Chapter 7 part B

Question 1

Maintaining Intensity

Answer 1

• The coupling between the strong winds in the boundary layer and the transfer of latent and sensible heat into the air as it spirals into the cyclone.
  
  • Frictional convergence of the tangential winds plays a key role in transporting latent heat into the core of the vortex.
  • The stronger the winds,
    • the greater the convergence into the core.
    • the greater the transfer of latent and sensible heat from the sea surface into the inflowing air.
• The formation of an anticyclone aloft by the heating aids in the upper‐level mass divergence which is necessary to sustain and intensify the cyclone.
• Tropical cyclones can maintain themselves as long as there is sufficient inflow of warm, moist air into the cyclone, and there is adequate outflow aloft.

Question 2

Factors that can cause a tropical cyclone to fluctuate in intensity (up/down) are:

Answer 2

• Variations in SST
• Interaction with land, which result in less evaporation & latent heat inflow.
• Enhanced or suppressed outflow
• Increased vertical shear

Question 3

Tropical Cyclone Motion

Answer 3

cumulative effect of the following three influences:

• Environmental steering (high importance)
• The beta effect (second high importance)
• Asymmetrical convection (little importance for long term motion)
  
  • mainly responsible for short term “eye wobble”

Question 4

Environmental steering:

To a first approximation, the tropical cyclones can be thought of as

Answer 4

merely advected by the mean environmental flow.

Question 5

The environmental flow can be represented by

Answer 5

various means, either through a single level, or through a mass‐weighted mean flow.

• A deep‐layer mean
  
  • best for intense, mature cyclones.
• A medium‐layer mean or even shallow‐layer mean
  
  • more suited to weak systems, especially if they are highly sheared.

Question 6

adjustments in order to determine the environmental flow

Answer 6

the cyclone circulation needs to be removed from the wind field

Question 7

…………………….. important for environmental steering currents.

Answer 7

Synoptic‐scale influences are important for environmental steering currents.

• An approaching trough can alter the steering current
  
  • cyclone recurves, or at least tracks more northerly.
• A strong subtropical ridge
  
  • keep a cyclone entrenched in the trade winds, and
  • tracking westward

Question 8

……………………. can influence the steering flow.

Answer 8

An adjacent tropical cyclone can influence the steering flow.

• this is called Fujiwhara effect:
  
  • can result in some unusual tracks as the cyclones spiral around each other, occasionally even merging

Question 9

Beta Effect

Answer 9

refers to the tendency of a circulation to move, even in the absence of a mean flow, due to the conservation of absolute vorticity.

Question 10

linear beta effect

Answer 10

The tendency for the cyclone to move westward in the absence of a mean flow

Question 11

Though the………………………………………, there are asymmetries in convection.

Answer 11

inner core of a tropical cyclone has large inertial stability

Question 12

The asymmetrical convection can be due to several factors, including:

Answer 12

• SST gradients
• Differential stress

Question 13

The asymmetrical convection can lead to:

Answer 13

• local pressure falls in the eyewall
  
  • can displace the eye in the direction of the pressure falls.
    
    • leads to short‐term “eye wobble” often seen in tropical cyclone tracks.

Question 14

Tropical Cyclone Intensity Estimation techniques:

Answer 14

• Dvorak Technique
  
  • Curved Band pattern
  • Shear pattern
  • CDO (Central Dense Overcast) pattern
  • Eye pattern (visible image)

Question 15

One important key for a successful prediction of the cyclone track and intensity is

Answer 15

an accurate analysis of the current position and intensity of the cyclone.

Question 16

Location is determined primarily by

Answer 16

• away from land
  
  • satellite imagery
• near land,
  
  • when radar and aircraft

Question 17

Intensity is determined by (near land)

Answer 17

• Doppler radar observations can be used to estimate intensity.
• In the absence of aircraft observations or Doppler radar observations, the Dvorak technique is the staple for determining intensity.

Question 18

Dvorak Technique

Answer 18

The Dvorak technique is based on appearance of the storm from IR and visible satellite imagery.

uses patterns and measurements from satellite imagery to estimate the strength of a tropical cyclone.

Question 19

The Dvorak technique explination:

Answer 19

assigns a T‐number based on the appearance from the satellite imagery.

• The T number scale runs from 0 to 8 in increments of 0.5.
• Minimal Tropical Storm intensity (35 kt) is T2.5
• Minimal Hurricane intensity (65 kt) is T4.0

Question 20

basic patter types (Dvorak):

Answer 20

Question 21

Curved Band pattern

Answer 21

Spiral Arc Distance of Curved Band (continuous deep convective spiral band) surrounding the Center.

• DT number determined by curvature of band around 10 log spiral.

Question 22

Shear pattern

Answer 22

Shear pattern for weak systems to 50 knots based on distance of Center from Edge of Deep Convective Clouds.

Distance: 1° latitude = 60 nautical miles (n mi.) = 111 km)

Shear pattern more obvious on visible imagery than on IR.

Question 23

CDO (Central Dense Overcast) pattern

Answer 23

• The CDO pattern is only used with VIS imagery.
• The CDO is a dense overcast mass of clouds that appears within the curve of the curved band axis and covers the cloud system center.

Question 24

CF number

Answer 24

• The size of the CDO and
• its boundary definition ie irregular or well‐defined

Question 25

DT number

Answer 25

derived by adding a BF‐number to the CF‐number according to the amount of banding observed around the CDO.

CF+BF=DT

Question 26

The banding feature (BF)

Answer 26

a measure of the amount of wide or narrow banding that wraps around the central features of the cloud pattern.

• The more an overcast band feature curves around the central features, the greater the banding feature (BF) addition.

Question 27

Eye pattern

Answer 27

(visible image)

• Distance of Eye from CDO Edge is defined as the “Embedded Distance” (CF).
• Extent of Surrounding Deep Convective Cloud Band (Banding Feature, BF).

DT number determined by DT = CF + BF

CF=CENTRAL FEATURE = E‐No + E‐Adj

Question 28

There are several different methods for predicting tropical cyclone tracks.

Answer 28

• persistence
  
  • useful in the first few hours of the forecast. (old)
• climatology
  
  • finding analogs, or past storms that had similar characteristics and synoptic environments.
• Statistical
  
  • use regression techniques to correlate information about the storm and its environment with the likely track.
  • CLIPER: statistical blend of persistence and climatology
    
    • had no dynamical input, and yet was very successful. It is often used as the reference by which new methods are judged.
  • statistical‐dynamical: use dynamical properties as independent variables
• Dynamical method
  
  • as understanding of the physical processes affecting motion was gained
  • BAM (Beta plus Advection Models)