Raman lidar in operational meteorology

Water vapor and temperature spatial distribution and their temporal evolution are among the most important parameters in numerical weather forecasting and climate models. The operational relative humidity/temperature profiling in meteorology is carried out mostly by radio sondes. Sondes provide profiles with high vertical resolution but suffer from systematic errors and low temporal resolution. The temporal resolution is also a limitation for the now-casting, which has become more and more important for meteorological alerts and for the aviation. Recently, some of national meteorological services have introduced Raman lidars for additional operational humidity/temperature profiling. The lidars allow monitoring of water vapor mixing ratio and temperature with high vertical and temporal resolutions. Here the design and measurement results from the Raman Lidar for Meteorological Observation (RALMO) developed by the Ecole Polytechnique Federal de Lausanne (EPFL) and operated by MeteoSwiss is presented as an illustration of the potential of Raman lidars in operational meteorology. The first applications of lidar data in numerical weather forecasting is also discussed.

Automated Raman Lidar for Day and Night Operational Observation of Tropospheric Water Vapor for Meteorological Applications

Todor Dinoev

Water vapor is a fundamental constituent of the atmosphere and is the most abundant green house gas thus having an important influence on climate. It is as well a key prognostic variable for numerical weather prediction models (NWP). Currently, the vertical profiles of tropospheric water vapor are provided by twice a day radiosondes. The routine observations have rather low temporal resolution that is insufficient to resolve fast-running meteorological phenomena. The aim of the thesis work was to design and build a Raman lidar instrument capable of continuous vertical profiling of tropospheric water vapor field with high temporal and vertical resolution. The provided observations will improve the database available for direct meteorological applications and could increase the accuracy of numerical weather prediction. RALMO – RAman Lidar for Meteorological Observations is developed as fully automated, eye-safe instrument for operational use by the Swiss meteorological service – MeteoSwiss. The lidar is able to provide vertical profiles of water vapor mixing ratio with time resolution from 5 to 30 min and vertical resolution from 15 m in boundary layer and 75 – 500 m in free troposphere. The daytime vertical operational range of the lidar extends from about 50 m to mid-troposphere and the detection limit is 0.5 g/kg. In night-time conditions the vertical operational range extends up to the tropopause with 0.01 g/kg detection limit. To allow daytime operation with extended vertical range and required detection limit the lidar is designed with narrow field-of-view receiver, narrow band detection, and it uses high pulse power laser with wavelength in the UV but out of solar blind region. The lidar transmitter uses flash-lamps pumped frequency tripled Nd: YAG laser generating 8 ns pulses with 0.3 J energy and 355 nm wavelength. To reduce the beam irradiance, required for eye-safe operation, and to reduce the divergence required for narrow field of view receiver, the laser beam is expanded by 15x refractive type expander. The backscattered laser light is collected by four 30 cm in diameter telescopes with focal length of 1 m. For better long term alignment stability the telescopes are tightly arranged around the beam expander in compact assembly. The field-of-view of the telescopes is reduced to 0.2 mrad to decrease the collected sky-scattered sunlight thus allowing daytime measurements up to mid troposphere. Fibers transmit the light collected by the telescopes to the lidar polychromator. Fiber coupling was preferred against free space connection because it separates the units mechanically and increases the overall lidar stability. In addition the fibers perform aperture scrambling which prevents range dependence of the receiver parameters. An additional "near range" fiber is installed in one of the telescopes to enhance the near range signal and to allow daytime measurements starting from approximately 50 m above the lidar. A high throughput diffraction grating polychromator is designed for narrow band isolation of water vapor, nitrogen, and oxygen Q branches of ro-vibrational Raman spectra. Water vapor mixing ratio is derived from the ratio of water vapor to nitrogen Raman signals and the oxygen signal is used to correct for aerosol differential extinction at water and nitrogen wavelengths. The water vapor detection channel passband is 0.3 nm. The narrow band detection increases the lidar sensitivity and operational range in daytime conditions. For maximum throughput of the polychromator, the exit and entrance slits are matched and the polychromator entrance accepts the divergent beam from the fibers without losses. Photomultipliers at the exit of the polychromator detect the Raman lidar signals, which are then acquired by transient recorders (Licel). The signals are simultaneously recorded in analog and photon-counting modes. The analog signals are used in daytime conditions with sky background signal that saturates completely the photon counter, whereas in night-time conditions, photon counting signals are used. Two computers provide for the automated operation of the lidar instrument. The first one controls all lidar units relevant to the lidar operation, including the laser and the data acquisition. For this purpose, Lidar Automat software has been developed under LabView and requires only activation by an operator. Automated data treatment software, developed under Matlab, is run on the second lidar computer. It reads the initial lidar data, treats them, and stores the final result in files ready for upload to the meteorological service. The files contain vertical profiles of water vapor mixing ratio and relative error. The lidar was completed in July 2007 and installed at the aerological station of MeteoSwiss in Payerne. Since October 2007 the lidar has been in experimental operation and the software for automated data treatment was completed. Since September 2008 the instrument has been fully operational, providing continuous vertical water vapor mixing ratio profiles uploaded to MeteoSwiss every 30 min. Regular comparisons with Vaisala RS-92 and Snow White® radiosondes were performed during the experimental lidar operation at Payerne. Long-term stability study of the calibration coefficient was performed as well.

EPFL2009

Land-Atmosphere Interactions Measured with Raman Lidar

Martin Froidevaux

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.

EPFL2010

Raman Lidar for Meteorological Observations, RALMO - Part 2: Validation of water vapor measurements

Bertrand Calpini, Valentin Simeonov

The Raman Lidar for Meteorological Observations (RALMO) was installed at the MeteoSwiss Regional Center of Payerne, Switzerland, in summer 2008. One of its aims is to provide continuous vertical profiles of tropospheric water vapor during day and night at a high temporal resolution. Twelve months (October 2009–September 2010) of lidar data are analyzed. During this period of time, the lidar produced 9086 profiles, representing 52.6% of the time (this figure reached 63.2% for the first 6 months of 2011). Under cloud-free conditions, half of the profiles reached more than 8610m above ground level at night, and 4050m during the day. In order to validate the capabilities of the instrument, the year of lidar data was compared to the collocated radiosondes. On average, lidar water vapor mixing ratio was found to be within 5 to 10% of radiosonde values up to 8 km at night, and within 3% up to 3 km during the day. Relative humidity results show an agreement within 2 and 5% for day and night, respectively. An integrated water vapor comparison also shows a good correlation with both radiosondes and GPS measurements: the lidar had a 4.2% dry bias compared to radiosondes and a 5.3% wet bias compared to GPS. These results validate the performance of the lidar and the humidity profiles with a 30 min time resolution.