WO2015067929A1

WO2015067929A1 - Airborne cloud and particle detector for use with a weather balloon

Info

Publication number: WO2015067929A1
Application number: PCT/GB2014/053268
Authority: WO
Inventors: Richard Giles HARRISON; Kerianne NICOLL
Original assignee: The University Of Reading
Priority date: 2013-11-05
Filing date: 2014-11-04
Publication date: 2015-05-14
Also published as: GB201319479D0

Abstract

A radiosonde system (2) is disclosed together with a ground station (12). The radiosonde system (2) comprises a radiosonde (4) suspended below a balloon (6) with a rope (8). The radiosonde (4) includes an optical cloud detector (30) with a plurality of LEDs (16, 18, 20, 22) and a photodiode (24). Light is emitted from the LEDs (16, 18, 20, 22) and back-scattered from particles in a cloud so that it is received by the photodiode (24). Each LED (16, 18, 20, 22) has a different optical characteristic, and differences in the back-scattered light from the respective LEDs is determined to be indicative of at least one property of the cloud particles.

Description

Airborne Cloud and particle Detector for use with a Weather Balloon

This invention relates to a cloud and particle detector for use with a weather balloon carrying a radiosonde. In particular, the invention relates to a device for detecting the size and/or shape of particles in a cloud.

Conventional radiosondes include a GPS receiver for determining position and additional sensors for detecting atmospheric properties. Typically, sensors are provided for measuring temperature, pressure and relative humidity (RH). The instruments on board the radiosonde take frequent measurements, and telemeter the data to a ground station by radio for analysis.

Radiosondes are generally not recovered after a single use. They are carried aloft by a weather balloon, which expands as it ascends, due to a decrease of

atmospheric pressure with height). When the balloon bursts the detector falls back to earth, slowed with a parachute. The disposable nature of radiosondes means that they are subject to stringent design criteria. Specifically, they must be inexpensive, small and lightweight. In addition, battery resources are limited, which means that any components must have low power consumption.

A ground station can infer the presence of cloud based on telemetry data transmitted by a radiosonde; this is based mainly on measurements of relative humidity (RH). Cloud detection using RH measurements is not always accurate due to the slow response time of capacitance humidity sensors, which worsens at low temperatures. For this reason detectors have been developed that include optical sensors for direct measurement of cloud droplets. One such cloud droplet detector is described in "Cloud droplet detector for radiosonde use", Proc. Aerosol Conf, 36-38, 2012 K.A. Nicoll and R.G. Harrison. The cloud droplet detector described in this document includes a low power, high brightness light emitting diode (LED) and a semiconductor photodiode that can detect radiation back-scattered from cloud droplets. Using this detector it is possible to determine the presence of cloud by analysing the photodiode response with height. An object of the present invention is to extend the functionality of cloud detectors so that they can reveal further information about the properties of particles in a cloud.

According to the present invention there is provided a cloud detector for use with a weather balloon comprising: at least one light source configured to emit a first light beam and a second light beam, wherein the first light beam has at least one optical characteristic that distinguishes it from the second light beam; a radiation detector configured to receive the first light beam and the second light beam, back-scattered from cloud particles; and a processing unit configured to extracta first signal related to the intensity of back-scattered light received in the detector from the first light beam and a second signal related to the intensity of back-scattered light received in the detector from the second light beam, wherein a difference between the first signal and the second signal is indicative of at least one property of the cloud particles. In this way the cloud detector can provide data that reveals properties of cloud particles, such as their size and shape. This is achieved by providing multiple detection channels that can determine the relative back-scatter for the different types of optical signal chosen to be emitted. Size information for cloud particles may be revealed by analysing the relative back-scatter intensity from multiple light beams with different wavelengths. Shape information may be revealed by analysing the relative back-scatter intensity using multiple light beams with different polarisation characteristics. The cloud detector can therefore provide new types of data about the properties and structure of droplet or particle clouds. The light source is preferably configured to emit visible and/or invisible light (invisible to the human eye). Preferably the light sources can emit radiation in the optical and/or infra-red parts of the spectrum. In some embodiments it may also be possible to emit ultra-violet light. The light source is preferably a LED which is selected to emit over a specific spectral range.

The airborne cloud and aerosol particles may include water droplets, ice crystals, dust and/or volcanic ash particles. It may be possible to determine the presence of one or more of these types of particles based on a difference in the properties of the first and second signals. It may also be possible to determine the concentration of particles based on an absolute measure of the back-scattered light intensity from the first and/or second light beam.

The cloud detector is suitable for use with a weather balloon. The cloud detector may also be mounted on other devices that allow it to be flown through clouds, including fixed or rotary wing aircraft or kites. In a further arrangement the cloud detector may be used with a parachute so it can descend through a cloud after it has been dropped from the balloon A typical water droplet in a cloud may have a diameter of around 0.02mm. These droplets tend to scatter all radiation in the optical, infra-red and ultraviolet parts of the spectrum. Moreover the scattering is largely frequency invariant. Thus, clouds tend to appear white. Clouds can also include smaller particles including nascent water droplets, ash particles and/or ice crystals. Some of these particles may have a diameter that is comparable with the wavelength of light in the UV, visible and infra-red parts of the spectrum (around 200 - 2,000nm). The scattering effect of these particles may be more pronounced when the wavelength of light is similar to the diameter of the particle. The first light beam and the second light beam are preferably provided with different wavelengths. Therefore, by analysing the relative back-scatter produced by the first and second light beams it is possible to determine information on the size of cloud particles. Preferably there are at least two light sources for the respective light beams. The light sources are preferably configured to emit the light beams simultaneously.

Additionally, the radiation detector is preferably configured to detect the light beams simultaneously. Preferably there are at least two light sources arranged to emit the first light beam and the second light beam respectively at different wavelengths. In one embodiment the two light sources may be arranged to emit filtered white light in order to produce different wavelengths. Thus, the first and second light beams will typically have respective spectral ranges with different peak wavelengths. One of the light beams may be infra-red and the other may be blue. This

arrangement may be capable of determining particle size over a broad range: around 400-2000nm.

Spherical cloud particles generally scatter incident light evenly, independent of its polarisation. However, particles with other shapes may scatter incident light differently depending on its polarisation. For example needle-shaped particles and prolate spheres may have a greater scattering effect where the polarisation of incident light is aligned with the orientation of the particle's long axis. There is often an orientation preference for particles with a long axis in a cloud because of airflow and/or electrostatic effects. Preferably the first light beam and the second light beam are provided with different polarisation characteristics. Therefore, by analysing the relative back-scatter intensity of the first and second light beams it is possible to determine this orientation preference together with information on the shape of cloud particles.

Preferably there are at least two light sources arranged to emit the first light beam and the second light beam respectively with different polarisations. In one embodiment light beams from the light sources may be passed through polarisers to produce a vertically polarised beam and a horizontally polarised beam. In other embodiments it may be possible to provide filters that produce different polarisation pairs, for example right-handed and left-handed circularly polarised light. Shape information could also be revealed by providing a first light source with linearly polarised light and a second light source with unpolarised light.

The first and second light beams may be emitted with linearly polarised light, with crossed orientations. This can yield vertically and horizontally polarised light, from the perspective of the light sources.

Preferably there are at least two light sources arranged respectively to emit modulated first and second light beams. In this way, the processing unit may be able to extract a first signal that shares the same modulation characteristics as the first light source, and a second signal that shares the same modulation characteristics as the second light source. This can remove any unmodulated background sunlight (or background light with a low modulation frequency) that could otherwise cause difficulties when detecting back-scattered light from the light source(s). Preferably the frequency of modulation is selected so that it is significantly different to the frequency with which background signals may change. When the cloud detector is assembled to a weather balloon the swinging of the detector on a cord below the balloon may cause the radiation detector to point in different directions. This may result in a variation in background light levels with a frequency in the region of 1 - 10Hz. These background variations can be filtered out by providing a frequency of modulation of around 1 kHz for the first and second light beams, and using

synchronous detection methods.

These techniques can facilitate use of the cloud detector during daylight conditions. This is possible because the combination of high pass filtering and synchronous detection can reject the effects of sunlight.

The first and second light beams preferably have different modulation frequencies and the processing unit is preferably configured to extract the first and second (or more) signals using at least two (or more) synchronous demodulation channels. Each demodulation channel can therefore be synchronised uniquely with a light source. This can be used to separate the back-scattered light beams received in the radiation detector. Square-wave amplitude modulation is preferably used. Thus, the first and second (or more) light beams can be operated simultaneously. The back-scattered signals can be separated in the frequency domain using respective synchronous detection channels. In an alternative arrangement the first and second light beams may be operated in a phased order so that only one light beam is in use at any time. In this arrangement the back-scattered light beams may be separated in the time domain using a single demodulation channel. The light beams may be modulated in any other convenient way that would occur to the skilled person. For example, it would be possible to apply frequency or phase modulation to the light beams so that respective back-scattered signals can be identified using appropriate demodulation.

In one arrangement there may be at least four light sources, each configured to emit a light beam with different optical characteristics. The four light sources may also be arranged around the detector. In this way a central detector can receive back- scattered light from each of the light sources. The detector can also receive back- scattered light from the same region of a cloud so that shape and/or size information can be provided for a specific and localised cloud region. The processing unit may include four demodulation channels for signals corresponding to the four light beams.

Preferably the detector and the at least one light source are configured to point downwards, in use. This can enable use of the cloud detector during daylight conditions. By pointing the detector downwards it is possible to minimise the amount of direct sunlight received in the radiation detector, which can help to avoid detector saturation. The cloud detector may have a housing portion with an upper end and a lower end. Attachment points for ropes may be provided at the upper end and the detector and the at least one light source may be provided at the lower end. In this way the cloud detector can be designed so that it reliably points downwards, in use. The centre of mass of the detector may also be provided towards the lower end of the housing to assist the detector and the at least one light source pointing downwards, in use. In one arrangement the cloud detector may be assembled to another unit that includes standard radiosonde detectors such as a GPS receiver, a temperature probe, a pressure probe and a relative humidity probe. The centre of mass of the overall unit is preferably provided towards its lower end to encourage the detector to point downwards.

Preferably the cloud detector comprises a transmitter configured to transmit data to a base station regarding the first signal and the second signal. In this way the cloud detector may be used to acquire data relating to the first and second signals. According to another aspect of the invention there is provided a cloud particle analysis system comprising the cloud detector as previously defined and a base station configured to receive data from the transmitter and to determine at least one property of the cloud particles based on a difference in the first signal and the second signal. The base station may be arranged to carry out more complex analysis to reveal shape and/or size information at different heights in a cloud. This helps to reduce the cost and power requirements of the cloud detector.

According to yet another aspect of the invention there is provided a method of determining at least one property of particles in a cloud, comprising the steps of: positioning a cloud detector into a cloud; emitting a first light beam and a second light beam from at least one light source in the cloud detector, wherein the first light beam has at least one optical characteristic that distinguishes it from the second (or more) light beams; receiving the first light beam and the second light beam in a single radiation detector in the cloud detector, wherein the received first and second light beams have been back-scattered by cloud particles; extracting a first signal related to the intensity of back-scattered light received in the detector from the first light beam and a second (or further) signal(s) related to the intensity of back-scattered light received in the detector from the second (or further) light beam(s); and determining at least one property of cloud particles based on a difference between the first signal and the second signal.

Apparatus features may be provided as corresponding method features and vice- versa.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic view of a radiosonde and a ground station, in an embodiment of the invention;

Figure 2 is a perspective view of a cloud detector for use with a weather balloon in an embodiment of the invention; and Figure 3 is a circuit diagram showing components in a cloud detector in an embodiment of the invention.

Figure 1 is a basic view of a radiosonde system 2 and a ground station 12. The radiosonde system 2 comprises a radiosonde 4 suspended below a balloon 6 with a rope 8. The radiosonde 4 includes a GPS receiver, a temperature sensor, a relative humidity sensor and a 9V battery which are enclosed within an outer housing. The radiosonde 4 also includes a radio transmitter 10 for transmitting telemetry data to a receiver 14 in the ground station 12.

The radiosonde system 2 also includes an optical cloud detector 30, further detail of which is shown in Figure 2. The optical cloud detector 30 is attached to the side of the standard radiosonde 4. A power connector extends from the optical cloud detector 30 to the battery in the radiosonde 4, and a data transfer cable is also provided so that telemetry data from the cloud detector 30 can be transmitted to the ground station 12, via the radiosonde 4.

The rope 8 is attached to an upper end of the radiosonde 4 to encourage the lower end of the optical cloud detector 30 to point downwards. This is also assisted by providing the centre of mass of the radiosonde 4 and optical cloud detector 16 towards the lower end of the overall unit.

In use the radiosonde system 2 is released into the atmosphere. The radiosonde system 2 then ascends at a roughly constant speed, due to the buoyancy of the balloon 6. Regular measurements are taken from the sensors in the radiosonde 4 and the optical cloud detector 30, and these measurements are transmitted to the ground station 12 using the transmitter 10.

Figure 2 is a detailed perspective view of the optical cloud detector 30. The detector can use any number of LEDs, but four are used here as an example. As an example these LEDS may transmit (unpolarised) red and infra-red light respectively. The detector includes a photodiode 24 and four LEDs 16, 18, 20, 22, arranged in two pairs. The first pair comprises the first LED 16 and the second LED 18. The second pair comprises the third LED 20 and the fourth LED 22. These LEDs respectively may transmit vertically and horizontally polarised yellow light. Light is emitted from the LEDs and back-scattered from particles in a cloud so that it is received by the photodiode 24. Figure 3 is a circuit diagram for a circuit installed in the optical cloud detector 30. The cloud detector 30 includes the LEDs 16, 18, 20, 22. Each LED is connected to a respective oscillator 32, 34, 36, 38, which is arranged to pulse the emitted light at a unique modulation frequency. The modulation frequencies for the four LEDs are in the kHz range, and they are chosen carefully so that they are not harmonically related. Although four LEDs are shown, any number may be used.

Back-scattered light from cloud particles is received in the photodiode 24 and the resultant electrical signal is amplified and passed through a high pass filter 40. The high pass filter rejects any signal components that do not have a modulation frequency below the kHz range. This is important for rejecting background light with a low frequency of variation. For example, this would reject signals associated with a constant solar background. It would also reject signals with a low frequency of around 1 -1 OHz that may be associated with a changing solar background due to the swinging of the cloud detector 16 on the rope 8 below the balloon 6.

The filtered signal from the photodiode 24 is then subjected to synchronous detection in four independent channels. Four synchronous detectors 42, 44, 46, 48 are provided; each connected to a respective oscillator 32, 34, 36, 38. Thus, each synchronous detector 42, 44, 46, 48 is configured to detect a feature in the signal from the photodiode 24 that shares the modulation characteristics of a corresponding LED 16, 18, 20, 22. The signal from each synchronous detector 42, 44, 46, 48 is then passed through a low pass filter 50, 52, 54, 56 to smooth the signal and eliminate switching noise from the demodulation; each signal is then is subjected to further amplification. Finally, each channel provides a signal V1 , V2, V3, V4 which is reflective of the back-scattered signal from each LED 16, 18, 20, 22. The signals V1 , V2, V3, V4 are transmitted to the ground station 12 by the transmitter 10 for analysis so that the size and shape of cloud particles can be analysed. Each LED 16, 18, 20, 22 has a different optical characteristic. In the first pair, the first LED 16 transmits red light and the second LED 18 transmits infra-red light. In the presence of cloud, the emitted light beams may be back-scattered so that they can be detected by the photodiode 24. A number of conclusions may be drawn from signals V1 and V2. In particular, a non-zero value in these signals may indicate the presence of back-scattering particles which can be used to infer the presence of cloud. The absolute value of signals V1 and V2 can also be used to determine the concentration of cloud particles. Information about cloud particle size can be determined from the relative strength of signals V1 and V2. In particular, the scattering effect of some small cloud particles has been found to be dependent on the wavelength of incident light, such that scattering is more pronounced when the wavelength of incident light is similar to the diameter of the particle. Therefore, by analysing the difference between V1 and V2 it may be possible to determine a difference in the concentration of cloud particles with diameters that are comparable to the wavelength of the incident light. As the radiosonde system 2 ascends, therefore, the ground station 12 may be able to determine a distribution for differently shaped particles at different heights in a cloud. Further conclusions may be drawn from signals V3 and V4. In particular, these signals may also be used to indicate the presence of back-scattering particles and to determine their concentration. Additionally, the relative strength of signals V3 and V4 can be used to determine information about cloud particle shape. In particular, it has been determined that cloud particles may scatter incident light in dependence on its polarisation. For example particles having a longer axis and a shorter axis may have a greater back-scattering effect where the polarisation of incident light is aligned with the orientation of the particle's long axis. There is often an orientation preference for particles with a long axis in a cloud because of airflow and/or electrical effects. By analysing the difference between V3 and V4 it is possible to determine an orientation preference in a cloud together with information on the shape of cloud particles. This may be useful for identifying the presence of volcanic ash and/or ice particles in a cloud. The ground station 12 may be able to determine the presence of these particles as well as information on their distribution at different heights in the cloud.

Claims

1 . A cloud detector for use with a weather balloon comprising:

at least one light source configured to emit a first light beam and a second light beam, wherein the first light beam has at least one optical characteristic that

distinguishes it from the second light beam;

a radiation detector configured to receive the first light beam and the second light beam, back-scattered from cloud particles; and

a processing unit configured to extract a first signal related to the intensity of back-scattered light received in the detector from the first light beam and a second signal related to the intensity of back-scattered light received in the detector from the second light beam, wherein a difference between the first signal and the second signal is indicative of at least one property of the cloud particles.

2. The cloud detector of claim 1 wherein the first light beam and the second light beam have different wavelengths.

3. The cloud detector of claim 2 comprising at least two light sources arranged to emit the first light beam and the second light beam respectively at different wavelengths.

4. The cloud detector of any of the preceding claims wherein the first light beam and the second light beam have different polarisation characteristics.

5. The cloud detector of claim 4 comprising at least two light sources arranged to emit the first light beam and the second light beam respectively with different

polarisations

6. The cloud detector of claim 5 wherein the first and second light beams are emitted with linearly polarised light, with crossed orientations.

7. The cloud detector of any of the preceding claims comprising at least two light sources arranged respectively to emit modulated first and second light beams.

8. The cloud detector of claim 7 wherein the first and second light beams have different modulation frequencies and the processing unit is configured to extract the first and second signals using at least two synchronous demodulation channels.

9. The cloud detector of claim 8 wherein the first light beam is modulated at a frequency that is not harmonically related to the frequency of the second light beam.

10. The cloud detector of any of the preceding claims comprising at least four light sources, each configured to emit a light beam with different optical characteristics, wherein the four light sources are arranged around the detector.

1 1 . The cloud detector of any of the preceding claims wherein the detector and the at least one light source are configured to point downwards, in use.

12. The cloud detector of any of the preceding claims comprising attachment points for ropes so that the cloud detector can be attached to a weather balloon.

13. The cloud detector of claim 12 comprising a housing portion having an upper end and a lower end, wherein the attachment points for ropes are provided at the upper end and the detector and at least one light source are provided at the lower end.

14. The cloud detector of any of claim 1 1 comprising a housing portion having an upper end and a lower end, wherein the centre of mass of the detector is provided towards the lower end of the housing to assist the detector and the at least one light source pointing downwards, in use.

15. The cloud detector of any of the preceding claims further comprising a transmitter configured to transmit data to a base station regarding the first signal and the second signal.

16. A cloud particle analysis system comprising the cloud detector of claim 15 and a base station configured to receive data from the transmitter and to determine at least one property of the cloud particles based on a difference in the first signal and the second signal.

17. A method of determining at least one property of particles in a cloud, comprising the steps of:

positioning a cloud detector into a cloud;

emitting a first light beam and a second light beam from at least one light source in the cloud detector, wherein the first light beam has at least one optical characteristic that distinguishes it from the second light beam;

receiving the first light beam and the second light beam in a radiation detector in the cloud detector, wherein the received first and second light beams have been back- scattered by cloud particles;

extracting a first signal related to the intensity of back-scattered light received in the detector from the first light beam and a second signal related to the intensity of back- scattered light received in the detector from the second light beam; and

determining at least one property of cloud particles based on a difference between the first signal and the second signal.