AMTAtmospheric Measurement TechniquesAMTAtmos. Meas. Tech.1867-8548Copernicus GmbHGöttingen, Germany10.5194/amt-8-1073-2015Field-deployable diode-laser-based differential absorption lidar (DIAL) for profiling water vaporSpulerS. M.spuler@ucar.eduRepaskyK. S.MorleyB.MoenD.HaymanM.NehrirA. R.National Center for Atmospheric Research, Earth Observing Lab, Boulder, CO 80307, USAMontana State University, Electrical and Computer Engineering, Bozeman, MT 59717, USANASA Langley Research Center, Hampton, VA 23681, USAS. M. Spuler (spuler@ucar.edu)4March2015831073108726August201418November20144February20155February2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://amt.copernicus.org/articles/8/1073/2015/amt-8-1073-2015.htmlThe full text article is available as a PDF file from https://amt.copernicus.org/articles/8/1073/2015/amt-8-1073-2015.pdf
A field-deployable water vapor profiling instrument that builds on the
foundation of the preceding generations of diode-laser-based differential
absorption lidar (DIAL) laboratory prototypes was constructed and
tested. Significant advances are discussed, including a unique shared
telescope design that allows expansion of the outgoing beam for eye-safe
operation with optomechanical and thermal stability; multistage optical
filtering enabling measurement during daytime bright-cloud conditions; rapid
spectral switching between the online and offline wavelengths enabling
measurements during changing atmospheric conditions; and enhanced performance
at lower ranges by the introduction of a new filter design and the addition
of a wide field-of-view channel. Performance modeling, testing, and
intercomparisons are performed and discussed. In general, the
instrument has a 150 m range resolution with a 10 min temporal resolution;
1 min temporal resolution in the lowest 2 km of the atmosphere is
demonstrated. The instrument is shown capable of autonomous long-term field
operation – 50 days with a > 95 % uptime – under a broad set of
atmospheric conditions and potentially forms the basis for a ground-based
network of eye-safe autonomous instruments needed for the atmospheric
sciences research and forecasting communities.
Introduction
The planetary boundary layer, the lowest part of the troposphere,
contains the majority of the atmospheric water vapor (hereafter WV). The
rapidly changing spatial and temporal distribution of WV influences dynamical
and physical processes that drive weather phenomena, general circulation
patterns, radiative transfer, and the global water cycle. The ability to
measure the WV distribution continuously within the lower troposphere has
been identified as a high-priority measurement capability needed by both the
weather forecasting and climate science communities. In particular, two
studies by the list
high-resolution vertical profiles of humidity in the lower troposphere as one
of the four highest-priority observations needed for a national mesoscale
weather observation network. Accurate, high-resolution continuous
measurements of WV remain a key observational gap for the mesoscale weather
and climate process studies communities. Such observations are particularly
important to the National Weather Service and other federal agencies for
evaluation of forecast impact in severe weather and quantitative
precipitation forecasts.
Conventional balloon-borne radiosonde soundings combined with global coverage
of low-vertical-resolution state parameters via satellite-based measurements
currently form the backbone of observations used for weather forecasting.
However,
the limited spatial and temporal resolution of these technologies is
inadequate for reliable forecasts of high-impact weather events like
thunderstorms. Passive remote sensors such as microwave radiometers are useful
at low ranges close to the surface but in general provide low vertical
resolution. The atmospheric emitted radiance interferometer (AERI) is
a passive infrared remote sensing instrument that utilizes an interference
technique to retrieve atmospheric emitted radiance
. Starting with
initial temperature and humidity profiles based on statistical models, an
iterative solution to the radiative transfer equations is utilized to
reproduce the measured atmospheric emitted radiance. This iterative solution
provides the final temperature and WV profiles for clear sky conditions up to
approximately 3 km. During cloudy conditions, retrievals are
sensitive to the cloud properties leading to larger errors in the temperature
and WV profiles. During cloudy conditions a separate measurement of the cloud-base height is often required.
Raman lidar is an active remote sensing technique that is capable of
monitoring WV in the troposphere . The Raman
lidar technique utilizes a high-power pulsed laser transmitter to illuminate
the atmosphere. The frequency shift due to the nonlinear Raman scattering
allows the scattering molecule to be identified while the intensity of the
scattered signal can be used to determine the WV mixing ratio. Raman lidars
typically require high laser pulse energy and large receiver aperture due
to the small scattering cross section associated with nonlinear Raman
scattering. Furthermore, Raman lidars typically require a calibration
technique based on an ancillary measurement for quantitative WV retrievals.
Differential absorption lidar (DIAL) is another active remote sensing
technique. It utilizes a laser transmitter capable of operating at two
closely spaced wavelengths: one wavelength is located at or near the
absorption feature for the molecule of interest, referred to as the online
wavelength, and the other is located away from the same absorption feature,
referred to as the offline wavelength. If the online and offline wavelengths
are closely spaced, then the only difference between the return signals
results from molecular absorption. Having a priori knowledge of the
differential absorption cross section of the molecule of interest, e.g., via
the HITRAN molecular spectral database , the ratio of the
online and offline return signals over a selected range within the atmosphere
is related to the molecular number density. The advantages of the DIAL
technique include no calibration or ancillary measurements and a direct
retrieval of the WV number density. However, the DIAL technique requires
a pulsed laser with high spectral fidelity and frequency agility, capable
of operating at two separate wavelengths.
The DIAL technique has been successfully demonstrated for ground-based water
vapor profiling based on injection-seeded Ti:sapphire laser systems as
discussed in , ,
, and . Some of these
instruments are capable of autonomous operation and provide a wide range of
daytime measurements. However, to realize the potential societal benefits
envisioned by a nationwide water vapor profiling network, a less cost
prohibitive approach is needed. A significantly lower-cost profiler may be
achieved by the development of diode-laser-based DIAL systems albeit with
reduced performance compared to Ti:sapphire-based DIAL systems. Reliable,
autonomous, low-cost transmitters may be enabled by diode lasers operating in
the spectral region most commonly used for water vapor DIAL (700 to
950 nm), and eye safety may be brought about by the inherent low
pulse energies and expansion of the laser beam.
Initial modeling by indicated that low pulse energy, high
pulse repetition rate DIAL measurements of WV in the lower troposphere are
feasible. This modeling of micropulsed DIAL instruments resulted in the
first development of a diode-laser-based WV-DIAL instrument by
that utilized a distributed feedback laser to
injection-seed a tapered semiconductor optical amplifier (TSOA) to achieve
the needed spectral fidelity for DIAL measurements.
demonstrated nighttime water vapor profiles up to 2.5 km with
30 min integration periods. Building from this initial modeling and
instrument development, researchers at Montana State University (MSU)
developed a series of diode-laser-based WV-DIAL instruments. The first
generation utilized an external cavity diode laser (ECDL) and a passively
pulsed amplifier . A second-generation instrument achieved
a factor of 10–20 greater energy (1–2 µJ) with a pulsed dual
stage TSOA in a master oscillator power amplifier (MOPA) configuration
. This version provided the first demonstration of daytime
water vapor measurements with a diode-laser-based DIAL. A third generation,
utilizing a pair of ECDLs connected via a fiber-coupled
microelectromechanical system (MEMS) switch to an improved single-stage TSOA,
achieved an even greater pulse energy of 7 µJ.
In all of these versions, the receiver utilized a commercial telescope to
direct the collected light to an avalanche photodiode (APD) operating in the
Geiger mode. Two narrowband filters, each with a 250 pm full width
half maximum (FWHM) bandpass, were used to filter solar background allowing
clear-sky daytime measurements to approximately 3 km.
Schematic of the WV-DIAL system. BS is the beam splitter,
T/R is transmit/receive, and I/O is input/output.
Since the summer of 2011, the researchers at MSU
and the National Center for Atmospheric Research (NCAR) have collaborated to
advance and evaluate the capability of the diode-laser-based WV-DIAL
technique. In 2012, the MSU third-generation WV-DIAL was modified to allow
for unattended operations in a laboratory environment: replacing the external
cavity diode lasers with more temperature-robust distributed Bragg reflector
(DBR) lasers and expanding the beam to be eye-safe at the exit port. The
redesigned transmit path for this temporary prototype used a series of large
turning mirrors to reflect the expanded eye-safe beam into the receiver
field-of-view (FOV) and was subject to pointing instability with environmental
temperature fluctuations. Nevertheless, the modified prototype was field
tested over a wide range of atmospheric conditions to evaluate its
performance. The evaluation indicated that the technology was well suited for
autonomous, long-term measurements of water vapor; however, as noted in
, improvements to the instrument were needed to achieve
continuous atmospheric coverage, particularly during bright-cloud
conditions and at lower ranges.
In this paper – having addressed the needed advancement in performance given
by – a next-generation (fourth) diode-laser-based WV-DIAL
is presented which is capable of autonomous long-term field operation under
an expanded set of atmospheric conditions. Major improvements with respect to
the older versions of the system are discussed and include
a unique shared telescope design that allows expansion of the outgoing beam for eye-safe operation with mechanical and thermal stability
improved performance during daytime and cloudy conditions by introducing a high-finesse etalon into the receiver optical train
improved performance during rapidly changing atmospheric conditions through increased switching rates between the online and offline wavelengths, and
improved performance at short range by the introduction of a near-range, wide field-of-view channel.
This paper is organized as follows. Section contains
a complete description of the instrument highlighting differences between the
third and fourth generations. A performance model of the diode-laser-based WV-DIAL
instrument is presented in Sect. . In Sect. ,
field data and intercomparisons are presented from when the instrument was
operated at the Boulder Atmospheric Observatory (BAO) during the
Front Range Air Pollution and Photochemistry Experiment
between 1 July 2014 and 19 August 2014. Data from this field campaign are
presented to demonstrate the capability of the WV-DIAL to provide continuous
water vapor profiles for extended periods of time in a variety of atmospheric
conditions. Furthermore, data collected during this observation period are
used to validate the instrument performance model described in
Sect. . Some brief concluding remarks are presented in
Sect. .
Description of the field prototype
The instrument, shown schematically in Fig. , utilizes
a diode-laser-based MOPA-configured transmitter capable of rapid wavelength
switching (red shading), a shared telescope transmitter and receiver to
achieve optomechanically stable eye-safe operation (purple shading), and
a multistage filtered two-channel receiver for the near- and far-range
returns (blue shading). The system parameters are outlined in
Table .
Instrument parameter list.
Seed lasersTwo DBR diode lasers (electrically pumped with 140 mA, nominal)AmplifierSingle-stage TSOA (electrically pumped with 10 A,1 µs pulses)Transmitted pulse duration900 nsTransmitted pulse energy5 µJPulse repetition rate9 kHzMax range16.5 kmLaser wavelength830 nmTransmitted beam M22×6Laser divergence56 µradTransmitted beam diameter114 mm effective (axicon shaped to 180 mm with 70 mm hole)Lidar design geometryT/R partitioned shared telescopeTelescope typef/3 NewtonianTelescope diameter406 mmEffective receiver collection area935 cm2Field-of-view115 µrad (far channel), 451 µrad (near channel)Detector active area diameter105 µm diameter fiber-coupled (far channel), 180 µm diameter free space (near channel)Photon-detector module20 ns dead time (<5Mcs-1), saturation at 40 Mcs-1Quantum efficiency of detector0.45Efficiency of the receiver0.23 (far channel), 0.04 (near channel)Daylight blocking filters750 pm two-cavity interference filter (far and near) 2.5 pm fringe etalon (far and near) 500 pm two-cavity interference filter (far only)Combined filter width20 pm (near), 14 pm (far)Online–offline switch speed (type)0.3 µs (ElectroOptic)Online–offline switching rate100 HzPhoton countingMultichannel scaler card (four-channel output)Data collection duty cycle100 %Data collection rate1.1 sWater vapor averaging time1–10 min (typical)Online and offline wavelength locking±0.05 pm (absolute accuracy ±0.2 pm with ±0.03 pm repeatability)
Careful absorption line selection is critical to DIAL performance (e.g., the
line should be insensitive to expected atmospheric temperature variations).
The line strength for the water vapor absorption feature used for the
measurements presented in this paper is S=1.477×10-23cm-1(molcm-2)-1, with a
ground-state energy of E=212.2cm-1. A complete discussion of
the line section criteria and other key spectroscopic parameters for this
lower troposphere a ground-based instrument is provided by .
Laser transmitter
The laser transmitter utilizes two DBR laser diodes (Photodigm, Inc.,
PH828DBR100TS): one operating at the online wavelength (828.2 nm) and
the other operating at the offline wavelength (828.3 nm) These lasers
operate in the continuous wave (cw) mode, produce up to 80 mW of
power, and have a measured linewidth less than 1 MHz. Previous
horizontal path, hard target spectral purity measurements indicate that the
spectral purity of the pulsed laser transmitter exceeds 99.96 % as
discussed in . The output from each diode is collimated
using an aspheric lens and passes through a Faraday isolator to prevent
unwanted feedback from affecting the power output and spectral stability.
This seed laser light is then fiber coupled to allow for splitting and
switching prior to pulsed amplification in the TSOA.
The third-generation WV-DIAL instrument used a fiber-coupled MEMS switch,
which exhibited a 1 ms 10/90 switching time as discussed in
, to alternate between the online and offline wavelength.
The data acquisition system utilized a single channel of a four-channel
scaler photon-counting card which had two buffers to allow for continuous
read and write operations to occur. To avoid mixing the signals within the
data acquisition system, one laser frequency was transmitted for 3 s,
followed by a 3 s dead time when the wavelength switch was changed,
and then the other laser frequency was transmitted for 3 s. This
switching method worked well, but it created several performance limitations.
First, the dead time resulted in a 60 % duty cycle (i.e., the
reported water vapor integration time of 20 min was equivalent to
a 33 min temporal resolution). Second, wavelength switching on
timescales of several seconds can result in errors due to decorrelation
between the online and offline signals from fluctuations in the backscatter
coefficient, as discussed in . Therefore, the
next-generation instrument was designed to use faster optical switches
(Agiltron, NanoSpeed); these electro-optic-based components are capable of
100 ns90/10 switching times. The new switching method – combined
with radio frequency (RF) switches in the receiver (details in the receiver
section to follow) – allows the instrument to run at nearly 100 %
duty cycle and thereby enables a significant improvement in temporal
resolution. A pair of fast optical switches are used for each wavelength:
a 1×1 switch is used to turn on the cw seed lasers only when the TSOA
current is applied and a 2×1 switch is used to alternate the modulated
online and offline seed signals to the TSOA at high repetition rates. The
switches were connected as shown in Fig. . This two-stage
optical switch arrangement provides 40 dB isolation between the
online and offline (each switch has -20 dB crosstalk) to maintain
a high spectral purity in the transmitted pulses. The instrument has
typically operated with the two wavelengths interleaved at 60 to
100 Hz.
The TSOA stage for this instrument is the same as in its third-generation
predecessor . The output of the seed laser delivery fiber
is collimated and passed through a Faraday isolator. A half-wave plate is used
to set the polarization needed by the TSOA. This light is incident on an
aspheric lens and focused into a 4 mm long TSOA (Eagleyard Photonics,
EYP-TPA-0830) used to amplify and pulse the laser transmitter. The TSOA is
driven with a commercial pulsed current driver (Directed Energy, Inc.,
PCX-7420) with a programmable pulse duration, pulse repetition rate, and peak
pulse current. Standard setting for this instrument used a pulse current of
10 A with a 1.1 µs pulse duration and a 9 kHz
pulse repetition frequency. The pulsed current driver is used as the master
clock for the DIAL instrument. The leading edge of the current pulse to the
TSOA is used to simultaneously trigger the 1×1 switches, the I/O
digital counter (which in turn triggers the 2×1 switch and RF switches
in the receiver ), and the multichannel scaler card (MCS). Therefore,
although a 1.1 µs current pulse is applied to the TSOA, the
resulting amplified laser pulse has a 900 ns duration due to the
200 ns rise time of the 1×1 switch driver. Because of the
astigmatic geometry of the output facet of the TSOA, the output requires
a beam-shaping pair of lenses (spherical and cylindrical) to achieve
a nominally circular collimated beam. Following the collimation optics,
a window is used to direct approximately 4 % of the light to
a photodetector to monitor the average output power from the laser
transmitter.
Shared telescope
The fourth-generation instrument uses a shared telescope (400 mm
diameter, f/3 Newtonian) for transmission and receiving. This provides the
system greater stability and offers a method for larger beam expansion for
eye safety. The design is a factor 20 times more optomechanically stable
than the preceding co-axial design because the beam is expanded by the
telescope after the transmit mirror. Although there are examples of shared
telescopes in the lidar community, they often illuminate the entire primary
mirror to expand the beam with the telescope and in the process lose light
that is blocked by the secondary mirror. Because this system uses a low-power
laser transmitter, the spatial distribution of the outgoing beam is shaped
for efficient transmission through the shared telescope. Starting with the
collimated output from the TSOA, the laser transmitter is expanded using
a two-times beam expander. The beam is incident on a matched pair of
conical (axicon) lenses to create a collimated annular beam with an outside
diameter dependent on the axicon spacing and wedge angle, while the inside
diameter is controlled by the incident beam diameter. The beam is not
expanded to the full diameter of the telescope – unlike many shared
telescope designs in the lidar community which use a polarization T/R
switch to separate the transmit from the receive path – but instead use an
annular mirror to divide the telescope primary. This provides better
isolation between transmit and receive paths. The annular-shaped beam passes
through a 10 mm hole bored through the center of an elliptic mirror
and is then incident on a lens with a 60 mm focal length. The f
number for the laser transmitter beam matches the f number of the telescope
to maximize efficiency of the lens, to avoid vignetting, and to collimate the
outgoing laser beam as it exits the telescope. The transmitted beam is
approximately 180 mm in diameter (roughly half the diameter of the
primary mirror) with a 70 mm hole; it clears the secondary mirror as
it exits the telescope and minimizes losses in the laser transmitter pulse
energy. In this manner, the inner half of the telescope is used to expand the
outgoing laser transmitter pulse for eye-safe operation while the outer half
of the telescope is used to collect the scattered return signal.
The transmitted beam for this instrument was designed to be eye safe at the
exit of the telescope as defined by standard Z136.1-2007 given by the
. As discussed in the second- and third-generation MSU WV-DIAL instruments were not eye-safe at the exit aperture.
It is imperative for an autonomous lidar system to be eye-safe at all ranges
to allow for unattended operation. It is also an enabling feature for
a network of instruments which would require oversight and compliance with
Federal Aviation Administration. The ANSI regulation sets the
single-pulse maximum permissible exposure (MPE) for a 1 µs
duration pulse at the 830 nm wavelength at 900 nJcm-2
(from Table 5a in Z136.1-2007). A repetitively pulsed lidar system must be
less than both the multiple energy and multiple power MPEs, which for this
system, operating at the 9 kHz repetition rate, is 52.0 nJcm-2
and 0.47 mWcm-2, respectively. For eye safety at the exit port
(i.e., a 0 m range MPE), the TSOA amplified beam was expanded
two times, shaped with an axicon pair, and expanded an additional
20 times with the shared receiver optics. The 1/e diameter was measured
before the axicons and telescope expansion to be 5.7 mm. After the
20-times telescope expansion, the transmitted beam has an effective
diameter of 114 mm (180 mm diameter with a 70 mm
hole.) The entire transmitted beam area was checked with
a 1cm×1cm square calibrated detector to verify that the
0.47 mWcm-2 MPE was not exceeded.
Optical receiver
The multistage optical filtering enabling measurement of water
vapor during daytime bright-cloud conditions. (a) The
interference filter transmission as a function of wavelength: the solid black
(blue) line is a fit to the measured filter transmission for the FWHM
bandpass of 750 pm (500 pm). A double cavity design was used
for both filters to provide a more flat passband. The wider filter is common
to both the near- and far-range channels while the narrower filter is used
only in the far-range channel. (b) The etalon transmission as
a function of wavelength; the green circles represent measured values while
the solid black line represents a fit. (c) The combined
passband for the far-range channel including both interference filters and
the etalon.
Light scattered in the atmosphere is collected by the telescope and is
incident on the 60 mm focal length lens which collimates the light
collected from infinity. The received light is then incident on the elliptic
mirror with the bored hole. This mirror only collects light from a diameter
of greater than 200 mm at the primary mirror of the telescope,
providing excellent isolation from the transmitted beam. The ability of the
receiver to discriminate signal photons against a bright background requires
a state-of-the-art multistage filter that suppresses the background while
minimally affecting the signal. This is a difficult task during daytime and
cloudy conditions where the high background signal can easily saturate the
photon-counting detector. For this instrument, the background suppression is
achieved through a combination of interference filters and an etalon. The
passband of the filters, the etalon, and the combined far-range filter–etalon
passband are shown in Fig. .
Figure a shows the transmission curve for the
500 pm (blue) and 750 pm (black) FWHM bandpass filters. The
transmission of the 750 pm filter remains relatively constant between
828.2 and 828.4 nm to both maximize the online and offline
throughput and minimize angle tuning effects for the filter transmission
at lower ranges that can affect the accuracy of the number density retrieval
as discussed in and .
Figure b shows the etalon transmission measurements and
a fit to the data based on
Tetalon=(1-R)21+R2-2Rcosθ,
where R is the etalon mirror reflectivity and θ=4πnLλ is the round-trip phase accumulation. The free
spectral range (FSR) of the etalon is related to the optical cavity spacing
by FSR=c2nL and was measured to be 0.0994 nm
(43.3 GHz). The etalon was designed to allow transmission of the
online and offline radiation in adjacent cavity modes. The finesse (F) of
the etalon is related to the mirror reflectivity by F=πR1-R, yielding a finesse of F=43.
Figure c shows the combined passband for the far-range
receiver channel including both interference filters and the etalon.
The etalon (manufactured by Light Machinery) is housed in a temperature-controlled mount that integrates with the tube assembly housing of the
receiver optics. A change in temperature of 22.4 ∘C is needed
to tune the etalon through a full free spectral range. The operating
temperature of the etalon is controlled via a thermoelectric cooler
using a commercial temperature controller with a temperature stability of
0.01 ∘C. The operating temperature of the etalon is adjusted
to be resonant with the transmitted wavelengths. A plot of the resonant
wavelength as a function of temperature is shown in the left panel of
Fig. . The black circles represent measured values and
the red line represents a linear fit showing a temperature tuning rate of
dλ/dT=0.00441nm∘C-1. A plot
of the cavity transmission as a function of finesse is shown on the right
panel of Fig. . The locking stability of the injection-seeding laser and the temperature stability of the etalon affects the etalon
transmission. The effects of the locking stability on the cavity transmission
can be modeled using the above equation with
θ=πΔλFSRλ, where Δλ
is the detuning from the resonant peak in wavelength and
FSRλ is the free spectral range in wavelength. The
temperature stability of the etalon can be modeled using the measured
temperature tuning rate, dλ/dT, through the
above equation as well with θ=2πdλ/dTΔTFSRλ, where
T=0.01∘C is the temperature stability for the etalon.
Etalon temperature tuning and stability. Left panel: resonant
wavelength as a function of etalon operating temperature. The black circles
represent measured values while the red line is a linear fit yielding
a temperature tuning rate of 0.0041 nm∘C-1. Right panel:
the cavity transmission as a function of finesse for a locking stability of
0.0002 nm (solid blue line) and a temperature stability of
0.01 ∘C (dotted red line).
Following the filters, 90 % of the received light is directed to
a narrow FOV fiber-coupled detector module (Excelitas,
SPCM-AQRH-13-FC) and 10 % of the light to a free space receiver
(Excelitas, SPCM-AQRH-13) using a beam-splitting cube (as shown in
Fig. ). The simplicity and relatively low cost of these
commercial off-the-shelf (COTS) single-photon-counting modules, coupled with
the low-power diode-laser-based transmitter, makes them a good choice for the
instrument – especially since the end goal of this research is to develop a
low-cost, reliable device that would participate in a national-scale network.
The light transmitted through the beam-splitting cube is incident on
a 20 mm focal length lens focusing it onto a free-space avalanche
photodiode. The active area of the APD acts as the field stop resulting
in a 451 µrad field-of-view. The light reflected from the
beam-splitting cube passes through an interference filter with
a 500 pm FWHM passband (passband is shown as the blue curve in
Fig. a). The diameter of the beam is reduced
approximately four times with a beam-reducing pair of optics (80 mm
and 19 mm focal length) to not exceed the numerical
aperture (NA) of the fiber, then focused with an 11 mm focal length
lens into an multimode optical fiber with a 105 µm core diameter
and a NA of 0.22. The optical fiber acts as the field stop producing a FOV of
115 µrad. The optical fiber guides the received light to a fiber-coupled APD.
Overlap function for the far- and near-range receiver channels
calculated from an optical model of the instrument.
For each detector module, full overlap occurs when the image of transmitted
beam diameter is less than the diameter of the field stop. As shown in
Fig. , the narrow field-of-view receiver (far-range
channel) has full overlap at ranges greater than 2.75 km, whereas the
wide field-of-view receiver (or near-range channel) achieves full overlap at
approximately 700 m. The collected light has a r-2 dependence;
therefore the near-range channel will have a larger signal at low ranges.
However, it will also have substantially higher noise during daytime as it
collects 16 times more background light compared to the far channel (solid
angle ∝FOV2). Thus only the lowest range gates of the wide
field-of-view channel are useful during daytime. As discussed later, this
receiver channel is most useful when operating the instrument at short
temporal and spatial resolutions (e.g., 1 min and 75 m,
respectively).
Model atmosphere parameters used to calculate the performance.
Daytime sky radiance1.15×10-3Wcm-2µm-1sr-1Nighttime sky radiance5×10-5Wcm-2µm-1sr-1Aerosol lidar ratio50 srMolecular lidar ratio8/3π srMolecular backscatter coefficientFig. 5 (left panel)Aerosol backscatter coefficientFig. 5 (left panel). Data from . Wavelength scaled lower decile values from Table 4 on page 944.Water vapor concentration profileFig. 5 (right panel). Data from .Data acquisition and post-processing
The output signal from the photon-detection modules are connected to RF
switches as shown in Fig. . The RF switch is used to
separate the signals generated from the photon-counting module between the
online and offline transmitted laser pulses so as to eliminate the buffer
crosstalk problem previously mentioned for the third-generation system.
A digital I/O counter tracks the number of pulses from the currentpulse
driver and sends a transistor–transistor logic state-change to the pair of
RF switches after a prescribed number of pulses are counted at each
wavelength. In the standard configuration, the current pulse generator
operates at 9 kHz, so there are 150 pulses at each wavelength with
60 Hz switching. The outputs of each RF switch are connected to two
separate channels on the MCS. For near- and far-range all four channels of
a 20 MHz MCS (Sigma Space Corporation) are used. 10 000 samples are
accumulated with a bin duration of 500 ns and 220 bins for a maximum
range of 16.5 km with 1.1 s acquisition time. The approximate
1 µs pulse duration is over-sampled by the MCS, yielding two data
points per 150 m, which corresponds to a sampled vertical range
resolution of 75 m. The summing of these 75 m bins is performed
during post-processing where two bins are grouped together to yield a 150 m
range resolution for the DIAL measurement.
Note that the large number of scattered photons from the outgoing pulse
prohibits measuring the atmospheric return during the initial 1.1 µs (i.e., while the current pulse is applied to the TSOA). Therefore, in the
standard operational configuration the lowest usable range gate is
225 m. However, the current driver is easily reconfigurable; thus, for
example, if it were programmed for a 500 ns duration pulse, the
lowest usable gate could be reduced to 75 m range. The amplified
laser pulse would be 300 ns in duration with roughly one-third of the
energy per pulse.
Model performance of a photon-counting DIAL
In the following, we consider a photon-counting DIAL system. The precision of
the water vapor measurement can be estimated by the propagation of
independent errors in the DIAL equation given by
nwv(r)=12Δr(σon(r)-σoff(r))lnNon(r)Non(r+Δr)Noff(r+Δr)Noff(r),
where nwv is the number density of water vapor, Δr is the
range bin size, σ is the absorption cross section at the online and
offline wavelength (subscripts on and off, respectively), and N is number
of online and offline (subscripts on and off, respectively) backscattered
photons received.
The number of signal counts is given by
Ns(r,λ)=Eλ2hArr2βa(r)+βm(r)ηrηdO(r)exp-2∫0r′(αa(r)+αm(r)+(σ(r,λ))nwvdr′,
where E is the pulse energy of the laser, λ is the laser
wavelength, h is the Planck constant, Ar is the area of the
receiver telescope, β is the backscatter coefficient for aerosol and
molecular (subscripts a and m, respectively), η is
the efficiency of the receiver and detector (subscripts r and
d, respectively), O(r) is the overlap function, and α is the
extinction coefficient of the aerosol, molecular and water vapor (subscripts
a, and m, respectively).
The number of background counts is given by
NB=SbΩrΔfArηrηdλhc,
where Sb is the sky radiance, Ωr is the
receiver field-of-view solid angle, Δf is filter bandpass
width, Ar is the area of the receiver telescope, η is the
efficiency of the receiver and detector, and c is the speed of light.
The DIAL random relative error is given by
σnnwv(r)=12Δr(σon(r)-σoff(r))nwv1mk0.5×NS.on(r)+NBNS.on(r)2+NS.on(r+Δr)+NBNS.on(r+Δr)2+NS.off(r)+NBNS.off(r)2+NS.off(r+Δr)+NBNS.off(r+Δr)20.5,
where m is the number of range bins averaged, k is the number of
samples/profiles averaged for the online wavelength (assumes a 50 %
duty cycle), σon is the online absorption cross section,
σoff is the offline absorption cross section, and Δr
is the range bin size. It should be noted that the equation assumes the times
of
transmitting and receiving the online and offline wavelengths are equal.
However, asynchronous wavelength switching – spending more time transmitting
and receiving the online wavelength – could provide an enhancement in
performance.
This random-error equation can be used to provide performance estimates for
a diode-laser-based WV-DIAL. The performance estimate includes the instrument
parameters given in Table , the atmospheric
parameters summarized in Table , overlap functions
shown in Fig. , and the backscatter coefficients
and water vapor number density shown in
Fig. .
Backscatter coefficients (left panel) and water vapor number density
(right panel) used in the performance model.
The expected instrument performance for day and night conditions with
a 10 min integration time and 150 m range resolution, when
tuned to a two-way online column optical depth (OD) of 1.5, is shown in
Fig. . Using 10 % error as a target,
the maximum range of the instrument is expected to vary from about
2.5 km to 3.25 km between day to night, respectively. At these
spatial and temporal resolutions, the near channel would provide a redundant
measurement extending to about 2.25 km range at night and about
1.5 km range during the day except during periods of bright clouds
where the count rate in this channel would exceed the linear count rate of
the photon-counting module. The maximum range can be marginally increased by
tuning the online wavelength to provide a lower column OD although this
introduces more error close the ground. A more effective method is to
process the data above the boundary layer with a larger range bin size. As
shown in Fig. , a two-way column OD of 0.6
and range bin of 600 m increases the maximum range of the instrument
to about 4 km for a 10 % error.
Performance estimate for day and night with 150 m range
resolution and 10 min averaging for the near- and far-range channels
for an online column OD of 1.5. For a 10 % error, the instrument has
a typical daytime range of ≈ 2.5 km with this spatial and
temporal averaging.
A practical limit to the maximum range for the ground-based DIAL results from
the small differential optical depth associated with the water vapor
absorption in the free troposphere. The differential optical depth,
Δτ, needed to retrieve the water vapor number density
is Δτ=nσonΔR≈0.03–0.1, where n is the water vapor number density, σon
is the absorption cross section at the online wavelength, and R is the
range bin size. For a maximum range bin size of 1 km, the maximum
range at which the water vapor profile can be retrieved approaches
7 km.
Figure shows the model results for
a case with higher temporal resolutions of 1 min and 10 s.
The performance is expected to be useful for low-level boundary layer
studies as the instrument can be operated with a resolution of 150 m
and 1 min as the combined near- and far-range channels provide
< 10 % error for the lowest 2 km. Since the data is collected at
approximately 1 Hz, any data set can be processed to high temporal
resolution a posteriori. Improved spatial resolution would be possible by
shortening the pulse length, for example to 75 m, although the
duration of the TSOA current pulse would be reduced in half from 1000 to
500 ns resulting in a pulse energy 2.6 µJ. It is possible
that the pulse energy could be increased from the standpoint of eye-safety
restrictions; however, exceeding pulse currents of 10 A to the TSOA
may adversely affect transmitter lifetimes. Therefore, reducing the pulse
duration, and subsequently the power, to achieve higher spatial resolution
needs to be considered more carefully.
Daytime performance estimate for resolutions of 150 m,
300 m, and 600 m with integration time of 10 min and
column optical depth at 5 km range of 0.6 and 1.5.
Daytime performance estimate in relative error (%) for
temporal resolutions of 10 s, 1 min, and 10 min with
a spatial resolution of 150 m for an online column OD of 1.5 The
model results indicate that 1 min, 150 m resolution may be
useful for boundary layer studies.
Error from temperature uncertainty was not included in the analysis above.
However, a temperature-sensitivity study of the absorption features used in
this work was done by , which showed the number density error
as a function of temperature to be very low in the boundary layer and lower
troposphere (typically 1 to 1.5 orders of magnitude below the overall error
in the water vapor measurement). Detector dark counts were also not included
in this error analysis. The manufacturer specifies the dark count rate for
the photon-detector modules used in this instrument at 250 Cs-1.
This is well below the measured nighttime background count rate of 2000 to
3000 Cs-1 (note this can be seen in bottom panel of
Fig. , which is discussed in the next section).
The instrument does not have a detector-limited background but, instead, a
signal-limited background that comes from backscattered photons from a range
of 16.5 km, which is the maximum range that can be reached with the
system running at 9 kHz. The background overestimation leads to a
small systematic bias. To minimize this bias, the instrument can be operated
at a pulse repetition frequency of 7 kHz, extending the maximum range to 21 km so
that the expected background count rate is comparable to the detector dark
count rate. As a final caveat, Rayleigh broadening errors were not included
in this analysis. Although beyond the scope of this effort, the most complete
analysis that should account for this error is described by
.
Top panel: 1 min, 150 m resolution relative backscatter
from the range of 0–14 km; middle panel: 1 min, 150 m resolution
water vapor in gm-3; 300 and 600 m smoothing were
applied to the data from 2 to 3 km and above 3 km,
respectively. The dashed black lines overlain on the top and middle panels
indicate times when sondes were launched. Bottom figures: individual water
vapor concentration profiles for the sonde (blue) and WV-DIAL with
25 min average profiles for far-range (red) and near-range channels
(green) in gm-3.
Data examples and intercomparisons
The fourth-generation diode-laser-based DIAL was constructed at the NCAR lab
in Boulder, CO, in four phases between October 2013 and June 2014,
implementing (1) the shared telescope, (2) two-channel receiver, (3) fast-switching transmitter and receiver, and (4) the optimization of the
background suppressing filters and etalon. During the 8-month period of
development, the instrument was run almost continuously with sonde comparisons
done at each stage of the development. The completed instrument was moved to
the BAO for operation during the Front
Range Air Pollution and Photochemistry Experiment (FRAPPE) between 1 July
and 19 August 2014. Data from this field campaign are presented to
demonstrate the capability of the WV-DIAL in continual operation for an
extended period of time in a variety of atmospheric conditions.
For the results in this paper, the Voigt differential absorption cross
section of the water molecule was calculated from parameters contained in the
2008 High-Resolution Transmission (HITRAN) molecular spectroscopic database
. Spectral parameters for the H2O molecule,
including all isotopologues, from 824 to 832 nm (592 spectral lines
in total) were used from the database to calculate the absorption cross
section at the online and offline wavelengths. The numerically calculated
Voigt absorption cross section is a convolution of the Lorentz and Doppler
line shapes, which are a function of temperature and pressure. A standard
atmosphere with surface T=25C and P=0.83atm was used to
correct the temperature- and pressure-dependent terms as a function of range.
Previous MSU WV-DIAL prototypes and publications used the HITRAN 2000
database parameters, which does not include correction terms for air pressure
shift. For those reported results (), only the main absorption line at
828.187 nm was corrected for the air pressure shift. The updated
version of the database includes a parameter to correct for the air-broadened
pressure shift of each line. The pressure shift corrections and other updated
line parameters included the 2008 database and should provide more accurate
results.
For the previous generations of MSU WV-DIAL prototypes, sonde comparisons
were made for both day and night conditions as shown in and . However, these instruments
switched from online to offline at timescales of several seconds and could
suffer from insufficient background suppression during daylight bright-cloud
sky conditions. This made it difficult to achieve continuous water vapor
measurements. Therefore, an important goal of this research effort was to
demonstrate that the next-generation instrument was, indeed, capable of
making
accurate continuous water vapor measurements during daytime cloudy sky
conditions, thereby operating over a more complete range of atmospheric
conditions.
Sondes were released at the BAO in close proximity to the WV-DIAL
instrument: 11 ozone radiosondes launched by NOAA from 10 to 16 July and 34
Vaisala RS92 radiosondes launched by the University of Wisconsin staff from
17 July to 17 August 2014. The period from 10 to 13 July had a high frequency
of radiosondes launched. The relative backscatter measured by the WV-DIAL
over the course of these 3 days is shown in the upper panel of
Fig. at 1 min
temporal resolution. Frequent periods of daytime clouds are evident – shown
in the figure as high backscatter regions (red) from 12:00 to
24:00 UTC on each day. The corresponding water vapor during this
period is shown in the middle panel at a temporal resolution of
1 min. The sonde launch times are indicated by the black lines
overlaid on these two panels. The comparison profiles between the sondes
(blue curves) and the WV-DIAL (near channel in green and far channel in red)
are on the figure bottom. The measurements agree well with each other for the
far-range channel from a lower range of 300 m extending to
3–4 km depending on atmospheric conditions. The near-field channel
provides a comparable measurement of water vapor from 300 m to
1–2 km during the daytime. The only nighttime launch in this series
on 11 July at 04:07 UTC (22:07 LT on 10 July) demonstrates that the
near-field channel extends to the same range as the far-field channel without
the solar background. The 4 km range limit was imposed by clouds in
this particular case. The increasing water vapor in the lower atmosphere –
from 6 to >12gm-3 – over the 3-day period was roughly
captured by the sonde profiles. However, the continuous WV-DIAL time series
offers an accurate and more detailed picture of the atmospheric water vapor
distribution. For example, the organization of filaments of water vapor with
wave-like structure is evident in the lower 2 km between
12:00 UTC on 11 July to 12:00 UTC on 12 July.
Sonde- to DIAL-measured water vapor in percent error. The mean (solid
line) and SD (dashed line) percent errors are shown for all 45 sondes
launched from 10 July through 19 August. The spatial resolution of the DIAL
profiles are 150 m below 2 km and 600 m above
2 km. The DIAL profiles were processed with at 1 min temporal
resolution and averaged for 25 min starting at the sonde launch
time.
During the FRAPPE campaign, 45 sondes were released (43 daytime sondes and 2
nighttime) between 10 July and 18 August 2014. A plot of the percent error of
WV-DIAL-measured water vapor concentration compared to the sondes is shown in
Fig. . A spatial averaging of 150 m was
used (with 600 m smoothing applied to the data above 2 km)
with 1 min temporal resolution profiles averaged for 25 min
starting at the sonde launch time. The mean relative error is less than
10 % from the lowest gate at approximately 300 m to
4 km for the far-range channel. For this predominantly daytime set of
comparisons, the near-range channel provides a slightly lower error below
500 m range. The SD is ≈10% above the mean at the
surface and increases to ≈20% at 1.5 km for the
near-range channel and 4 km for the far-range channel. A small number
of comparisons were greater than three times the SDs and were removed as outliers
from the calculation of the mean. There are several potential reasons for
discrepancies between the absolute humidity measured by the WV-DIAL and the
radiosondes. The sondes inevitably drift with the wind as they rise and are
generally not in close proximity to the lidar beam. Spatiotemporal matching
was not employed at this phase in the analysis as done in
. Additionally, a strong wet bias often occurs in the
WV-DIAL at the cloud edges, which is not filtered out in the current
post-processing – as evidenced in the time vs. height profiles shown in
Fig. . These spatially
localized biases can dominate the statistics given the relatively small
number of comparisons. More work will be done to remove these small regions
of bias in the future. Finally, there may be systematic instrument bias
or errors within the HITRAN 2008 molecular spectroscopic database parameters
used to calculate the differential absorption cross section of the water
molecule. To improve performance, detailed studies of the molecular
spectroscopic parameters have been performed for other DIAL systems – such
as those presented by , , and
– and may be required to improve the accuracy of this
system. With these caveats, it seems reasonable to claim that the
measurements of water vapor provided by the WV-DIAL agree well (with less
than 10 % mean error) with the radiosonde measurements of water
vapor over a wide range of atmospheric conditions.
Two weeks of near continuous data collected 1–14 July 2014. Top
panel: 1 min, 150 m resolution relative backscatter from 0 to
14 km range for the far-range channel; second panel from top:
10 min, 150 m resolution water vapor in gm-3 from 0 to
7 km range for the far-range channel – with 300 and 600 m
smoothing applied to the data from 3 to 5.25 km and above
5.25 km, respectively, second panel from bottom: measured two-way
optical depth at 2.5 km (blue) and 5.0 km (black) range for
the far-range channel; and bottom panel: background in counts per second
(C s-1) for the near-range (red) and far-range (black) channels.
The instrument was operated unattended for 50 continuous days during the
FRAPPE field campaign from a mobile laboratory container with approximately
±5∘C temperature stability. The device was aligned once
(at setup only) and there was no detectable degradation in performance during
the project or evidence of temperature cycling in the data. The instrument
operated reliably, providing >95% data coverage. A subset –
a 2-week period from 1 to 14 July 2014 – of near continuous data is shown in
Fig. . The top panel shows the relative
backscatter at 1 min, 150 m vertical resolution for distances
between 0 to 14 km range. The image is a composite of the near- and
far-range channels below 3 km and the far-range channel only above
3 km. The second panel from the top shows the measured absolute
humidity in gm-3 at 10 min, 150 m vertical resolution
from 0 to 7 km (data from the near-range channel). This unique
image of the water vapor distribution in the lower atmosphere is only
possible with high-vertical-resolution continuous profiling. The second panel
from bottom shows the measured two-way column optical depth at 2.5 km
(blue) and 5.0 km (black) range for the far-range channel for this
time period. The wavelength of the instrument was not adjusted for this time
span, so this is representative of the natural atmospheric variability and can
be converted to a water vapor column content and used to compare with other
instrumentation. The bottom panel shows the measured background in counts per
second (C s-1) for the near-range (red) and far-range (black) channels.
To measure water vapor accurately, the photon count rates need to be within
the linear response range of the photon-counting modules – below
5 MCs-1 for the modules used in this instrument. As seen in the
plot, the far-range channel has a maximum count rate of 1 MCs-1
during the brightest of atmospheric conditions. This is a significant
improvement over the previous-generation receivers where the count rate would
be in excess of 10 MCs-1 under daytime cloudy conditions, as
shown in Fig. 9 of . The near-range channel, with its wide
field-of view and one less filtering stage, approaches 100 MCs-1
under these bright conditions and is not capable of accurately measuring
water vapor during these time periods.
At FRAPPE, the diode-laser-based WV-DIAL was co-located with several active
and passive remote sensing instruments. A quantitative comparison between
other instruments is beyond the scope of this instrument paper. However,
a detailed intercomparison study highlighting the different spatial and
temporal resolutions between the WV-DIAL and the AERI using the FRAPPE
data set is planned for a separate paper. In the future, more focus is needed
on improved algorithms to process the data (e.g., masking the data at cloud
boundaries, improving low signal-to-noise thresholds, and allowing for variable
temporal and spatial resolution with range). Also, real-time algorithms
are desired to adjust the online wavelength (and corresponding offline and
etalon temperature) to maintain optimal optical depth and performance for
extended autonomous field operations. Tests should be performed at shorter
pulse duration to investigate reducing the lowest usable gate a.g.l. to below
300 m. Currently the transmitter is capable of higher output power
and is limited by eye-safety regulations; the shared transmit–receive path
could be redesigned (albeit with some added complexity) to allow the beam to
be expanded to the full telescope diameter, which would allow higher output
power and improved performance. Also, asynchronous wavelength switching could
be tested as a means to improve performance without increasing the
transmitted power level. The near-range channel could be modified to have
a slightly reduced field-of-view to avoid nonlinear detector response at
high count rates during bright-cloud conditions. Finally, a more portable
environmental enclosure should be designed and constructed to make fielding
the instrument easier and less costly in the future.
Conclusions
A field-deployable, high-vertical-resolution water vapor profiling instrument
has been constructed and tested. It was built on the success of previous
diode-laser-based prototypes and advances the technology to provide
measurements over a broadened range of atmospheric conditions. The
optomechanically stable, eye-safe design provides improved temporal
resolution, reduced errors at short range, improved daytime performance, and
reduced errors when clouds are present. This next-generation water vapor
profiler has been demonstrated to be capable of reliable operation in the
field and shown to provide data that compare favorably with sondes over
a wide range of atmospheric conditions. The instrument has the potential to
provide a priority observation needed for national mesoscale weather
observation networks, the National Weather Service, and other federal
agencies. The demonstrated performance and reliability suggest that this
technology may be suitable for a future network of water vapor profiling
instruments.
Acknowledgements
The authors affiliated with Montana State University would like to
acknowledge the support of the National Science Foundation grant number
1206166. The National Center for Atmospheric Research is sponsored by the
National Science Foundation. Component manufactures were included to help
other researchers reproduce this work but are not meant as an endorsement by
the authors. The NCAR authors thank Rich Erickson for technician support and
Richard E. Carbone and Tammy M. Weckwerth for helpful discussions pertaining
to the atmospheric science applications and internal review of the
paper.Edited by: G. Ehret
References
American National Standard Institute: American National Standard for Safe
Use
of Lasers, in: Z136.1-2007, edited by Laser Institute of America, Orlando,
FL, USA, 2007.
Behrendt, A., Wulfmeyer, V., Riede, A., Wagner, G., Pal, S., Bauer, H., and
Späth, F.: Scanning differential absorption lidar for 3D observations of
the atmospheric humidity field, in: International Laser Radar Conference,
p. 3, St. Petersburg, Russia, 2010.
Bösenberg, J. and Linné, H.: Continuous Ground-Based Water Vapour
Profiling using DIAL, in: International Laser Radar Conference, pp.
679–682, Nara City, Japan, 2006.
Ertel, K., Linné, H., and Bösenberg, J.: Injection-seeded pulsed
Ti:sapphire laser with novel stabilization scheme and capability of
dual-wavelength operation, Appl. Optics, 44, 5120–5126, 2005.
Feltz, W., Smith, W., Howell, H., Knuteson, R., H., W., and Revercomb, H.:
Near-continuous profiling of temperature, moisture, and atmospheric
stability using the atmospheric emitted radiance interferometer (AERI),
J. Appl. Meteorol., 42, 584–597, 2003.Goldsmith, J. E., Forest, M., Blair, H., Bisson, S. E., and Turner., D. D.:
Turn-Key Raman Lidar for Profiling Atmospheric Water Vapor, Clouds, and
Aerosols, Appl. Optics, 37, 4979, 10.1364/AO.37.004979, 1998.
Grossmann, B. E. and Browell, E. V.: Spectroscopy of water-vapor in the 720
nm
wavelength region – line strengths, self-induced pressure broadenings and
shifts, and temperature dependence of linewidths and shifts, J. Mol.
Spectrosc., 136, 264–294, 1989:.
Ismail, S. and Browell, E. V.: Influence of Rayleigh-Doppler Broadening on
the
Selection of H2O DIAL System Parameters, in: International Laser Radar
Conference, p. 3, Toronto, Canada, 1986.
Knuteson, R., Revercomb, H., Best, F., Ciganovich, N., Dedecker, R., Dirkx,
T.,
Ellington, S., Feltz, W., Garcia, R., Howell, H., Smith, W., Short, J., and
Tobin, D.: Atmospheric emitted radiance interferometer. part II: Instrument
performance, J. Atmos. Ocean. Technol., 21, 1777–1789,
2004a.
Knuteson, R., Revercomb, H., Best, F., Ciganovich, N., Dedecker, R., Dirkx,
T. P., Ellington, S., Feltz, W., Garcia, R., Howell, H., Smith, W., Short,
J., and Tobin, D.: Atmospheric emitted radiance interferometer. Part I:
Instrument Design, J. Atmos. Ocean. Technol., 21,
1763–1776, 2004b.
Lisak, D., Havey, D. K., and Hodges, J. T.: Spectroscopic line parameters of
water vapor for rotation-vibration transitions near 7180 cm-1, Phys. Rev. A,
79, 1–10, 2009.Machol, J. L., Ayers, T., Schwenz, K. T., Koenig, K. W., Hardesty, R. M.,
Senff, C., Krainak, M. A., Abshire, J. B., Bravo, H. E., and Sandberg, S. P.:
Preliminary Measurements with an Automated Compact Differential Absorption
Lidar for the Profiling of Water Vapor, Appl. Optics, 43, 3110–3121,
10.1364/AO.43.003110, 2004.
National Research Council: Observing Weather and Climate from the Ground
Up:
A Nationwide Network of Networks, The National Academies Press, Washington,
DC, 2009.
National Research Council: When Weather Matters: Science and Service to
Meet
Critical Societal Needs, The National Academies Press, Washington, DC, 2010.NCAR Atmospheric Chemistry Division: FRAPPÉ – Front Range Air
Pollution
and Photochemistry Experiment, available at:
https://www2.acd.ucar.edu/frappe (last access: 1 October 2014), 2014.
Nehrir, A. R.: Development of an Eye-Safe Diode-Laser-Based Micro-Pulse
Differential Absorption Lidar (MP- DIAL) for Atmospheric Water vapor and
Aerosol Studies, Ph.D. thesis, Montana State University, 2011.Nehrir, A. R., Repasky, K. S., Carlsten, J. L., Obland, M. D., and Shaw,
J. A.:
Water Vapor Profiling Using a Widely Tunable, Amplified Diode-Laser-Based
Differential Absorption Lidar (DIAL), J. Atmos. Ocean.
Technol., 26, 733–745, 10.1175/2008JTECHA1201.1, 2009.Nehrir, A. R., Repasky, K. S., and Carlsten, J. L.: Eye-Safe
Diode-Laser-Based
Micropulse Differential Absorption Lidar (DIAL) for Water Vapor Profiling in
the Lower Troposphere, J. Atmos. Ocean. Technol., 28,
131–147, 10.1175/2010JTECHA1452.1, 2011.
Nehrir, A. R., Repasky, K. S., and Carlsten, J. L.: Micropulse water vapor
differential absorption lidar: transmitter design and performance, Optics
express, 20, 137–151, 2012.NOAA Physical Sciences Division: The Boulder Atmospheric Observatory,
available at: http://www.esrl.noaa.gov/psd/technology/bao/ (last
access: 1 October 2014), 2014.
Pike, E. and Sabatier, P.: Scattering, Two-Volume Set: Scattering and inverse
scattering in Pure and Applied Science, Elsevier Science, 2001.
Ponsardin, P. L. and Browell, E. V.: Measurements of H216O Linestrengths and
Air-Induced Broadenings and Shifts in the 815 nm Spectral Region, J.
Molecular Spectr., 185, 58–70, 1997.Reagan, J. A., Cooley, T. W., and Shaw, J. A.: Prospects for an economical,
eye-safe water vapor LIDAR., in: International Geoscience and Remote Sensing
Symposium, Better Understanding of Earth Environment, 872–874, IEEE,
Kogakuin University, Tokyo, Japan, 10.1109/IGARSS.1993.322198, 1993.Repasky, K., Moen, D., Spuler, S., Nehrir, A., and Carlsten, J.: Progress
towards an Autonomous Field Deployable Diode-Laser-Based Differential
Absorption Lidar (DIAL) for Profiling Water Vapor in the Lower Troposphere,
Remote Sens., 5, 6241–6259, 10.3390/rs5126241, 2013.Rothman, L., Gordon, I., Barbe, A., Benner, D., Bernath, P., Birk, M.,
Boudon,
V., Brown, L., Campargue, A., Champion, J.-P., Chance, K., Coudert, L., Dana,
V., Devi, V., Fally, S., Flaud, J.-M., Gamache, R., Goldman, a., Jacquemart,
D., Kleiner, I., Lacome, N., Lafferty, W., Mandin, J.-Y., Massie, S.,
Mikhailenko, S., Miller, C., Moazzen-Ahmadi, N., Naumenko, O., Nikitin, A.
V., Orphal, J., Perevalov, V., Perrin, A., Predoi-Cross, A., Rinsland, C.,
Rotger, M., Šimečková, M., Smith, M., Sung, K., Tashkun, S.,
Tennyson, J., Toth, R., Vandaele, A. C., and Vander Auwera, J.: The HITRAN
2008 molecular spectroscopic database, J. Quant. Spectr.
Radiat. Trans., 110, 533–572, 10.1016/j.jqsrt.2009.02.013,
2009.
Turner, D. D. and Löhnert, U.: Information Content and Uncertainties in
Thermodynamic Profiles and Liquid Cloud Properties Retrieved from the
Ground-Based Atmospheric Emitted Radiance Interferometer (AERI), J.
Appl. Meteorol. Climatol., 53, 752–771, 2014.
Turner, D. D., Ferrare, R. A., Brasseur, L. A. H., Feltz, W. F., and Tooman,
T. P.: Automated Retrievals of Water Vapor and Aerosol Profiles from an
Operational Raman Lidar, J. Atmos. Ocean. Technol., 19,
37–50, 2002.Vogelmann, H. and Trickl, T.: Wide-range sounding of free-tropospheric water
vapor with a differential-absorption lidar (DIAL) at a high-altitude
station, Appl. Optics, 47, 2116, 10.1364/AO.47.002116, 2008.Vogelmann, H., Sussmann, R., Trickl, T., and Borsdorff, T.: Intercomparison
of atmospheric water vapor soundings from the differential absorption lidar
(DIAL) and the solar FTIR system on Mt. Zugspitze, Atmos. Meas. Tech., 4,
835–841, 10.5194/amt-4-835-2011, 2011.
Wulfmeyer, V. and Bösenberg, J.: Ground-based differential absorption
lidar for water-vapor profiling: assessment of accuracy, resolution and
meteorological applications, Appl. Optics, 37, 3825–3844, 1998.Wulfmeyer, V. and Walther, C.: Future performance of a ground-based and
airborne water-vapor differential absorption lidar. I. Overview and theory,
Appl. Optics, 40, 5304–5320, 10.1364/AO.40.005304,
2001a.
Wulfmeyer, V. and Walther, C.: Future performance of ground-based and
airborne
water-vapor differential absorption lidar. II. Simulations of the precision
of a near-infrared, high-power system, Appl. Optics, 40, 5321–5336,
2001b.