centure7-ga
You have interesting but complex questions.
It would be helpful (and it may increase the rate at which your questions
are answered) if you would rate the answers.
Rating helps researcher to gage how useful was his/her the
contribution to the answer in those situations when full, exhaustive
answer is not possible.
(This is one of those situations ;-)
I will focus on the physical processes which determine the rate of evaporation.
That will provide a handle on, what the units are and will provide a
complement to the empirical study referenced in the comment.
But first, some actual numbers for the ocean:
-------------------------------
" The average amount of evaporation F.e from the sea
surface is about 120 cm/yr, i.e. the equivalent of the sea surface
sinking by that amount. Local values range from an annual minimum of
as little as 30 to 40 cm/yr in high latitudes to maxima of 200 cm/yr
in the tropics associated with the trade winds. This decreases to
about 130 cm/yr at the equator where the mean wind speeds are lower.
How is the evaporation rate Fe determined? A direct method is to
determine the rate of water loss from a pan of water, but this has
serious practical difficulties. For large area estimates and for
prediction, a formula using easily measured parameters is desirable.
Evaporation is basically a diffusive process that depends on how water
vapor concentration changes with height above the sea surface and on
the processes that cause diffusion. In section 5.1.3, we discussed
"eddy diffusion" which is analogous to molecular diffusion, except
that the turbulence in the air or water is considered to be the
process that diffuses properties, rather than movement of individual
molecules. The air turbulence is the process that controls the
diffusion that creates evaporation, and this air turbulence depends on
wind speed. Therefore we expect the evaporation rate to depend on
wind speed. (This explains why we feel cooler when the wind is
blowing which creates the effect known as 'wind chill' "
This is quote from section 5.4.6 of excellent text:
CHAPTER 5 Water, Salt and Heat Budgets of the Oceans
http://www-pord.ucsd.edu/~ltalley/sio210/pickard_emery/chapter_5_917.htm
whole book
http://www-pord.ucsd.edu/~ltalley/sio210/pickard_emery/
and books
http://oceanworld.tamu.edu/ocean410/ocng410_text_book.html
and projects described on the site , such as
http://www-pord.ucsd.edu/po/research/TS.html
may be of interest for this and the for the related question
http://answers.google.com/answers/threadview?id=514897
I now want to focus on the theoretical aspect of the evaporation:
----------------------------------------------
What does it mean " Evaporation is basically a diffusive process?"
The search term for the fundamental mechanism is: Stefan Problem
named after Slovenian physicist ( J. Stephan)
http://www.answers.com/topic/joseph-stefan
The quote from his book, given in the above link:
"There always something will remain, that we shall not know, why?"
is appropriate comment (sigh) to your question.
One reason 'why' is that even if we would know all the physical
parameters of the phase transition (evaporation, melting, .. ) that is
latent heat of the transition, diffusion rates,heat conductivity, ...
,we still would need to solve the transport equations, to determine
the rate of transition, speed of the boundary.
OK - let me try to put it more simply:
The (textbook) example of the Stefan problem is an ice cube, melting
in the glass of water.
Melting is phase transition (just like evaporation) and the boundary
between the phases (surface of the cube) is moving (in either
direction, depending on temperatures). The speed of the moving
boundary depends on how quickly the fluid phase can supply the heat
(and in same cases material) needed for the transition. This is how
winds into play. Heat comes by conduction, radiation (sun rays) and
convection and water vapour is carried away by wind (convection) and
diffusion (condiction).
When you look at the 'big' picture:
http://www.physicalgeography.net/fundamentals/8b.html
you will notice that while water evaporates and vapor is carried away
by the wind, some of that water returns to the ocean right away. Only
small part comes back through rains over land and runoff.
How much? That depends on the local conditions -- it is different near
coast and in the open sea.
For large scale (global) model (cumulative) numbers are known:
361 Tm^3 evaporates and 324 returns directly (by rain over the
ocean), according to
http://ww.mines.edu/Academic/ courses/dc/dcgn101/StreamsRivers.doc
Here Tm^3 means Tera-cubic-meters; T= E12 is SI prefix for million of
millions as defined here
http://physics.nist.gov/cuu/Units/prefixes.html
So, at one spot, water may evaporate and be carried away by winds; at
another spot it may rain all the time and that same water is returned
to the ocean.
This distances (from one sot to the other spot) affects the 'net evaporation':
The scale of the model and the grid (cell size) needs to be specified
before 'net evaporation rate' is well defined.
So, while local studies, such as the one cited in the comment are
possible, and constants determined empirically may reflect local
miroclima, and have some temporary and limited validity, true answers
are tied to global circulation models (both water and atmosphere),
which are a topic of an ongoing research,
are far from completion and very complex.
That is one reason why Stephan's sigh is still valid.
Hope this is useful. Questions welcome, rating appreciated.
Hedgie |
