Scales of Atmospheric Motion. The atmosphere features a wide range of circulation types, with a wide variety of different behaviors

Transcription

1 Scales of Atmospheric Motion The atmosphere features a wide range of circulation types, with a wide variety of different behaviors Typically, the best way to classify these circulations is according to: - Their size (or spatial scale); and/or - Their oscillation period or duration (or time scale)

2 Scale Category microscale mesoscale synoptic scale planetary scale Time Scale seconds to minutes Spatial Scale meters to 1 km minutes to kilometers to hours to 1 day hundreds of km days to weeks weeks to months thousands of km global Examples turbulence, small cumulus clouds thunderstorms, sea breezes, mountain circulations fronts, cyclones, anticyclones planetary waves, el niño

3 Scale Category microscale mesoscale Time Scale seconds to minutes minutes to hours to 1 day synoptic scale days to weeks planetary scale weeks to months Spatial Scale meters to 1 km kilometers to hundreds of km thousands of km global Examples turbulence, small cumulus clouds thunderstorms, sea breezes, mountain circulations fronts, cyclones, anticyclones planetary waves, el niño

4 Example Circulation: Microscale turbulence in the boundary layer small cumulus clouds / turbulent eddies

5 Scale Category microscale mesoscale Time Scale Spatial Scale Examples seconds to minutes meters to 1 km turbulence, small cumulus clouds minutes to kilometers to hours to 1 day hundreds of km synoptic scale days to weeks planetary scale weeks to months thousands of km global thunderstorms, sea breezes, mountain circulations fronts, cyclones, anticyclones planetary waves, el niño

6 Example Circulation: Mesoscale thunderstorms and collections of thunderstorms individual storms and their component parts

7 Example Circulation: Mesoscale mountain circulations (lee vortices in this case)

8 Example Circulation: Mesoscale sea breeze circulations

9 Scale Category Time Scale Spatial Scale Examples microscale seconds to minutes meters to 1 km turbulence, small cumulus clouds mesoscale minutes to hours to 1 day kilometers to hundreds of km thunderstorms, sea breezes, mountain circulations synoptic scale days to weeks planetary scale weeks to months thousands of km global fronts, cyclones, anticyclones planetary waves, el niño

10 Example Circulation: Synoptic scale high and low pressure systems, warm and cold fronts most of what we consider day-to-day weather

11 Scale Category Time Scale Spatial Scale Examples microscale seconds to minutes meters to 1 km turbulence, small cumulus clouds mesoscale minutes to hours to 1 day kilometers to hundreds of km thunderstorms, sea breezes, mountain circulations synoptic scale planetary scale days to weeks weeks to months thousands of km global fronts, cyclones, anticyclones planetary waves, el niño

12 Example Circulation: Planetary scale climate patterns (e.g., el niño / la niña) planetary-scale waves

13 Scale Category Time Scale Spatial Scale Examples microscale mesoscale synoptic scale synoptic and planetary scales together planetary scale are often referred to as large-scale

14 and if you want even bigger... Jupiter's Great Red Spot (3x the size of Earth)

15 The Geostrophic Balance For large-scale (synoptic and planetary) circulations, it turns out that the PGF and Coriolis force (CF) are often very nearly in balance. Why?

16 The Geostrophic Balance For large-scale (synoptic and planetary) circulations, it turns out that the PGF and Coriolis force (CF) are often very nearly in balance. Why? Well, large-scale flows evolve very slowly, so there's lots of time for the forces to equilibrate (Note: this isn't usually true for mesoscale and microscale flows.)

17 To make the balance work, we'll need the following to be true:

18 To make the balance work, we'll need the following to be true: The PGF acts perpendicular to the isobars (on a surface map) or the height contours (upper-level chart), directed from high values to low values

19 To make the balance work, we'll need the following to be true: The PGF acts perpendicular to the isobars (on a surface map) or the height contours (upper-level chart), directed from high values to low values To be in balance, the Coriolis acts opposite the PGF

20 To make the balance work, we'll need the following to be true: The PGF acts perpendicular to the isobars (on a surface map) or the height contours (upper-level chart), directed from high values to low values To be in balance, the Coriolis acts opposite the PGF The resulting wind must be parallel to the isobars or height contours, with higher values to the right (in the Northern Hemisphere)

21 One important consequence of this balance is: wind L PGF CF H CF PGF wind

22 At large scales, circulation is counter-clockwise (or cyclonic) around low pressure centers in the NH. But in the SH...

23 In the SH, everything switches and the flow becomes clockwise around lows. But we still call it cyclonic...circulation around lows is always cyclonic.

24 Friction Near the ground, we have one additional force to worry about: friction. It turns out that friction is only important in the lowest km or so near the ground

25 Friction Near the ground, we have one additional force to worry about: friction. It turns out that friction is only important in the lowest km or so near the ground (For full disclosure, what we're calling friction here is not really friction...it's turbulence. But the effects of turbulence are very similar to the effects of friction.)

26 The net effect of friction is to: L PGF CF wind H

27 The net effect of friction is to: (i) slow down the wind, which... L PGF CF wind H

28 The net effect of friction is to: (i) slow down the wind, which... (ii) weakens the Coriolis force, so that... L PGF CF wind H

29 The net effect of friction is to: (i) slow down the wind, which... (ii) weakens the Coriolis force, so that... (iii) the PGF dominates and the wind veers toward lower pressure L PGF CF wind H

30 The end result is that air converges into low pressure centers and diverges away from high pressure centers at the ground L H But keep in mind, friction is only important in the lowest km or so (i.e., not on upper-level pressure surfaces)