The RMR lidar at IAP Kühlungsborn

Purpose

The multiwavelength RMR lidar of the IAP is used to measure temperatures and aerosol parameters in the atmosphere. The system is operated since June 1997. It was rebuilt and extended several times in the last years. At the moment the following measurements are possible:

Temperature soundings

Temperature profiles between 1 and 90 km are observed. They are derived on the one hand from the Rayleigh scattering and on the other hand from the rotational Raman scattering. In the first case the detection of the elastic (Rayleigh) scattering is proportional to the density in the aerosol-free part of the atmosphere. With the ideal gas law and the assumption of hydrostatic equilibrium temperature profiles are calculated between 20 and 90 km. (A detailed description is given on the web pages of the ALOMAR RMR lidar.) The upper boundary is given by the low air density which does not produce enough backscattering. The lower boundary is given by the increasing number of aerosols. Below 34 km the effect of the aerosol is corrected using the inelastic Raman scattering. The temperature between 1 and 20 km is calculated from the spectral form of the rotational Raman spectrum of the air molecules. The used signal ratio between 529.1 und 530.4 nm can be derived even if aerosols are present. In combination with the measurements of the potassium lidar the altitude range between 1 and 105 km are covered at night-time for the first time (cf. figure below). The big advantage of the combination of the two lidars measuring at the same time and place is that the absolute temperatures at an altitude about 90 km are measured by the potassium lidar. This temperature gives the start value for the calculation of temperatures from the Rayleigh scattering at lower altitudes.

Temperature profile between 1 and 105 km on 5 October 2005 integrated over one hour

Aerosolsoundings

In summer occassionally the RMR lidar detects aerosol backscatter from about 83 km altitude in addition to the Rayleigh backscatter. The aerosol backscatter is due to noctilucent clouds (NLC). The soundings at the most sensitive wavelength 532 nm allow conclusions on the existence of ice particles, which may be combined with the temperature soundings of this and the K lidar. Stronger NLC can also be detected at the other two Nd:YAG wavelengths and the wavelengths of the K lidar and the metal lidar. With this up to six wavelengths detailed conclusions on the aerosol particle properties are possible, e.g. on number, size and size distribution.

Futhermore, temperatures, aerosol parameters and water vapour concentrations are observed in the troposphere by the combination of Rayleigh scattering, N2 and H2O vibrational Raman scattering and rotational Raman scattering signals.

The transmitter

The lidar uses a Nd:YAG laser emitting simultaneously the fundamental (1064 nm) and also the second and third harmonic wavelengths (532 nm und 355 nm). The technical data are summarised in the table.

Laser type

"seeded" Nd:YAG

Wavelengths

355, 532, 1064 nm

Pulse energies

200, 400, 500 mJ

Repetition rates

30 pulses per second

Pulse lengths

10, 13, 18 ns

The receiver

The light backscattered from molecules and particles in the atmosphere is received by the telescopes. The light is reflected by mirrors and transmitted via glass fibres to the optical bench (cf. schematic drawing). Depending on the measurement problem up to 8 telescopes with 50 cm diameter each are combined (cf. photo in right column). The RMR lidar beam is coaxially transmitted to the atmosphere above the middle of the 5-mirror-system. Due to the geometry the full geometric overlap between the laser beam and field of view is reached as at low altitudes as possible.

Type of telescope

8 parabolic mirrors

Focal length

120 cm

Telescope diameter

50 cm each

Fiber diameter

1.0 mm & 0.6 mm

Num. aperture

0.22

Field of view

0.83 mrad & 0.5 mrad

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Beam of RMR lidar

The main building of the IAP with the green lidar beam of the RMR lidar (photo: G. Baumgarten)

Telescope room

Telescope room with the beeam of the RMR lidar and the metal lidar (photo: G. Baumgarten)

The detector

Schematic drawing of the lidar detector (status: Jan. 2006)