Validating EOS/Aura/OMI UV Products
David R. Brooks, Principal Investigator
Drexel University, Philadelphia, PA.
GLOBE Aerosols, Water Vapor, and UV-A Protocols
brooksdr@drexel.edu
Julie F. Westerlund, Project Collaborator
Texas State University – San Marcos, TX.
jw33@txstate.edu
Would you like your students to participate in real-world science, be able to
contribute to cutting-edge science and communicate their results to scientists? We are
looking for a few research participants and/or schools around the Seguin, Texas area to
participate in an exciting project with NASA that measures UV-A radiation. The recently
launched AURA satellite passes overhead each day taking UV-A measurements. Ground
measurements of UV-A are needed to validate the measurements taken by AURA.
Benefits of Participation
 1) Participation in a real scientific collaboration to collect needed scientific data to
improve the UV monitoring capability of the Aura satellite,
 2) Recognition of your involvement in NASA research,
 3) Publication of your research, and
 4) Acquisition of scientific instrumentation at no cost.
Free Radiometer to measure UV-A radiation in
Texas State, Texas Christian University and Area Schools Collaboration
Imagine standing outside in the early afternoon and looking up through a
cloudless deep blue sky. Even though you can’t see it, the Aura spacecraft flies overhead.
Onboard Aura, the Ozone Monitoring Instrument (OMI) measures ultraviolet (UV)
radiation from the sun reflected from Earth and its atmosphere. Back on the ground,
scientists use these data to calculate how much UV radiation reaches the surface. This is
an important question because of the effects of UV radiation, both good and bad, on you
and your environment. With the global coverage possible from a satellite, measurements
from OMI are used to produce global maps of UV radiation at Earth’s surface that would
be impossible to obtain just from surface-based measurements.
What’s wrong with this picture? The “cloudless deep blue sky” assumption is
definitely a problem. Under these ideal conditions, scientists can do an excellent job of
predicting surface UV based on sunlight reflected from the Earth/atmosphere system.
However, these conditions hardly ever occur! Real atmospheres contain clouds, water
vapor, and aerosols. When scientists try to interpret OMI measurements made in the real
world, there are many difficulties. What does Earth’s surface actually look like as seen
from space? What are the effects of aerosols, water vapor, and clouds on the reflection of
UV radiation back to space and its transmission through the atmosphere to the surface?
Now let’s imagine how you might help OMI scientists. Instead of just standing on
the ground looking up, you could measure the actual UV radiation coming through the
atmosphere as Aura flies overhead. Then scientists can compare this value against the
value predicted by OMI. Such measurements can be used to evaluate and, over time,
perhaps improve the performance of OMI algorithms. Mission accomplished? Not so
fast...
When OMI looks down at the Earth/atmosphere system, it sees not just a single
“point,” but an area at least 13x24 km. This is called the instrument “footprint” – a
measure of its spatial resolution. The UV values calculated by OMI are, essentially,
averages over this footprint. However, an area of this size, and the atmosphere over this
area, are not homogeneous. All the important atmospheric properties – clouds, aerosols,
and water vapor – can change significantly within this footprint. OMI’s algorithms try to
take this variability into account. For example, scientists have built mathematical models
of the effects of clouds on surface UV radiation. Depending on their size, shape, and
composition, clouds can sometimes even increase the amount of UV radiation reaching
the surface, compared to a cloud-free sky. Especially when the sky is partly cloudy, the
relationship between the actual average UV radiation over an entire OMI footprint and a
simultaneous ground measurement at a single point within that footprint is very
complicated and still very uncertain.
This fact brings us to a critical conclusion: In order to evaluate OMI UV
products, several simultaneous measurements must be made within a single OMI
footprint during overflights of the OMI instrument. It is only in this way that spatial
variability in atmospheric properties can be related to the calculations that OMI makes.
In addition, it is not sufficient just to make a single measurement simultaneously
at several points within a footprint. UV radiation varies with time as well as spatially.
One of the OMI products is the total daily UV radiation dose; this so-called “erythemal
dose” is related to the UV index published in newspapers and elsewhere. So, not only
must a single OMI measurement over a footprint be accurately converted to a spatial
average over that footprint, it must also be related to an entire day’s worth of UV
radiation.
Because UV radiation varies in both space and time, and because much of this
variation is due to the movement of clouds, some scientists believe that it may be possible
to relate spatial variability in radiation, based on simultaneous measurements at different
places, to temporal variability, based on continuous measurements made at the same
place. This is a hypothesis that can be tested only by collecting a time series of
measurements at several places within an OMI footprint. The practical effect of this
requirement is that instruments for recording UV radiation must be attached to data
loggers that record measurements starting well before and ending well after an Aura
overflight – perhaps even during an entire day.
These requirements for properly validating OMI measurements pose daunting
challenges. High-quality instruments for measuring UV radiation are very expensive to
purchase, operate, and maintain. In fact, it is generally agreed that it is prohibitively
expensive to collect proper validation measurements with such instruments. This is
where you can make a contribution! Scientists who have studied this problem have
concluded that a reasonable approach to validating OMI measurements involves using a
combination of one high-quality instrument, to provide a reliable measurement and
calibration standard, and several “simple radiometers” (to quote one author writing on
this topic) distributed around an OMI footprint.
There is one additional restriction on these simple radiometers. Scientists would
like to have ground measurements taken at a wavelength that is unaffected by varying
amounts of ozone in the atmosphere, thus removing one variable from their calculations.
Ozone absorbs significant amounts of solar radiation across most of the UV part of the
solar spectrum. (This is why the well known atmospheric “ozone hole” affects the
amount of UV radiation reaching Earth’s surface.) At higher UV wavelengths, in the UVA part of the spectrum, ozone absorption is negligible.
There are several relatively inexpensive commercial instruments for measuring
UV radiation. However, they do not measure just that small part of the UV spectrum that
is unaffected by ozone. In cooperation with NASA’s Goddard Space Flight Center,
GLOBE scientist David Brooks and his colleague Forrest Mims have developed an
inexpensive radiometer that measures UV radiation in a narrow band of wavelengths
centered around 372 nm, in the desired part of the UV-A spectrum. The cost of such an
instrument is about 100 times less than a high-quality reference instrument! If several of
these instruments are deployed around an area equal in size to an OMI footprint, the
requirements for evaluating OMI UV measurements can be met in a very cost-effective
way.
Free Radiometer to measure for UV-A radiation in
UV-A Collaboration
This is an ideal project for scientist/teacher/student partnerships. It addresses a
well defined science need that, as a practical matter, cannot be met in any other way. It is
a true partnership in which teachers and students work together with scientists to develop
an OMI validation network within a limited region. The ground and space-based
measurements address important environmental issues and help tie together several
GLOBE atmosphere protocols. The required equipment will be provided at no cost to
participants. The experiment protocols offer considerable flexibility about when
measurements are taken. (It is not necessary to collect data every day.) The qualitative
descriptions of sky and cloud conditions that only a human observer can provide are
essential to the OMI validation process.
We look forward to your participation in this exciting new project!
Dr. David R. Brooks
Dr. Julie F. Westerlund