Temperature Measurements with Lidar

Abstract

Temperature is a key parameter of the state of the atmosphere. Temperature data play an important role in such fields as atmospheric dynamics, climatology, meteorology, and chemistry, to name just a few. In addition to these direct geophysical applications, the atmospheric temperature profile is necessary as an input parameter to many remote-sensing tech-niques including lidar for the determination of other quantities. Examples are the measurement of the particle backscatter and particle extinction coefficients with Raman lidar [1], particle polarization [2], water-vapor mixing ratio with Raman lidar [3], or the measurement of trace-gas concentrations with differential-absorption lidar. Until now, temperature profiles of a model atmosphere or the results of radiosonde soundings (in the free troposphere and lower stratosphere) have usually been taken for this purpose. It is obvious that the quality of the results is considerably improved when the atmospheric temperature profile is measured at the same location during the same time interval. 10.2 Overview on Temperature Lidar Techniques Today, lidar techniques for the remote sensing of atmospheric tem-perature profiles have reached the maturity necessary for routine observations. Stable and rugged systems have been employed success-fully and advanced the understanding of atmospheric processes and climatology. At present, there are three lidar techniques available for 274 Andreas Behrendt routine observations. Together, they cover a height range from the ground to the lower thermosphere: rotational Raman (for observations from the ground to the upper stratosphere), the integration technique (from the lower stratosphere up to the mesopause), and the resonance fluorescence technique (from the mesopause region to the lower thermosphere). As any optical measurement in the atmosphere that does not depend on the sun or some other celestial object as a radiation source, these techniques work best at nighttime when the background noise is low. However, high-power systems with spectrally narrow signal detection have to date been set up that also perform well under daytime conditions. Other lidar techniques for temperature profiling are under development such as high-spectral resolution lidar (HSRL) and differential absorption lidar (DIAL). In this chapter, lidar techniques for temperature-profile measure-ments are reviewed. The reader will find a detailed description of integration lidar and rotational Raman lidar. The remaining techniques for lidar temperature profiling are covered in detail in other chapters of this book: the resonance fluorescence technique in Chapter 11, the HSRL technique in Chapter 5 and temperature measurements with DIAL in Chapter 8. An overview of the techniques presently available for temperature profiling with lidar is given in Table 10.1. The integration technique uses a molecular backscatter signal. This signal can be either the Rayleigh band, the Cabannes line (sometimes also called " Rayleigh line "), a temperature-independent fraction of the pure rotational Raman band, a vibrational Raman band or line, or part of the central (" Gross ") Brillouin line. The intensity of a molecular lidar signal is proportional to the number density N of atmospheric molecules at height z, N(z). Under the assumption that the atmosphere is in hydro-static equilibrium and with the initialization at a reference height, the temperature profile can be derived from N(z) and the ideal-gas law. To extend the height range of the integration technique downward below ∼30 km where particles are present in quantities sufficient to severely perturb Rayleigh integration lidar, an inelastic lidar such as the vibra-tional Raman signal of N 2 can be used instead of the Rayleigh signal [4–6]. However, when aerosol extinction becomes significant compared with molecular extinction, this technique also fails. Here rotational Raman (RR) lidar is the method of choice. With RR lidar, temperature measurements can be carried out not just in the clear atmosphere, but in aerosol layers and optically thin clouds as well.