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Mobile Aerosol and Temperature lidar (AT)

General

Instrument Description

It is is a combined Rayleigh-Mie, rotational Raman, and vibrational lidar, with the instrumentation installed in a mobile trailer. Three wavelengths are transmitted from a high-power, injection-seeded Nd-YAG laser at 1064, 532 and 355 nm. This laser operates at a 50 Hz pulse rate, and is rated at 60 W in the fundamental. Prior to transmission into the atmosphere, the wavelengths are separated into two beams (one at 355 nm and the other at 1064 and 532 nm), expanded through a pair of 5X refractive telescopes and then transmitted co-linearly approximately 1 m from the center of the 91 cm receiver telescope. Six backscattered wavelengths are detected: 1064, 607, 532, 408, 387, and 355 nm; 1064 is collected using analog detection and the others use fast photon counting techniques. Spectral separation is accomplished using beamsplitters and interference filters. Because of signal dynamic range issues, the signals at each wavelength are power split into as many as three individual detectors. In addition, a 10 cm telescope provides near-field return signals at 355, 378 and 408 nm. In total the lidar retrieves data in 16 photon counting channels and one analog channel.

Measurement Technique

Temperature is retrieved using three different techniques. Stratospheric temperature above 28-30 km is retrieved from the backscattered radiation at 355 nm. Above this altitude aerosols are considered to be negligible, and so the lidar return is a function of atmospheric density and well known geometric parameters. From this data a relative density profile can be formed and from that an absolute temperature can be extracted. Below 28-30 km, aerosols become more important and prevent accurate temperature retrieval from the elastically backscattered radiation. Between 10-12 and 28-30 km, the Raman backscattered radiation at 387 is used. This signal is a purely molecular signal and minimizes the affects of aerosol scattering. This signal is a purely molecular signal and minimizes the affects of aerosol scattering. This Raman return can be normalized to the elastic return at 355 nm to generate the lower altitude atmospheric density profile. In the troposphere, however, a rotational Raman technique is better suited for the retrieval of temperature. A frequency-doubled, line-narrowed Nd-YAG laser is used for this measurement at 532 nm. In this technique, very narrow filters are used to separate out two regions of the rotational Raman backscatter from the atmosphere. This is very close spectrally to the transmitted wavelength. One Raman region is chosen because it is nearly insensitive to changes in temperature, and the other is chosen because of its sensitivity to temperature. The ratio of these two return signals can then be normalized to a known temperature, and from that a vertical profile of temperature can be retrieved. This technique, because of the weaker nature of the Raman returns requires integration over fairly long time intervals of 2 hours or more. In the case of polar stratospheric cloud studies, all night integrations are required to reach the necessary altitudes.

Vertical profiles of aerosol backscatter and microphysical properties can be retrieved from the ratio of the elastic backscatter and the Raman backscattered returns. The elastic signal contains both molecular and aerosol backscatter signals, while the Raman return contains only molecular scattering. The Raman signal can be normalized to the elastic signal in a region where aerosols are negligible and the ratio is equal to one.

Water vapour is a recent addition to the vertical profiles retrieved by this instrument. This is achieved from the ratio of backscattered return signals at 407 nm (Raman scattered from H2O), and 387 (Raman scattered from N2). Since 407 represents the concentration of water vapour in the atmosphere, and the 387nm return is representative of the atmospheric density as a whole, the ratio of these two signals can be calibrated to yield the mixing ratio of water vapour in the atmosphere. This measurement is a new one within the NDACC and is not yet archived.

Data Products

The table below provides an overview of the products that can be obtained with AT and their corresponding altitude ranges:

• Stratospheric temperature profile: ~ 12 km to above 75 km
• Tropospheric temperature profile: ~ 12 km to above 75 km
• Stratospheric aerosol backscatter ratios (1064, 532 and 355 nm): ~ 10 km to 25 km
• Tropospheric aerosol backscatter ratios 1064, 532 and 355 nm): ~ 1 km above site to 15 km
• Aerosol depolarisation ratio: ~ 1 km above site to 25 km
• Tropospheric water vapour mixing ratio: ~ 1 km above site to 14 km

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