|
|
|
 |
EDUCATION - Stuff for Kids
|
Mark Williams by the NASA research aircraft that carries the Lidar.
|
USING LASERS TO UNDERSTAND OCEAN COLOR
Greetings! I'm Mark Williams from the Wallops Flight Facility located on
Virginia's Eastern Shore. I work with a team of experimenters headed by Dr.
Frank Hoge, who work with NASA's Airborne Oceanographic Lidar, a
remote sensing instrument generally referred to as simply as the AOL. We
use the AOL to look at the surface layer of the ocean to tell us how much
chlorophyll is in the water. Chlorophyll is a substance found in tiny green
ocean plants called phytoplankon and is important because of its role in
photosynthesis, an activity which uses sunlight and carbon dioxide to produce
sugar and release oxygen.
|
Figure 1. A diatom, which is a type of photoplankton that has a silica skeleton.
|
Knowing the amount of chlorophyll in the ocean is important
to scientists studying the global carbon cycle. As you
probably already know, the amount of carbon dioxide gas in
the air has greatly increased over the last 100 years as
mankind has burned fossil fuels such as coal and oil to
produce heat and to provide energy for other uses. There is
concern that this increased amount of carbon dioxide may
lead to global warming by trapping heat that would normally
leave the earth and go into space, a process known as
the greenhouse effect. There is considerable debate among
scientists about the amount of global warming to expect
over the next century. Part of this uncertainty is due to the
not knowing how much of the newly formed carbon dioxide
will be naturally removed from the atmosphere and oceans.
The photosynthetic activity in ocean phytoplankton is a
major "sink" which removes carbon dioxide, we just don't
know how much yet [Fig. 1].
|
Figure 2. One of the NASA research aircraft at Wallops Island, VA. These aircraft carry instruments such as the Airborne Oceanographic Lidar on research missions worldwide. The four engine P3-B Orion aricraft shown here is very well suited for flying instruments over the ocean as it can carry a heavy load over the ocean at low altitudes.
|
How does the AOL work? The AOL pumps blue-green
laser pulses into the ocean from an aircraft [Fig. 2] that flies about 500 feet above the ocean surface. The
blue-green laser pulses are absorbed by the phytoplankton which then give off a red colored fluorescence
as part of the energy absorbed from the
laser pulse is released. The AOL captures
the red fluorescence through a telescope
which views the "footprint" where the laser
pulse struck the surface of the water and the
intensity of the fluorescence is recorded on
a computer for post mission analysis. The
intensity of the fluorescence is directly
related to the amount of chlorophyll within
the footprint. A photograph of the AOL
[Fig. 3] shows the laser used to make the
measurements inside the aircraft on which
the sensor is flown. Results [Fig. 4] from a
flight from the Delaware Bay into the North
Atlantic Ocean are shown on a graph where the chlorophyll is plotted
with distance from the Delaware Bay.
|
Figure 3. The Lidar in the P3-B Orion aircraft.
|
As you see by comparing the AOL data with the satellite view
[Fig. 5], the AOL could only measure chlorophyll over a small part
of the vast oceans which cover nearly seven tenths of the world's
surface. That would not give much of an answer for scientists needing
to know how much carbon dioxide is removed by the phytoplankton.
Other scientists are trying to look at all of the oceans using
satellite ocean color images from the SeaWiFS sensor that was
launched in July, 1997. The satellite measures chlorophyll using a
different method which is less certain at this time. The AOL is flown
under the satellite in different portions of the ocean [Fig. 6]. Scientists
try to improve on the accuracy of their methods of measuring
chlorophyll from the satellite images by comparing their results with
those from the AOL. If you want to know how chlorophyll measurements
from space are used, check with my colleague, Mary-Elena
Carr, on the Visit to an Ocean Planet CD-ROM.
|
Figure 4. Graphs showing some of the data collected during a flight over a
warm core. The amount of chlorophyll (red line) is plotted along a flight
line from the Delaware Bay to the Gulf Stream. The sea surface
temperature (blue line) was measured by a radiometer on the same
aircraft. If you look closely you will see two red lines and two blue lines
because data was taken on the way out and the way back. These
graphs show the relationship between physical properties and
photoplankton which is represented by chlorophyll content.
|
|
Figure 5. An image from the SeaWiFS sensor, which measures ocean
color. Red indicates areas of high pigment concentration and blue
indicates areas of low concentration.
|
Figure 6. Map showing the flight line for the warm core mission. A warm
core is an ocean feature, often found near the Gulf Stream. They are
formed when a mass of warm water spins off of the warm current and
goes into the cold water area.
|
|
|
|
|
|
|
|
|
|
|
|
|
