[MITgcm-support] The Net Heat Flux

Tue May 30 18:12:32 EDT 2017

Hi Saeid

Have you read this external forcing documentation page?

http://mitgcm.org/sealion/online_documents/node261.html

Anybody please correct me if I am wrong.

If I read that right, energy/heat fluxes (SW, LW, SH, LH)
are positive if they are upward (out of the ocean).
In this convention energy losses by the ocean are positive.
I.e., if you define energy flux as the dot product of an energy current
(such as the Poynting Vector for SW and LW) with the unit normal
vector to the surface, then the MITgcm convention is that the
unit normal vector points upward (out of the ocean).
Formally:
Qnet = SW + LW + LH + SH

By contrast, mass fluxes, don't seem to follow such an uniform
sign convention.
Precipitation and runoff are positive when they add mass to the ocean 
(as they normally do), i.e., lead to a decrease in salinity.
However, evaporation is positive if it removes
mass from the ocean (as it usually does), i.e. leads to a salinity increase.
So, mass fluxes are not simply characterized by a dot product of a mass 
current with a normal vector to the surface. (Or, you would
have to switch the direction of that vector,
depending on whether you're dealing with precip & runoff,
or dealing with evap.)
Formally:
EmPmR = Evaporation - Precipitation - Runoff

In any case, you need to check the data that you're using
to produce your Qnet (and EmPmR),
because it may or may not follow the same sign convention
used by the MITgcm.

***

There is also a Matlab script
(written by Martin Losch, I think) that calculates Qnet,
and EmPmR from data.
The script is here:
http://wwwcvs.mitgcm.org/viewvc/MITgcm/MITgcm/verification/tutorial_global_oce_latlon/diags_matlab/ncep2global_ocean.m?revision=1.3&view=markup

[I think the script uses the old style of Matlab netCDF functions,
so it needs an update, but that is beyond the point.]

The script has these lines of code:

*******************************
evap = lhflx/ql_evap/rho_fresh;

qnet = solr + lwflx + lhflx + shflx;

emp = evap - precip;
% substract the runoff (doesn't lead anywhere, unfortunately)
empmr = emp - change(runoff,'==',NaN,0)/rho_fresh/onemonth;
runoff(find(isnan(runoff)))=0;
empmr = emp - runoff/rho_fresh/onemonth;

******************************

Again, this boils down to

Qnet = SW + LW + LH + SH

and

EmPmR = Evaporation - Precipitation - Runoff

In an 2007 email to this list, Martin indicates that he used NCEP
data comes from the IRI Data Library:

http://mitgcm.org/pipermail/mitgcm-support/2007-February/004598.html

Specifically, that email points to this data source:

http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/.MONTHLY/.Diagnostic/.surface/

which turns out to be NCEP Reanalysis data.

However, there are downward, upward, and net versions of shortwave and 
longwave, and the email is not specific about which were used.

I assume that the net SW and LW were used to calculate Qnet.
With this assumption, the NCEP Reanalysis data seem to match the
MITgcm sign convention.
I.e. SW has negative values (solar radiation enters the ocean),
and LW has (mostly) positive values (the ocean emits infrared radiation):

SW:
http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/.MONTHLY/.Diagnostic/.surface/.net/.solr/figviewer.html?map.url=X+Y+fig-+colors+coasts+lakes+-fig

LW:
http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/.MONTHLY/.Diagnostic/.surface/.net/.lwflx/figviewer.html?map.url=X+Y+fig-+colors+coasts+lakes+-fig

In addition, the latent heat flux is mostly positive in the tropics
(where the ocean loses heat by evaporation),
and negative in high latitudes.
The sensible heat flux is mostly positive over
the ocean in the winter hemisphere (where the ocean warms up the 
atmosphere above), but may be negative in the summer hemisphere.
Hence, NCEP Reanalysis LH and SH also seem to match the MITgcm sign 
convention (ocean energy losses are positive).

LH:
http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/.MONTHLY/.Diagnostic/.surface/.latent/.heat_flux/figviewer.html?map.url=X+Y+fig-+colors+coasts+lakes+-fig

SH:
http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/.MONTHLY/.Diagnostic/.surface/.sensible/.heat_flux/figviewer.html?map.url=X+Y+fig-+colors+coasts+lakes+-fig

Hence, with this data, which follows the MITgcm sign convention,
the Qnet formula is a straightforward sum of four items (although some
of the items may have negative values in some locations and times).

Qnet = SW + LW + LH + SH

**

Now for the EmPmR.

Evaporation is calculated from the latent heat above, hence it matches 
the MITgcm sign convention, i.e. positive when the ocean loses mass
(by evaporation and latent heat loss) and the salinity increases.

NCEP Reanalysis precipitation is all positive, which again matches the 
MITgcm convention, i.e. positive when the ocean gains mass and salinity
decreases.

NCEP Reanalysis runoff is all positive, which again matches the MITgcm 
convention, i.e. positive when the ocean gains mass and salinity decreases.

Precipitation rate (prate):
http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/.MONTHLY/.Diagnostic/.surface/.prate/figviewer.html?map.url=X+Y+fig-+colors+coasts+lakes+-fig

Runoff:
http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP-NCAR/.CDAS-1/.MONTHLY/.Diagnostic/.surface/.runoff/figviewer.html?map.url=X+Y+fig-+colors+coasts+lakes+-fig

Therefore, with this data that follows the MITgcm sign convention,
the formula is simply the same as above:

EmPmR  = Evaporation - Precipitation - Runoff

By the way, note that the NCEP Reanalysis runoff data exists only over 
land (and sea-ice).
That may explain the comments in Martin's script:
"% substract the runoff (doesn't lead anywhere, unfortunately)"
NCEP Reanalysis may not be the best runoff data set
for ocean modeling work.
Other data sets exist, for example, 
http://data1.gfdl.noaa.gov/nomads/forms/mom4/COREv2/CIAF_v2.html

**

Again, by all means, check the data that you're using, to make sure
it matches the MITgcm sign convention. If not, flip the specific
sign or signs in the corresponding formula above.

By the way, you can download that NCEP Reanalysis
data from the IRI Data Library (links above, click on the "Data Files" 
tab) or from NCEP 
(https://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.html)

I hope this helps,
Gus Correa

On 05/30/2017 01:01 PM, Dimitris Menemenlis wrote:
> A very useful description of surface forcing MITgcm diagnostics is here:
> http://wwwcvs.mitgcm.org/viewvc/MITgcm/MITgcm/doc/diags_changes.txt
> 
>> On May 29, 2017, at 2:01 PM, smaeilpour <saeid_gal at yahoo.com 
>> <mailto:saeid_gal at yahoo.com>> wrote:
>>
>> Hi everyone,
>>
>> for calculating The net heat flux at the ocean-atmosphere interface, 
>> which two equations below are correct?
>>
>> (1) QNet=Qshortwave-Qlongwave-Qlaten-Qsensible
>> (2) QNet=Qshortwave+Qlongwave+Qlaten+Qsensible
>>
>> because references have mentioned both of them.
>> the values of my data flux into all Q are either negative or positive. 
>> should i have to add them as Algebraic sum [as (1) equation]? or i 
>> have to subtract them from each other [as (2) equation]?
>> Thank you in advance for any help.
>>
>> cheers,
>> saeid
> 
> 
> 
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