Turbulence in the Hurricane Outflow Layer By David

Turbulence in the Hurricane Outflow Layer By David Vollaro, John Molinari, and Patrick Duran Tropical Lunch: Sept 20 th 2013

Big whirls have little whirls which feed on their velocity. Little whirls have lesser whirls and so on to viscosity…. -L. F. Richardson

Turbulence Generation Mechanisms n n n Mountain waves Kelvin-Helmholtz Instability Convection Gravity waves Other

Goals n n Explore how common is turbulence in the hurricane outflow layer? Possible mechanism for generation of turbulence in the hurricane outflow layer.

G-IV dataset n n n 32 Storms over 7 seasons between 1997 -2005 Total sondes R = 0 -1000 km: 2577 Upshear: 1186 Downshear: 1391 TD = 3% TS = 25% Cat 1+2 = 32% Cat 3 -5 = 46% Too few sondes for analysis inside of R=100 km

Dropsonde Processing • Dropsondes were processed by EDITSONDE, a program created by the Hurricane Research Division. • Data(T, Td, winds etc) output every 0. 5 s from flight level(~150200 h. Pa) down to splash point. • Interpolated data to 100 m levels. • Corrected gaps in the data of up to 400 m using linear interpolation. • Eliminated all data where QC flags = poor quality. • Eliminated 1 st 30 sec (~600 m) from flight level to allow for sonde to adjust to environment. • Applied Bogner et al. (2000) correction to saturated near-surface layers with unrealistically large lapse rates

• Gray=0 -100 km Rings=200 -1000 km inc=200 km Distribution of G-IV sondes N=2577

Number of sonde observations

Mean Storm-Relative Vr and Vl Vr Contour inc=1 ms-1 Vl Contour inc=3 ms-1

Bulk Richardson Eq. • RB has no critical value for turbulence. • RB < 1 indicates that turbulence is possible. • RB < 0. 25 indicates that turbulence is certain. • Stability term and a Shear term. • High shear and low stability ideal for turbulence.

Percentage RB < 1 Red: Upshear Blue: Downshear Black: All sondes

Percentage of RB < 1. 0 Upshear Downshear Upshear: High % of RB<1 mainly confined to inner radii Downshear: High % RB<1 extends to all radii Mean

Percentage of observations of Turbulent RB GIV(solid) n=2577 Ivan(dash) n= 322 RB <1. 0 = Red RB < 0. 25 = Blue

2156 UTC 6 Sept 2004 Ivan Cat-2 Magenta “X”: drop Location Blue: Stability Term Red : Shear Term Pink bands: RB<0. 25 Cyan bands: RB<1. 0

Studies of Below Cloud Turbulence n n Kudo(2013) – Case study of turbulence below a mid-level cloud base associated with an upper-level front during recurvature of a Typhoon. Luce. et. al(2010) - Case study of turbulence below a cirrus deck.

Sounding observed at Tateno 1200 UTC May 12 2008. q Vert Shear Windspeed Direction Pink bar indicates layer with turbulence reports Kudo 2013

Sounding from Shigaraki MU Observatory 2357 LT 7 Jun 2006 Ri = Black Speed = dash Stability=Gray Shear =black Dir = gray Luce. et. al 2010

Low RB Composite n n n 12 sondes in Ivan(2004) having RB <0. 25 in outflow layer Radial range: 270 -880 km Storm Intensity: Cat 2 -4 Composited around level of minimum RB Level of minimum RB ranged from 8700 -10700 m

Composite Potential Temperature RB <1: Yellow Bar RB <0. 25 Magenta Bar * Composite level set at Z=9200 m Kudo 2013

Composite RB, Shear and Stability Terms Kudo 2013 RB : black Shear: Red Stability: Blue

Composite Relative Humidity

Summary n n n A well-defined cirrus base exists with relatively dry air underneath. A stable layer exists near cloud base. A near dry neutral layer exists below the stable layer Small values of RB occur within the low-stability layers. A sharp shear maximum, often exceeding 20 m s-1 km 1 in individual sondes, occurs directly above the stable layer.

Physical processes leading to the generation of below cloud turbulence

Summary n Turbulence is a fairly common occurrence in the tropical cyclone outflow layer. n There is a weak relationship between turbulence and shear as R B <1. 0 are more common downshear in association with the extended cirrus layer. n n The tropical cyclone cirrus deck creates its own stability field as a result of cloud-top cooling, diabatic warming (during the day), and sublimation underneath. Turbulence (RB < 0. 25) does not appear to be associated with the large shear layers, but rather is found largely in the near dry adiabatic layers associated with instability and mixing driven by cirrus diabatic effects. The shear layers do have RB < 1 almost always.

Questions?

Percentage of RB < 1. 0 Upshear Downshear Upshear: High % of RB<1 mainly confined to inner radii Downshear: High % RB<1 extends to all radii Down - Up Mean

Storm-relative Radial Wind Upshear Downshear

Pilot Reports between 0500 and 0700 UTC 12 May 2008. * Tateno * Kudo 2013

Physical processes leading to the generation of below cloud turbulence

Composite Lapse Rate Kudo 2013

Composite Windspeed

Control experiment 77 min 104 min 125 min

Time–height cross sections from 2200 LT 7 Jun 0715 LT 8 Jun 2008 at Shigaraki Observatory. Reflectivity (d. B) measured by VHF MUradar after Capon processing. Vertical velocity ms-1 Luce. et. al 2010

Storm-relative Windspeed mean

Flight data recorded from XAC to TATEYAMA around 0615 UTC 12 May 2008. Variables with vertical differentials were calculated using dz=300 m within +/150 m in height.

Sept 7, 2057 UTC Magenta “X”: drop Location Blue: Stability Term Red : Shear Term Pink bands: RB<0. 25 Cyan bands: RB<1. 0

Storm-relative Windspeed

Relative Humidity Comparison RHice Black RHwater Blue

Storm-relative Tangential Wind