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vibrational Raman shifts for water vapour,
S
H2O
and nitrogen,
S
N2
. The measure-
ment is made with laser transmission at visible (532 nm) and ultraviolet (266 nm)
wavelengths. The visible measurement (660/607 nm) is available at night and the
ultraviolet measurement (294/284 nm) is available day and night. The ultraviolet
measurement is limited to the first 3 km of the atmosphere because of signal loss
due to the large scattering cross section. The specific humidity
q
(
z
) is determined
from the relationship,
K
S
H
2
O
(
z
)
S
N
2
q
(
z
)
=
,
(4.14)
where the calibration factor,
K
, is verified by comparison with radiosonde mea-
surements when the instrument is moved to a new site (Philbrick and Mulick
2001
). Figure
4.24
shows an example of a humidity profile obtained with LAPS
(Pennsylvania State University Laser Atmospheric Profile Sensor, Philbrick
2002
).
The humidity profile data from the Raman LIDAR coincide well within 10% with
Fig. 4.24
Specific humidity profile from Raman LIDAR measurements (
diamonds
) with LAPS
(Lidar Atmospheric Profile Sensor) compared to thethered balloon measurements (
bold line
in the
lower 400 m) and aircraft measurements (
thin full line
). From Philbrick (
2002
)
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