1. Yumin Moon & David S. Nolan (2014)
Spiral Rainbands in a Numerical Simulation of Hurricane Bill(2009). Part I: Structures and Comparisons to Observations
Yumin Moon & David S. Nolan (2014)
Michaela Rosenmayer

2. Rainband Classification
Types of Rainbands:
Principal
Secondary
Distant
Inner
Houze (2010)

3. Methodology
Hurricane Bill was a category 4 hurricane that occurred in 2009
The simulation covered a 3 day period from 1200 UTC 18 August to 1200 UTC 21 August during which the hurricane was at maximum intensity over the Atlantic Ocean.
WRF was used with the following parametrizations: WRF single-moment 6-class microphysics, Yonsei University planetary boundary layer, Goddard shortwave and Rapid Radiative Transfer Model longwave radiation schemes

4. Principal Rainband Scattered convective cells, not a continuous line
Bounded on radial outward side by dry air
Slightly lower θe & θv on radial inward side

5. Principal Rainband: Idealized Convection
Figure C. Vertical cross section of the downdraft region in the center of a principal rainband. (Houze 2010)
Figure B. Vertical cross section of the updraft region in the center of a principal rainband. (Houze 2010)

6. Principal Rainband: Updraft
Radially outward tilt in the vertical
Max vertical velocities slightly inward compared to reflectivity
Overturning circulation: inflow on outward side below 3 km and outflow at 8 km
Higher tangential velocity at 4km on outward side
Higher values of θe in the updraft

7. Principal Rainband: Downdraft
Descending radial inflow from 2-4 km heights on the left side.
Lower θe near location of downward motion
Overturning circulation, on right graphs, at the radial inward side with outflow at z=7km and radial inflow below 2 km
No enhanced tangential velocity at low levels
Less frequent

8. Principal Rainband: Idealized Stratiform
Bright band signal near melting layer
Weak upward and downward motions centered around z=4km
Descending inflow from radial outward sides driven by radial buoyant gradients
Line arrows are vortex scale motions and thick arrows are mesoscale
(Didlake and Houze 2013b)

9. Principal Stratiform Tilted radially outward with height
Bright band at 5km
Upward and downward motion centered about 4km
Radial inflow from outward side starts at heights of 3km then moves to 2 km on inward side
Lower than 5km
Lack of outflow near 2 km above the inflow
Tangential velocity enhanced at 4km
Low θe on the outward side associated with downdrafts

10. Secondary Rainband Located between eyewall and principal rainband
Divided into five regions

11. Secondary Rainband: Updraft
Strongest upward motion radially inward of largest reflectivity values
Highest upward motion inward of max reflectivity
Overturning circulation on outward side
θe minimum at about 8 km
Bright band at 5 km : fully developed convection
Reflectivity values at higher heights
Overturning circulation on outward side
Not as strong of convergence at base of updraft
Enhanced tangential velocity from 2-6 km

12. Secondary Rainband: Downdraft
E-H: overturning circulation on inward side
Enhanced tangential velocity at 2 km unlike principal band
Less frequent
A-D: descending radial inflow at 2km from outward side
Slight correlation with lower θe

13. Distant Rainband Locally dense air Surface cold pools
Lower values of θ , water vapor mixing ratio, and θe

14. Distant Rainband Tilts radially inward unlike principal and secondary
Updraft on the radially outward edge of rainband and maximum reflectivity
Strong convergence at base of updraft with shallow inflow
Convection is correlated with outward motion and low θe

15. Inner Rainband Forms just outside the eyewall
Upward motion on radial outward side and downward motion on radial inward side
No significant virtual or equivalent potential temperature anomalies

16. Inner Rainband Shallow reflectivity up to 5km with outward tilt
Vertical velocity tilts inward
θe minimum at 4km
Lower radial inflow and tangential velocity along with higher cloud water mixing ratio and θe at 1 km on radially inward side

17. Conclusions Distant rainbands Principal and secondary rainbands
Differ from principal and secondary rainbands by:
Radially inward tilt
Less affected by inner core vortex dynamics; structure similar to squall lines
Linked with surface cold pools and move predominately radially outward with time
Principal and secondary rainbands
Results that agreed with past observations:
Tilt radially outward with height
Overturning secondary circulation or descending midlevel radial flow
Tangential velocity maximum on the outward side
Main finding that was unique to this simulation was fewer instances of overturning circulations compared to descending midlevel radial flow
Inner rainbands
Shallow convection
Possible origination from eyewall that is investigated in Part II
No observational studies have been done to verify simulation

18. References
Barnes, G. M., E. J. Zipser, D. Jorgensen, and F. Marks Jr., 1983: Mesoscale and convective structure of a hurricane rainband. J. Atmos. Sci., 40, 2125–2137, doi: / (1983)040,2125: MACSOA.2.0.CO;2 Didlake, A. C., and R. A. Houze, 2013b: Dynamics of the stratiform sector of a tropical cyclone rainband. J. Atmos. Sci., 70, 1891–1911, doi: /JAS-D Hence, D. A., and R. A. Houze, 2008: Kinematic structure of convective-scale elements in the rainbands of Hurricanes Katrina and Rita (2005). J. Geophys. Res., 113, D15108, doi: /2007JD Houze, R. A., 2010: Clouds in tropical cyclones. Mon. Wea. Rev., 138, 293– 344, doi: /2009MWR Moon, Y., and D. S. Nolan, 2015: Spiral Rainbands in a Numerical Simulation of Hurricane Bill (2009). Part I: Structures and Comparisons to Observations. J. Atmos. Sci., 72, Samsury, C. E., and E. J. Zipser, 1995: Secondary wind maxima in hurricanes: Airflow and relationship to rainbands. Mon. Wea. Rev., 123, 3502–3517, doi: / (1995)123,3502: SWMIHA.2.0.CO;2. Wang, Y., 2009: How do outer spiral rainbands affect tropical cyclone structure and intensity? J. Atmos. Sci., 66, 1250–1273, doi: /2008JAS Willoughby, H. E., 1988: The dynamics of the tropical cyclone core. Aust. Meteor. Mag., 36, 183–191.