Raman Lidar Measurements of Atmospheric Temperature during International H2O Project

1. Leipzig, Germany, September 14th-20th, 2003 Raman Lidar measurements of atmospheric temperatureduring the International H2O ProjectPaolo Di Girolamoa, Rocco Marchesea, David N. Whitemanb, Belay B. DemozbaDIFA, Università degli Studi della Basilicata, C.da Macchia Romana, 85100 Potenza, Italy bNASA/GSFC, Mesoscale Atmospheric Processes Branch, Greenbelt, MD 20771, USA

2. Measurements of atmospheric temperature: • Highaccuracy • High time and spaceresolution • Global coverage. Comprehension of meteorological processes and climate trends • Observational requirements for networks of ground-based and satellite remote sensors • World Meteorological Organization (WMO) Measurements of atmospheric temperature: Vertical extent = up to the LS Accuracy = 0.7 K Vertical resolution = 0.1 km Temporal resolution = 15 min Globally distributed Leipzig, Germany, September 14th-20th, 2003

3. Lidar systems have the potential to achieve these obs requirements • Lidar measurements of atmospheric temperature: • Combined Rayleigh-vibrational Raman scattering technique (Hauchecorne and Chanin, 1980) • Differential absorption technique • (Mégie, 1980; Theopold and Bösenberg, 1993) • Rayleigh backscatter spectral width measurement technique (Fiocco et al., 1971) • Pure rotational Raman technique • (Cooney, 1972) Measurements reported in this presentation make use of the pure rotational Raman (RR) technique in the UV region. All pure rotational Raman lidar measurements reported in literature have been performed in the visible domain. Leipzig, Germany, September 14th-20th, 2003

4. Advantages of the use of UV laserlight instead of visible • increase the precision • achievebetter daytime performances due to reduced sky background • safer in terms of hazard for eye injury • Threshold for thermal retinal damage: • 3 orders of magnitude lower than in the visible • Maximum allowed exposition of human eye (for laser pulses 1 to 100 ns in duration): • 30 J/m2 in the spectral region 280-400 nm • 5 mJ/m2 in the 400-700 nm region (EN 60825-1, 2001). UV laser beams used in most lidar applications result to be eye-safe within few hundred meters from the laser source Leipzig, Germany, September 14th-20th, 2003

5. NASA Scanning Raman Lidar, SRL • Mobile system in a environmentally controlled trailer • Nd:YAG laser • 0.76 meter telescope • Large aperture scanning mirror • Outfitted with a UV rotational Raman temperature measurement capability prior to IHOP Nd:YAG laser: Single pulse energy @ 354.7 nm = 300 mJ Pulse repetition rate = 30 Hz Linewidth (FWHM)=1 cm-1 Frequency stability<0.5 cm-1 Beam divergence=250 mrad Leipzig, Germany, September 14th-20th, 2003

6. Filter assembly: based on interference filters (IFs) • Sensitivity study • careful analysis of the temperature dependence of rotational lines • maximising measurement precision • maximising measurement sensitivity • minimize potential sources of contamination, as RR scattering from water vapour Filters specs

7. International H2O Project (IHOP) Location: Southern Great Plains (USA) Period: May-June 2002 Main goal: study the role of water vapour in convection initiation and to improve on quantitative precipitation forecasting (QPF). SRL deployment: Homestead site, Western Oklahoma • SRL operated for approximately 35 days during IHOP • Most of the measurements were carried out in vertically pointing mode Approx. 200 hours of SRL data Radiosonde launch station next to SRL 148 radiosondes launches Leipzig, Germany, September 14th-20th, 2003

8. CALIBRATION FUNCTION • exactly valid for two individual lines (Arshinov et al., 1983) • can be assumed valid also for portions of RR spectrum Low-J filter: 4 rotational lines J=5 from O2, J=7 from O2, J=4 from N2, J=5 from N2, High-J filter:17 rotational lines J=14-23 from O2, J=19-32 from O2 Systematic error assuming calibration analytical expression valid for portions of RR spectrum < 1.5 K. Leipzig, Germany, September 14th-20th, 2003

9. Calibration constantsa and b determined through comparison with simultaneous radiosondes. • 6 lidar-radiosonde comparisons • Inclusion of both night-time and twilight cases • a = -758 ± 6 andb = 0.95 ± 0.02 Systematic error associated with indetermination of calibration constants max 2 K • Systematic error associated different overlap functions in the two RR channels • near range, < 1-2km max 2 K Systematic error associated with laser frequency looking accuracy/stability < 0.5 cm-1 max 0.5 K Assuming the different sources of systematic error to be independent max 3 K z < 2 km max 2 K z > 2 km Overall systematic error

10. MEASUREMENTS NIGHT TIME MEASUREMENT • Error bars statistical uncertainty only • Ended 1/2 hour before sunrise • almost clear sky conditions • Lidar measurements • up to approx. 23 km (rand. error > 5 K) 9 June 2002 Rand.err.@1.5 K at 15 km • Lidar-radiosonde comparison • Good agreement • Deviations • < 2 K up to 14 km • < 3 K up to 17 km (max. sonde height) • Average bias = 0.5 K • RMS deviation = 1.2 K Leipzig, Germany, September 14th-20th, 2003

11. MEASUREMENTS TWILIGHT MEASUREMENT • started 1 hour before sunset (twilight conditions) • almost clear sky conditions 2 June 2002 • Lidar measurements • up to approx. 14 km • smaller vertical extent vs night-time • day-dusk transition (lidar performances degraded by solar background noise • Lidar-radiosonde comparison • Good agreement • Average bias = 0.2 K • RMS deviation = 1.8 K Leipzig, Germany, September 14th-20th, 2003

12. Simulations→ quantify measurement precision of RR technique • 355 and 532 nm • nigh-time and daytime operation Behrendt and Reichardt, 2000 • Poisson statistics for backscatter and background signals • Pressure, temperature and humidity from US standard atm (1976) • Aerosol extinction data from the ESA ARMA (1999), median model • No clouds Filters specs at 532 nm were defined in order to isolate the same rotational Raman lines as at 355 nm (same quantum numbers). • Overall rec. efficiency @ 355 nm=0.055 • receiving optics reflectivity (0.9) • filter transmission (0.3) • detector quantum efficiency(0.2)) • Overall rec. efficiency @ 532 nm=0.055 • receiving optics reflectivity (0.9) • filter transmission (0.5) • detector quantum efficiency(0.12)) Same power-aperture product @ 355 and 532 nm, as SRL • Daylight background • Mainly due to scattering of sunlight • Determined from Modtran database • Sun zenith angle = 400 • bk355=0.15 bk532 Leipzig, Germany, September 14th-20th, 2003

13. Daytime simulations • Two spectral selection configurations: • use of IFs only • use of combination of a Fabry-Perot interferometer and IFs • to reduce sky background(Arshinov et al., 2001; Bobrovnikov et al., 2002) • Gain in signal-to-background ratio @65 (ratio between average separation between adjacent lines (3.3 cm-1) and the spectral width of individual lines (0.05 cm-1). SIMULATION Vertical resolution = 100 m (to fit WMO requirements) Night-time DT355 < DT532 20-50 % Dt=1 h DT < 0.4 K Dt=15 min DT < 0.7 K z<15 km @ 355 and 532 nm Satisfies target observational requirements from WMO

14. Day-time IFs only SIMULATION DT355 @ 0.2 DT532 Dt=1 h, z<15 km DT355 < 2.5 K Fabry-Perot + IFs DT355 @ 0.2 DT532 Dt=1 h, z<15 km DT355 < 0.7 K Leipzig, Germany, September 14th-20th, 2003

15. Conclusions and Future Plans • Measurements of atmospheric temperature in the UV have been performed based on the application of the pure rotational Raman technique • First successful attempt to perform RR temperature measurements throughout the tropospherein the UV region • eye-safe concerns are less stringent than VIS and IR • increase the precision • betterdaytime performances due to reduced sky background • Simulation have been performed in order to quantify the potentialities in terms of measurement precision of the RR lidar technique both in the visible and UV • Simulations reveal that night-time measurements satisfy target observational requirements from WMO Future:Implement high resolution spectral detection based on FP+IFs

17. Photomultipliers: • included inside unshielded housings • performances altered by the laser induced electromagnetic noise (SIN) • reduction in photon count rates (both low-J and high-J chns) • system configuration unoptimized for temperature measurements. signal discrimination level for photon counting increased (2mV → 3 mV) Leipzig, Germany, September 14th-20th, 2003

18. BACK-UP SLIDES

19. Cirrus cloud between 10.5-12 km Peak scattering ratio = 10

20. Raw lidar data vertical resolution = 30 m • Vertically smoothing = 600 m in order to reduce signal statistical fluctuations • Smoothing procedure • binning • assigning equal weight to each data point Leipzig, Germany, September 14th-20th, 2003

21. LASER Nd:YAG laser: Single pulse energy @ 354.7 nm = 350 mJ Pulse repetition rate = 30 Hz Unseeded Linewidth (FWHM)=1 cm-1 Frequency stability=0.5 cm-1 Beam diverengence=250 mrad According to manufacturer specifications, laser fluctuations resulting from thermal drifts inside the laser cavity are expected to guarantee a frequency looking accuracy/stability better than of 0.5 cm-1. Consequent changes in amplitude of detected signals, primarily the high J signal, may lead to a systematic error which has been estimated to not exceed 0.5 K. 3.1GHz/K

22. FILTER ASSEMBLY A filter blocking at the laser wavelength of 10-6 has been estimates to prevent from contamination due to elastic echoes from aerosol/cloud structures with a scattering ratio up to 10.

23. Gain in signal-to-background ratio @ 65 (ratio between average separation between adjacent lines (3.3 cm-1) and the spectral width of individual lines (0.5 cm-1).

24. Leipzig, Germany, September 14th-20th, 2003

25. Single attempt to use the RR technique in the UV region: • Agnew and Twort (2002) Refractivity measurements up to 3 km obtained from the combination of RR temperature measurements with vibrational Raman water vapour measurements.

26. Measured parameters • water vapor mixing ratio • particle extinction, backscattering and depolarization • cloud liquid water, cloud droplet radius and number density Exclusively during IHOP: temperature profile through the rotational Raman technique

27. International H2O Project (IHOP) Location: Southern Great Plains (USA) Period: May-June 2002 Main goal: study the role of water vapour in convection initiation and to improve on quantitative precipitation forecasting (QPF). SRL deployment: Homestead site, Western Oklahoma • SRL operated for approximately 35 days during IHOP • Most of the measurements were carried out in vertically pointing mode Leipzig, Germany, September 14th-20th, 2003

28. SRL outfitted with a UV rotational Raman temperature measurement capability prior to the field campaign. • Filter assembly developed at University of Basilicata. • Filters’ specifications resulted of a detailed sensitivity study • based on a careful analysis of the temperature dependence of rotational lines, considering different temperature regimes; • maximising measurement precision.

29. HEIGHT TEMPERATURE

30. Target requirements from: • Global NWP • RegionalNWP • Synoptic meteorology • Nowcasting, • Global climate modelling • SPARC First successful attempt to perform RR temperature measurements in the UVthroughout the troposphere. The use of the alternative the analytical expression leads to slightly smaller systematic errors not exceeding 1 K

31. Lidar-radiosonde comparison • Good agreement • Deviations • < 3 K • Average bias = 0.2 K • RMS deviation = 1.8 K • Larger RMS vs night-time due to larger statistical uncertainty

32. Night-time measurements • lidar up to approx. 23 km • Lidar-radiosonde comparions • Deviations < 2 K up to 14 km • Average bias = 0.5 K • RMS deviation = 1.2 K Night-time Day-time Day-time • DT355 < DT532 20-50 % • Dt=1 h, z<15 km • DT355,DT532 < 0.4 K • Dt=15 min, z<15 km • DT355,DT532 < 0.7 K IFs only Fabry-Perot+IFs DT355 @ 0.2 DT532 DT355 @ 0.2 DT532 Dt=1 h, z<15 km DT355 < 2.5 K Dt=1 h, z<15 km DT355 < 0.5 K