KEE

1
Simulations and Observations of Extreme Low-Level
Updrafts in Hurricane Isabel
Daniel P. Stern1, David S. Nolan1, and Sim D.
Aberson2 1Rosenstiel School of Marine and
Atmospheric Science, University of
Miami 2Hurricane Research Division, AOML/NOAA

• Introduction
• Extreme low-level updrafts have previously been
  found to be a ubiquitous feature of intense
  tropical cyclones (Stern and Aberson 2006).
  Herein, we demonstrate that high resolution
  simulations of Hurricane Isabel produce similar
  features, and we investigate their cause.
• There are numerous unanswered questions
  regarding low-level updrafts within tropical
  cyclones. These include their origin, spatial
  distribution, and relationship to larger scale
  structure and intensity. Some studies have
  concluded that eyewall updrafts are generally
  forced by local buoyancy (Braun 2002), while
  others (Zhang et al. 2000) have found the forcing
  to be due to dynamic pressure gradient forces.
  Numerous studies have found the environmental
  vertical wind shear to be critical to the
  organization of updrafts within tropical cyclones
  (e.g. Corbosiero and Molinari 2003), with
  updrafts found preferentially downshear-left or
  left of shear, depending on the study. Other
  studies have also found motion-induced frictional
  asymmetry to be important, with updrafts located
  preferentially in the right-front quadrant
  relative to storm motion. Most of these studies
  were not looking at low-level updrafts in
  particular, however. It is possible that these
  features are dynamically distinct from mid/upper
  level updrafts, in which case their cause and the
  mechanisms which control their distribution may
  be different.

• Summary
• We have shown that high-resolution simulations
  of Hurricane Isabel are able to produce very
  strong low-level updrafts, which compare
  favorably to observations, in magnitude and
  spatial distribution. By tracking an individual
  extreme updraft at high temporal resolution, we
  have begun to elucidate some aspects of the
  complex dynamics involved in producing these
  features. In particular, we have demonstrated
  that the forcing for the tracked updraft cannot
  be local buoyancy. This very likely holds true
  for other similar simulated features, which
  requires that dynamic nonhydrostatic pressure
  gradient forces must be the forcing. While this
  is more difficult to demonstrate in the
  observations, we should note that the same rough
  estimate of the thermal perturbations required to
  generate such updrafts applies equally well. 15
  m/s updrafts have been observed at and below 500
  m height, and a 13.1 m/s updraft was observed at
  140 m in Isabel on the 12th. Such an updraft
  would require a 19 K temperature perturbation if
  produced by buoyancy! Therefore, the observed
  upsondes are also very unlikely to be driven by
  buoyancy.
• It appears that some of the strongest simulated
  horizontal winds are found slightly upstream,
  radially outward, and below the extreme updrafts
  (not shown). This is consistent with the
  production of vertical vorticity by the updraft
  in a region of very strong radial shear of the
  mean tangential winds. The extreme low-level
  updrafts are potentially an important mechanism
  by which the strongest horizontal windspeeds in
  tropical cyclones are produced. This is also
  consistent with the strong overlap between the
  observed upsondes and the set of all sondes in
  which 90 m/s horizontal winds are found. This
  may have implications for the mechanism by which
  extreme wind damage is produced in landfalling
  major hurricanes.

• Evolution of a Single Extreme Updraft

• Sensitivity to horizontal resolution and to
  boundary layer parameterization scheme

• To examine sensitivity of updraft strength to
  resolution we compare the simulation with 444 m
  resolution to one with 1.33 km resolution. To
  ensure that differences are dynamically
  meaningful, the simulations are compared on the
  same 1.33 km grid. Additional simulations were
  performed at 444 m resolution, but with the drag
  coefficient at 80 of its original value, and
  with the depth of the boundary layer reduced to
  80 of its original value.
• Shown to the right are plots of the vertical
  velocity of the strongest updrafts versus height.
  The resolution of the simulation is given in
  the legend as 1.33km or 444m. d03 refers
  to output on the 1.33 km grid while d04 refers
  to output on the 444 m grid. The upper plot is
  of the maximum vertical velocity anywhere in the
  domain, at each height, and so is not indicative
  of the structure of actual individual updrafts.
  The lower plot is of the vertical velocity at
  each height following the track of the strongest
  updraft in the domain at a height of 1.5 km. In
  both plots, data are averaged over the hourly
  output at each height.
• The updrafts are substantially stronger when
  simulated at higher resolution.
• The height at which the maximum vertical
  velocities are found is lower for the simulation
  at higher resolution, with a sharper peak. When
  following individual features, the heights of the
  maximum low-level updrafts are the same.
• For Cd.8, the maximum vertical velocities are
  weakened by several m/s, while their height
  remains unchanged.
• For PBL.8, the maximum vertical velocities are
  strengthened by several m/s, and the height of
  the maximum lowers by 500-750 m.
• The height of the individual extreme updrafts is
  only 250 m lower in PBL.8. This difference is
  partly because there are some extreme 500-1000 m
  updrafts in PBL.8 which are not the strongest
  feature at 1.5 km, but that are the single
  strongest feature at any height.

• A single updraft was tracked from 20 second
  output, from 180000-181340 UTC. The maximum
  intensity was 27.5 m/s at 1500 m height at
  180620 UTC. The vertical velocity is plotted
  below as a function of height, tracking the
  maximum in space and time.

Storm Relative Trajectory of Updraft

• Model
• We use WRF version 2.2 to simulate Hurricane
  Isabel (2003) from 00Z 12th until 00Z 14th. The
  initial and lateral boundary conditions are
  provided by the GFDL 6-hourly analyses. There
  are 40 vertical levels, equally spaced in
  pressure. For the control simulation we use 4
  nested grids with horizontal resolutions of 12,
  4, 1.33, and .444 km. The YSU PBL scheme is
  used, with a modified drag coefficient based on
  the results of Donelan et al. (2004). The WSM
  5-class scheme was utilized for microphysics
  (Hong et al. 2004), while for radiation the RRTM
  longwave (Mlawer et al. 1997) and Goddard
  shortwave (Chou et al. 1998) schemes were used.
  Output was saved at hourly intervals, except
  18-19Z 12th, when 20 second output was saved.

Vertical Vorticity (colored) and 10 m/s Vertical
Velocity (contoured)

• Intensity and Location of Simulated and
  Observed Updrafts
• In Stern and Aberson, observed updrafts were
  identified as extreme when vertical velocity
  exceeded the terminal fall speed of the dropsonde
  (roughly 12-14 m/s). In our simulations, there
  are updrafts which exceed this criterion at
  almost all times, at all levels between 500 m and
  5 km (which is the highest level we examine).
  The strongest updrafts are generally found
  between 1-2 km height. The maximum simulated
  updraft on the 12th is 28.0 m/s. By comparison
  the maximum observed updraft from a dropsonde in
  any storm is 25 m/s (Ivan). The strongest
  simulated downdraft is -18.7 m/s, compared to
  -17.7 m/s from a dropsonde in Hurricane Mitch.
• In Isabel on the 12th , extreme updrafts were
  observed by 6 dropsondes at 3 different times.
  The sondes first encountered the updrafts at
  various heights between 140 m and 1500 m. The
  maximum vertical velocities were between 13.1 and
  17.3 m/s. The upsondes were located 21-28 km
  from the center.
• For comparison, the storm relative locations of
  all simulated updrafts exceeding 15 m/s at a
  height of 1 km from 15-23Z are plotted (blue)
  along with the upsondes (red). During this time,
  the observed vertical shear was from the north at
  12 kt, while the storm motion was towards the
  west at 4 kt. The simulated and observed
  updrafts are located in regions favored by shear,
  but not motion. The simulated extreme updrafts
  are located 10-15 km outward from the observed,
  which is consistent with the simulated storm
  being too large by the same amount. Roughly 85
  of the simulated 15 m/s updrafts below 2 km
  height occur in the left of shear semicircle (not
  shown).

• Acknowledgments
• D. Stern has been supported through a University
  of Miami Graduate Fellowship, and D. Nolan was
  supported by NSF grant ATM-0432551.

Vertical Velocity at 1, 3, and 5 km

• The maximum vertical velocity increases from 11.9
  to 27.5 m/s in just over 6 minutes, with a 9.5
  m/s increase in just 3 minutes.
• The updraft decays to 7.3 m/s at z1.5 km in the
  following 7 minutes, with a 12 m/s decrease in
  the final 3 minutes.
• The discontinuity evident at upper levels
  beginning at 1808 is indicative of the lack of a
  coherent updraft above these heights (it cannot
  be tracked properly). The updraft appears to
  weaken and subsequently disappear from above.
• As the updraft decays, it moves inward at low
  levels by about 3 km over a 5 minute period.
• The updraft tilts outward with and slightly
  cyclonically with height. At 30 km radius, the
  azimuthal tilt is 250 m/km, while the radial
  tilt is 2-3 times as large.

• References
• Aberson, S. D. and D. P. Stern, 2006 Extreme
  horizontal winds measured by dropwindsondes in
  hurricanes. Preprints, 27th AMS Conference on
  Hurricanes and Tropical Meteorology, Monterey,
  CA, April 2006.
• Braun, S. A., 2002 A cloud-resolving simulation
  of Hurricane Bob (1991) Storm structure and
  eyewall buoyancy. Mon. Wea. Rev., 130, 1573-1592.
• Chou, M.-D., M. J. Suarez, C.-H. Ho, M. M.-H.
  Yan, and K.-T. Lee, 1998 Parameterizations for
  cloud overlapping and shortwave single-scattering
  properties for use in general circulation and
  cloud ensemble models. J. Climate, 11, 202-214.
• Corbosiero, K. L. and J. Molinari, 2003 The
  relationship between storm motion, vertical wind
  shear, and convective asymmetries in tropical
  cyclones. J. Atmos. Sci., 60, 366-376.
• Donelan, M. A. et al., 2004 On the limiting
  aerodynamic roughness of the ocean in very strong
  winds. GRL, 31, L18306.
• Eastin, M. D., W. M. Gray, and P. G. Black, 2005
  Buoyancy of convective vertical motions in the
  inner core of intense hurricanes. Part I General
  statistics. Mon. Wea. Rev., 133, 188-208.
• Hong, S.-Y., J. Dudhia, and S.-H. Chen, 2004 A
  revised approach to ice microphysical processes
  for the bulk parameterization of clouds and
  precipitation. Mon. Wea. Rev., 132, 103-120.
• Mlawer, E. J., S. J. Taubman, P. D. Brown, M. J.
  Iacono, and S. A. Clough, 1997 Radiative
  transfer for inhomogeneous atmospheres RRTM, a
  validated correlated-k model for the longwave.
  JGR, 102(D14), 16663-16682.
• Shapiro, L. J., 1983 The asymmetric boundary
  layer flow under a translating hurricane. J.
  Atmos. Sci., 40, 1984-1998.
• Stern, D. P. and S. D. Aberson, 2006 Extreme
  vertical winds measured by dropwindsondes in
  hurricanes. Preprints, 27th AMS Conference on
  Hurricanes and Tropical Meteorology, Monterey,
  CA, April 2006.
• Zhang, D.-L., Y. Liu, and M. K. Yau, 2000 A
  multiscale numerical study of Hurricane Andrew
  (1992). Part III Dynamically induced vertical
  motion. Mon. Wea. Rev., 128, 3772-3788.

• 7. What is the significance of these extreme
  low-level updrafts?

• The simulated extreme low-level updrafts
  apparently decay rapidly with height above
  slightly above the inflow layer, and are
  essentially non-existent above 3 km. Strong
  updrafts of 10-20 m/s are simulated above 3 km,
  but these are not contiguous with the extreme
  low-level updrafts, are azimuthally very broad,
  and are apparently entirely different features.
  However, the location and timing of these
  updrafts indicates that they may be somehow
  dynamically related.
• Shown below are radius-height cross sections
  through the azimuth of the maximum updraft at 750
  m height. To the right is an azimuth-height cross
  section at the radius 1 km outward of the maximum
  updraft at 750 m.
• The updrafts are tightly coupled to very intense
  vertical vorticity maxima, which lie just
  radially inward of the vertical velocity maxima.
  The maximum vorticity is found at the lowest
  level.
• There is often a very strong downdraft located
  just azimuthally upstream of the extreme
  updrafts.
• There are extreme horizontal wind maxima
  associated with the updrafts, and these are found
  outward of and below the strongest vertical
  velocities.

Tangential Wind (color), W (contoured every 2
ms-1, zero omitted)

• Are the extreme low-level updrafts buoyant?

• At a minimum, parcels must accelerate from 0-15
  m/s between the surface and 500 m height. They
  must further accelerate to 25 m/s between
  500-1000 m.
• If the forcing were constant with height/time,
  then the acceleration from 0-15 m/s would be
  152/(2500).225 m/s2
• If buoyancy were the source of acceleration, then
  the virtual potential temperature perturbation
  would be roughly .225300/9.81 6.87 K. This is
  very large!
• Perturbations in the simulation are an order of
  magnitude smaller, and are not even clearly
  positive at the location of the updraft.
• The extreme low-level updrafts therefore must be
  due to dynamic nonhydrostatic pressure gradient
  forces.

Azimuth-height cross section of W (color), Zeta
(contoured every .005 s-1, negative dashed, zero
omitted)
Virtual Potential Temperature (colored) and
Vertical Velocity (contoured every 5 m/s starting
at 10 m/s)
Zeta (color), W (contoured every 5 ms-1, zero
omitted)
Tangential Wind (color), W (contoured)
Radial Wind (color), W (contoured)
11. Corresponding Author Daniel
Stern RSMAS/MPO 4600 Rickenbacker
Causeway Miami, FL 33149 Email
dstern_at_rsmas.miami.edu