336
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
14.1 INTRODUCTION
Water vapor is the most important greenhouse gas, much more effective than CO 2 . It
governs the atmospheric water cycle and is a key component in atmospheric chem- chem- chemistry. The frequent occurrence of phase transitions from vapor to liquid water or ice
crystals further enhances the importance of atmospheric humidity. Cloud formahumidity. Cloud formahumidity. Cloud formation and the various forms of precipitation certainly belong to the most important
weather phenomena. The strong temperature dependence of the saturation vapor
pressure in combination with vertical transport processes causes a large variability
of the atmospheric humidity, which exists on practically all scales from turbulence
to global distribution.
In view of its importance, the detection and observation capabilities for atmoof its importance, the detection and observation capabilities for atmoof its importance, the detection and observation capabilities for atmodetection and observation capabilities for atmoobservation capabilities for atmospheric water vapor are far from suffi cient, both for the operational global observawater vapor are far from suffi cient, both for the operational global observawater vapor are far from suffi cient, both for the operational global observafar from suffi cient, both for the operational global observasufficient, both for the operational global observation system and for detailed process studies. Most routine observations are still made
using in situ sensors on radiosondes. Standard capacitive humidity sensors (e.g.,
Vaisala RS80-H and RS90), which act as the backbone of weather forecast centers,
do not provide reliable measurements in the upper troposphere and above at temperatures below –40°C. More accurate measurements of low humidity in this region
can be performed using sophisticated in situ instruments that have been developed
for operation on balloons or aircraft. They are the frost-point hygrometers (Oltmans
1995; Ovarlez 1991; Busen and Buck 1995), Lyman-α hygrometer (Zöger et al. 1999),
and tunable-diode-laser hygrometer (May 1998; Sonnenfroh et al. 1998). However,
data provided by these instruments are limited to one- dimensional vertical (balloons) and horizontal (aircraft) profiles. Apart from the problems caused by the sencaused by the sencaused by the sensor properties, the sampling strategy is limited by typically only two instantaneous
measurements per day at a relatively small number of stations worldwide, which does
not permit a characterization of the water vapor distribution. Satellite-borne passive
remote sensing instruments, in contrast, provide global observations. Unfortunately,
current satellite instruments such as SAGE II (Mauldin III et al. 1985) and HALOE
(Russell et al. 1993) have insufficient vertical resolution in many cases and tend to
have large error bars for data collection in the upper troposphere. HALOE is capable
of observing water vapor from the troposphere up to the upper mesosphere with a
vertical resolution of approximately 2 km and with an accuracy of ±10% between
0.1 and 100 mb, rising to ±30% at the boundaries of the observational range (Harries
et al. 1996).
For process studies, the vertical structure of the atmosphere is of great importance. An airborne differential absorption lidar (DIAL) can provide two-dimensional water vapor measurements along extended cross sections with high accuracy
and spatial resolution and, thus, fills the gap between existing in situ instruments and
passive remote sensing sensors. During the first two decades of DIAL development,
the basic theory was initialized, the relationship between signal-to-noise ratio and
detection limits was elucidated, and DIAL systems in both the visible and infrared
spectral regions were developed (Collis and Russell 1976; Killinger and Mooradian
1983; Measures 1984). In the early days, building DIAL was difficult. The systems
were complex and costly, and their long-term stability was bad. They required highly
trained operators and frequent adjustments. These difficulties severely limited the
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
14.1 INTRODUCTION
Water vapor is the most important greenhouse gas, much more effective than CO 2 . It
governs the atmospheric water cycle and is a key component in atmospheric chem- chem- chemistry. The frequent occurrence of phase transitions from vapor to liquid water or ice
crystals further enhances the importance of atmospheric humidity. Cloud formahumidity. Cloud formahumidity. Cloud formation and the various forms of precipitation certainly belong to the most important
weather phenomena. The strong temperature dependence of the saturation vapor
pressure in combination with vertical transport processes causes a large variability
of the atmospheric humidity, which exists on practically all scales from turbulence
to global distribution.
In view of its importance, the detection and observation capabilities for atmoof its importance, the detection and observation capabilities for atmoof its importance, the detection and observation capabilities for atmodetection and observation capabilities for atmoobservation capabilities for atmospheric water vapor are far from suffi cient, both for the operational global observawater vapor are far from suffi cient, both for the operational global observawater vapor are far from suffi cient, both for the operational global observafar from suffi cient, both for the operational global observasufficient, both for the operational global observation system and for detailed process studies. Most routine observations are still made
using in situ sensors on radiosondes. Standard capacitive humidity sensors (e.g.,
Vaisala RS80-H and RS90), which act as the backbone of weather forecast centers,
do not provide reliable measurements in the upper troposphere and above at temperatures below –40°C. More accurate measurements of low humidity in this region
can be performed using sophisticated in situ instruments that have been developed
for operation on balloons or aircraft. They are the frost-point hygrometers (Oltmans
1995; Ovarlez 1991; Busen and Buck 1995), Lyman-α hygrometer (Zöger et al. 1999),
and tunable-diode-laser hygrometer (May 1998; Sonnenfroh et al. 1998). However,
data provided by these instruments are limited to one- dimensional vertical (balloons) and horizontal (aircraft) profiles. Apart from the problems caused by the sencaused by the sencaused by the sensor properties, the sampling strategy is limited by typically only two instantaneous
measurements per day at a relatively small number of stations worldwide, which does
not permit a characterization of the water vapor distribution. Satellite-borne passive
remote sensing instruments, in contrast, provide global observations. Unfortunately,
current satellite instruments such as SAGE II (Mauldin III et al. 1985) and HALOE
(Russell et al. 1993) have insufficient vertical resolution in many cases and tend to
have large error bars for data collection in the upper troposphere. HALOE is capable
of observing water vapor from the troposphere up to the upper mesosphere with a
vertical resolution of approximately 2 km and with an accuracy of ±10% between
0.1 and 100 mb, rising to ±30% at the boundaries of the observational range (Harries
et al. 1996).
For process studies, the vertical structure of the atmosphere is of great importance. An airborne differential absorption lidar (DIAL) can provide two-dimensional water vapor measurements along extended cross sections with high accuracy
and spatial resolution and, thus, fills the gap between existing in situ instruments and
passive remote sensing sensors. During the first two decades of DIAL development,
the basic theory was initialized, the relationship between signal-to-noise ratio and
detection limits was elucidated, and DIAL systems in both the visible and infrared
spectral regions were developed (Collis and Russell 1976; Killinger and Mooradian
1983; Measures 1984). In the early days, building DIAL was difficult. The systems
were complex and costly, and their long-term stability was bad. They required highly
trained operators and frequent adjustments. These difficulties severely limited the