The Tropical Cyclone Boundary Layer 1: Introduction and Observations

The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

The Tropical Cyclone Boundary Layer1: Introduction and Observations

Jeff Kepert

Head, High Impact Weather Research

Oct 2013

www.cawcr.gov.au

The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology

The boundary layer - definition

The boundary layer is the lowest 1-2 km of the atmosphere, the region most directly influenced by the exchange of momentum, heat and water vapour at the earth’s surface. Turbulent motions on time scales of an hour or less dominate the flow in this region, transporting atmospheric properties both horizontally and vertically through its depth. The mean properties of the flow in this area – the wind speed, temperature and humidity – experience their sharpest gradients in the first 50 – 100 m, appropriately called the surface layer. Turbulent exchange in this shallow layer controls the exchange of heat, moisture and momentum at the surface and therefore the state of the whole boundary layer.

Kaimal and Finnegan (1994)


Hurricane Guillermo (1997)


Importance of the TC BL

• We live in the boundary layer• Potential intensity theory

• Energy source (latent and sensible heat flux)

• Energy sink (frictional dissipation)

• Ocean response• Waves

• Storm surge

• Major hazards, often more so than wind (e.g. Katrina)

• Engineering design, characteristics of the mean winds and turbulence• Definition of cyclone intensity


Structure of lecture series

1. Introduction, observed structure

2. Dynamics of the tropical cyclone boundary layer

3. More on the dynamics

4. Thermodynamics

5. Air-sea fluxes and turbulence

6. Secondary eyewalls and the boundary layer


Surface winds in Hurricane Frederic

• Maximum winds in right forward quadrant• Maximum inflow angle to right rear, sometimes weak outflow to left• Mean inflow angle 20 – 25o

• Powell (1982, MWR)

5 m s-1


NW Pacific Typhoon, Japanese Navy

Arakawa and Suda (1953, MWR)


Waves in the Typhoon

Arakawa and Suda (1953, MWR)


Wright et al. 2001, JPO


Surface Wind Factor

• The surface wind factor is the ratio of the surface wind speed to that at some higher level

• Typically to gradient wind equation, or to observed flight level winds

• Traditional values range from 0.7 – 0.8

• Widely used• to estimate surface winds from aircraft data

• as a very crude parameterisation of the boundary layer for storm surge or wind risk modelling


Surface vs gradient winds

• Typhoon Vera, 1977• Observations from flat coral atoll,

south of Japan• Significant variation in surface

wind factor near eyewall• Rapid reduction in radial flow near

eyewall

• Mitsuta, Y., T. Suenobu and T. Fujii, 1988: Supergradient surface wind in the eye of a typhoon. J. Meteor. Soc. Japan, 66, 505-508.

Mitsuta et al. (1988)


Winds in Hurricane Katrina

Powell et al. (2009)


SWF from buoy and aircraft data

Powell and Black (1990, JWEIA)


Hurricane Mitch: Mean Winds by Annulus

• Speed maximum below 500 m inside RMW.

• Increases rapidly in height across RMW.

• In 15-25 km band, max shear in layer above jet.

• Strongest inflow near RMW, with marked outflow above.

• Kepert (2006b)

0-15km

15-25km

25-40km40-100km

0-15km

15-25km

25-40km40-100km

Azi

mu

thal

Win

dR

adia

l Win

dAzimuthal

Radial


Variation in wind structure within storm

• Mean wind profile in many hurricanes from dropsonde observations

• Normalised by 700-hPa wind• Substantial difference between

eyewall and outer vortex

• Franklin et al (2003, W&F)


SWF’s from dropsonde data

Franklin et al. (2003, W&F)


Interstorm variability

• Mean normalised wind speed profiles in the eyewall of seven hurricanes

• Significant differences between storms

• Structure of profile

• Surface wind factor 0.82 to 0.96


0.82 0.96


The logarithmic surface profile

• Below the wind maximum, the mean wind profile is approximately logarithmic

• Slight curvature in lowest 20 – 30 m



Radius-height sections

Frank (1984)

Max.updraft


Hurricane Fabian cross-sections

• Hurricane Fabian mean axisymmetric wind on 3 days

• Dropsonde and Doppler radar data

• Shading: mean azimuthal wind

• Arrows: radial and vertical flow

• Contours: radial flow at 5 m/s intervals

• Bell et al. (2012, JAS)


Hurricane Isabel cross-sections

• Hurricane Isabel mean axisymmetric wind on 3 days

• Dropsonde and Doppler radar data

• Shading: mean azimuthal wind

• Arrows: radial and vertical flow

• Contours: radial flow at 5 m/s intervals

• Bell et al. (2012, JAS)



Composite r-z sections• Obs show that the well-mixed (constant θ) layer is half or less the depth

of the inflow layer in TCs.

• Zhang et al (2011, MWR) composite r-z sections in N Atlantic hurricanes.

Azimuthal wind

Potential temperature

Radial wind

Top of inflow layer


Photo: Frank Marks, HRD




Observations

• Heavy reliance on research aircraft• Doppler radar

• Tail-mounted, horizontal axis, forward-and-aft scanning (FAST)

• Step frequency microwave radiometer (SFMR)• Downward-looking, passive microwave, 6 frequencies

• Emissivity depends on white cap fraction, which depends on wind speed

• Extensive calibration against dropsonde and buoy data

• Uhlhorn and Black (2003, J.Tech)

• GPS Dropsonde• Radiosonde-like package dropped from aircraft

• Measures wind, p, T, q at 2 Hz (~ 6 m vertical resolution)

• Revolutionised understanding of boundary layer in tropical cyclones

• Hock and Franklin (1999)

• Data is available from the HRD website http://www.aoml.noaa.gov/hrd/



P3 Dropsonde Workstation


Summary

• Max winds in right forward quadrant

• Low level jet• Strong inflow near surface, strong

updraft at eyewall, outflow aloft• Variation within and between

storms• Logarithmic profile near surface

Documents

The Tropical Cyclone Boundary Layer 1: Introduction and Observations