Remote Sensing of Planetary Boundary Layer Temperature and Water Vapor Using Near-, Mid- and Thermal-Infrared Measurements

The planetary boundary layer (PBL)—the turbulent layer adjacent to the ocean, land and ice surface that mediates the interactions between the surface and the troposphere—is at the heart of several key climate science challenges, including cloud-climate feedbacks, extreme weather and energy exchange. The PBL is a global feature, including the world’s oceans and remote land regions where in-situ observations are rare. For a global characterization of the PBL, space-based observations are essential. Since the PBL is often dominated by convective mixing, the PBL temperature and water vapor vertical structure determine and characterize many of the key physical processes, such as entrainment and clouds. To realistically characterize the PBL from a global perspective, there is an urgent need for more accurate observations of the vertical profiles of PBL temperature and water vapor. Current infrared (IR) and microwave sounding has fairly coarse horizontal resolution and broad weighting functions in the PBL. The utility of radio occultation is challenged by an additional dependence on pressure and temperature, sampling issues and an even coarser horizontal resolution.

We propose to perform information content analysis to evaluate the sensitivity of different spectral regions in the near, mid and thermal IR to PBL water vapor and temperature, in order to determine specifications for a future instrument to profile these key quantities. For this purpose, we utilize multiple H₂O absorption bands with different temperature sensitivities. The retrieval technique is demonstrated by (1) conducting a series of sensitivity experiments to select the best combination of H₂O absorption bands that show high information content in retrieving H₂O and temperature within the boundary layer; (2) conducting retrievals using simulated synthetic spectra and satellite observation geometries over different surface types to ensure the robustness of the proposed technique; and (3) testing the proposed technique using real measurements from the California Laboratory for Atmospheric Remote Sensing. The results show that the temperature and water vapor gradient between the bottom and top of the Los Angeles city boundary layer may be measurable by a sufficiently sensitive instrument.

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