Some Preliminary Modeling Results on the Upper-Level Outflow of Hurricane Sandy (2012)

JungHoon Shin and Da-Lin Zhang

Department of Atmospheric & Oceanic Science, University of Maryland

On the Genesis of Hurricane Julia (2010)

• Cecelski, S., and D.-L. Zhang, 2013: Genesis of Hurricane Julia (2010) within an African Easterly wave: Low-level vortices and upper-level warming. Journal of the Atmospheric Sciences, 70, 3799-3817.

• Cecelski, S., D.-L. Zhang, and T. Miyoshi, 2014: Genesis of Hurricane Julia (2010) within an African easterly wave: Developing and non-developing members from WRF-LETKF ensemble forecasts. Journal of the Atmospheric Sciences, in press.

• Cecelski, S., and D.-L. Zhang, 2014: Genesis of Hurricane Julia (2010) within an African easterly wave: Sensitivity analysis of WRF-LETKF ensemble forecasts. Journal of the Atmospheric Sciences, in press.

• Cecelski, S., and D.-L. Zhang, 2013: Genesis of Hurricane Julia (2010) within an African easterly wave: Sensitivity to ice microphysics. To be submitted to Monthly Weather Review.

Outline

• Overview • Model description and verification• Upper-level outflow, inertial instability, and

hurricane reintensification• Summary and conclusions

DateOct 24 25 26 27 28 29

Rad

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Hwind dataMaximum windSea level pressure

Time series of the best track (PMIN in blue, and VMAX in red) and the RMW from Hwind analysis.

Stage 1 Stage 2 Stage 3 Stage 4

2220Z 24 (Stage 1) 1040Z 26 (Stage 2)

1400Z 27 (Stage 3) 1330Z 29 (Stage 4)

Satellite image showing the evolution of Sandy at 2220 UTC 24, 1040 UTC 26, 1400 UTC 27, and 1330 UTC 29 October 2012.

NCEP analysis at 00Z 29 Oct, when Sandy underwent re-intensification.

(a) 850 hPa wind vectors and isotherm (red: °C) with hourly accumulated precipitation from TRMM (shaded, mm)

(b) 200 hPa wind vectors, wind speed (shaded, m/s) and geopotential height (blue, m)

The thick solid line denotes the best track of Sandy, and black squares give Sandy’s location at each 00 UTC.

850 hPa

200 hPa

Questions to be addressed:

(i) How is the convectively generated upper-level outflow defined with respect to the eyewall convection and spiral rainbands?

A conceptual model of the hurricane’s inner-core structures

From Liu, Zhang and Yau (1999)

Questions to be addressed (continues): (ii) Was the re-intensification of Sandy caused by the approaching upper-level jet stream or the low-level forcing, including frontal lifting? If yes, how to prove it? If not, why could Sandy reintensify in a less favorable environment compared to its earlier stages?

(iii) How was the upper-level outflow related to the secondary circulations of Sandy? Are the upper outflow changes in intensity and coverage passive or active during its reintensification stage?

Model description and verification

Time period 12 UTC 23 ~ 00 UTC 29 Oct 2012 (132 h)

Cumulus parameterization Betts-Miller-Janic

Micro physic WSM5

PBL scheme YSU scheme

Radiation RRTM

Initial and boundary conditions

NCEP/GFS analysis (constant SST from NECP analysis)

Grid size & dimension 27 km (400 × 300), 9 km (262 × 262), 3 km (373 × 373) and 1 km (502 × 502)

WRF domains and SSTs.

-Blue simulated WRF track-Black: best track

Sandy’s location is marked at every 00UTC.

WRF physics options

Date

Oct 24 25 26 27 28 29

Cen

tral

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Pa)

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NHC best trackWRF simulation200-850 hPa VWS

DateOct 24 25 26 27 28 29

Max

imum

win

d sp

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(m s

-1)

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NHC best trackWRF simulation

Stage 1 Stage 2 Stage 3 Stage 4Stage 1 Stage 2 Stage 3 Stage 4

(a) Simulated (blue) and observed (red) PMIN, plus the 200-850 hPa vertical wind shear (1000 km × 1000 km).

(a) (b)

(b) Simulated (blue) and observed (red) VMAX.

Time-radius cross section of the azimuthally averaged 900 hPa tangential wind speed (shaded, m/s) from the western semicircle. Dashed lines are for the radius of maximum wind.

850 hPa wind vectors, isotherm (red, 3°C intervals), and hourly rainfall (shaded, mm)

200 hPa wind vectors, wind speed (shaded, m s-1) and geopotential height (blue).

(a) 06Z 26 Oct (b) 12Z 27 Oct (c) 12Z 28 Oct

(a) 06Z 26 Oct (b) 12Z 27 Oct (c) 12Z 28 Oct

900-hPa radar reflectivity (shaded, dBZ). The storm is centered in the domain (960km × 960 km).

900 hPa wind vectors with speeds shaded (m/s), and isobar (contoured at 5 hPa intervals). The storm is centered in the domain (960km × 960 km).

(a) 04Z 25 Oct (b) 06Z 26 Oct (c) 12Z 27 Oct (d) 12Z 28 Oct

a. 04Z 25 b. 06Z 26 c. 12 Z 27 d. 12 Z 28

Roles of the upper-level inertial instability in the reintensification of Sandy

• We have shown that the simulated re-intensification begins around 18Z 27 Oct, which coincides with the enhanced across-isobaric outflow into the upper level jet. It is caused by stronger pressure gradient force aloft. That is, we may consider the outward acceleration of air parcels caused by imbalance between centrifugal, pressure gradient and Coriolis force.

• The outward radial acceleration (dUrad/dt) can be calculated by

rrVfV

dtdU rad

2

Time-azimuth distribution of the 175-275 hPa averaged & radial averaged (450~500 km) outward acceleration due to inertial instability (shaded: × 104, m s-2) and the 175-275 hPa averaged radial mass flux [contoured: × 102, kg/(m2 s)] at R= 450 km from the storm center.

Time - azimuth distribution of the 500-hPa vertical mass flux [× 102, kg/(m2 s)] that is radially averaged from R= 50 to 250 km.

Summary and conclusions

• Sandy was a well-predicted storm;• The upper-level outflow appears to be more

associated with spiral rainbands than the eyewall convection; and

• The presence of upper-level inertial instability coincides with large outward mass fluxes, indicating the favorable conditions for deep convection.

Future work

• More analyses are needed to reveal the three-dimensional structures of the upper outflow layer and their relation to deep convection in the eyewall and spiral rainbands, CAPE and moisture source;

• Piecewise PV inversion will be used to reduce the influence of the upper-level jet stream to see how it would affect Sandy’s reintensification;