Title: Falcon H20 DIAL Data [DLR]
CONTACTS:
Dr. Gerhard Ehret (PI)
DLR Oberpfaffenhofen
Institut fuer Physik der Atmosphaere / Lidar
Muenchnerstr. 20
82234 Wessling
Voice: +49 8153 28 2509
Email: gerhard.ehret@dlr.de
Christoph Kiemle (Co-I)
DLR Oberpfaffenhofen
Institut fuer Physik der Atmosphaere / Lidar
Muenchnerstr. 20
82234 Wessling
Voice: +49 8153 28 2525
Email: christoph.kiemle@dlr.de
Dr. Gorazd Poberaj (Co-I)
DLR Oberpfaffenhofen
Institut fuer Physik der Atmosphaere / Lidar
Muenchnerstr. 20
82234 Wessling
Voice: +49 8153 28 1817
Email: gorazd.poberaj@dlr.de
1.0 DATA SET OVERVIEW
DLR's Water Vapor DIAL system aboard the Falcon 20 was operated during
the International H2O Project (IHOP_2002) field experiment from May 17
through June 15, 2002.
2.0 INSTRUMENT DESCRIPTION
2.1 Instrumentation
An airborne DLR Water Vapor Differential Absorption Lidar (H2O-DIAL)
was developed for high resolution measurements of humidity and aerosols
in the troposphere and lower stratosphere. The system was designed to
operate on the German meteorological aircraft Falcon 20. Its
transmitter is based on an injection-seeded KTP optical parametric
oscillator (OPO) pumped by the second harmonic of a Q-switched, diode
pumped single-mode Nd:YAG laser at a repetition rate of 100 Hz. The OPO
is optimized for operation in the spectral region between 920-950 nm
with an average output power of 1.2-1.8 W and a spectral purity higher
than 99%. The system is also designed to perform simultaneous
polarization-sensitive backscatter measurements at 532 nm and 1064 nm
for aerosol detection. A detailed system description and assessment of
its accuracy for measuring lower water vapor contents in the tropopause
region can be found in [1].
The H2O-DIAL aboard the Falcon could be positioned to look either
downward or upward. Its broad tunability in the spectral region around
935 nm enables to reach a variety of water vapor lines with different
line-strengths ranging over three orders of magnitude. In the past, the
system was deployed mainly to measure low water vapor contents in the
upper troposphere and lower stratosphere [e.g. 2,3].
During IHOP 2002 campaign the system was deployed for the first time to
measure water vapor in the atmospheric boundary layer. For these
measurements we selected a water vapor absorption line at 926.87403 nm
(10788.95078 cm-1), which has a line strength of 3.96E-24 cm, linewidth
of 0.079 cm-1, and lower energy state of 1282.9191 cm-1 (data from
HITRAN 2001).
Water vapor number density fields below flight tracks are calculated
using a DIAL equation with an effective differential absorption cross
section. This takes into account the spectral modification of the on-
and off-line DIAL signals due to absorption by water vapor and due to
Doppler-broadened Rayleigh backscattering. A slight temperature
sensitivity of the absorption line strength was corrected using
atmospheric temperature profiles obtained by the dropsondes, whenever
possible. Atmospheric temperature and pressure profiles obtained by
dropsondes are used also for calculation of water vapor mixing ratio
fields.
In addition, simultaneous atmospheric backscatter measurements were
performed at 926 nm (off-line) and 1064 nm (s and p-polarization).
Measurements at 532 nm were not possible due to eye-safety
requirements.
3.0 DATA COLLECTION AND PROCESSING
DLR H2O-DIAL was operated aboard the German research aircraft Falcon 20
in the nadir looking mode. From a total of 21 flight missions (75
hours), 52 hours of lidar data were collected in the IHOP region:
BL Heterogeneity: 11 flights for a total of 26 h 4 min
BL Evolution: 2 flights for a total of 5 h 24 min
Convective Initiation: 4 flights for a total of 12 h 36 min
Low-level-jet: 4 flights for a total of 7 h 39 min.
3.1 Quality of data
a. Water vapor mixing ratio:
High quality data (9 flights):
- BL Heterogeneity: 17, 21, 28 and 29 May
- BL Evolution: 14 June (1st flight)
- Convective Initiation: 24 May, 2 and 15 June
- Low-level-jet 9 June
Data of moderate quality (4 flights):
- BL Heterogeneity: 20 and 25 May
- BL Evolution: 14 June (2nd flight)
- Low-level-jet 3 June (1st flight)
b. Atmospheric Backscatter:
All backscatter data at 926nm and 1064 nm are of high quality.
3.2 Data Resolution:
a. Water vapor mixing ratio measurements:
- horizontal resolution is 0.5s to 2s or ~75m to 300m, typically
- vertical resolution is 150m to 300m, typically
b. Atmsopheric backscatter at 1064nm:
- horizontal resolution is 0.5s or ~75m
- vertical resolution is 15m
Actual information on the measurement resolution and data grid distance
for a specific data set is included in the corresponding file header.
4.0 DATA FORMAT AND FILE NAMING
4.1 Data format
Evaluated water vapor and aerosol measurements are stored in the ASCII
format. Each text file begins with a header including aircraft and
lidar specific data, plus data processing information like spatial
range and resolution. The header is followed by a series of data blocks
corresponding to vertical profiles from a two dimensional data field.
The top and bottom height of the data profiles correspond to altitudes
above the sea level. The flight altitude of the Falcon, which was used
as a reference, was determined using the GPS. Each profile is preceded
by a line indicating the profile number, UTC time in seconds, latitude,
longitude, and also UTC time in the hh:mm:ss format. Missing or bad
data are marked by "-9999.". They can be caused by optically thick
clouds leading to strong absorption of the backscatter signal, or
detector saturation due to strong backscatter, but also due to unstable
instrument operation.
The accompanying IDL program "Read_DIAL.pro" (attached to the data
files) will read formatted H2O-DIAL water vapor and aerosol profile
data.
4.2 File Naming Conventions
Data Files
a. H2O_YYMMDD_Leg#.txt
Water vapor mixing ratio, YY=year, MM=month, DD=day, Leg No.
b. ABS_YYMMDD_Leg#.txt
Atmospheric BackScatter at 1064nm, YY=year, MM=month, DD=day, Leg No.
BSR_YYMMDD_Leg#.gif
BackScatter Ratio at 1064nm, YY=year, MM=month, DD=day, Leg No.
(backscatter ratio is here defined as a ratio between a total
backscatter and Rayleigh backscatter)
5.0 DATA REMARKS
DLR H2O-DIAL IHOP_2002 data set release history
a. "Quick-look", preliminary data
Preliminary data were produced and distributed into the JOSS Catalog
during the IHOP_2002 field experiment during May - June 2002. These
data include only images and can be used for "quick-look" purposes.
Atmospheric backscatter ratio data (total backscatter to Rayleigh
backscatter) were evaluated mostly for the wavelength at 926 nm (DIAL
off-line). In many cases normalization of the total backscatter to
Rayleigh bacskactter could not be performed accurately due to low
flight levels and presence of clouds. Also the aerosol extinction was
not taken into account.
b. Final processed data
Final data processing is in progress. The final data sets include
images as well as text files with water vapor mixing ratio profiles and
atmospheric backscatter profiles at 1064 nm.
6.0 REFERENCES
[1] G. Poberaj, A. Fix, A. Assion, M. Wirth, C. Kiemle, G. Ehret:
"Airborne all-solid-state DIAL for water vapour measurements in the
tropopause region: system description and assessment of accuracy",
Appl. Phys. B 75, 165-172 (2002)
[2] G. Ehret, K. P. Hoinka, J. Stein, A. Fix, C. Kiemle, and G.
Poberaj: "Low stratospheric water vapor measured by an airborne DIAL",
J. Geophys. Res., 104, D24, 31,351-31,359 (1999)
[3] K.P. Hoinka, E. Richard, G. Poberaj, R. Busen, J.-L. Caccia, A.
Fix, H. Mannstein: "Analysis of a potential vorticity streamer crossing
the Alps during MAP IOP-15 on 6 November 1999", Q. J. R. Meteorol.
Soc., Vol. 129, 609-632 (2003)