Rain Rate

Cross-band Velocity

Along-band Velocity

Cold Pool

Pressure

Mixing ratio

θ

θe

Δθe“Gap” stations exhibited

minimal Δθe decrease

“Wake” stations reachedmax Δθe about 60 km after RBLE

“Convective” stations had

max Δθe > 4 K about 20 km

after RBLE

Ahead of Leading Edge

Behind Leading Edge

Δθ

Mean cold pool maximum was 2-3Kabout 20 km behind the RBLE

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 ObjectivesSurface 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 TC-tornadoes (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:

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”.

Tropical Storm Hanna – 5-6 September 2008 – A Tale of Two Rainbands

Environment

Coastal MesonetVaisala (RENCI) Davis Instruments (UNCC)

UNCCRENCIASOS

Methods and DefinitionsAfter removing any significant biases from individual station time series (identified using non-convective 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 passageof the subjectively-identified, quasi-continuous 30-dBZ isoline (from LTX) as each rainband moved over the mesonet. This time closely corresponds tothe first measured precipitation at each station. Alltime series when then adjusted to this common reference frame with respect to their RBLE.

Next, a band-relative coordinate system was definedwith down-band flow along the band’s major axis as itspirals toward the TC center (toward the southwest in this case), and cross-band flow perpendicular to theband’s major axis (along a southeast – northwest axis).

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.

1800 UTC

Modestdry air

MesonetDomain

MHX

Rainband #1

Modestdry air

Rainband Initiation

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

Rainband #2 (1930-2230 UTC)

Representative Surface Station Time Series

Summary of Common Structure

Cold Pool Source

Rainband #1 (1630-1930 UTC)

Δθ

Mean cold pool maximum was 2-4Kabout 20-30 km behind the RBLE

“Convective” stations had

max Δθe > 6 K about 15 km

after RBLE

“Gap” station exhibitedmodest Δθe decrease

“Wake” stations reachedmax Δθe about 30 km after RBLE

Δθe

References and Additional Reading

Rain Rate

θ

Pressure

Cold Pool

Mixing ratio

θe

Along-band Velocity

Cross-band Velocity

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.

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)

ColdPool

Wake

ColdPool

Band / Cell Motion

CrossBand

DownBand

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).

Barnes, G. M., J. F. Gamache, M. A. LeMone, and G. J. Stossmeister, 1991: A convective cell in a hurricane rainband. Mon. Wea. Rev., 119, 776-794.

Eastin, M. D., and M. C. Link, 2009: Miniature supercells in an offshore outer rainband of Hurricane Ivan (2004), Mon. Wea. Rev., 137, 2081 – 2104.

Engerer, N. A., D. J. Strensrud, and M. C. Coniglio, 2008: Surface characteristics of observed cold pools. Mon. Wea. Rev., 136, 4839-4849.

Knupp, K. R., J. Walters, and M. Biggerstaff, 2006: Doppler profiler and radar observations of boundary layer variability during the landfall of Tropical Storm Gabrielle. J. Atmos. Sci, 63, 234-251.

Powell, M. D., 1990: Boundary layer structure and dynamics in outer hurricane rainbands. Part II: Downdraft modification and mixed layer recovery. Mon. Wea. Rev., 118, 918-938.

Rotunno, R., J. B. Klemp, and M. L. Weisman, 1988: A theory for strong, long-lived squall lines. J. Atmos. Sci., 45, 463-485.

Skwira, G. D., J. L. Schroeder, and R. E. Peterson, 2005: Surface observations of landfalling hurricane rainbands. Mon. Wea. Rev., 133, 454-465.

Xie, L., and Y.-L. Lin, 1996: A numerical study of airflow over mesoscale heat sources with application to Carolina coastal frontogenesis. Mon. Wea. Rev., 124, 2807-2827.

Zipser, E. J., 1977: Mesoscale and convective-scale downdrafts as distinct components of squall-line circulation. Mon. Wea. Rev., 105, 1568-1589.

Profiles of θe from MHX soundings

Minimum θe in RB-1 θe at RBLE in RB-1

Minimum θe in RB-2θe at RBLE in RB-2Cross

Band

DownBand

RBLE

LTX