Intensification and weakening; Formation of eye and evolution

of inner core

TC Intensification

Rapid Intensification Factors:• High SST/OHC, Moist troposphere- esp.

low and mid levels, Cold UL temps • Low VWS, Relative Eddy Flux

Convergence (REFC) supportive of UL Div, UL Div, cyclonic mid and UL PV, mod- fast forward speed of storm

• Far from MPI?, Julian Day- near peak

Eye Formation• Inner core = eye and eyewall region• Tropical depressions and weak tropical storms

may exhibit strong convection near the center of the storm that has warm air rising upwards (direct thermal circulation, or DTC) that will induce subsidence away from the heat source.

• The horizontal length scale required for the subsidence is relatively large for low wind speeds. Even for a vortex wind profile, subsidence will not be effective at the center, as it’s too close to eyewall convection.

Eye Formation

Eye Formation• As you increase wind speeds in area of latent

heating, scale needed for sinking motion decreases. An eye will eventually form.

• The eye features warm air sinking, an indirect thermal circulation (ITC) imposed by eyewall convection (DTC). This further lowers the pressure of the hurricane, increases the pressure gradient just inside the radius of maximum winds (contributors to SLP min in eye: ~70% from eyewall LH release, ~30% from warming due to dry subsidence in eye).

• The wind speed increases, especially just inside the RMW. Intensification and contraction of the eyewall.

Eye Flow

Very clear for an eye- typical of intensification

TC Intensification

• Need enough latent heating to overcome inertial stability generated by AAM gradient. If that happens, radius of maximum winds will contract by height falls within eye and eyewall produced by eyewall latent heating.

• If they are in equilibrium and the synoptic environment is relatively negligible, eyewall radius stabilizes.

Dvorak Technique (DT)• Dvorak Technique (DT): An attempt to estimate

tropical cyclone intensity objectively via IR satellite imagery.

• Hurricane appearance on IR imagery correlated with intensity.

• Accurate to within 20 kt roughly 90% of the time, probably more so for meteorologists who have much practice using DT.

• T-number assigned, where higher number correlates with higher sustained wind speed value.

DT• Dvorak Technique (DT): Increased symmetry and

increased cold convective cloudtops around center indicative of intensification.

• Increase in temperature difference between eye and eyewall indicative of intensification.

• Ability for automated estimates, without use of humans to determine a value.

• Trends important.• Climatologies from different basins have been

and are being created and updated.

DT

Cat-2 Julia

Cat-4 Igor

Microwave Sensing Satellites• Microwave satellites can better pierce clouds

(not only see cloudtops) to more accurately portray precipitation fields.

• Give a better portrayal of 3-D structure, latent heat release and lack thereof.

• Better resolve secondary eyewall structures.• Microwave satellites can give more detailed

views of circulation field and radius of 34-kt winds, correlated with intensity and area of high seas.

Secondary Eyewalls

• Secondary eyewall formation.• AAM loss to sea surface via friction. Inward

radial winds, though still a greater cyclonic component. Convective bands form away from center; cut off warm, moist inflow. Reduce heating of inner eyewall, induce pressure falls outside of inner eyewall.

• Intensify at expense of inner eyewall and contract to replace old eyewall.

Eyewall Evolution

Energy for Waves via PV Gradients• Polygonal eyewalls and mesovortices.• Energy from vorticity gradient. Above features end up mixing

the gradient, spinning up the eye at expense of the eyewall. This reduces the vorticity gradient, killing off vortex Rossby waves and mesovortices. Vortex may re-intensify, creating a new, strong PV gradient maximized at the inner edge of the eyewall once again if synoptic conditions are favorable.

• Usually result in weaker IS due to increased eye-eyewall mixing. Generates weaker subsidence-> reduces warming and SLPs will increase, and inversion in eye will ascend.

• Generally, vortex expands with time, as gets older and moves to higher latitudes (larger f). Also, annulus of high PV around center is an unstable configuration- usually results in mixing.

Inner Core PV Structures

Vertical Wind Shear

• Wavenumber-1 asymmetry from vertical wind shear.

• Increases ventilation of system to the point that it reduces available heating-> generally results in weakening.

• Deepest convection typically on downshear left side.

Midlatitude Troughing

Troughs, especially at upper levels:• Can intensify storm if UL Div and cyclonic vort

enhancement dominates over increased VWS• Weaker values of inertial stability at upper

levels in hurricanes makes them sensitive to wind shear aloft; VWS with high AAM values from large scale of trof or outer environment can destroy heat (too much ventilation) and momentum fields at upper levels first.

TC Death• Decay as move to higher latitude… cooler water and

higher VWS most of time.• R# = U/foL -> decreases with greater latitude… size of

storm increases. Latent heating also decreases. Pressure gradient and tangential winds decrease.

• Extratropical transition possible in N Atlantic and NW Pacific.

• Die over land due to loss of LH flux, often also SH flux. Increased friction reduces wind speed and may cause too much of a rapid increase in LL mass convergence for adequate evacuation via UL Div.

Size Differences

Extratropical Transition• PV and UL Div from upper trough may enhance

TC.• Typically occur in September and October.• Winds increase aloft due to colder mid and upper

levels reducing the warm core of the TC. • Precipitation shield on left and poleward side,

winds on equatorward side.• Fast forward speed and large, broad wind field

may produce very large waves.• Generally becomes increasingly baroclinic and

cold core.

Extratropical Transition

• End presentation.