Tuesday, 13 January 2009: 2:00 PM
Planetary Boundary Layer dynamic measurements with new compact long range wind Lidar Windcube™ WLS70
Room 122A (Phoenix Convention Center)
INTRODUCTION The WindCube™ WLS70 is a new generation wind Lidar developed by LEOSPHERE for meteorological applications. The Lidar is derived from the commercial WindCube widely used used for autonomous and very accurate measurements by the wind industry. It has been modified increasing the range up to 4 km. First results of the measurement campaign which took place in Orsay, France in August 2008 put in evidence both vertical wind speed and atmosphere structure (PBL height , clouds top and base) derived from Lidar data with good time resolution (30s per profile), good range resolution (50m from 100m to 4000m), and good velocity resolution (0.2m/s). These measurement data play a key role in many meteorological applications, and can be used as input for forecast mathematical models. MEASUREMENT SET-UP WindCube™ WLS70 was deployed in Orsay (48.7N 2.2W) in the beginning of August 2008. The WLS70 Lidar instrument was placed close together with an EZ Lidar™ ALS450. EZ Lidar™ ALS450, is a rugged and compact eye safe aerosol Lidar, that uses a tripled pulse laser source Nd:YAG at 355nm wavelength with an energy of 16mJ and pulse repetition frequency of 20 Hz. Both analog and photon counting detection are available. The Lidar system, validated through several measurement campaigns, provides a real time measurement of backscattering and extinction coefficients, aerosol optical depth (AOD), automatic detection of the planetary boundary layer (PBL) height and clouds base and top from 100m up to more than 20 km depending on atmospheric conditions and time of the day. The WindCube WLS70 is a wind Lidar developed for meteorological applications. The Lidar is derived from the commercial WindCube WLS7 as described in  with some major changes: The conical scan has been removed in order to measure only the vertical wind speed. The beam is then transmitted vertically in starring mode. The optical set up, the pulse length and the number of averaged pulses have been modified to increase the range greater than 4000 meters and however maintain vertical wind velocity resolution better than 0.4 m/s The WLS70 has been integrated in the same compact casing than the WLS7 WindCube. The table below summarizes the WindCube WLS70 parameters Parameter WindCube WLS7 WindCube WLS70 Conical scan Yes Under Development Vertical speed Yes Yes Horizontal speed and direction Yes Available Soon Range 40-200m 100-4000m Range resolution (FWHM) 20m 50m Number of range gates 10 70 Velocity resolution 0.2m/s 0.2m/s Nb of averaged shots/profile 10000 20000 Measurement time 1.5s 30s (10s average, 20s computing time) Table 1 Major difference between WLS7 and new long range WLS70 MEASUREMENT DATA The WLS70 measures both the amplitude and spectral content of the backscattering signal. From raw data, the embedded signal processing software performs the computation of the aerosol backscattering coefficient and vertical wind speed profile. The data are stacked in 2D arrays with horizontal axis corresponding to elapsed time and vertical axis corresponding to altitude. Normalized Relative Backscatter (NRB) and Vertical Wind Speed are coded in false colors, from blue for lower values (down) to red for higher values(up). NRB and velocity data are stored in binary files. WLS70 software transforms these data into ASCII format for further processing. CET(Central European Time) time has been selected for timestamps. Similarly, EZ Lidar™ data are plotted in 2D array. MEASUREMENT RESULTS On 1st August 08, the temporal plots of the Vertical Wind Speed and normalized relative backscattering (NRB) measured by WindCube™ WLS70 and EZ Lidar™ in the same spatial-temporal frame are plotted in Figure 1. Higher values of NRB are proportional to higher aerosol concentration. At 1540 nm, molecular scattering is negligible; it is then possible to directly retrieve the Planetary Boundary Layer height evolution observing the height at which the WindCube NRB drops drastically. In Figure 1, where on top is shown Vertical Wind Speed, in the middle the WindCube NRB and on bottom the EZ Lidar NRB, we can observe the evolution of the PBL with the classical late afternoon decrease of the residual layer to the stable layer in the night hours. Vertical wind speed explains PBL dynamics. Strong vertical air motion and turbulence can be observed until 11pm. The NRB profile retrieved with EZ Lidar™ put in evidence the same textures. The higher response to the aerosol load from 10pm indicates a non turbulence layer with relative stability. In these conditions the pollutants do not disperse in the atmosphere but they stay then increasing the level health risks. This is confirmed also from the wind measurement where after 10pm the vertical wind is negligible. . 5. CONCLUSIONS WindCube™ WLS70 is a pulsed Doppler Lidar based on fiber-optic technology transferred from Onera (French Aerospace Lab) and has been experimentally tested and validated through EZ Lidar™ ALS450. The system was designed for long-range, range-resolved, atmospheric wind measurements. Due to its robustness and fully transportability, together with velocity resolution, range and temporal resolutions of 0.4 m/s , 50 m, and 30s are potentially useful for a range of boundary layer meteorology applications where punctual measurements in the micro and mesoscale are required. Simultaneous horizontal speed and direction measurements will be soon available on these lidars.