Nearcasting Convection using
GOES Sounder Data
ROBERT M. AUNE
AND
RALPH PETERSEN
NOAA/ASPB/STAR
JORDAN GERTH
AND
SCOTT LINDSTROM
SSEC / CIMSS
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Requirement, Science, and Benefit
Requirement/Objective
•
Mission Goal: Weather and water
– Increase lead time and accuracy for weather and water warnings and forecasts
– Improve predictability of the onset, duration, and impact of hazardous and severe
weather and water events
– Increase development, application, and transition of advanced science and
technology to operations and services
Science
• Can observations from a geostationary IR sounder be used
to predict severe weather outbreaks 1 to 6 hours in
advance, filling the gap between radar nowcasts and NWP
models?
Benefits
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•
•
Reduce loss of life, injury and damage to the economy
Better, quicker, and more valuable weather and water information to support
improved decisions
Increased customer satisfaction with weather and water information and services
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Nearcasting uses GOES Sounder Data
• The GOES Sounder includes three separate
water vapor channels
• The water vapor channels have weighting
functions that peak in different parts of the
troposphere (longer wavelengths see farther
down into the atmosphere)
• Therefore have a three-dimensional look at
atmospheric moisture
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Note how sounder yields
data at three levels!
http://cimss.ssec.wisc.edu/goes/wf/faq.html
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Note that the peak in the weighting
function descends as the sounding
dries out – you are looking at the
radiation emitted by water vapor.
As the sounding dries, less water
vapor aloft to emit, so the sensor
‘sees’ farther down into the
atmosphere
(compare this page with
the previous page)
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Imager Water Vapor for weighting function slides
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Imager Water Vapor for weighting function slides
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Imager Water Vapor for weighting function slides
Note Brightness Temperature values at CHS and LBF
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Imager Water Vapor for weighting function slides
Note Brightness Temperature values at CHS and LBF
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Nearcasting uses GOES Sounder Data
• Retrievals transform observed Sounder
radiances to more common meteorological
variables (e.g. temperature, dewpoint) that can
then be used to compute other variables (e.g.
Lifted Index, CAPE)
• Retrievals require clear skies
• Is there a way to ‘move’ the clear pixels now to
future positions that may be cloudy?
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Nearcasting Severe Convection
Using the GOES Sounder
• GOES sounder provides hourly snapshots of layeraveraged stability parameters (for example, Qe).
These observations can be assimilated at multiple
levels into a Lagrangian model to provide fast, shortterm projections of atmospheric stability.
• Lagrangian model uses model winds (u,v) and
geopotential heights to guide motion of observations.
• Model output and sounder retrievals are blended
together to yield t = 0 observations – thus, there is
more horizontal coverage at t = 0 than just from
sounder retrievals alone (cloudy regions and eclipse
regions can be included)
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Premise: Sounder gives information on distinct layers in atmosphere at observation time
Winds from a numerical model can move those slabs of moisture around
Question: Where does Convective Instability develop because of the moving slabs?
Very Dry Layer
Somewhat Moist Layer
Very Moist Layer
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Observations at this time are
limited over the East Coast
by plenty of cloudiness
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00-h fields include information
from previous runs; areal
extent of information on East
Coast is greater.
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Cloud-free observations
inside black curve – other
obs are from earlier runs
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Observations shown again
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How is nearcasting done?
fcst time increasing
Data
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obs time increasing
Data
Start at an initial time.
Use a Lagrangian model.
Step forward 6 hours.
Output hourly forecasts
Use hourly output as input
into later forecasts
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Data
Data
Data
Data include winds
and sounder observations
of qe and qe that has
moved to a point at time=0
and geopotential heights at
t=0, 3 and 6h
Data
etc
etc
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etc
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Benefit
As clouds develop in the daytime heated boundary layer,
you still can track information from earlier observations.
Retrievals aren’t made when clouds appear, but earlier
information is still present in the advected fields
There will be more coverage in the 00-h image than a
sounder dataset for that same time because the 00-h fields
include output from (up to) the previous 6 runs.
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Example:
• Yazoo City, MS tornado from 24 April 2010
• Supercell developed in a region of extensive
cloudiness, making Sounder data sparse
• However, available data and nearcast model
output did suggest a region of strong convective
instability in the region of tornadogenesis
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Tornado on ground in northeast Louisiana
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Tornados on ground in northeast Mississippi
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Sounder data ignored in the presence of
clouds, but information still there in the
holes in the cloud deck and in regions
where data has moved from earlier times
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Minimum in
stability indicated
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Convective Instability indicated
(Tornado location and eventual track shown)
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Forecasts for 1800 UTC show
excellent run-to-run continuity
(See next six slides!)
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Next Case:
• 2009 Case over NW Iowa
• Convection aligned in an arc similar in structure
to that suggested by region of instability in
nearcast output
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How does this area
of instability evolve?
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Nearcasting uses GOES Sounder Data
Conclusions
• Lagrangian moisture transport controlled by
RUC winds/height fields and GOES-East
Sounder moisture.
• Moisture information is from three different
levels in the atmosphere for Sounder, and for
ABI (vs. 1 for Imager)
• Information can be moved into regions not
covered by sounder because of clouds, eclipse,
KOZ, etc.
– Thus, you have information where you need it!
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Hail case from Wisconsin, 2006
• Presented as a difference in Precipitable water
between the two layers
• 6-h forecast shows strong increase in drying
aloft in region where severe weather was
reported
• Very large hail in Madison -- >2” in diameter
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How do individual points move in the
Lagrangian model?
• Note regions of convergence – points are
coming closer together – and regions of
divergence – points are moving farther apart
with time. Lagrangian model is interpolated to a
grid for hourly output
• Only one level is shown. Differing rates of
convergence and divergence at different levels.
• For product to be computed, must have data at
both levels
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How do individual points move in the
Lagrangian model?
• In regions of divergence, number points in
Lagrangian model output should decrease.
Subsequent interpolation from Lagrangian
Model to Grid used for output: Requires at
least 2 Lagrangian points near the Grid to be
considered ‘valid’
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What are error sources?
• Winds in the Lagrangian model are from the
RUC. The level chosen to move moisture
around must be correct
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In this 6-h forecast,
note that the axis of
instability is near TUL
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However, the axis of observed convection
at that time is still back in central Oklahoma
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Forecasts for 2200 UTC
do converge to a solution
showing max instability
where observed.
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