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Surface Cold Pools in the Outer Rainbands of Tropical Storm Hanna (2008)
Matthew D. Eastin, Tiffany L. Gardner, M. Christopher Link, and Kelly C. Smith
Department of Geography and Earth Sciences, University of North Carolina at Charlotte
Motivation and Objectives
Tropical Storm Hanna – 5-6 September 2008 – A Tale of Two Rainbands
Surface cold pools are known to play a significant role in the evolution and organization of
mesoscale-convective systems (Zipser 1977; Rotunno et al. 1988), tropical cyclone (TC) rainbands
(Barnes et al. 1991), and mid-latitude supercells (Doswell and Burgess 1993). Furthermore, cold
pools can impact TC intensity (Powell 1990) and tornadogenesis (Markowski et al. 1998).
However, near surface cold pools have not been well documented in landfalling TCs, particularly
those associated with the outer rainbands that often contain miniature supercells and spawn TCtornadoes (Eastin and Link 2009).
The objectives of this study are to document the surface characteristics of outer rainbands in
landfalling TCs as they pass over a surface mesonet situated within a gap of the existing NOAA
network. Specifically we wish to:
Environment
Rainband #1 (1630-1930 UTC)
Rainband #2 (1930-2230 UTC)
Hanna’s two outer rainbands exhibited several similar
characteristics as they passed over the coastal mesonet
(see summary figure below). Each band did not contain
a single continuous cold pool, but rather distinct pockets
of cold air. These most intense cold pools (Δθ > 2K)
were located immediately behind the most intense
convective cells (> 50 dBZ) where cross-band surface
convergence was also most intense. The cold pools
exhibited cross-band expansion and down-band
advection, producing prominent “wake” signatures at
several downwind stations.
1800 UTC
Rainband #1
MHX
Mesonet
Domain
Modest
dry air
1. Document the structure and evolution of the surface flow within and adjacent to outer
rainbands soon after they moving onshore.
2. Establish the frequency of prominent surface outflow events, as well as the cell and
environmental characteristics during such events.
During the 2008 Atlantic season, the University of North Carolina at Charlotte (UNCC) and the
Renaissance Computing Institute (RENCI) deployed three Davis Instrument Vantage Pro and five
Vaisala WXT-510 surface stations across Brunswick County, NC. On 5-6 September three outer
rainbands passed over the mesonet. Here we present “the tale of two rainbands”.
Vaisala (RENCI)
Summary of Common Structure
Δθ
Δθ
Ahead of
Leading Edge
Behind
Leading Edge
Mean cold pool maximum was 2-4K
about 20-30 km behind the RBLE
Davis Instruments (UNCC)
Cold pool intensities (Δθ or Δθe) were similar to those
documented in several offshore TC rainbands (see
Barnes et al. 1991) as well as the few onshore TC cases
(Skwira et al. 2005; Knupp et al. 2006) . However, the
cold pools were less intense than those often observed in
mid-latitude convection (e.g. Engerer et al. 2008)
Coastal Mesonet
Rainband Leading Edge
Band / Cell
Motion
Cold
Pool
Mean cold pool maximum was 2-3K
about 20 km behind the RBLE
Cross
Band
UNCC
RENCI
ASOS
Down
Band
Δθe
Methods and Definitions
Next, a band-relative coordinate system was defined
with down-band flow along the band’s major axis as it
spirals toward the TC center (toward the southwest in
this case), and cross-band flow perpendicular to the
band’s major axis (along a southeast – northwest axis).
Assuming no dilution, comparison of the minimum θe
observed at each surface station with the vertical profiles
of θe from MHX suggest the source of the cold pool air
was ~1 km above the surface (or higher with dilution).
“Gap” station exhibited
modest Δθe decrease
Modest
dry air
“Convective”
stations had
max Δθe > 4 K
about 20 km
after RBLE
“Wake” stations reached
max Δθe about 60 km after RBLE
“Convective”
stations had
max Δθe > 6 K
about 15 km
after RBLE
“Wake” stations reached
max Δθe about 30 km after RBLE
RBLE
Profiles of θe
from MHX soundings
LTX
Representative Surface Station Time Series
Rainband Initiation
Cross
Band
Down
Band
Finally, each station’s time series was classified as one of following three categories based on the
presence, intensity, and/or timing of any significant cold pool passage:
Convective: Time series exhibits distinct minima in both θ and θe (with Δθ > 1 K and
Δθe > 4 K relative to their respective values at the RBLE) after passage
of the RBLE, and minima occur ±30 min of the rainfall maximum.
Wake: Time series exhibits distinct minima in both θ and θe after passage of
the RBLE, but minima occur >30 min after the rainfall maximum.
Gap: Time series does not exhibit a distinct minimum in either θ or θe after
passage of the RBLE.
Cold
Pool
Cold Pool Source
“Gap” stations exhibited
minimal Δθe decrease
After removing any significant biases from individual station time series (identified using nonconvective time periods as well as pre- and post-season “buddy” checks) and adjusting the winds
to a standard 10-m height for “open” exposure, a rainband passage time (relative to the leading
edge) was determined in order to have a common frame of reference for any cold pools as they
moved over the mesonet.
Rainband Leading Edge (RBLE): First passage
of the subjectively-identified, quasi-continuous
30-dBZ isoline (from LTX) as each rainband moved
over the mesonet. This time closely corresponds to
the first measured precipitation at each station. All
time series when then adjusted to this common
reference frame with respect to their RBLE.
Δθe
Wake
Animated satellite imagery
indicates that the rainbands
developed (or were enhanced)
along the northern thermal gradient
of the Gulf Stream (e.g. Xie and Lin 1996)
and then moved to the west-northwest
Minimum θe in RB-1
θe at RBLE in RB-1
Rain Rate
Minimum θe in RB-2
θe at RBLE in RB-2
Rain Rate
Pressure
Pressure
θ
θ
Cold Pool
Cold Pool
References and Additional Reading
Mixing ratio
Mixing ratio
θe
θe
Along-band Velocity
Along-band Velocity
Cross-band Velocity
Cross-band Velocity
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