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# 

# Storm Model

# Weather Product Description

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The Storm Model Weather Product is derived from the NEXRAD level three
data. The following sections describe how the product is derived, what
it represents in terms of weather, the model parameters and how they are
used to generate the model.

## The Model Product

The Storm Model Weather Product is a set of two-dimensional polygons,
where each polygon has an assigned altitude, intensity value and object
Id. A polygon represents a 2D slice, at a particular altitude*,* through
a 3D region where the precipitation is of a particular intensity. Each
3D region is assigned an object Id; all the polygons representing the
region will have the same intensity value and object Id (see Figure 1).

![](media/image3.jpeg){width="5.5in" height="2.6326388888888888in"}

*Figure 1 -- Polygon Slices of a 3D Region.*

The polygons in aggregate represent samples of a *storm cell*. The
intensity value corresponds to the strength of the radar return from the
precipitation in the air in the region. For the Storm Model product, the
storm cell regions are not defined by sampling because the radar
coverage is too sparse; rather the cell locations and structure (derived
by algorithmic processing of the radar data) is extracted from the
NEXRAD product files and used to model the storm cells. The product's
polygons are generated from the modeled cells.

## Model Inputs

Each storm cell is a simple model based on an idealized
conceptualization of a typical supercell thunderstorm. Figure 2
illustrates a thunderstorm with considerable overhang, surrounded by the
signature anvil cloud. In the figure, the numbers assigned to the banded
regions are typical radar reflectivity values. This is the kind of cell
that would be detected by the NEXRAD radar and be a data point in the
NEXRAD product files. The NEXRAD product files are inputs into the cell
modeling process as shown in Figure 3.

![](media/image4.png){width="4.0in" height="1.8166666666666667in"}

*Figure 2 -- Vertical Cross Section of Typical Supercell (source:
\[1\])*

The NEXRAD Storm Structure File (Product 62) identifies each storm cell
detected by the radar at a particular site. It contains the location of
the cell center (in Az/Range from the radar), the height of the cell
base, the height of the cell top, and the highest point of maximum
reflectivity. It also contains the values for vertically integrated
liquid (VIL) and the maximum reflectivity (in dBz)--only the later is
currently used in the modeling.

![](media/image5.jpeg){width="4.501388888888889in" height="1.7875in"}

*Figure 3 -- Modeling Inputs*

The NEXRAD Storm Track Information File (Product 58) contains the speed
and direction of cell movement, and is correlated with the Storm
Structure file by cell Id. The Storm Track file also contains predicted
cell positions for one hour in fifteen minute increments, but these are
currently not used.

*Table 1 -- Cell Information Sources*

Table 1 summarizes the information utilized from the NEXRAD product
files. This information is used to construct simplified models of the
storm cells, which are then used to generate the Storm Model Product
polygons. Figure 5 illustrates the simplified model used to generate the
product polygons. Only the precipitation regions are modeled; the clouds
associated with the storm cells are not necessarily detected by radar
and not modeled at this time.

Presently, no attempt is made to model the lifecycle stage of the cells.
Figure 4 illustrates the typical stages for a convective cell. We are
modeling cells after they have been detected by NEXRAD and flagged as
storms--probably from the late Cumulus states through the Mature and
possibly into the early dissipating stage.

![](media/image6.jpeg){width="4.771527777777778in"
height="2.001388888888889in"}

*Figure 4 -- Storm Lifecycle Stages (source: \[3\])*

For the purposes of the Storm Model Product, each cell consists of up to
three concentric "shells", where each shell encapsulates a region of
storm intensity. NEXRAD reflectivity values as measured may range from 0
to 75dBz. Most end-user applications map the dBz value to an integer
value for intensity, usually from 0 to 15. For this model, the intensity
values are further "bucketized" into severe, moderate and light. Table 2
lists the intensity buckets and the values to which they are mapped.

![](media/image7.jpeg){width="4.15625in" height="2.4159722222222224in"}

*Figure 5 --Vertical Cross Section of Modeled Cells*

*Table 2- Intensity Mappings*

  ------------------- ------------------------- -------------------------
  **Level**           **Intensity Range**       **Mapped Value**

  Severe              10 - 15                   11

  Moderate            7 - 9                     8

  Light               4 - 6                     5
  ------------------- ------------------------- -------------------------

The starting point for the modeling process is the value of the maximum
reflectivity in the storm cell. The innermost shell is given the
intensity value to which this reflectivity is mapped. The surrounding
shells are each assigned an intensity value that is a percentage of the
inner shell's value. Any shells that have values that fall below the
minimum threshold (4) are dropped. This means that not all cells may
have three shells.

The maximum diameter of the each shell (cell width) is determined by
computing an offset from the cell location point. This offset is a
heuristic derived by taking a fixed percentage of the distance from the
maximum reflectivity height to the cell top height--the assumption being
that the width of a storm bears some relationships to its height. For
the outermost shell the percentage is 100% so that the top of the shell
corresponds with the top of the storm cell. The offset for each of the
other shells is a proportionally lesser percentage. The bottom for each
shell is similarly computed from the difference between the cell base
height and the maximum reflectivity height. Table 3 gives the offsets
for each of the three shells computed for the Storm Model Product.

Table 3 -- Cell Offsets from Max Refl Height

  ------------------ --------------------------------- ------------------
  **Cell**           **Intensity**                     **Offset**

  Inner              Severe (11)                       25%

  Middle             Moderate (8)                      50%

  Outer              Light (5)                         100%
  ------------------ --------------------------------- ------------------

The shape of a shell is initially modeled by computing a sphere with the
shell top height as its diameter. The equator of the sphere is set to
ground level. At each of the predefined flight levels (2 Kft to 60 Kft
in 2 Kft increments), the circle that intersects the sphere defines the
polygon boundary.

Most storm cells that last longer than 10 minutes will develop a tilt.
Otherwise, the downdraft from the precipitation will interfere with the
uplift that is feeding the storm and cause the cell to dissipate (see
Figure 6). To model this tilt, a tilt-magnitude parameter is used to
progressively offset the center of the polygons computed in the upper
half of the range between the storm cell bottom and storm cell top. The
current tilt setting is .02 degrees and is used for both latitude and
longitude.

![](media/image8.jpeg){width="2.0in" height="3.3291338582677166in"}

*Figure 6 -- Supercell Airflow (source: \[2\])*

The downdraft from a storm cell can disturb the air, triggering the
development of a new cell, whether or not the first cell dissipates.
Figure 7 illustrates a cell with a defined tilt and an adjacent cell
developing at its edge.

As storm cells dissipate and develop, the storm system moves in the
direction of cell formation. The direction of movement may not be
correlated with the wind direction. For this model, the storm track
heading for a storm cell is assumed to be indicative of the leading edge
of the cell. The offsets for shell tilt are calculated so that the tilt
is in the direction of the storm track. No use is made of storm speed at
this time. Figure 8 illustrates how the storm heading and the tilt
magnitude are used to define the tilt for the top half of the cell
models.

![](media/image9.jpeg){width="4.5409448818897635in"
height="2.5700787401574803in"}

*Figure 7 -- Two Storm Cells (source: \[2\])*

![](media/image10.jpeg){width="5.104166666666667in"
height="2.3958333333333335in"}

*Figure 8 -- Cell Tilt Computation*

## Cell Merging

Shells of the modeled cells may intersect shells of adjacent cells. When
this happens, the intersecting shells are merged. The shells may in fact
intersect at only one or two altitude levels, however all the polygons
from all altitude levels are assigned the Object Id of one (arbitrarily
chosen) of the shells and are thereafter treated as a single object.

Generally, due to the implicit separation of the storm centers, only the
outermost shells will intersect with adjacent cells. This is consistent
with observed NEXRAD data, where large, irregular regions of low
intensity surround kernels of high intensity reflections.

## Conclusion

Basically the model is one of idealized thunder storm cells based on
storm structure and track data from two NEXRAD level III products. The
model uses some heuristics, describe herein, to arrive at a plausible
model of the weather that could produce the NEXRAD products. As the RUC
data comes on-line it may prove useful in improving the future fidelity
of the model.

## References

1.  NWS Louisville, *Structure and Dynamics of Supercell Thunderstorms*,
    <http://www.crh.noaa.gov/lmk/soo/docu/supercell.htm>

2.  Pani, Eric A., *Thunderstorm Structure and Evolution,* PowerPoint
    Presentation, University of Louisiana at Monroe

3.  Wolfson, Marilyn, *Terminal Transitional En Route Convective Weather
    Forecast*, MIT Lincoln Laboratory Presentation, 26-May-2000