Modeling of a Semiconductor Based Differential Absorption Lidar for Temperature Profiling in the Lower Troposphere

Temperature and water vapor are two fundamental thermodynamic variables that help define the state of the atmosphere.  The atmospheric temperature and water vapor fields influence many important atmospheric processes including radiative transfer, the global energy and water cycles, mesoscale circulation, convective initiation, and vertical stability, and land surface-atmosphere feedback.  While space-borne measurements of temperature and water vapor profiles, referred to as thermodynamic profiles, provide vital measurements for applications such as global numerical weather prediction, they lack the spatial and temporal resolution within the lower troposphere needed for weather and climate research studies.  The need for improved thermodynamic profiling in the lower troposphere by the research community has been documented through recent National Research Council (NRC) reports, reports to the National Science Foundation and National Weather Service, and in the literature. 

     Differential absorption lidar (DIAL) can be used for measuring both temperature and water vapor in the lower troposphere.  DIAL utilizes a pulsed laser transmitter operating at two closely space wavelengths, one associated with an absorption feature for the molecule of interest called the online wavelength and the second minimally affected the absorption feature called the offline wavelength.  The backscatter signal from the online and offline wavelengths can then be used to retrieve either the number density profile as a function of range for water vapor if a temperature insensitive absorption feature is chosen or the temperature profile if a temperature sensitive absorption feature is chosen for a molecule with a well know atmospheric mixing ratio such as oxygen (O₂).

     Researchers at Montana State University (MSU) and the National Center for Atmospheric Research (NCAR) have developed a field deployable semiconductor-based micropulse DIAL instrument for water vapor profiling in the lower troposphere.  The laser transmitter is based on two distributed Bragg reflector (DBR) diode lasers and a tapered semiconductor optical amplifier (TSOA) operating near 828.2 nm while the DIAL receiver utilizes a shared telescope design and an avalanche photodiode (APD) operating in the photon counting mode.  These components are available over a wide range of wavelengths in the near infrared (IR) spectral region allowing the DIAL architecture to be used for a variety of applications – including temperature profiling in the lower troposphere.  Building on our semiconductor-based water vapor DIAL instrument architecture, a design for an eye-safe O₂ DIAL instrument operating near 769.2 nm for temperature profiling in the lower troposphere has been developed.  

    A performance model of this temperature profiling lidar instrument in the lower troposphere will be presented.  A differential error analysis will be utilized to estimate the error in the differential absorption coefficient.  Combining the estimated error in the retrieved absorption, with the change in the absorption coefficient as a function of temperature and altitude, the error in the retrieved temperature profile can be estimated.  The model assumes a separate channel is available for measuring partitioning of the molecular and aerosol backscattered light.  Initial calculations indicate that with 10 minute (20 minute) averaging time and a range resolution of 150 m, an error of less than 1 C in the retrieved temperature profile can be maintained below 3 km (3.5 km) for daytime operations.