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

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

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 3.2.1 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

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

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)

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

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

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)

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

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

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

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

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

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

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

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

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

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