Land-Atmosphere Interactions Measured with Raman Lidar

The EPFL water vapor and temperature scanning solar blind Raman lidar is a research tool developed for atmospheric boundary layer studies. The lidar has raw spatial and temporal resolutions of 1.25 meters and 1 second respectively and offers a new vision of the atmosphere. This study is dedicated to the validation of its design and investigates some of the potential applications and capacities of such a remote sensing technique. The use of four telescopes is evaluated with preliminary indoor experiments. The calibration of the system, obtained with point sensors along the beam is necessary to retrieve the absolute values of water vapor mixing ratio and temperature. The accuracy of water vapor measurement is below 0.32 g/kg with 2 min by 3.75 m averaged signals, from 15 up to 500 meters. The temperature measurements needs more work to be fully satisfying. The quality of the lidar data is verified with a signal to noise ratio threshold method. In order to test the lidar ability to detect various atmospheric boundary layer phenomena, three sites are studied with different scanning strategies. Vertical soundings and scans allow the measurement of stable atmospheric situations as well as low level clouds, front passages, convective cells, nocturnal jets and internal boundary layers. Street canyon effect and coherent patterns of humidity are studied with horizontal soundings. The different analysis performed support the hypothesis that coherent structures in the humidity time-space field are signatures of low-speed fluid cores induced by nested packets of hairpin vortices. It is the first time that such observations are reported with lidar measurements. Comparing the energy spectra from lidar and eddy covariance system measurements allows for the definition of the smallest turbulent scales that the lidar can confidently detect in a static orientation. Such scales range from 14 to 25 seconds below 150 meters from the light source and support the use of lidar measurements to retrieve fluxes.


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