Raman Lidar measurements of atmospheric temperature during the International H 2 O Project Paolo Di Girolamo a, Rocco Marchese a, David N. Whiteman b, Belay B. Demoz b a DIFA, Università degli Studi della Basilicata, C.da Macchia Romana, Potenza, Italy b NASA/GSFC, Mesoscale Atmospheric Processes Branch, Greenbelt, MD 20771, USA Leipzig, Germany, September 14th-20th, 2003

Measurements of atmospheric temperature: High accuracy High time and space resolution Global coverage. Leipzig, Germany, September 14th-20th, 2003 Observational requirements for networks of ground- based and satellite remote sensors World Meteorological Organization (WMO) Comprehension of meteorological processes and climate trends 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

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) Leipzig, Germany, September 14th-20th, 2003 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 Advantages of the use of UV laser light instead of visible increase the precision achieve better 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/m 2 in the spectral region nm 5 mJ/m 2 in the nm region (EN , 2001). UV laser beams used in most lidar applications result to be eye-safe within few hundred meters from the laser source

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

Filter assembly: based on interference filters (IFs) Filters specs 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 Filter assemblyLow-JHigh-J CWL (nm) FWHM (nm) CWL transmission30 % 355 nm10 -6

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 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 Approx. 200 hours of SRL data Radiosonde launch station next to SRL 148 radiosondes launches

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 O 2, J=7 from O 2, J=4 from N 2, J=5 from N 2, High-J filter: 17 rotational linesJ=14-23 from O 2, J=19-32 from O 2 Systematic error assuming calibration analytical expression valid for portions of RR spectrum < 1.5 K. CALIBRATION FUNCTION Leipzig, Germany, September 14th-20th, 2003

Calibration constants a and b determined through comparison with simultaneous radiosondes. 6 lidar-radiosonde comparisons Inclusion of both night-time and twilight cases a = -758 ± 6 and b = 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 Overall systematic error max 3 K z < 2 km max 2 K z > 2 km

MEASUREMENTS 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) 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 NIGHT TIME MEASUREMENT Rand.err.  1.5 K at 15 km 9 June 2002 Leipzig, Germany, September 14th-20th, 2003

TWILIGHT MEASUREMENT MEASUREMENTS started 1 hour before sunset (twilight conditions) almost clear sky conditions 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 2 June 2002 Leipzig, Germany, September 14th-20th, 2003

Simulations → quantify measurement precision of RR technique 355 and 532 nm nigh-time and daytime operation 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 Behrendt and Reichardt, 2000 Overall rec. 355 nm=0.055 receiving optics reflectivity (0.9) filter transmission (0.3) detector quantum efficiency(0.2)) Overall rec. 532 nm=0.055 receiving optics reflectivity (0.9) filter transmission (0.5) detector quantum efficiency(0.12)) Same power-aperture 355 and 532 nm, as SRL Filters specs at 532 nm were defined in order to isolate the same rotational Raman lines as at 355 nm (same quantum numbers). Leipzig, Germany, September 14th-20th, 2003 Daylight background Mainly due to scattering of sunlight Determined from Modtran database Sun zenith angle = 40 0 bk 355 =0.15 bk 532

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 ). Vertical resolution = 100 m (to fit WMO requirements) Night-time  T 355 <  T %  t=1 h  T < 0.4 K  t=15 min  T < 0.7 K z< and 532 nm SIMULATION Satisfies target observational requirements from WMO

Day-time  T 355  0.2  T 532  t=1 h, z<15 km  T 355 < 2.5 K IFs only  T 355  0.2  T 532  t=1 h, z<15 km  T 355 < 0.7 K Fabry-Perot + IFs SIMULATION Leipzig, Germany, September 14th-20th, 2003

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 troposphere in the UV region  eye-safe concerns are less stringent than VIS and IR  increase the precision  better daytime performances due to reduced sky background Future:Implement high resolution spectral detection based on FP+IFs Simulations reveal that night-time measurements satisfy target observational requirements from WMO 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

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

BACK-UP SLIDES

Cirrus cloud between km Peak scattering ratio = 10

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

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. Nd:YAG laser: Single pulse nm = 350 mJ Pulse repetition rate = 30 Hz Unseeded Linewidth (FWHM)=1 cm -1 Frequency stability=0.5 cm -1 Beam diverengence=250  rad LASER 3.1GHz/K

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

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

RequirementHor. Res. (km) Vert. Res. (km) Obs. Cycle (h) RMS (K)UserApplication Lower troposph.10/5000.3/30.5/120.5/3WMORegional NWP Upper troposph.10/5001/30.5/120.5/3WMORegional NWP Lower tratosph.10/5001/30.5/120.5/3WMORegional NWP Lower troposph.50/5000.3/31/120.5/3WMOGlobal NWP Upper troposph.50/5001/31/120.5/3WMOGlobal NWP Lower tratosph.50/5001/31/120.5/3WMOGlobal NWP Lower troposph.20/2000.1/23/120.5/3WMOSynopt. Meteor. Upper troposph.20/2000.1/23/120.5/3WMOSynopt. Meteor. Lower tratosph.20/2000.1/23/120.5/3WMOSynopt. Meteor. Lower troposph.5/2000.5/10.25/10.5/2WMONowcasting Upper troposph.5/2001/30.25/11/2WMONowcasting Lower troposph.50/5000.5/26/720.5/1WCRPSPARC Upper troposph.50/5000.5/26/720.5/1WCRPSPARC Lower tratosph.50/5000.5/26/720.5/1WCRPSPARC Lower troposph.50/5000.3/33/120.5/3WCRPGlobal Modelling Upper troposph.50/5001/33/120.5/3WCRPGlobal Modelling Lower tratosph.50/5001/33/120.5/3WCRPGlobal Modelling Leipzig, Germany, September 14th-20th, 2003

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.

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

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 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 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.

TEMPERATURE HEIGHT

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

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

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  T 355 <  T %  t=1 h, z<15 km  T 355,  T 532 < 0.4 K  t=15 min, z<15 km  T 355,  T 532 < 0.7 K Night-timeDay-time  T 355  0.2  T 532  t=1 h, z<15 km  T 355 < 2.5 K IFs only Day-time  T 355  0.2  T 532  t=1 h, z<15 km  T 355 < 0.5 K Fabry-Perot+IFs