The commercial SnowWhite radiosonde is complemented by a home-made sensor named COBALD (Compact Optical Backscatter AeroloL Detector). The principle to characterize aerosol or larger particles by their optical backscatter is well established (Rosen,_ApplOpt, 1991). But the mass of the Wyoming sonde described in this article (6 kg) prohibits its use for our application. With the authors’ consultation a much more compact device has been developed that weighs about ten times less.
The first picture shows COBALD next to the Wyoming sonde. The light from two high power LEDs providing typically 250 mW optically at 455 nm and 870 nm is scattered back by air molecules and particles in front of the sonde and measured by a silicon photo detector placed between the LED emitters. A range extending 0.5 m to 5 m in distance from the sonde is probed. The sensitivity required limits the application to night time only. The COBALD technical details are summarized in its data_sheet. Data measured by COBALD is included in the telemetry transmitted by the SnowWhite sonde. The combined SnowWhite COBALD setup is shown in the second and third pictures.
The data interface is flexible, however, and has been extended to other radiosonde and sensor configurations allowing to add COBALD to the Boulder frost point hygrometer (FPH) or the cryogenic frost point hygrometer (CFH) developed by Holger Vömel.
Optical backscatter data is commonly expressed as backscatter ratio taking the signal fraction caused by the air molecules alone (Rayleigh level) as reference. This altitude dependent Rayleigh level can be computed from prior sonde calibration and the measurement of pressure and temperature during the sounding. The backscatter ratio excess with respect to unity results from scattering particles and is thus termed aerosol backscatter ratio. Dividing the red channel aerosol backscatter ratio (which in general is the larger one since the red Rayleigh level is much lower) by that of the blue channel provides the color index revealing information on the particle size: particles much smaller than the optical wavelengths involved behave like Rayleigh scatters yielding a unity color index, whereas particles much larger than the wavelengths approach the geometrical limit to which the scattered intensity is equal for both colors, thus the color reaches the ratio of the Rayleigh levels, approximately a value of 15 for the selected wavelengths.
Data from one of the first test flights in November 2007 launched from the roof of the institute building were used to analyze the COBALD performance, the result is shown below. Vertical optical backscatter profiles for the red and blue channels are averaged for 100 m altitude bins (typically 20 values, right panel each). Backscatter ratio of unity corresponds to the Rayleigh level caused by molecular backscatter only. The harp on the left hand in the figures indicated the noise expected from laboratory characterization at the given altitude (blue color). The yellow and red counterparts show standard deviation and standard error of the atmospheric measurement. From this at 15 km altitude a noise limit of 10% of the Rayleigh value for the red and of 1 % for the blue channel can be derived.
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