Severe Weather Study Shows Potential of GNSS-RO
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Authors: S. Mark Leidner, Thomas Nehrkorn, John Henderson and Marikate Mountian;
Date: January 5, 2015; Type: Report
In a novel application of space-based atmospheric measurements, Atmospheric and
Environmental Research (AER) of Lexington, Massachusetts, in collaboration with GeoOptics
Inc. of Pasadena, California, is investigating the use of radio occultation (RO) measurements to
improve severe weather forecasting. Pairing state-of-the-art, high-fidelity weather models with
the rapidly evolving technology of miniaturized satellites in a scientific investigation is unique
and new. So, the results of AER’s study are not only intriguing from a science/discovery point
of view, but compelling also, with the very distinct possibility of becoming a reality in the next
few years.
The challenges of severe weather forecasting
Detailed severe weather forecasting continues to be a vexing problem for meteorologists. There
are simply not enough measurements of atmospheric temperature, moisture and pressure to start
up, or initialize, today’s detailed weather models accurately. The start-up problem for such
models is “underdetermined,” in mathematical terms.
Applying established techniques in a new, unique way
Radio occultation is a well-established technique today for measuring planetary atmospheres,
first used in the 1960’s to probe the atmosphere of Venus from Earth. The technique involves
two orbiting satellites exchanging radio signals along a straight line that passes through the edge,
or limb, of Earth’s atmosphere. The signals that pass from one to the other are called “signals of
opportunity,” because they already exist as part of the Global Navigation Satellite System
(GNSS). As the two satellites move with respect to each other, the straight line between them
either cuts down further into the atmosphere (known as a “setting occultation”) or moves from
Earth’s surface up and out of the top of the atmosphere (known as a “rising occultation”). The
Earth occludes or blocks the two satellites from seeing one another at the beginning or end of an
observing sequence, so these measurements are referred to as “occultations.” Subtle changes are
induced in the radio signals passed between the two satellites by that portion of the atmosphere
through which the signals pass. Those signal changes contain markers for atmospheric
temperature and water vapor. In the last decade, a partnership between US and Taiwan space
agencies has flown a 6-satellite constellation to measure Earth’s atmosphere using radio
occultations, the COSMIC/FORMOSAT-6 mission. Measurements made by the COSMIC
mission have proven to be invaluable to meteorologists world-wide. The six orbiting COSMIC
satellites currently gather about 1,400 vertical occultation profiles every day globally. The
COSMIC follow-on constellation of radio occultation satellites, COSMIC-2, when fully
deployed (12 satellites) will produce 8,000 - 10,000 global profiles/day.
Over the next few years, GeoOptics expects to launch a constellation of miniature radio
occultation satellites that will yield about 50,000 vertical occultation profiles every day globally,
Severe Weather Study Shows Potential of GNSS-­‐RO -­‐ webpage -­‐ final.docx p.1/4 the front end of their CICERO constellation. This significant increase over COSMIC and
COSMIC-2 observation yields should produce measurable improvements in global weather
forecast models, but perhaps only very modest improvements in small-scale regional weather
models where the sampling requirements in space and time are much more stringent.
In AER’s study, we are considering the potential impact of constellations of radio occultation
satellites on weather forecasting that are very much larger than COSMIC, COSMIC-2 or the
initial CICERO deployment, comprised of hundreds to a thousand orbiting/measuring units
instead of six, twelve or twenty-four. The CICERO constellations AER is investigating can yield
up to 2.5 million vertical occultation profiles every day globally. For comparison purposes, the
COSMIC mission gathers about 1 vertical radio occultation profile over the state of Oklahoma
every other day. A future constellation that yields 2.5 million vertical profiles globally means 3550 vertical profiles over the state of Oklahoma every hour.
Because these are potential future satellite constellations, actual observations from these
constellations don’t exist for testing. So we conduct our study “in simulation,” using a scientific
method known as Observing System Simulation Experiments, or OSSEs. In an OSSE, the true
atmospheric state is calculated by a high-fidelity weather model, and experiments using
simulated observations employ a lower resolution model. The experiments fuse simulated
observations with a best guess atmosphere in a repeated cycle every hour. We use a data fusion
technique that relies on an ensemble of forecasts to get the best estimate of uncertainty in our
first guess, or a priori, atmospheric states. To test the impact of the new observations, a
“Control” data assimilation experiment is generated. A Control experiment is a baseline result,
without any of the new observation type – radio occultations in our case. Further experiments
that add varying amounts of the new observation type to the control set of observations provide a
way to gauge the impact as more and more of the new observation type are used.
Significant results, plus more studies planned
In our OSSE, we chose a severe weather case in Oklahoma that produced an EF3 tornado and
flash flooding in the Oklahoma City metro area, late in the afternoon and early evening of May
31, 2013.
Figures 1-3 show snapshots of a key ingredient for severe weather, water vapor in the lower part
of the troposphere, from different treatments in our OSSE. All are valid at the same time, about
12 noon local time, May 31, 2013 (i.e., 4-5 hours before severe weather impacted Oklahoma
City), and at the same level in the atmosphere, about 4,000 feet above ground level. Figure 1
shows water vapor from the Nature Run (high-fidelity; our OSSE’s “truth”). Figures 2 & 3
compare the effects of using conventional weather observations versus conventional weather
observations plus CICERO radio occultation profiles. The data fusion experiment that includes
the CICERO observations (Fig. 3) produces a water vapor analysis that is very much closer to
the Nature Run snapshot (Fig. 1) than the data fusion experiment that uses only conventional
weather observations (Fig. 2). These results show that CICERO data has significant potential to
produce more accurate analyses in severe weather environments, and therefore, better
characterize the environment for severe weather convective initiation. More accurate 3D
analyses of temperature, water vapor and pressure in the lower troposphere is critical to
Severe Weather Study Shows Potential of GNSS-­‐RO -­‐ webpage -­‐ final.docx p.2/4 improved severe weather forecasts. But the proof awaits in the forecasts started from these
analyses.
Our severe weather OSSE study will continue to measure the effects of CICERO data on
subsequent severe weather forecasts. We are using updraft helicity (UH) and accumulated
precipitation to verify the forecasts against the Nature Run. Also, by examining the effects over
a range of constellation sizes, 15,000 - 2,500,000 global profiles/day, we can perhaps find an
inflection point in the impact of these data. That is, is there a point at which the cost of adding
more orbiting satellites does not produce a correspondingly valuable improvement in severe
weather forecasts?
Figure 1. “Nature Run” atmospheric water vapor at about 4,000 feet above the ground. The yellow-tored color scale (bottom of figure) indicates how much water vapor is present, i.e., yellow is dry and red is
moist. This realization of atmosphere moisture during an Oklahoma severe weather outbreak in May
2013 is the yardstick against which our assimilation experiments are compared for realism. It has a
horizontal resolving power of about 1-1/4 mile (2 km).
Severe Weather Study Shows Potential of GNSS-­‐RO -­‐ webpage -­‐ final.docx p.3/4 Figure 2. Atmospheric water vapor analysis using conventional observing system. Valid time, vertical
level and color scale are the same as in Figure 1. Note that the data fusion experiments use a bigger grid
than the Nature Run (Figure 1) with a horizontal resolving power of about 11 miles (18 km).
Figure 3. Atmospheric water vapor analysis using conventional observing system + CICERO radio
occultation observations. The distribution of water vapor in this analysis is much closer to the Nature
Run (Fig. 1) in pattern and magnitude than the Control analysis (Fig. 2). Severe Weather Study Shows Potential of GNSS-­‐RO -­‐ webpage -­‐ final.docx p.4/4