Wide-Angle Imaging Lidar (WAIL) for Active Cloud Probing ... · F igu r e4 .WAIL nh acm t s: db p...
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Wide-Angle Imaging Lidar (WAIL) for Active Cloud Probing: Instrument Enhancements and New Data Analyses Steven P. Love, Thomas C. Hale, and Anthony B. Davis Space and Remote Sensing Sciences Group (ISR-2), Los Alamos National Laboratory Los Alamos, NM 87545, USA Igor N. Polonsky Department of Atmospheric Sciences, Colorado State University Fort Collins, CO 80523 Airborne THOR : •Direct measurement of Cloud Top •Derived Cloud Thickness, Bottom Ground-based WAIL: •Direct measurement of Cloud Bottom •Derived Cloud Thickness, Top Figure 3. WAIL data and retrieved cloud thickness validation. Left: Sample frames from WAIL datasets obtained at the Oklahoma CART site during the WAIL/THOR cross-validation experiment in March 2002. Shown here are sequences obtained using three different background suppression filters that optimize for different ranges of observation angle. The first (535 nm band center) emphasizes the central region, the second (540 nm) emphasizes intermediate angles, and the third (546 nm) emphasizes large angles. Time delay after the laser pulse is noted on each frame. In this 2002 implementation of WAIL, a complete dataset is obtained only after combining these three restricted- angle sets. This rather awkward procedure prompted us to re-evaluate the need for such narrow-band interference filters. Our revised instrument uses a wider-bandpass filter, enabling us to capture the full angular range at once. Right: Comparison of cloud top and cloud bottom positions determined by WAIL and THOR, as well as by the CART site’s micropulse lidar and ceilometer, showing excellent agreement between the WAIL and THOR direct and derived cloud interface positions. Gated/Intensified CCD (Roper Scientific “PI-Max”) Figure 1. The Wide-Angle Imaging Lidar (WAIL) concept, contrasted with standard lidar. Standard lidar collects returning light from only a very narrow angular range about the laser beam; light scattered out of the beam is intentionally ignored and is considered to be extinguished. For optically dense clouds, this limits standard lidar to measuring the range to the cloud bottom. WAIL, in contrast, focuses its attention specifically on the light scattered out of the beam, taking high-speed, wide-angle “movies” of the spatial distribution of returning light as a function of time after the laser pulse. This multiply scattered light samples the interior of even optically dense clouds, all the way to the top of the cloud. By measuring both the spatial and temporal distributions of returning light, WAIL can determine both the physical thickness and optical density of the cloud, and hence estimate liquid water content. At right is some typical early WAIL data, with images above and below showing the spatial distribution of returning light at various times after the laser pulse, modified by the spatial response of the background-suppression interference filters. Each image spans approximately 60˚. Graphs show the spatially-integrated signal as a function of time. The cloud deck for this example was rather complex, consisting of several thin layers beneath a denser upper layer. Figure 2. The WAIL/THOR cross-validation experiment. WAIL operated from the ground and thus provided a direct measurement of the cloud base height; THOR, airborne above the cloud, similarly provides a direct measurement of the cloud top height. Each could thus verify the other’s derived far- side interface position determination. At right is a photograph showing the WAIL instrument deployed at the SGP CART site. The laser transmitter is a doubled Nd:YAG (532 nm) operating at a repetition rate of 4 to 12 kHz, a 30 ns pulse width, and roughly 4W average power. The receiver is a gated intensified CCD camera with a time resolution as fast a 1 ns (we typically use 50 ns or longer). A WAIL “movie” is collected by sequentially advancing the CCD’s gate delay w.r.t. the laser pulse. In 2002, WAIL and THOR were deployed simultaneously at the SGP CART site in a mutual validation exercise. While successfully demonstrating the power of the off-beam lidar concept and validating retrieval algorithms, this experiment also highlighted some aspects of the hardware that could be improved. WAIL Cloud Top WAIL Cloud Base THOR Cloud Top Micropulse Lidar Cloud Top Micropulse Lidar Cloud Base Ceilometer Cloud Base Figure 4. WAIL enhancements: Wider bandpass and wider field of view. Two things became apparent during the WAIL/THOR experiment: (1) The 53.6˚ field of view, though “wide,” wasn’t wide enough for the low cloud ceilings during that campaign, leading to truncation of our spatial distribution data. This necessitated abandoning our moment- based analysis and inventing a new retrieval method [Polonsky, et al. 2005]. (2) Our use of very narrow-band background suppression filters necessitated an awkward dataset splicing process, but in practice, background light wasn’t really a limiting factor in a typical rural environment. So an obvious improvement is to increase the f.o.v. and avoid the splicing by widening the bandpass. These two laboratory images, illuminated by diffusely- scattered 532nm laser light, compare the angular acceptance of our old system (with 10nm-bandpass filter at 540nm displaying annular acceptance ) on the left, and the new system (with 50nm bandpass filter admitting the entire angular range). Fields of view are 53.6˚ square and 88.3˚ square, respectively. WAIL is the ground-based progenitor of a new class of active cloud probes that also includes the airborne THickness from Off-beam Returns (THOR) system developed at NASA Goddard and the in-cloud lidar developed by the U. of Colorado and SPEC. This class of instruments exploits the highly scattered light from a pulsed laser illuminating the cloud at a non-absorbing wavelength, and can probe optically thick clouds using VIS-NIR light which in traditional lidar is all but extinguished (i.e. scattered out of the beam) within a few mean- free-paths. In contrast with the direct beam, the multiple-scattering Green function probed by WAIL permeates the whole cloud. Thus, both the physical thickness of the cloudy medium and its optical thickness (equivalently, volume-averaged extinction) can be determined from the spatial and temporal data. Collectively, these new instruments rejuvenate optical cloud remote sensing at a time when ambitious climate monitoring programs such as ARM have identified the need for breakthrough in observational techniques to reach their common goal to advance predictive climate science. Both WAIL hardware and analysis methodology have evolved in the last year or so towards a configuration where it can sooner be incorporated into cloud instrument suites at ARM facilities. Figure 5. SNR enhancement using image intensifier. WAIL’s gated CCD is equipped with an image intensifier that, at least for the extremely dim long-time, large-scattering-angle returns, offers significant SNR improvement. Here we show a laboratory test comparing a very weak optical signal imaged with no intensification on the left, and maximum intensi- fication on the right. Artifacts make intensifi- cation less desirable for stronger signals. Figure 6. New Analysis Techniques. Here we show the angular dependence of the normalized WAIL signal for selected time- slices mapped to altitudes above ground. Below the cloud, we have 180 m (), 293 m ( ), and 406 m (). Notice that for the in- cloud range of 594 m () this angularly- resolved signal is significantly flatter than for the below-cloud ranges. The multiple- scattering retrieval method by Bissonnette et al. [2005], based on the small-angle approx- imation, will enable us to extract new information about both aerosol and cloud particle size distributions from these near-axis WAIL data. References Polonsky, I. N., S. P. Love, and A. B. Davis, 2005: “Wide-Angle Imaging Lidar deployment at the ARM Southern Great Plains site: Intercomparison of cloud property retrievals,” J. Atmos. Oceanic Technology 22, 628-648. L.R. Bissonnette, G. Roy, and N. Roy, 2005: “Multiple-scattering-based lidar retrieval: Method and results of cloud probings,” Appl. Opt., 44, 5565-5581.
Transcript of Wide-Angle Imaging Lidar (WAIL) for Active Cloud Probing ... · F igu r e4 .WAIL nh acm t s: db p...
Wide-Angle Imaging Lidar (WAIL) for Active Cloud Probing:Instrument Enhancements and New Data Analyses
Steven P. Love, Thomas C. Hale, and Anthony B. DavisSpace and Remote Sensing Sciences Group (ISR-2), Los Alamos National Laboratory
Los Alamos, NM 87545, USA
Igor N. PolonskyDepartment of Atmospheric Sciences, Colorado State University
Fort Collins, CO 80523
Airborne THOR:•Direct measurement of Cloud Top•Derived Cloud Thickness, Bottom
Ground-based WAIL:•Direct measurement of Cloud Bottom•Derived Cloud Thickness, Top
Figure 3. WAIL data and retrieved cloud thickness validation. Left: Sample frames from WAILdatasets obtained at the Oklahoma CART site during the WAIL/THOR cross-validation experiment inMarch 2002. Shown here are sequences obtained using three different background suppression filtersthat optimize for different ranges of observation angle. The first (535 nm band center) emphasizes thecentral region, the second (540 nm) emphasizes intermediate angles, and the third (546 nm)emphasizes large angles. Time delay after the laser pulse is noted on each frame. In this 2002implementation of WAIL, a complete dataset is obtained only after combining these three restricted-angle sets. This rather awkward procedure prompted us to re-evaluate the need for such narrow-bandinterference filters. Our revised instrument uses a wider-bandpass filter, enabling us to capture the fullangular range at once. Right: Comparison of cloud top and cloud bottom positions determined byWAIL and THOR, as well as by the CART site’s micropulse lidar and ceilometer, showing excellentagreement between the WAIL and THOR direct and derived cloud interface positions.
Gated/Intensified CCD(Roper Scientific “PI-Max”)
Figure 1. The Wide-Angle Imaging Lidar (WAIL) concept, contrasted with standard lidar. Standard lidarcollects returning light from only a very narrow angular range about the laser beam; light scattered out of thebeam is intentionally ignored and is considered to be extinguished. For optically dense clouds, this limitsstandard lidar to measuring the range to the cloud bottom. WAIL, in contrast, focuses its attention specificallyon the light scattered out of the beam, taking high-speed, wide-angle “movies” of the spatial distribution ofreturning light as a function of time after the laser pulse. This multiply scattered light samples the interior ofeven optically dense clouds, all the way to the top of the cloud. By measuring both the spatial and temporaldistributions of returning light, WAIL can determine both the physical thickness and optical density of thecloud, and hence estimate liquid water content. At right is some typical early WAIL data, with images aboveand below showing the spatial distribution of returning light at various times after the laser pulse, modified bythe spatial response of the background-suppression interference filters. Each image spans approximately 60˚.Graphs show the spatially-integrated signal as a function of time. The cloud deck for this example was rathercomplex, consisting of several thin layers beneath a denser upper layer.
Figure 2. The WAIL/THOR cross-validation experiment. WAIL operated from the ground and thusprovided a direct measurement of the cloud base height; THOR, airborne above the cloud, similarlyprovides a direct measurement of the cloud top height. Each could thus verify the other’s derived far-side interface position determination. At right is a photograph showing the WAIL instrument deployedat the SGP CART site. The laser transmitter is a doubled Nd:YAG (532 nm) operating at a repetitionrate of 4 to 12 kHz, a 30 ns pulse width, and roughly 4W average power. The receiver is a gatedintensified CCD camera with a time resolution as fast a 1 ns (we typically use 50 ns or longer). AWAIL “movie” is collected by sequentially advancing the CCD’s gate delay w.r.t. the laser pulse.
In 2002, WAIL and THOR were deployed simultaneously at theSGP CART site in a mutual validation exercise. While successfullydemonstrating the power of the off-beam lidar concept andvalidating retrieval algorithms, this experiment also highlightedsome aspects of the hardware that could be improved.
WAIL Cloud Top
WAIL Cloud Base
THOR Cloud Top
Micropulse Lidar Cloud Top
Micropulse Lidar Cloud Base
CeilometerCloud Base
Figure 4. WAIL enhancements: Wider bandpass and wider field of view. Two thingsbecame apparent during the WAIL/THOR experiment: (1) The 53.6˚ field of view, though“wide,” wasn’t wide enough for the low cloud ceilings during that campaign, leading totruncation of our spatial distribution data. This necessitated abandoning our moment-based analysis and inventing a new retrieval method [Polonsky, et al. 2005]. (2) Our use ofvery narrow-band background suppression filters necessitated an awkward dataset splicingprocess, but in practice, background light wasn’t really a limiting factor in a typical ruralenvironment. So an obvious improvement is to increase the f.o.v. and avoid the splicingby widening the bandpass. These two laboratory images, illuminated by diffusely-scattered 532nm laser light, compare the angular acceptance of our old system (with10nm-bandpass filter at 540nm displaying annular acceptance ) on the left, and the newsystem (with 50nm bandpass filter admitting the entire angular range). Fields of view are53.6˚ square and 88.3˚ square, respectively.
WAIL is the ground-based progenitor of a new class of active cloudprobes that also includes the airborne THickness from Off-beamReturns (THOR) system developed at NASA Goddard and the in-cloudlidar developed by the U. of Colorado and SPEC. This class ofinstruments exploits the highly scattered light from a pulsed laserilluminating the cloud at a non-absorbing wavelength, and can probeoptically thick clouds using VIS-NIR light which in traditional lidar isall but extinguished (i.e. scattered out of the beam) within a few mean-free-paths. In contrast with the direct beam, the multiple-scatteringGreen function probed by WAIL permeates the whole cloud. Thus,both the physical thickness of the cloudy medium and its opticalthickness (equivalently, volume-averaged extinction) can bedetermined from the spatial and temporal data. Collectively, these newinstruments rejuvenate optical cloud remote sensing at a time whenambitious climate monitoring programs such as ARM have identifiedthe need for breakthrough in observational techniques to reach theircommon goal to advance predictive climate science. Both WAILhardware and analysis methodology have evolved in the last year or sotowards a configuration where it can sooner be incorporated into cloudinstrument suites at ARM facilities.
Figure 5.SNR enhancement usingimage intensifier. WAIL’sgated CCD is equippedwith an image intensifierthat, at least for theextremely dim long-time,la rge-sca t te r ing-anglereturns, offers significantSNR improvement. Herewe show a laboratory testcomparing a very weakoptical signal imaged withno intensification on theleft, and maximum intensi-fication on the right.Artifacts make intensifi-cation less desirable forstronger signals.
Figure 6. New Analysis Techniques. Here weshow the angular dependence of thenormalized WAIL signal for selected time-slices mapped to altitudes above ground.Below the cloud, we have 180 m (), 293 m (), and 406 m (). Notice that for the in-cloud range of 594 m () this angularly-resolved signal is significantly flatter than forthe below-cloud ranges. The multiple-scattering retrieval method by Bissonnette etal. [2005], based on the small-angle approx-imation, will enable us to extract newinformation about both aerosol and cloudparticle size distributions from these near-axisWAIL data.
ReferencesPolonsky, I. N., S. P. Love, and A. B. Davis, 2005: “Wide-Angle Imaging Lidar deployment at the ARM Southern GreatPlains site: Intercomparison of cloud property retrievals,” J. Atmos. Oceanic Technology 22, 628-648.
L.R. Bissonnette, G. Roy, and N. Roy, 2005: “Multiple-scattering-based lidar retrieval: Method and results of cloudprobings,” Appl. Opt., 44, 5565-5581.
