Use of the UW-NMS to Simulate the Flow Around Ross Island on 3 September 2003

1
Use of the UW-NMS to Simulate the Flow Around
Ross Island on 3 September 2003 Amanda S. Adams
and Gregory J. Tripoli Department of Atmospheric
and Oceanic Sciences, University of
Wisconsin-Madison
Observations from Automated Weather Stations near
Ross Island
University of Wisconsin-Madison Nonhydrostatic
Modeling System (UW-NMS)
Figure 4 (left) and Figure 5 (right) Meteograms
from Willie Field and Cape Bird automated weather
stations on 3 September 2003. Characteristics of
the windward (Willie Field) and leeward (Cape
Bird) sides of Ross Island were observed. (Cape
Bird is not completely on the leeward side of
Ross Island, but the lack of AWS on the leeward
side, make Cape Bird the closest representation
of the leeward side.) Ross Island has a local
effect on the surrounding atmosphere which is
evident in the temperatures and pressures
observed. The tall peaks of Mt. Erebus and Mt.
Terror on Ross Island provide a blocking to the
stable flow approaching the island, causing a
localized build up of mass and a locally higher
pressure on the windward side. The pressures at
Willie Field and Cape Bird were initially
comparable, but while the pressure dropped at
both stations as a result of the approaching
cyclone, the localized effect of the topography
caused the pressure at Cape Bird to drop
significantly more than that at Willie Field.
The warmer temperatures observed at Cape Bird can
be explained by the stations proximity to the
leeward side of Ross Island. The warmer
temperatures can be explained by both adiabatic
downsloping on the leeward side, as well as the
damning of the cold air on the windward side.
Cape Bird and Willie field show a significant
temperature difference between the windward and
leeward sides of Ross Island.
The UW-NMS is a nonhydrostatic,
quasi-compressible, enstrophy conserving model
formulated in the non-Boussinesq framework
(Tripoli, 1992). This three dimensional, fully
scalable numerical weather prediction model is
capable of multiple two-way interactive grid
nesting. One of the truly unique features of the
UW-NMS that makes it suitable for modeling the
atmosphere over Antarctica is the use of a
variable step topography system. Unlike other
topography systems such as terrain following or
step topography, the UW-NMS system is based on a
surface coordinate step of variable depth, chosen
to exactly match surface elevation (Figure 1).
This allows the UW-NMS to represent slopes as
steep as 90 degrees while also being capable of
representing even the most subtle topography.
Terrain following systems, while well suited for
subtle topography changes, are limited to one
vertical increment per horizontal grid increment,
limiting the horizontal resolution possible in
areas of steep topography. While step topography
systems capture steep topography well, the
requirement of stepping in one discrete vertical
increment makes the subtle topography nearly
impossible to represent. Finite differencing
advection cast in vorticity/kinetic energy form
and directly specifying vorticity and kinetic
energy at topographical boundaries (Figure 2)
ensures the UW-NMS of a vectorally consistent
numerical treatment of flow interacting with
topographical or structural barriers. As a
result, competent simulations of flow around
topographical obstacles are possible even in the
severe Antarctic flow regimes, where both steep
and subtle topography must be handled competently
due to the presence of the Transantarctic
Mountains low Froude number flow. A
topographical dataset with 1km resolution is used
with the UW-NMS.
Results from the UW-NMS

• The UW-NMS was initialized from the 0000UTC 3
  September 2003 run of the GFS. Initially a
  series of three nested grids was used, then 12
  hours into the simulation two more grids were
  added. The inner most grids were positioned
  specifically to capture the flow around Ross
  Island (Figure 6). The variable step topography
  system used in the UW-NMS does not restrict the
  horizontal resolution in areas of steep
  topography, and thus a high horizontal resolution
  was able to be applied with the innermost grid
  having a horizontal grid spacing of 500m. While
  the splitting of low level flow around Ross
  Island was captured by the course outer grids
  (Figure 7), the inner grids showcase the truly
  small scale influence that the topography of Ross
  Island has on the wind, temperature, and pressure
  patterns. Some of the unique meteorological
  features created by Ross Island are presented
  below
• As low level flow approached Ross Island it was
  unable to go over Ross Island, and thus slowed
  down, and created a localized area of high
  pressure on the windward side (Figure 7).
• The near surface flow behaved differently on the
  eastern and western sides of Ross Island. The
  flow directed around the eastern side of Ross
  Island did not experience any additional
  topographic barriers. The flow that was directed
  around the western side of Ross Island interacted
  with Hut Point Peninsula and Cape Bird, causing
  the flow to be slower than around the eastern
  side and experience a vertical component (Figure
  8). Notice on Figure 8 that the pressure change
  along the trajectories on the eastern side of
  Ross Island are due to the pressure gradient
  across the island, while the trajectories that
  flow around the western side of the island change
  pressure due to changes in elevation.
• Ross Island is a tall enough barrier that it
  causes cold air damning on the windward side
  (Figure 11 and 12). The cold air builds in depth
  until it is deep enough to spill over between the
  peaks of Mt. Erebus and Mt. Terror. Underneath
  the damned-up cold air, a rotor like circulation
  develops as the air tries to go over Ross Island.
  
• The downsloping on the leeward side of Ross
  Island is analogous to a hydraulic jump, and
  transports high momentum air from aloft down to
  the surface (Figures 11, 12, and 13). The
  downward transport of momentum is what allowed
  for fast low level flow to come out of the
  leeward side of Ross Island (Figure 9), an area
  of localized low pressure.
• The downsloping on the leeward side produces
  localized areas of warming due to the adiabatic
  descent (Figure 10). Of note is the donut shaped
  warming located at the center of the leeward side
  of Ross Island. Vertical streamlines across Ross
  Island (Figure 11) show that the downsloping air
  does not immediately descend all the way to the
  surface, possibly due to an isolated pocket of
  cold air on the leeward side that is cut off from
  the flow due to the geometry of the leeward side.
  
• Rotor like circulations develop on the leeward
  side of Ross Island due to Cape Bird. These
  circulations are depicted in the streamlines and
  vertical streamlines of Figure 9.
• Small scale horizontal vortices are also created
  due to Ross Island and are visible in the surface
  streamlines (Figure 12) on the eastern leeward
  side of Ross Island and in Wohlschlag Bay between
  Mt. Erebus and Mt. Bird on the western side of
  Ross Island.

Figure 8 (above) Grid 5 trajectories show the
asymmetry to the flow as it splits around Ross
Island (contours of pressure in 1mb increment).
Figure 1 (above) Depiction of various systems
used to handle topography in numerical weather
prediction models where the gray shading
represents hypothetical topography that has both
steep and subtle elevation changes. Figure 2
(left) Zoomed in view of the UW-NMS variable
step topography, including relative location of
variables on the grid.
Figure 6 (above) A series of five nested grids
were used to model the flow around Ross Island on
3 September 2003.
Figure 9(above) Grid 5 trajectories and
streamlines are placed to illustrate the small
scale circulations induced by Ross Island.
3 September 2003
At 0000UTC on 3 September 2003 a strong surface
cyclone was located just off of the Ross Ice
Shelf, with a central pressure of 962mb. By
1000UTC the cyclone had moved over the Ross Ice
Shelf and intensified to a central pressure of
957mb. The winds created by the strong pressure
gradient of the cyclone, when combined with the
barrier wind created by the Transantarctic
Mountains resulted in extremely windy and
hazardous conditions near McMurdo. Observations
at Ferrell AWS reported winds at 46.6 ms-1.
Figure 7 (above) Grid 3 plot of surface pressure
in 1mb increments demonstrates the local effect
of Ross Island on the pressure field. Near
surface trajectories (colored by speed), show the
flow splitting around Ross Island.
Figure 10 (above) Grid 4 surface temperature
(oC) and streamlines show localized areas of
warming on the leeward side of Black, White, and
Ross Islands.
Figure 11 (above) Grid 4 cross section (location
of cross section shown in Figure 7) of
streamlines and potential temperature illustrates
the cold air damning on the windward side of Ross
Island and downsloping between Mt. Erebus and Mt.
Terror.
Figure 12 (above) Grid 5 view from the north
side of Ross Island showing the sfc streamlines
and the 260K potential temperature surface shaded
by speed (m/s).
Figure 13(above) Grid 5 view of sfc streamlines.
Trajectories (colored by pressure) illustrate
that fast surface winds on the leeward side are
from downsloping.
Figure 3 (left) Infrared satellite image valid
at 1800 UTC on 3 September 2003, shows the
cyclone located over the Ross Ice Shelf.
Reference Cited
Acknowledgements
Tripoli, Gregory J. 1992 A Nonhydrostatic
Mesoscale Model Designed to Simulate Scale
Interaction. Monthly Weather Review Vol. 120,
No. 7, pp. 13421359.
Thank you to AWS and AMRC for providing the
surface observations and satellite data.