Dear All,
There are two figures attached that show the isobars of atmospheric circulation on TP. For the green one, arrows indicate the wind field in JJA on the ground (1000hPa). For the other one, there are no arrows (700hPa,equals to ~3000m a.s.l.). Could someone give some advice on why the wind field on TP is different altitude? Thanks.
If you use surface wind data, there will be wind vectors over the TP. If you use pressure-level (e.g., 1000 hPa) wind data, there should not be any wind vectors over TP. Any wind vectors appear over TP at lower pressure levels may result from extrapolation from the air into the land and make little sense. A common way is to mask them out such as the upper figure you've shown.
you should be careful when interpreting both figures. The wind vectors are not measured data but calculated from the 1000 hPa layer pressure field (reanalysis NCEP/NCAR data) and averaged over 60 years. I cannot figure out for what altitude the streamlines are valid. The 1000 hPa layer (about 110 m MSL typically) is much lower in altitude than the TP.
Hi Dr.Qian, thank you very much for your answer. I wanna make sure if I understand this clearly. As you said, for surface wind data, wind vectors make sense, such as the lower figure (1000 hPa, the green one). While for upper atmosphere, such as 700 hPa, arrows seems meaningless.
Best regards,
Mingda
I agree with the comments given previously:
These are modelled winds - based on computed temperature / pressure fields.
the real wind field would be more complex.
This is particularly so when the pressure level or altitude modelled is below the maximum altitude of the near-by terrain (i.e. very high mountains)!
Physically, friction with the ground surface causes a rotation of the wind vectors at low levels, away from the Coriolis wind field found at higher altitudes (more from high pressure to low pressure - rather than perpendicular to the pressure gradient.
Other important and real factors are that water vapour distributions are very complex and create vertical pressure distributions that are more complex than the models can properly assimilate - due to lack of real high-resolution global measurements. Increased (vertical) complexity of the real pressure distributions will cause real vertical variations of wind speed. However, detailed measurements, not assimilative models, would be required. As yet, the global measurement capability is not available to fill this requirement!
If possible, try to use the lowest level in sigma coordinates. Depending on the model or dataset, it is often sigma 0.995 or so (often 30-40 m above ground). The levels adapt to the topography and you don't end up with "sub terrain" fake winds.
http://upload.wikimedia.org/wikipedia/commons/a/a1/Sigma-z-coordinates.svg
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