1. INTRODUCTION
The
Forecast System Laboratory (FSL), in cooperation with the National Weather
Service (NWS), has developed the Display 3-Dimensional workstation (D3D). D3D allows users to view real-time
meteorological model data in a three-dimensional interactive display. D3D has evolved from the WFO-Advanced D2D
system, which is currently the operational interactive display software on the
Advanced Weather Interactive Processing System (AWIPS) installed in all NWS
forecast offices (WFOs).
Since
atmospheric processes are inherently three-dimensional, it was natural for FSL
to extend its D2D and WFO-Advanced system to three dimensions. At the core of D3D capability is the Vis5D
software developed at the University of Wisconsin.
The
National Weather Service (NWS) has begun establishing D3D test sites at several
WFOs in the NWS Southern Region, as well as Regional Centers. D3D was developed primarily on HP
workstations, but now has been ported to PC/Linux platforms, a convenient and
low-cost workstation.
The NWS Forecast Office in
Tallahassee has been given the opportunity to evaluate D3D. Since Tallahassee is located along the
northeast Gulf of Mexico coast, and has coastal waters forecast
responsibilities, it is only fitting that we examine D3D in a tropical
framework. This paper will show how D3D
might be used in a tropical environment.
Normally, the computer
models available to the WFO, cannot resolve the mesoscale features of the
tropical cyclone. Little can be
ascertained about the intensity of this type of disturbance. However, synoptic features, which may affect
the storm’s environment can be evaluated.
Through the use of D3D, the 3-dimensional structure of a tropical
cyclone can be viewed by examining the moisture and vorticity fields. A tilted system would be indicative of an
environment, which is unfavorable for development. Kinematic fields (such as
divergence) may also be useful. Viewing
divergence in 3 dimensions may show instances where upper level troughs lead to
development (or demise) of tropical systems.
2. TROPICAL
STORM GABRIELLE
The National Hurricane Center classified an
area of disturbed weather in the southeast Gulf of Mexico as a tropical
depression on Tuesday, 11 September 2001.
Ship observations as well as satellite estimates supported the upgrade
of this area of low pressure and disturbed weather to tropical storm Gabrielle
on Thursday morning, 13 September.
Model guidance showed diverse solutions and uncertainty to where
Gabrielle would go.
Some
strengthening was expected during the next 24 hours, as diffluence was observed
over Gabrielle from an advancing trough to the west and northwest. The system became more sheared Thursday
afternoon and evening, and the center appeared to reform more to the northeast
closer to the southwest Florida coast.
Late Thursday evening, the short wave trough began to move rapidly
southeastward, influencing the already disorganized core of Gabrielle. Early Friday morning, hurricane
reconnaissance aircraft reported an 81-kt wind aloft, and a surface pressure
that had deepened to 980 mb, indicative of a category 1 hurricane. However, surface reports indicated winds of
only 40 to 60 kts as Gabrielle neared the coast (see Fig. 1). Research radar in the Venice area reported
near surface winds of about 32 m s-1 (64 kts), with the eye making
landfall at Venice, FL at 1200 UTC Friday morning, 14 September (Personal
Communication, Mike Biggerstaff).
Tropical Storm Gabrielle moved ashore and quickly lost much of its deep
convection, as midlevel dry air was injected into the storm.
Much
of Florida was drenched with rain for at least 5 days, both before Gabrielle
came ashore, and while it slowly meandered across the Peninsula. Up to 11 inches of rain was reported in
southwest Florida. Central and
northeast Florida also reported large amounts of precipitation.
Figure 1. KTBW radar
reflectivity at 1308 UTC 14 September 2001.
3. D3D
AND GABRIELLE
We
will use D3D to analyze the situation at 0000 UTC 14 September 2001, 12 hours
before tropical storm Gabrielle made landfall.
In D3D, numerical model data can be selected using the D3D Volume
Browser window (Fig. 2). NCEP models,
as well as local models can be used in D3D.
In this case, we will only examine the Eta model.
Three
dimensional rendering can be obtained by selecting isosurface in the 3D
Volume Browser (Fig. 2). An isosurface
is the 3D contour surface of a field at a particular value. It depicts the volume bounded by that value,
allowing one to visually depict the field’s 3D structure at any desired viewing
angle. Isosurface skin values and
presentation colors can easily be modified.
Other
fields that can be visualized are also shown in Fig. 2. Planview permits viewing of a
horizontal section of data. Cross
section allows vertical cross sections of data to be viewed. Volume rendering is a technique for
displaying a 3D field as a semi-transparent colored fog. Surface allows viewing data on the 3D
topography surface, and wind allows viewing of horizontal and vertical
cross sections of wind data, both in the 3D volume and at the surface.
Figure 2. D3D Volume
Browser window showing model source and available fields.
Examples
of isosurfaces and surface fields are presented in Figs. 3 through 6. The 50-kt windspeed isosurface is shown in
Fig. 3 as dark gray. Ridging can be
seen across much of the central U.S., while a trough is located along the
FFCantic seaboard. A small jetstreak is
observed off the southeast U.S. coast carving out a ridge to the south of
hurricane Erin, located to the northeast of Bermuda.
The
most identifiable features in Fig. 3 are the vorticity isosurfaces. Three cylindrical mounds of vorticity are
observed, tropical storm Ivo located off Baja California, tropical storm
Gabrielle in the eastern Gulf of Mexico, and hurricane Erin on the eastern
boundary of the Eta analysis volume.
Looping images (not shown) through the forecast cycle shows that Ivo
quickly dissipates over the next 48 hours, while Gabrielle moves across the
Florida peninsula and on into the FFCantic.
The vorticity isosurface immediately to the north of Gabrielle is
actually northwest of Gabrielle. This
vorticity isosurface will continue on a southeastward track, encircling the
upper vorticity cylinder of Gabrielle.
This feature was probably a
factor in weakening Gabrielle over the next 24 hours.
Figure
4 looks down on the analysis volume from the southeast, to examine isosurfaces
of convergence (dark gray) and divergence (light gray). Surface pressure is hidden by the
convergence isosurface in Gabrielle’s center.
Three-dimensional imaging shown here in this paper does not do D3D
justice. A large elongated divergence
isosurface is located above Gabrielle.
Due to parallax, it appears to be over the upper Midwest, while it is
actually over the tropical storm. Only
by examining top views, side views, etc., can the forecaster really know where
the isosurface is in relation to other meteorological features and geographical
locations. However, there appears to be
more divergence aloft over Gabrielle than there is convergence near the
surface. Therefore, pressure in the
storm should continue to fall, at least according to the Eta.
Figure
5 is a somewhat strange view, looking down into the analysis volume from the
west. The isosurface we examine in Fig.
5 is vertical velocity, 24 hours after the initial time of 0000 UTC 14
September. Notice that Gabrielle is
predicted to be along the east coast of Florida, and the vertical velocity
isosurface just above the pressure center is the vertical motion associated
with Gabrielle. It is interesting to
note that no vertical velocity is shown over Ivo located off Baja California,
which at this time has been downgraded to a tropical depression by the National
Hurricane Center.
The
question also arises as to the extratropical characteristics of tropical storm
Gabrielle, as it approached southwest Florida.
An old stationary front was situated across the state of Florida,
extending northeast into the FFCantic Ocean.
Figure 6 examines the 299º K isentropic surface across the southeast
U.S. and near shore waters. With pressure
and winds superimposed on the isosurface, it is clear that much of the precipitation
across Florida, and northeast of Gabrielle, was forced by upglide motion along
isentropic surfaces. In Fig. 6, the Eta
suggests that the 299º K surface was
lifted about 100 mb from the southern tip of Florida northward to the center of
the state.
4.
SUMMARY
A presentation in a preprint volume,
cannot adequately depict this visually powerful tool. With the use of a mouse, any perspective can be examined and
shifted fluidly and continuously to another perspective. The forecaster can look around corners, zoom
in, zoom out, loop, change colors, change parameter settings, and even change
the opaqueness of a field.
Only a small portion of the 3D Volume
Browser has been examined in this paper.
Cross section and plan views can be easily shifted from one level to
another. Other sampling techniques
include a sounding and hodograph viewing mode.
Selecting the probe mode makes it possible to inspect data values at any
location in the grid. Finally, wind
trajectories can trace air motions in time through the 3D volume.
The NWS Tallahassee has placed the D3D PC/Linux
machine in a prominent place in the operations area. D3D has been found to be most useful during the winter and
transition months. It has not become a
replacement for D2D, but is used as a complement to D2D. We look forward to integrating D3D into
office operations, and hope to see D3D as an option on all of the WFO AWIPS
workstations in the very near future.
Figure 3. D3D
rendering of the Eta 16 x 10-5 s-1 vorticity isosurface
(rust) and 50-kt windspeed
isosurface (grey) at 0000 UTC 14
September 2001. Surface pressure
contours at 1-mb increments are also shown.
Figure 4. D3D
rendering of the Eta –2 x 10-5 s-1 divergence isosurface
(yellow) and 2 x 10-5 s-1 divergence isosurface (red)
at 1200 UTC 14 September 2001. Surface
pressure contours at 1-mb increments are also shown.
Figure 5. D3D
rendering of the Eta 0.05 m s-1 vertical velocity isosurface at 0000
UTC 15 September 2001. Surface pressure
contours at 1-mb increments are also shown.
Figure 6. D3D rendering of the Eta 299º K potential temperature isosurface at 0000 UTC 14 September 2001. Pressure and wind barbs are shown on the 299º K surface. Surface pressure contours at 1-mb increments are also shown.