The role of wavegroup velocity in the blocking problem |
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| Authors: | Martin Dunst |
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| Institution: | (1) Meteorologisches Institut, Universität Hamburg, BRD |
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| Abstract: | Considering the blocking problem as a baroclinic instability problem in a dispersive wave system with diabatic heating effects, it is of great interest to investigate the role of wavegroup velocityv
gr in blocking processes, becausev
gr controls the energy transfer in the wave field. Using a  Newtonian Cooling —type of forcing with a phase differencek to the main field and taking the linearized version of a two-level model, the phase speedc
r, the group velocityv
gr and the growth ratekc
i have been obtained as analytical functions of the mean zonal windU, the thermal windU
T, the coefficient of diabatic heating x, the phase differencek and the wavelengthL. Now the hypothesis is introduced, that a blocking should be expected, ifv
gr has a maximum value in the vicinity ofL
o, for whichc
r vanishes and thee-folding timet=1/kc
i (kc
i>0) is smaller than 6 days (see condition (20) in the text). One finds, that for special parameter combinations (U
T, U,  ), where 15 m/sec U
T25m/sec,U=10m/sec, 0.8·10–51.5·10–5 sec–1], certain valuesL
o with an appropriate phase differencek exist, which satisfy the conditions mentioned above (for values see Table 2). TherebyL
o varies within the range 8500 km <L
o<11000 km corresponding to the preferred planetary blocking wavenumber 2 in middle latitudes 50°< <70° N. |
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| Keywords: | Blocking Dynamic meteorology Planetary waves |
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