In this NASA shuttle photo of a sunset over South America, a pink layer, attributed to sulfuric acid droplets and ammonium sulfate particles, begins at the tropopause and extends upward into the stratosphere to 19 kilometers. (NASA)

Water-Vapor Lidar Extends to the Tropopause

John Wessel and Robert W. Farley

Lidar's role in obtaining accurate measurements of water vapor in the upper troposphere is becoming increasingly important as the issue of global warming heats up.

The question of whether Earth is dangerously heating up has become a subject of debate in our time. But is global warming fact or fiction? One thing is certain: The surface temperature of Earth has increased 0.45 to 0.6 degrees Kelvin in the past century. A recent study supported by The National Science Foundation predicts our planet will warm by 2 degrees Kelvin in the 21st century. Recent research conducted by The Aerospace Corporation to validate Defense Meteorological Satellite Program (DMSP) measurements could shed some light on global warming issues.

Global warming takes place when heat becomes increasingly trapped in Earth's atmosphere. It is now widely believed that "greenhouse gases" (so called because these atmospheric constituents produce a "greenhouse effect" over Earth) contribute to this warming. Water is the most influential component of the greenhouse gas mixture: Water vapor absorbs infrared radiation emitted from Earth's surface and lower atmosphere more than any other constituent, thereby trapping heat best. Accurate knowledge of the amount of atmospheric water must be obtained to improve and test global-warming models.

A radiosonde is an instrument package carried by a balloon that ascends to altitudes of 20 to 30 kilometers. It measures temperature, humidity, and pressure in the atmosphere and broadcasts the information back to a ground station. The Global Positioning System is used to record the trajectory during ascent to determine wind speed and direction.

Aerospace recently made significant advances in the ability to measure the distribution of water vapor in the upper troposphere (upper portion of the lower atmosphere). Using a ground-based lidar (light detection and ranging) system, which operates much like radar, the researchers discovered significantly more water content in the upper reaches of the troposphere than was previously thought to exist. This capability was developed to improve calibration of U.S. Air Force meteorological satellites.

Lidar vs. Weather Balloons

The combined use of lidar and satellites provides many advantages over conventional balloon-borne radiosondes, which measure humidity, temperature, and air pressure. First, radiosondes do not operate correctly at the low temperatures typically encountered above an altitude of 8 kilometers. Second, they are one-shot attempts at measuring atmospheric conditions, and they take one hour to reach their zenith.

The lidar system measures water vapor continuously over the entire altitude range. This is important because the lidar can capture data from a satellite as the satellite moves overhead, and satellites are only in view for five minutes. During this brief time, the "ground footprint" of the satellite must be in line with the lidar. The lidar then calibrates the data derived from the satellite. Essentially, it verifies whether or not a satellite is measuring water vapor properly. (See sidebar "How Lidar Works")

Finally, water-vapor data can now be derived from multiple satellites that measure water vapor all over the world. Before the advent of satellites, data were derived from radiosondes routinely launched over land by national weather services.

Improving Computer Models

Combining accurate ground-based lidar measurements with high-quality imagery obtained by satellites promises to improve both global-climate and weather-forecast modeling. One way to determine the causes (and the only way to predict the future extent) of global warming is to have accurate models, which presupposes having accurate knowledge of the initial atmospheric conditions.

Global-climate-change studies rely heavily on computer-generated models that predict the future state of the atmosphere based on initial data retrieved from ground- and satellite-based weather measurements. Although these models are largely based on thoroughly tested principles of physics, a number of simplifications are used to improve calculation speed and bypass scientific problem areas. In addition, adequate computer power is not yet available to process the volume of data required for accurate prediction. The result is that computer-generated weather-forecast and global-climate-change models yield oversimplified results. As more computer power becomes available, more complete data on initial conditions can be processed, resulting in improved models.

The most reliable calibration occurs when the lidar measures moisture in the satellite "ground footprint" area as the satellite passes overhead. A lidar set up over water works best during calibration of satellite data because the water's surface emits less interfering microwave energy than land. For this reason, setting up the lidar on a small island or on a shipboard platform is ideal.

Microwave Sounders vs. Radiosondes

DMSP recognized in 1979 the need for accurate data to feed numerical (computer) global-weather-prediction models and pioneered a microwave-sounding instrument, the SSM/T-1 (Special Sensor Microwave/ Temperature), to measure temperature in the atmosphere. Until then, radiosondes alone were relied upon to gather data about atmospheric temperatures.

Microwave sounders are passive devices, radio receivers that listen for emissions at various frequencies. Water vapor emits microwaves, the intensity of which is used to estimate water content in the atmosphere, particularly over oceans, where conventional methods of obtaining measurements are in short supply. (Radiosondes, which provide a more direct means of measuring atmospheric properties, are in widespread use over land.)

In 1991, DMSP launched the SSM/T-2, which measures water vapor. SSM/T-1 and SSM/T-2 now serve as eyes on worldwide weather, providing the data needed to initialize the computer models. Although the SSM/T-1 temperature sounder has a long history of success, the SSM/T-2 is newer and has had limited development.

Because water vapor is highly variable in the atmosphere, measurements of it are generally neglected in computer-generated forecast models. This is unfortunate because water provides the principal means of energy transport in the troposphere and plays a critical role in global warming. Additionally, water vapor induces cloud formation and violent weather events, determines atmospheric visibility, causes icing, and influences aircraft contrail formation.

In processing water-vapor data derived from SSM/T-2 measurements, serious discrepancies were observed between microwave and radiosonde water-vapor data. The problem was traced to radiosonde humidity transducers. Errors show up in the water-vapor data derived from satellites because satellites are calibrated against radiosondes.

Water-Vapor Lidar

Raman lidar is a specialized type of lidar named after Sir Chandrasekhara Raman, who won the Nobel Prize for physics in 1930 for his discovery of the shifts in the wavelength of light that occur when a light beam interacts with molecular vibrations and rotations. Whereas lidar typically measures light that remains at one frequency, Raman lidar can measure wavelength-shifted light. Because each constituent of the atmosphere correlates to a characteristic wavelength shift, Raman lidar is useful in measuring the constituents of the atmosphere.

Raman lidar was initially developed at NASA's Goddard Space Flight Center. Aerospace confirmed its feasibility for use in DMSP applications in experiments performed during 1993 at the Air Force Malabar facility. Plans were then made to develop a mobile lidar system capable of calibrating satellites from a variety of remote locations, and Aerospace designed and constructed the system in-house. This mobile lidar is housed in a surplus Air Force transportable radar container.

Setting up the Aerospace mobile lidar are Steven Beck, Yat Chan, and Jerry Gelbwachs. It was first used for satellite calibration at Kauai, Hawaii. The container was equipped with wheels and towed to the final site. The structure on the left front of the container is the elevator that raises the beam director periscope (located on top of the container). The beam director is stored below the roof line so the container can fit inside an aircraft.

Steven Beck, Jerry Gelbwachs, and John Wessel preparing to launch a radiosonde near the Aerospace transportable lidar system at Kauai. The radiosonde, contained in a white plastic-foam package, is suspended below the balloon.

Experimentation on Kauai

The new Raman lidar system was first put to use during the calibration and validation of the DMSP F-14 satellite. A sea-level island location with an airport and controlled air space, clear dark sky, and, of course, a moist atmosphere were required for the demonstration. The Navy's Pacific Missile Range Facility on the Hawaiian island of Kauai was chosen. The Hawaii Air National Guard provided transportation.

Calibration was highly successful and showed significant improvement over radiosonde calibration. During a two-week period, 10 lidar measurements were taken that closely matched measurements gathered by the SSM/T-2. However, the radiosonde measurements suggested considerable instrument error. Aerospace concluded that radiosonde water-vapor measurements taken above 8 kilometers altitude are incorrect most of the time because the water-vapor transducers carried by radiosondes become unresponsive at the low temperatures encountered at high altitudes.

High-Altitude Water Vapor

High-altitude water-vapor measurement is a key element in modeling global warming because water has a much greater influence on Earth's tropospheric energy balance than trace gases such as carbon dioxide. However, water vapor is not accurately monitored, and little is known about its influence on global climatic change. Aerospace learned from the Kauai experiment that the lidar's high-altitude accuracy needed to be improved to accurately validate SSM/T-2 for upper-tropospheric water-vapor measurement. A new high-altitude detector system was incorporated into the lidar, which allowed measurements above 10 kilometers. When this new system was used at the U.S. Navy's Pacific Missile Range Facility at San Nicolas Island, off the coast of California, SSM/T-2 upper-atmosphere water-vapor measurements were validated.

(Left) Mixing ratio of water vapor to air, measured by lidar on September 25, 1998. Wind direction in degrees from north is 10 times the value shown on the wind speed scale. (Right) Average mixing ratios observed by Aerospace lidar at San Nicolas and the Table Mountain Observatory June–October. NASA Sage II instrument midlatitude measurements were lower than Aerospace averages, Oltmans-Mastenbrook measurements above Colorado were much lower, and Mastenbrook's, in summer over Washington, D.C., and Trinidad, V.I., were closest to Aerospace measurements. Most high-quality ground-based measurements for the upper tropopause were made in equatorial regions; the Kley points are typical of these. (The mixing ratio is the ratio of water-vapor pressure to air pressure in the atmosphere.)		Relative humidity measured at San Nicolas Island Oct. 7, 1998. Lidar measurements are appreciably higher than corresponding radiosonde measurements.

The picture of upper-tropospheric water vapor observed from San Nicolas Island was much different than expected based on prior NASA satellite measurements. The lidar measurements indicated that, on average, four times more water vapor than expected lies in thin layers near the tropopause (top of the troposphere).

A second set of data acquired in 1999 from the NASA Jet Propulsion Laboratory Smithsonian Table Mountain Observatory site, located at 2,300 meters in elevation at Wrightwood, California, confirms this figure. The high location provided very clear skies and reduced the range to the tropopause, thereby improving measurements. Data from the two sites were similar, confirming that the data represent weather in the vicinity of the local tropopause.

If the results of these experiments are broadly descriptive of the midlatitude atmosphere, they may add to our understanding of global warming. Before now, few high-altitude measurements had been taken at those latitudes using precise methods.

Brightness-temperature mappings recorded in the SSM/T-2 upper tropospheric channel for satellite overpasses, made during San Nicolas Island lidar measurements. Blue corresponds to low-brightness temperature, which indicates the presence of moisture and signifies that emissions are primarily from the cold upper troposphere. Red corresponds to high-brightness temperature. High-brightness temperatures occur when microwave emissions originate from low altitudes, which are normally warmer, and the middle and upper troposphere are dry. Otherwise, the microwave emissions would be absorbed by high-altitude water vapor. In these images, San Nicolas Island, indicated by a small white triangle, lies near the boundary between moist and dry upper layers.

Atmospheric Circulation and Hadley Cells

The combination of lidar data, wind data, and SSM/T-2 upper-atmosphere water-vapor imagery provided information that supports the current scientific understanding of general atmospheric circulation. Global atmospheric circulation is caused by the uneven heating of Earth's surface. Lower latitudes receive more radiation from the sun than do higher ones. To understand atmospheric circulation, various models have been developed.

One such model is the Hadley circulation model. The sun shines approximately overhead at the equator. This heats surface regions, causing air to rise and cool. The cool air loses moisture in the process. Once cool, it moves north and south, descending toward midlatitudes and then returns at low levels back to the equator where it gets reheated. (See sidebar, "The Hadley Circulation Model").

SSM/T-2 routinely identifies large midlatitude regions, including locations such as San Nicolas Island, that have very dry upper air. Dry regions are surrounded by moist high-altitude regions. The dry air presumably came from very high altitudes near the equator and subsided to a lower altitude upon reaching midlatitudes. The lidar and radiosonde data can be used to estimate how long ago this subsidence occurred. Thin moist layers are usually observed by lidar in regions that characteristically have dry air.

In the case of San Nicolas Island, we hypothesized that wind shear carries moisture from moist regions into neighboring dry areas. When we combined the wind-shear velocities with the distance between the lidar location and the surrounding moist region, we estimated that the air subsided within 2 to 20 hours of our measurements. The overall picture of dry high-altitude equatorial air moving north and subsiding at our midlatitude is consistent with the Hadley circulation model, which predicts that equatorial air moves toward the poles.

Applying Our Knowledge

Future research will extend water-vapor measurements over regions representative of the global atmosphere, and the capability of measuring temperature will be added to the lidar. The new DMSP Special Sensor Microwave Imager Sounder (SSMIS) instrument will measure temperatures up to 80 kilometers altitude, which will require a new validation method. This instrument is being developed by Aerojet-Gencorp and is scheduled for launch in November 2000. It combines the features of the current-generation SSM/T-1, SSM/T-2, and SSM/I (SSM/Imager) instruments with increased horizontal resolution and the capability of measuring temperatures at high altitudes.

Some researchers use Rayleigh lidar to measure temperature up to 80 kilometers altitude. We currently measure temperature to an altitude of 40 kilometers using both Rayleigh and rotational Raman lidar, and expect to reach 80 kilometers in 2001. Lidar will then provide the data needed to validate all SSMIS atmospheric measurements.

The U.S. Navy Pacific Missile Range Facility on the island of Kauai. Because a minimum of land in view is desired when a satellite aligns with the lidar, an island setting was ideal for The Aerospace Corporation's test of its new Raman lidar system. In addition, Kauai provided the necessary moist environment for the experiment. (R. L. Jones, Sandia National Laboratories)

The U. S. Navy Pacific Missile Range Facility, San Nicolas Island, off the California coast, where Aerospace used lidar with a new high-altitude shutter system to validate SSM/T-2 upper-atmosphere water-vapor measurements. (Naval Air Warfare Center)

In general, lidar is an excellent method for calibrating satellite microwave sensors. It accurately depicts the atmosphere those sensors view. Enhancing lidar's capabilities will contribute to our increasing understanding of the atmosphere, which may one day in the near future lead us to solving the puzzle of why Earth is getting hotter.

Further Reading

1. M. A. Janssen, Atmospheric Remote Sensing by Microwave Radiometry (Wiley-Interscience, New York, 1993).
2. T. R. Karl and K. E. Trenberth, "The Human Impact on Climate," Scientific American, December 1999, pp. 100–105.
3. R. M. Measures, Laser Remote Sensing (Wiley-Interscience, New York, 1984).
4. S. H. Melfi and D. N. Whiteman, "Observation of Lower Atmospheric Moisture Structure and Its Evolution Using a Raman Lidar," Bulletin of the American Meteorological Society, Vol. 66, 1282–1292 (1985).
5. D. Rind, "Just Add Water Vapor," Science, Vol. 281, 1152–1153 (1998).
6. F. W. Taylor, "The Greenhouse Effect and Climate Change," Reports on Progress in Physics, Vol. 54, 881–918 (1991).
7. J. Wessel, S. M. Beck, Y. C. Chan, R. W. Farley, and J. A. Gelbwachs, "Raman Lidar Calibration for the DMSP SSM/T-2 Microwave Water Vapor Sensor," IEEE Transactions on Geosciences and Remote Sensing, Vol. 38, 141–154 (2000).

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