A RAMAN LIDAR WITH A DEEP ULTRAVIOLET LASER FOR CONTINUOUS WATER VAPOR PROFILING IN THE ATMOSPHERIC BOUNDARY LAYER

A Raman lidar with a deep ultraviolet laser was constructed to continuously monitor water vapor distributions in the atmospheric boundary layer for twenty-four hours. We employ a laser at a wavelength of 266 nm and detects the light separated into an elastic backscatter signal and vibrational Raman signals of oxygen, nitrogen, and water vapor. The lidar was encased in a temperature-controlled and vibration-isolated compact container, resistant to a variety of environmental conditions. Water vapor profile observations were made for twelve months from November 24, 2017, to November 29, 2018. These observations were compared with collocated radiosonde measurements for daytime and nighttime conditions.


INTRODUCTION
Water vapor, one of the most variable atmospheric constituents, plays an important role in atmospheric processes such as the atmospheric energy budget and the global water cycle. The distribution of water vapor is associated with that of clouds and rainfall through the vertical stability of the atmosphere caused by a large amount of latent heat related to the phase changes of water. Therefore, water vapor is a key parameter in understanding the localized extreme weather events associated with severe weather disasters such as torrential rain and floods. Information on the spatiotemporal distribution of water vapor is highly beneficial for improving the accuracy of weather forecasts made by mesoscale numerical weather prediction models.
The Raman lidar technique is a wellestablished tool for measuring the water vapor mixing ratio in the atmosphere 1 . While most of the Raman lidar previously used for the monitoring and field observation of water-vapor distribution employ a laser wavelength of 355 nm, Renaut et al. (1980Renaut et al. ( , 1988 and Lazzarotto et al. (2001) demonstrated the Raman lidar with a 266 nm laser and a grating polychromator to conduct daytime observations during periods of high sky radiance 2,3 . The use of the deep ultraviolet wavelengths is convenient because of the low background noise during the daytime, since most of the solar radiation in the wavelength range below 300 nm is absorbed by the ozone layer in the stratosphere. Recently, the improved performance of UV optical components has led to the development of enhanced Raman lidar systems with a deep ultraviolet lidar. In this study, we constructed a Raman lidar by employing a laser at a wavelength of 266 nm and an interference-filter-based polychromator.

Water vapor profiles from Raman lidar measurements
The water-vapor mixing ratio m at range R is obtained from the Raman backscatter signals of water vapor H 2 O , nitrogen N 2 , and oxygen O 2 as follow: where Km is the calibration constant and commonly evaluated by comparing the results of an independent measurement (e.g., radiosonde) of the water vapor mixing ratio; σ O 3 ( ) is the ozone absorption cross section at Raman-shifted wavelength of species x; and ∆ indicates the difference in transmission of each Raman-shifted wavelength due to Rayleigh and Mie scattering processes.

System setup
The experimental setup of the Raman lidar is shown in Fig. 1. We employed the fourth harmonic output (266 nm) of an Nd:YAG laser (Continuum Surelite III-10, USA) and operated with the pulse energy of < 50 mJ. The beam divergence of the outgoing laser beam after passing the beam expander is 0.17 mrad. A custom-made Cassegrain telescope (Raymetrics, Greece) with a primary mirror diameter of 200 mm and a focal length of 1000 mm was used as the optical receiver. The backscattered radiation was separated by the dichroic beam splitters and the interference filters to elastic and inelastic wavelengths of 266 nm for the elastic scattering, and 277.5 nm, 283.6 nm, and 294.6 nm for the vibrational Raman scattering of oxygen, nitrogen and water vapor molecules, respectively. Three photomultiplier tubes (PMT) were used for detecting each backscatter light (R9880U, Hamamatsu, Japan). The data acquisition was performed using a four-channel transient recorder (TR 20-16bit, Licel, Germany) to record the analog and photon counts in the signal pulses. The lidar was encased in a temperaturecontrolled and vibration-isolated compact container with laser and quartz windows for transmitting the laser beam and the scattered light from the atmosphere, respectively. Water vapor mixing ratio profiles from the radiosonde and the Raman lidars for selected cases during nighttime and daytime are shown in Fig. 2

CONCLUSIONS
We have constructed a Raman lidar for observing long-term monitoring of water vapor mixing ratio profiles in the atmospheric boundary layer for twenty-four hours. The Raman lidar employs a laser at a wavelength of 266 nm, characterized by the low background noise during daytime, and an interference-filter-based polychromator. The lidar was encased in a temperature-controlled and vibration-isolated compact container. We have demonstrated the potential of the developed lidar by collecting observations over a whole year at Shigaraki MU radar observatory in Japan. The lidar worked well throughout the twelve months. Water vapor mixing ratio profiles from the radiosonde and the Raman lidars agreed well up to about 1500 m which is the top of the atmospheric boundary layer. The variations of the calibration constant, as well as the derived water vapor profiles, were small before and after replacing the flashlamp every 2-4 months.