Aircraft observations of rapid intensification

Robert RogersNOAA/AOML Hurricane Research Division

Collaborators: Paul Reasor, Jun Zhang, Sylvie Lorsolo

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•Advances in forecasts of tropical cyclone (TC) intensity, and rapid intensity change (RI) in particular, lag advances in TC track forecasts

• Multiscale interactions major reason for this – environmental to microscale•RI forecasts (e.g., RI index) based largely on environmental-scale fields generally

explain about 35% of skill in RI forecasts

Motivation

• To what extent can processes smaller than the environmental scale explain remainder of skill?• Aircraft data is a valuable tool for potentially detecting differences in inner-

core kinematic and thermodynamic structure for rapidly intensifying and steady-state TC’s

(Kaplan et al. 2009)

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Radial wind (m s-1)

Vertical vorticity(x 10-4 s-1)

Composite results

• IN cases have ring-like eyewall vorticity structure; weaker vorticity, lower inertial stability outside RMW• IN cases have a deeper inflow layer

40 eyewall passes from 8 intensifying TC’s, 53 passes from 6 steady-state TC’s

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Percentiles of eyewall (0.75 < r/RMW < 1.25) vertical velocity

• IN cases have stronger updrafts above freezing level for extreme portions of updraft spectrum (top 1% and greater) • no significant differences in profiles for weaker portions of spectrum, or for downdrafts• stronger deep convection, and more convective bursts, distinguish intensifiers from steady-state

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Composite results

Convective-scale comparisons

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Lower fuselage imagery of Hurricane Guillermo (1997)

EDOP cross sections of Hurricane Dennis (2005)

Reasor et al. 2009

Guimond et al. 2010

Composite resultsExamples of convective bursts in intensifying TCs

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Radial distribution of convective bursts

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vorticity (x 10-4 s-1)

• IN cases show radial distribution of convective bursts that peaks inside RMW compared with outside RMW for SS cases• relationship between radial location of diabatic heating and intensification consistent with

balance model studies (Vigh and Schubert 2009, Pendergrass and Willoughby 2009, others)

IN SS

Composite resultsConvective-scale comparisons

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Portion covered by P-3 flights considered here

On station times for P-3 flights

•Long-lived TC developed in eastern Atlantic, underwent RI near Leeward Islands•Intensified from 25 m s-1 to 60 m s-1 in 36 h; RI onset at 06 UTC August 29•Sampled by NOAA P-3’s, G-IV; NASA DC-8, Global Hawk; Air Force C-130’s

• Several studies have looked at observations of this case (Rogers et al. 2014, Stevenson et al. 2014, Susca-Lopata et al. 2014)

•Moderate (8 m s-1) northeasterly shear at RI onset, became weak by third P-3 flight•Focus here on inner-core structure and evolution

• airborne Doppler and dropsonde observations (Rogers et al. 2014)

Track Intensity

Case Study: Hurricane Earl (2010)

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Storm-relative wind speed at 2 km altitude (shaded, m s-1)

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Radius-height plot of axisymmetric vorticity (shaded, x 10-4 s-1) and tangential wind (contour, m s-1)

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RI onset RI continuing

Two stages of Earl’s RI: • RI onset – shallow symmetric vortex, broad RMW, RI onset in moderate shear• RI continuing – deep symmetric vortex, contracting eyewall, majority of intensification in low shear

Case Study: Hurricane Earl (2010)

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RI onset: Convective bursts and vortex alignment

• Displaced circulation center during first flight• Convective bursts tracked around storm left of shear vector during first flight, inside RMW; evolution

supported by lightning data (not shown)• RI began ~ 6 h after end of first flight• Vortex had aligned by time of second flight

Reflectivity (shaded, dBZ) from lower fuselage radar at ~ 3 km

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2-km wind speed (shaded, m s-1) and 8-km wind vectors (m s-1)

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Rogers et al. 2014

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RI continuing : Convective bursts located inside RMWTwo examples: Earl 2010 (RI) and Gustav 2008 (SS)

Wind speed at 2 km (shaded, m/s) and burst locations

Radial distribution of convective bursts

RMW

• both storms ~ 50 m/s hurricane, RMW of ~35 km• burst distribution peaked inside RMW for Earl, outside

RMW for Gustav

shearmotion

13:40 UTC Aug 30 2010

shearmotion

22:26 UTC Aug 31 2008

What governs radial location of convective burst activity relative to RMW?• radial profiles of boundary layer convergence • outer-core inertial stability, radial inflow depth• slopes of updraft and reflectivity cores

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r / RMW r / RMW

RI continuing: Radial profiles of boundary layer convergence Boundary-layer profiles of axisymmetric fields from dropsondes during Earl’s RI

Earl - 00 UTC Aug 30 2010 Earl - 00 UTC Aug 30 2010

• Low-level supergradient flow inside RMW• Convergence peaked below 0.3 km altitude inside RMW• Limited dropsondedata for SS cases (including Gustav)

Divergence (dVr/dr + Vr/r; 10-3 s-1)Agradient wind (Vag; m s-1)

Earl - 00 UTC Aug 30 2010 Earl - 00 UTC Aug 30 2010

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Axisymmetric inertial stability (I2, shaded, x 10-7 s-2) and tangential wind (contour, m s-1)RI continuing: Inertial stability, inflow depth differences

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• Lower outer-core inertial stability for Earl than for Gustav• Deeper inflow layer for Earl than Gustav

Axisymmetric radial flow (shaded, m s-1) and tangential wind (contour, m s-1)

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RI continuing: Updraft slope differencesDownshear vertical velocity (shaded, m s-1) and tangential wind (contour, m s-1)

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• Updraft core originates inside RMW for both cases• Slope of updraft core departs from M surface for Earl, parallels M surface for Gustav• Peak heating inside local RMW for Earl, outside for Gustav

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Slopes of reflectivity (dashed) and M (solid) as a function of shear-relative quadrant

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RI continuing: Comparison with slopes from composites36 Missions in 15 different TC’s

(Hazelton et al. 2014)

Paloma Ophelia

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Schematic of slope differences left of shear

Intensifying Steady-state

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Summary

• Characteristics of inner-core structure can distinguish RI from SS cases• axisymmetric structure• radial distribution of convective bursts

• Individual cases support this: Earl (2010)•RI onset: vortex aligned in moderate shear with bursts likely playing a role in alignment •RI continuing: RMW contracted with convective burst activity concentrated inside RMW

• Mechanisms explored to explain this radial distribution of convective bursts• PBL convergence • outer-core vorticity and inflow depth• slope differences in updraft

• Other cases also support these observations• Guillermo (1997), Dennis (2005), Gustav (2007), Paloma (2008),

Ophelia (2005), Ingrid (2013) 15

Future work• RI onset

• is alignment necessary for RI to begin? • what are mechanisms of alignment?

• RI continuing• what are causes of differences in radial location of convective bursts?• does buoyancy of inflowing air play a role?• what is relative importance of deep vs. moderate vs. shallow

convection (both onset and continuing)?

• Acquire additional datasets, esp. outside RMW, for analysis• dropsonde measurements for PBL convergence, buoyancy calculations• Hurricane Edouard (2014) excellent candidate for this analysis

• Analyze output from numerical models• 3-km HWRF forecast of Hurricane Earl (Chen and Gopal, 2014)• vorticity, PV budgets during early stage, buoyancy calculations from

model• RI vs. non-RI cases 16

Extra slides

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Intensifying Steady-state

Best track intensity history for each IOP (thick line is mean)

40 eyewall passes from 14 IOP’s in 8 intensifying TC’s 53 eyewall passes from 14 IOP’s in 6 steady-state TC’s

IOPIOP

• Many IOP’s in IN dataset occur in middle of intensification period

Composite study of airborne Doppler radar observations of vortex- and convective-scale structure and TC intensification

Composite results

Rogers et al. 2013

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Number and location of convective bursts(Bursts defined as locations where w > 5.5 m/s at 8 km altitude, i.e., top 1% of w)

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• IN cases have more convective bursts• greater proportion of bursts in downshear left quadrant

IN SS

Composite results

Convective-scale comparisons

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Stevenson et al. (2014) Susca-Lopata et al. (2014)

RI onset: Convective bursts and vortex alignment

• Lightning data shows a similar structure and evolution – rotating upshear inside RMW

WWLLN lightning frequency and time-radius Hovmollerof density

WWLLN lightning strike frequency relative to GFS shear vector from 21 UTC 28 – 06 UTC 29 August

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8 km RMW

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thermodynamic analyses (G-IV and P3 dropsondes)wind analyses (P3 TDR data)landfall 16/1115 UTC

A recent example: Ingrid (2013)Best Track intensity

Flight 2

Flight 5

• two flights compared here: one during intensification (flight 2) and one during steady-state, prior to weakening (flight 5)• three-dimensional analyses of kinematic and thermodynamic fields using SAMURAI (Bell et al. 2012)

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09/14/13 12 UTC09/14/13 10:20 UTC

09/16/13 00 UTC 09/15/13 21:45 UTC

Flight 2

Flight 5

Hodograph from GFS analyses 2-km wind speed (shaded, m/s) and 6-km wind vectors

• Moderate (~10 m/s) westerly shear during flight 2, increased to ~15 m/s by flight 5• Bulk of shear above 500 hPa• Vortex aligned during flight 2, showed ~25 km displacement left of shear by flight 5

Ingrid (2013) – Shear and tilt

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Reflectivity at 2 km during Flight 5 Theta-e at 0.5 km during Flight 5

• Channel of high PBL theta-e on southwest side of storm• Much lower PBL theta-e outside RMW on east and northeast side of storm, coincident with area of significant rainfall

Ingrid (2013) – Boundary layer thermodynamics

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