|
E. Linacre and B. Geerts |
10/'97 |
Data given by Linacre (1), summarised in Table 1.1 of the present book and expanded in Table A below, show typical timescales and spatial dimensions of various atmospheric processes. Corresponding figures have been given by Abbott et al. (2) for processes within the oceans (Table B). It is interesting to conmpare the atmospheric and marine data.
Table A. Scales of atmospheric processes
|
scale |
domain |
relevant features of the atmosphere |
typical horizontal dimension |
typical duration |
rate* (m/s) |
|
global |
Earth |
solar radiation, general circulation |
20,000km |
1 month |
7.7 |
|
synoptic |
continent |
frontal weather, weather forecasting, tropical cyclone |
1,000km |
3 days |
3.9 |
|
mesoscale |
region |
thunderstorm, seabreeze |
50km |
4 hours |
3.5 |
|
toposcale |
locality |
a thermal, cumulus cloud, rainfall |
2km |
1 hour |
0.6 |
|
microscale |
site |
irradiance, evaporation, cloud physics, gusts |
10m |
1 minute |
0.2 |
* the rate is calculated as typical horizontal dimension divided by duration; it is a measure of advection velocity causing the change
Table B. Scales of oceanic processes (2).
|
processes |
typical horizontal dimension |
duration |
rate (m/s) |
|
deep ocean ‘conveyor circulation’ |
10,000 km |
30 years |
1.1 x 10-2 |
|
ENSO |
1,000 km |
3 years |
1.1 x 10-2 |
|
upwelling, eddies |
10 km |
30 days |
3.9 x 10-3 |
|
turbulent diffusion |
100 m |
1 day |
1.2 x 10-3 |
|
turbulent mixing |
1.0 m |
2 hours |
1.4 x 10-4 |
|
dissipation |
0.1 m |
15 minutes |
1.1 x 10-4 |
The rates in Table B are much slower than those in Table A. Across all scales, atmospheric processes are about 3 orders of magnitude faster than those in the oceans. This is essentially because the air density about 1,000 times less than that of water. Also, both in the atmosphere and in the ocean, larger-scale phenomena tend to evolve and/or move faster.
References
(1) Linacre, E.T. 1992. Climate Data & Resources (Routledge) p.12.
(2) Abbott et al. 1996. Marine biotic responses to environmental change and feedbacks to climate. In Houghton et al. 1996, 483 - 516 (3).