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The Singular Vector(SV) technique seeks perturbations for wind, temperature and pressure that will maximize their impact on a 48 hour forecast as measured by the total energy over the hemisphere outside the tropics.  This maximization does not mean that the SV SVs only intensifies intensify weather systems; equally often it weakens them.  In addition, such systems can be displaced.  To specifically address just as often they weaken or displace them.  A special (tropical) version of the SV is created for use in the tropics to deal with uncertainties in the moisture processes typical of low latitudes, in particular of tropical cyclones, a special (tropical) version of the SV is created for use in the tropics.  These tropical SVs may also influence forecasts of extratropical developments, (e.g. when tropical cyclones enter mid-latitudes some days into the forecast and interact with the baroclinic developments in the westerlies).

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• they deliver rearward sloping baroclinic structures, which should generally be more realistic. 
  
• the spread of outcomes grows realistically.
• code is fairly inexpensive to run (it has a low resolution (320km) and there is no evidence yet that higher resolution SVs would improve ensemble skill).
  

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• they don’t directly take into account analysis uncertainty, nor observation coverage or accuracy.
• they produce rather localized variability (in the very short range).
• they under-develop variability in tropical regions (tropical singular vectors are employed to try to offset this problem).

Points to note:

• SVs (i.e. the SV perturbations applied) tend to be localized in areas of strong barotropic and baroclinic instability.
• Average SV total energy charts can be used to identify unstable regions. 
• SV growth tends to be characterized by an upscale and upward energy transfer (most rapid for smaller scales), with initial-time potential energy being converted, during the model run, into kinetic energy.
• Information for the final-time synoptic-scale structure is contained in the sub-synoptic scale SVs:
  
  • SVs with sub-synoptic-scale wavenumbers at initial time may grow very rapidly to reach synoptic scales.
    
  • SVs with large scale wavenumbers at initial time grow much more slowly than unconstrained, smaller-scale perturbations; the presence or absence of large scales has little effect on SV growth. 
• The atmospheric flow structure in winter tends to be more baroclinically unstable than it does in summer (and during winter the flow over Pacific and Atlantic sectors is on average more unstable than it is in other regions).

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