Two research tasks are described: (1) the improvement and enhancement of an existing mesoscale numerical simulation system, and (2) numerical diagnostic studies associated with an individual case of severe storm development (April 10, 1979 in the Red River Valley of Texas and Oklahoma)

Kaplan, M.

Zack, J.

English

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NASA Contractor Report 159282
SEVERE STORM
FORECAST SYSTEMS
M. Kaplan and J. Zack
(NASA-CR-159282) SEVERE STORMS FORECAST 	 N80- 24944
SYSTEMS Final .Report (Systecas and Applied
Sciences Corp.) 8 p HC A0 2/MF A01 CSCL 04 H
Unclas
G3/47 21628
Contract NAS1-15660
Final Report
May 1980
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Prepared for:
Langley Research Center
National Aeronautics & Space Administration
Hampton, Virginia 23665
Submitted by:
Systems & Applied Sciences Corporation
17 Research Drive
Hampton, Virginia 23666
rDuring the contract	 two basic research tasks were
undertaken: first, the improvement and enhancement of an
existing mesoscale numerical simulation system and second,
numerical diagnostic studies associated with an individual
case of severe storm development.
There were five areas in which the mesoscale numerical
simulation system was improved. Probably the most critical
improvement was made possible with the arrival. in March, 1980
at the Langley Research Center of the CDC CYBER-203 computer
replacing the CDC STAR-100 computer, With this new machine
it is now possible to significantly increase the size of the
matrices used in the numerical simulation model,. With the in-
core storage increasing from 500,000 to 1,000,000 64-bit words
it was possible for us to double the model matrix sizes by
changing the x and y-space dimensions from 90 and 64 to 128
and 90, respectively. This increase in matrix size from 5,760
to 11,520 32-bit words enables us to run the numerical simula-
tion system over an area equivalent to the continental United
States, northern Mexico, and southern Canada when we employ a
38.1 km grid mesh. This will, of course, greatly extend the
useful simulation (forecast) period fr7,ri approximately 12 to
between 18 and 24 hours. When we "nest" down to finer mesh
lengths, i.e., 19.05 and 9.525 km,the implications of this
larger matrix are further amplified by extending each area
of coverage and therefore increasing the simulation (forecast)
periods at each of these scales as well. A bonus associated
with the arrival of the CDC CYBER-203 computer was the system's
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capability for a five-fold increase in "scalar" computing-
speed. While the numerical model is efficiently coded, i.e.,
dominated by vector processing, the increased scalar processing
speed was still critically important in enhancing the speed with
which the model simulation was processed. This was so because
even the relatively minor amoi^nt of scalar computing was ex-
tremely slow on the STAR-100. Thus, even with the doubling
of the two-dimensional matrix sizes, there was only a 20-30
percent increase in total running time of the model due to the
elimination of the significant overhead caused by scalar pro-
cessing.
A second critical area of improvement involves the use of
mandatory and significant level radiosonde information in the
initialization process of the numerical simulation model. Pre-
viously we had been using only the National Weather Service's
Limited Fine Mesh analysis geopotential height, tomperaturel
and dew point temperature information vertically-spared on man-
datory pressure level surfaces to initialize the simulation
model. Since this information is analyzed only on the 1000,
850, 700, 500, 400, 300, 250 and 200 MB surfaces there is a
great deal. of information which is not included from the radio-
sonde reports which could be used to enhance the vertical structure
in the model's initial state. This vertical structure is par-
ticularly crucial ov,, , r the great plains and front range of the
Rocky Mountains where relatively shallow and very well-defined
inversion layers play a key role in establishing the hydro-
dynamical and -thermodynamical environment leading to severe
2
A
storm development. Often as many as 15 additional significant
levels of data are available in a radiosonde report and this
can occur in as many as 70 radiosonde reports on a given day.
We use this information by constructing interpolated level.
of information in the vertical every 25 MB from 1000 to 500 MB
based on the Limited Fine Mesh Analysis. We then combine this
information with the significant level radiosonde information
by applying a standard analysis technique. We then interpolate
to the simulation model's terrain-- following surfaces from
this vertically— denser hybrid analysis yielding a more compre-
hensive vertical structure of the atmosphere. Presently we
are using this process through the 500 MB lever only because
most of the numerical model surfaces are below that pressure
level. In the future this technique will be extended to 150 MB
as the number of vertical levels increases in 'the numerical
model above 500 MB.
With this increased low-level vertical data resolution
,also comes a requirement for improved surface data. Since the
model has excellent vertical resolution in the boundary layer
with levels at 250, 500, 750 and 1000 meters abovo the earth's
surface the addition of surface information could result in a
more complete vertical profile of boundary layer dependent
variables. Presently the source of surface data readily avail-
able from the Water and Power Resources data base are service
A surface station reports of temperature, pressure, wind speed
and direction, as well as dewpoint temperature. Because we do
not yet have implimented a dynamical. initialization technique
that enables us to use both wind and mass information we are
3
restricted to use the surface dewpoint temperature onl y. In
-the future we will be usin; all available surface data. The
surface dewpoint is used at initialization time by inputting
approximately 250 service A reports of dewpoint temperature
from surface observing stations between the Rocky Mountains
and the Mississippi River and between the borders of Canada
and Mexico, Basically this surface dew point temperature ob-
servation is built into the vertical lapse rate structure and
then interpolated horizontally such that it modifies the initial
dew point observation at the surface at each Limited fine Mesh
analysis grid point before data is interpolated to the 38.1 km
matrix.
The fourth improvement to the numerical, simulation system
concerns the development and implimentation of a nested grid
algorithm. This algorithm is designed to give us the capability
to specify a region in which -the hydrodynamical and thermodynamical
mesoscale structure is being optimized for the development of
severe local storms. By combining hydrodynamical and thermo-
dynamical forcing functions such as the time tendency of velocity
convergence, the moist static stability, and relative humidity
we can determine in space and time the optimal region for "building"
a finer-scale simulation mesh. More specifically, what we do is
to "freeze" the dependent variables centered upon the region where
the nesting index reaches a critical preset threshold value and
interpolate the data to one-half the previous grid mesh length.
This new data set then becomes the initial state for a shorter
time period and higher resolution simulation in an effort to better
define the environment where severe storms may best develop. This
4
process of establishing a new data set will continue as long as
the index threshold is exceeded thus selecting the bast candidate
for a simulation with a 19.05 km grid mesh. The index threshold
is tested every hour for a relative maximum value. The same pro-
cess is initiated during the course of the 19.05 km grid mesh
scale simulation for an even shorter period but higher resolution
simulation at 9.025 km. The t oast is done every 30 minutes for
the 9.025 km simulation. Typically, the 19.05 km simulation is
run to six hours of real time and the 9.025 km simulation to 2
hours of real time.
The fifth and final significant improvement to the numerical
simulation system concerns the linking of the simulation output
to a sophisticated and very useful graphics software package.
This package accesses a group of display routines resident on the
Langley computer system which enables us to project several criti-
cal fields for mesoscale and severe storm potential forecasting
onto a relocatable geographical background which is highly de-
tailed. The inherent flexibility in this software enables us
to contour output fields with a variable contour interval and
number as well as enabling us to modify the geographical display
by projecting place names at the appropriate latitude and longi-
tude. Such a package virtually eliminates any ambiguity that
might exist in associating a simulated field with its geographical
position and is extremely practical in facilitating the rapid
assimilation by users of any model product.
While the research and development effort was underway to
improve the numerical simulation system an effort was also under-
taken to study the dynamical adjustments associated with a case
of severe storm development. The case selected wa y April 10,
1979. There were two prominent reasons for selecting this caso.
First, it was an extremely severe outbreak day in which several
large tornado-producing thundarg torms caused extensive loss of
life and property in the Re ,"' Aver V,.Mey region of Texas and
Oklahoma. Second, an unprecedented amount of observational data
existed for comparison with model simulations. This data was
available durint the spring of 1979 as a result of the SESAME
data gathering experiment which was under-Laken over the southern
and central plains states, the Mississirpi, and the Missouri River
Valleys, Probably the most useful information made available to
researchers were three-hourly radiosonde balloon releases which
could give us a dramatically improved picture of the three-dimensional,
atmospheric structure in the vicinity of the severe storm occurrences.
Such information, which is, of course,far superior to the normal
radiosondes taken every twelve hours can tell us much more about
the evolving three-dimensional. atmospheric structure before and
during the tornadic storms and tell us to what extent the numerical
simulation system was capturing this evolution. with this informa-
tion available we could compare a variety of model-simulated and
three-hourly observed dynamical fields such as: velocity conver-
gence, vorticity, moist static stability, equivalent potential
temperature, individual wind components, surface pressure, and
many others. We could also examine more closely individual terms
in such critical dynamical equations as the Navier-Stokes, diver-
gence, pressure-tendency, and vorticity equations in an effort to
sort out the more subtle adjustments which lead to the evolution
of the important observed features on that particular day. As
6
an example, during the period immediately proceeding the onset
of tornadic activity a very pronounced mesoscale surge of southerly
momentum is observed to develop southeast of the tornadic region.
The existence of this feature is indisputable and is well-documented
from the SESAME data sets. By studying these data sets from the
perspective of a complete diagnostic "--nd dynamical breakdown of
dominant terms in the previously mentioned equations and then
comparing them to a similar breakdown in the model simulation
we can gain insights into the imbalances which lead to the develop-
ment of this feature and to what extent the model improvements can
enhance the accuracy with which we simulate it. The southerly
momentum surge appears to have been instrumental in producing
the transport of warm moist air northward into the region of
the Red River Valley. This low-level tongue of warm moist air
aloft led to a drastic reduction in the vertical thermodynamical
stability of the atmosphere in this region immediately prior to
tornadic development. This feature also appears to have been
responsible for amplifying the vertical wind shear and low-level
convergence which allows the realization of the increased potential
buoyancy implied by the reduction in the thermodynamical stability
in this region. As model development continues we may use such
an ideal test case as a control to examine. the implications of each
new addition to the modelling system.
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