Title: A Severe Weather Proxy Developed from the NOMADS Real Time Data Base of an Operational Model
1A Severe Weather Proxy Developed from the NOMADS
Real Time Data Base of an Operational Model
EMC
NCEP/Environmental Modeling Center
Jordan C. Alpert jalpert_at_ncep.noaa.gov Jun
Wang/SAIC jun.wang_at_noaa.gov
21st Conf on IIPS, 18.4, 85 Annual AMS Meeting,
9-13 January 2005, San Diego, California
where the nations climate and weather services
begin
2Motivation for a Severe Weather Proxy from an
Operational NCEP Numerical Weather Prediction
(NWP) Model
There are difficulties with NWP models predicting
severe weather such as
- Highest resolution Operational NWP model is not
able to resolve severe weather causing phenomena
that forecasters need to make accurate
predictions. This is because of the - Lack of horizontal and vertical scale resolution
for observations and models with sufficient
detail and physical knowledge. - Severe weather
- Tornado, damaging winds gt50kts or hail gt¾ (SPC
web page) - Resulting from severe or organized convection,
- Thunderstorms, super cells, squall lines,
multi-cell complexes - An array of indicators is used by forecasters to
analyze severe weather potential at or near
observation time and from forecasts - Lifted Index, Storm Relative Helicity, CAPE,
Vertical profiles of weather elements such as
temperature, dew point and wind.
3A challenge in reducing the severe weather risk
To improve the utilization of model output
signatures of severe weather.
Models represent (simulate) very well dynamic and
physical processes
- Wind convergence patterns,
- Often along frontal boundaries,
- Genesis of complex systems along fronts,
- Vorticity advection and vertical motions,
- Often occurring at scales of motion that are
barely resolvable by models and may have
difficulty reproducing the convection details. - For example
- Qualitatively the vertical extent of convergence
patterns associated with severe weather appears
to grow and be advected vertically. - We can not observe this phenomena directly.
- We suggest that the volume, shape or vertical
extent of the 3-D convergence patterns from
forecast models is a signature for sub-grid scale
convective outbreaks. - This was noticed by viewing Vis5D 3-D displays
of model convergence patterns.
4Interactive Visualization of Animated 3-D Model
Data
- In a previous report (19th Conf on IIPS 15.2,
AMS) it was shown that convergence pattern
signatures and other 3-Dimensional displays were
unique and useful, compared to 2-Dimensional
ones. - Display Three-Dimensions (D3D), Szoke, et. al.,
2001 software available for Advanced Weather
Interactive Processing System (AWIPS) under test
for forecasters at some Weather Forecast Offices.
- Using Vis5D, visualization 5-dimensional,
meaning 3 space, time and variable space, is an
interactive visualization system created at the
University of Wisconsin (Hibbard, 1994). - In use by the science, educational and by the
atmospheric science and NWP model research
community as well as other display systems. - Can View renderings of Iso-surfaces of
convergence patterns. - The basic visualization capabilities of these
systems are similar as D3D was created from
Vis5D.
53-dimensional rendering of the 21-hr NCEP
Operational forecast of convergence gt3X10-5 s-1 a
yellow iso-surface from IC of 00Z 17SEP2004
6A Model Severe Weather Proxy
- Convergence calculated from the motion fields of
the NCEP Global Forecast System - Test that the model associates these convective
areas with severe weather according to vertical
extent, shape and volume. - Can the proxy distinguish between events with no
severe weather? - Need to define a precise criteria
- Define a convergence amplitude criteria,
- Define a vertical extent as number of vertical
levels between the surface and 500 mb where
convergence amplitude criteria is met. - Qualitatively the convergence patterns grow in
vertical extent in proportion to increased
precipitation, winds, decreased surface pressure.
The NWP model may advect the convergence
patterns upward in response to unresolvable
meso-scale phenomena.
7Verification
A convenient verification for objectively
locating observed severe weather is the NCEP
Severe Storms Prediction Center (SPC) Severe
Weather Watch or Warning Box.
- The Watch means that severe weather has potential
in an area usually over a few hours, - The Watch is a manually prepared prediction from
conventional and non-conventional observations
such as radar and RAOB soundings. - The Warning means that severe weather is known to
have occurred in an area. - A Watch or Warning consists of a text file
indicating the location by latitude and
longitude pairs outlining a polygon area of
severe weather potential typically 25,000 square
miles. - The often used model forecasts of Best Lifted
Index, where negative values less than 4 are
considered an indicator of potential severe
thunderstorms, will also be verified with watch
box locations as a comparison. - Watch and Warning box areas will be used as the
verification area. Forecasted patterns meeting a
convergence criteria will be considered a success
(Hit) if it is forecast within the Watch or
Warning box.
8Convergence pattern (X10-5 s-1) at 1000mb for
17SEP04 21-h forecast and the SPC Watch box (in
black, middle Atlantic location) valid at the
same time and over the next 3 hours. The
verification area of interest is shown in red.
9Accessing the Data Using NOMADS servers
The NOMADS real time server was used to obtain
the wind fields needed to calculate the
convergence as well as the lifted index.
- Using the NOMADS OPEN-DAP/DODS server to download
the GFS model run history, including unpacking
the model GRIB files and organizing
projection/location information. - This is done using constrained queries to the
server of the form http//nomad2.ncep.noaa.gov90
90/dods/gfs/archive/gfs20040917/gfs/_00z.ascii?vgr
d10m77110140235300 - Replacing vgrd10m with vgrd would include the
vertical wind profile. - 77 indicates the seventh forecast time (3-hr
intervals) or 21z. - 110140235300 are the grid points used in
the eastern US domain chosen to encompass
inclusively an area where severe weather often
occurs. The region under consideration starting
with 110 is 100 degrees north of the south pole
and 235 represents 235 degrees east of 0
meridian for a 30 by 65, over 1200 grid point
area.
10Results of the Automated Convergence Verification
Program
The verification program considers the distance
from forecasted locations to the nearest point
within the watch box
- Many large distance errors in the location of the
severe weather box indicate a high false alarm
rate. - When the convergence criteria for severe weather
is met, but the verifying distance exceeds 1000
km, then the result does not apply (N/A) as there
may be synoptic weather from another system. This
is also done for the lifted index indicator. - Six criteria C1-C6 are chosen for the convergence
calculation for location and vertical extent. - When the convergence criteria is met the data is
distributed into 6 accuracy categories Hits,
lt100 km, 100-200 km, 200-300 km gt300 km and N/A,
as described above.
11The convergence is calculated at vertical levels
from the surface (10m wind), 1000, 925, 850, 700,
500 mb. Thus, if the convergence criteria is 6
vertical levels (e.g., C1), then all the vertical
levels must exceed the convergence amplitude
criteria.
- Criteria Convergence Number of Vertical
- Amplitude (10-5s-1) levels required to meet
- the criteria for severe weather
- C1 gt 3.0 6 (lt400 mb)
- C2 gt 3.0 5 (lt500 mb)
- C3 gt 5.0 6 (lt400 mb)
- C4 gt 5.0 5 (lt500 mb)
- C5 gt10.0 6 (lt400 mb)
- C6 gt10.0 5 (lt500 mb)
- For lifted index tests
- C7 lt -3
- C8 lt -4
- C9 lt -5
12Excerpt showing convergence criteria and distance
form the watch box in km. 0 km means the
convergence criteria points were within the watch
box (Hits).
13Total number of hits or distance from the watch
box for convergence exceeding the criteria for
all watch boxes. Same for Lifted index for C7
and C8 criteria. A six week period containing 47
watch boxes was used to gather the SPC Watch (no
Warnings were issued in this period) box
locations.
Criteria (km) C1 C2 C3 C4 C5 C6 C7 C8 Hits
0 27 0 25 0 13 17 7 lt100 4 0 3 0 2 0
3 3 100-200 5 0 3 0 1 0
9 13 200-300 14 0 11 0 7 0 6
7 gt300 13 12 11 6 13 6 6 12 N/A 11
8 19 16 24 28 6 5
14Summary of Results
- The criteria C2, C4 were superior with the least
distance errors compared to lifted index. - Requiring the vertical extent of the convergence
criteria to extend past 500 mb C1, C3, C5, does
not improve the accuracy. - We found the Best Lifted Index to have a larger
number of false alarms and distance errors larger
than 200 km. - A six week period containing 47 watch boxes was
used to gather the SPC Watch (no Warnings were
issued in this period) box locations. - Model initial conditions were set as the nearest
such that 24-h forecasts to the valid times
covered by the Watch box could be made. - NOMADS OPEN-DAP/DODS queries can be used to make
client (user) calculations.
15Conclusion
- Model predicted convergence patterns have
signatures that indicate the potential of severe
weather using the SPC Watch and Warning Box
product as verification. - The volume or shape as measured by vertical
extent and amplitude of the convergence pattern
was used to test the accuracy of predicting the
potential for severe weather. - Convergence amplitude and vertical extent appear
to be competitive with other stability measures
like Best Lifted Index. - Forecasters viewing the 3-Dimensional convergence
structures of model forecasted convergence
patterns could potentially improve their severe
weather predictions. - NOMADS OPEN-DAP/DODS servers can enhance user
access to NOAA model database. - Quantitative verification statistics will be
improved to include false alarm rates and threat
scores when more samples are used.
16Real Time Vis5D files from NCEP Model Forecasts
- http//www.emc.ncep.noaa.gov/gmb/wd23ja/vis5d.html
- http//www.emc.ncep.noaa.gov/gmb/wd23ja/vis5d/avn3
h - for the GFS over the CONUS
- Western and Northern hemisphere add /avn
- South America add /avnsa
- Africa add /gfsaf
- Improved vertical resolution over the CONUS add
gfsv - Eta 212 grid http//www.emc.ncep.noaa.gov/gmb/w
d23ja/vis5d/eta212 - Global Ensembles stacked in the vertical
dimension - http//www.emc.ncep.noaa.gov/gmb/wd23ja/vis5d/ens
- RSM over Hawaii at 10 km http//www.emc.ncep.noaa
.gov/gmb/wd23ja/vis5d/rsmh10