Transcript Document

Discriminating between Tornadic and Non-Tornadic Soundings in Tropical Cyclones
Matthew D. Eastin, Brian M. Hays, and M. Christopher Link
Department of Geography and Earth Sciences, University of North Carolina at Charlotte
• No tornado reported within 185 km
and 3 h of launch location/time
• Must exhibit non-zero ML-CAPE
• Total = 3956
Radius from storm center (km)
Radius from storm center (km)
SB-CAPE
Climatology of Onshore TC Environment – Earth - Relative
USED
ML-LFC
RH-24km
RH-46km
01-SHR
USED
03-SHR
06-SHR
BRN-SHR
01-CRH
USED
Radius from storm center (km)
• Use the instability, shear, and helicity parameters that best
discriminate between tornado proximity and non-proximity
soundings in tropical cyclones
• Following methods outlined in Thompson et al. (2003), we
develop a single, normalized, non-dimensional parameter
01km-CRH
  2000 MLLCL 
  

1400m
 

Radius from storm center (km)
Initial Performance
02-CRH
03-CRH
06-CRH
USED
• The TCTP outperforms SCP, STP, and BRN within the
developmental dataset, and it exhibits a strong spatial
correlation within the climatological assessment
SCP
STP
TCTP
BRN
TCTP
Future Work
BRN
Radius from storm center (km)
 03SHR   01CRH
  
TCTP  
1 
2 2
12
m
s
40
m
s

 
• An additional criteria is that the sounding must exhibit
non-zero ML-CAPE, or the TCTP is set to zero.
• As any one component decrease to zero, the TCTP → 0
• TCTP > 1.0 suggests the local environment is
supportive of TC tornadoes
m2/s2
STP
SCP
ML-LCL
Initial Formulation & Rationale
m/s
m/s
MLCAPE03
TC Tornado Parameter (TCTP)
%
BRN-SHR
03km-SHR
MU-CAPE
RH-46km
m
J/kg
ML-CAPE
Radius from storm center (km)
ML-LCL
Radius from storm center (km)
ML-CAPE
Composite Non-Proximity
Altitude (km) & Pressure (hPa)
Non-Proximity Soundings
EF-Sum
Radius from storm center (km)
Radius from storm center (km)
EF-Sum
Composite Proximity
Proximity Soundings
• Launched within 185 km and 3 h
of at least one reported tornado
• Must exhibit non-zero ML-CAPE
(located along/on the warm side
of any pre-existing boundary)
• Total = 184
S
EF-Sum
Proximity Soundings
Altitude (km) & Pressure (hPa)
Shear-Relative from TC
Radius from storm center (km)
Radius from storm center (km)
Radius from storm center (km)
E
Radius from storm center (km)
1. A total of 60 TCs were identified – only periods when a system was
classified as either a tropical storm or a hurricane are included.
2. A total of 958 TC tornadoes were reported with these systems. A
cursory examination of available surface analyses was performed to
ensure the tornadoes were embedded within the TC environment.
3. A total of 5601 TC soundings were identified. Soundings with
either (a) missing data at mandatory levels, (b) large dewpoint
spikes, or (c) super-adiabatic layers >100 m in depth were discarded
4. In a few cases, soundings did not reach 12 km AGL or exhibited
sensor wetting errors in their boundary layer data. Sounding tops
and wetting errors were removed following Bogner et al. (2000).
5. All soundings & tornadoes were placed in a TC-relative framework
(azimuth/radius) as function of either (a) true north (earth–relative),
(b) the storm motion (storm–relative), and (c) the 850-200 hPa
vertical shear (shear –relative) derived from the SHIPS database.
6. All sounding winds were converted to their cylindrical components
relative to the moving storm center.
7. A large number of instability (CAPE, CIN, LCL, LFC, EL, LI),
moisture (RH, θv, θe), vertical shear (mean, bulk, helicity), and
composite (BRN, EHI, VGP, SCP, STP) parameters were computed
for each sounding. Shears were computed through a multitude of
layers and in the cell-relative framework following McCaul (1991).
8. TC-relative composite maps were constructed for each sounding
parameter and tornado EF-Sum (McCaul 1991) using an objective
analysis with the Cressman weighting function and a 200 km cut-off
on a 800×800 km horizontal grid with 50×50 km grid spacing.
W
Radius from storm center (km)
Three primary databases were used for this study: the NHC-HURDAT
database was used to identify the track and intensity of all landfalling
tropical cyclones during 1997-2008; the SPC-ONETOR database was
searched to identify all tornadoes reported within 800 km of each TC
center; and the ESRL-RAOB database was searched to identify all
rawinsondes launched within 800 km of each TC. These data were
further quality-controlled and analysed in the following manner:
TC Tornado Proximity & Non-Proximity Soundings
N
Radius from storm center (km)
Methods and Definitions
Motion-Relative from TC
Radius from storm center (km)
The objectives of this study are to first discriminate between tornadic
and non-tornadic soundings associated with U.S. landfalling TCs based
on a comprehensive review of stability and vertical shear parameters;
and second, develop a new composite parameter, called the Tropical
Cyclone Tornado Parameter (TCTP), which effectively
identifies
regions within the TC environment most conducive to miniature
supercell formation and tornadogenesis. We anticipate that such a
parameter will enhance the situational awareness for those severe
weather forecasts unique to tropical cyclones.
Earth-Relative from TC
Radius from storm center (km)
Landfalling tropical cyclones (TCs) regularly spawn tornadoes, with the
majority of events occurring within 100 km of the coastline as the outer
rainbands (>200 km from center) of major hurricanes, but the threat can
persist for 2-3 days after landfall (Schultz and Cecil 2009). Many
tornadoes are spawned by “miniature supercells”, which are often
shallower, less intense, and shorter-lived than their midlatitude
counterparts (Eastin and Link 2009, and references therein). Moreover,
sounding-based composite parameters such as SCP and STP (see
Thompson et al. 2003) – designed to enhance the situational awareness
for midlatitude severe weather forecasting – have shown limited success
in TCs (Baker et al. 2009), perhaps in part, due to significant physical
differences between the respective supercells and the stability and
vertical shear profiles in their local environments (e.g., McCaul 1990,
Spratt et al. 1997; Bogner et al. 2000; Curtis 2004; Molinari and Vollaro
2010). Such differences would suggest that forecasting TC tornadoes
may require a unique set of forecasting tools and conceptual models.
TC Tornadoes (1997-2008)
Radius from storm center (km)
Motivation and Objectives
1. Stratify the proximity soundings with regard to their relative time and
location to the reported tornadoes (before/after and upwind/ downwind).
2. Perform a comprehensive statistical assessment of TCTP using the
classic 2×2 contingency table and its associated metrics.
3. Explore additional (more effective) formulations of the TCTP within the
context of the contingency table analysis.
4. Explore the role of dry air intrusions in TC tornado outbreaks and how to
include their impact in the TCTP formulation.
5. Assess TCTP performance using an independent dataset derived from the
2009-2011 landfalling TC cases (may need more cases).
6. Complete a climatological assessment of the near-shore (but offshore)
environment to determine the spatial evolution of those regions most
conducive to miniature supercell formation and tornadogenesis as storms
transition from offshore to onshore.
References and Additional Reading
Baker, A. K., M. D. Parker, and M. D. Eastin, 2009: Environmental ingredients for supercells and tornadoes within Hurricane
Ivan. Weather and Forecasting, 24, 223-244.
Bogner, P. B., G. M. Barnes, and J. L . Franklin, 2000: Conditional instability and shear for six hurricanes in the Atlantic Ocean.
Weather and Forecasting, 15, 192-207.
Curtis, L, 2004: Midlevel dry intrusions as a factor in tornado outbreaks associated with landfalling tropical cyclones from the
Atlantic and Gulf of Mexico. Weather and Forecasting, 19, 411-427.
Doswell, C. A. III, and D . M. Schultz, 2006: On the use of indices and parameters for forecasting severe storms. Electronic
Journal of Severe Storms Meteorology, 1(3), 1-22.
Eastin, M. D., and M. C. Link, 2009: Miniature supercells in an offshore outer rainband of Hurricane Ivan (2004). Monthly
Weather Review, 137, 2081-2104.
Edwards, R. and A. E. Pietrycha, 2006: Archetypes for surface baroclinic boundaries influencing tropical cyclone tornado
occurrence. 23rd AMS Conference on Severe and Local Storms, St. Louis, MO, P8.2.
Markowski, P. M., E. N. Rasmussen, and J. M. Straka, 1998: The occurrence of tornadoes in supercells interacting with boundaries
during VORTEX-95. Weather and Forecasting, 13, 852-859.
McCaul, E. W. Jr., 1991: Buoyancy and shear characteristics of hurricane-tornado environments. Monthly Weather Review, 119,
1954-1978.
Molinari, J., and D. Vollaro, 2010: Distribution of helicity, CAPE, and shear in tropical cyclones. Journal of the Atmospheric
Sciences, 67, 274-284.
Rasmussen, E. N., and D. O. Blanchard, 1998: A baseline climatology of sounding-derived supercell and tornado forecast
parameters. Weather and Forecasting, 13, 1148-1164.
Schultz, L. A., and D. J. Cecil, 2009: Tropical cyclone tornadoes, 1950-2007. Monthly Weather Review, 137, 3471-3484.
Spratt, S. M., D. W. Sharp, P. Welsh, A. Sandrik, F. Alsheimer, and C. Paxton, 1997: A WSR-88D assessment of tropical cyclone
outer rainband tornadoes. Weather and Forecasting, 12, 479-501.
Thompson, R. L., R. Edwards, J. A. Hart, K. L. Elmore, and P. Markowski, 2003: Close proximity soundings within supercell
environments obtained from the Rapid Update Cycle. Weather and Forecasting, 18, 1243-1261.