Ground-based calibration of rotational Raman lidar for profiling atmospheric temperature

Abstract:

Temperature profiling in the atmospheric boundary layer is vital for understanding urban climates, such as localized heavy rains causing water-related disasters and urban heat islands. Rotational Raman Lidar (RRL) has been developed to obtain temperature profiles with high spatiotemporal resolutions. However, we must obtain its associated calibration factors to estimate the temperature during the observations by comparing lidar signals with the temperature value of independent measurement techniques (e.g., radiosonde). This is the main factor limiting the multipoint deployment and long-term operation of RRL.

In this study, we aim to establish a versatile calibration method for Raman lidar with in situ observations. While conventional RRL methods detect the ratio of signals on two wavelengths with different temperature dependencies, our multispectral method can grasp the shape of the Rotational Raman Spectrum (RRS) to reduce the uncertainties in the optical alignment of the polychromator and in the stability of the laser wavelength. Considering these characteristics of our system, we propose a method to obtain the RRS in the laser irradiation area on the ground without combined measurements from other instruments before emitting into the atmosphere. A prototype unit of the ground-based compact calibration system was constructed by controlling the temperature in a small detection area surrounding the laser beam path to detect the RRS at each temperature using the lidar detector. The calibration factors were obtained in the equivalent of radiosonde measurements under ideal conditions, regardless of the weather and location. Ground-based calibration could be performed simultaneously with atmospheric measurements to observe the vertical temperature distribution. The unit will be applied for real-time continuous calibration, thereby providing a more accurate temperature estimation than conventional methods that calibrate based on comparison with intermittent radiosonde observations. We show the preliminary results of the RRL using the proposed ground-based calibration system, including a suitable analysis method for this system associated with temperature estimation.