uncertainties between the window cleanings are unavoidable if the dirt contamination is not or not sufficiently measured and corrected for.

This paper shall help the users to understand the natural limitations of certain measurement technologies in order to formulate realistic and technically and economically achievable requirements.