Add comments for coded tendency terms to aid interpretation

[BrodiePearson]

The current tendency terms are decomposed into various terms that arise from a range of physical processes, e.g.

MPAS-Model/src/core_ocean/shared/mpas_ocn_adcReconstruct.F

Lines 565 to 569 in 5dc8d6e

	w2tend1(k,iCell) = -wumd(k,iCell)**2.0_RKIND*( &
	Entrainment(k,iCell) + Detrainment(k,iCell))
	w2tend2(k,iCell) = (McMid(k-1,iCell)*(1.0_RKIND - 2.0_RKIND* &
	areaFractionMid(k-1,iCell))*wumdMid(k-1,iCell)**2.0 - McMid(k,iCell)* &
	(1.0_RKIND - 2.0_RKIND*areaFractionMid(k,iCell))*wumdMid(k,iCell)**2.0) / dzmid

Because these budgets involve several closure assumptions, the physical meaning of each of these tendency terms is not always obvious. We should comment the code to make it clear which physical processes are encompassed in different tendency terms. This will be based on @qingli411's commented code.

For future reference: changing the tendency terms, and adding them as save-able output variables requires the following steps:

1. Add the new tendency term in main ADC code:

   MPAS-Model/src/core_ocean/shared/mpas_ocn_adcReconstruct.F

   Line 566 in 103e68e

   w2tend1(k,iCell) = -wumd(k,iCell)**2.0_RKIND*( &

2. Add the new term in the ADC registry in 2 places:

   MPAS-Model/src/core_ocean/adc_mixing/Registry_adc_mixing_fields_opts.xml

   Lines 269 to 270 in 103e68e

   	<var name="w2tend1" type="real" dimensions="nVertLevelsP1 nCells Time" units="m^2/s^3"
   	description="entrainment production term -- similar to dissipation"

   MPAS-Model/src/core_ocean/adc_mixing/Registry_adc_mixing_fields_opts.xml

   Line 576 in 103e68e

   <var name="w2tend2"/>

3. Add the new term to mpas_ocn_turbulence.F in various places.
4. Add the new term to mpas_ocn_vmix_adc.F for deallocation:

   MPAS-Model/src/core_ocean/shared/mpas_ocn_vmix_adc.F

   Line 531 in 103e68e

   w2tend1, w2tend2, w2tend3, w2tend4, &

5. If the new term is an output variable, add it to the streams.ocean file of the forward run directory

[vanroekel]

@BrodiePearson are you able to address this? I think it would be good to have. Note that the terms are somewhat explained in the adc registry file.

[BrodiePearson]

@vanroekel Yes, and I had not noticed their registry appearance so thanks for that! I'll try and do this in the next few days. I do have a question:

@katsmith133 & I discussed re-arranging/naming the tendency terms based on their physics (i.e. w2tend_entrain, w2tend_transport, w2tend_buoyancy, w2tend_pressure_isotropy/w2tend_pressure_slow, w2tend_pressure_shear/w2tend_pressure_rapid, w2tend_splat). Would you support this approach? This would reduce the amount of commenting required and also make it easier to directly compare budgets between the LES and ADC scheme (e.g. Issue #13)

[BrodiePearson]

I'm going to hold off on this issue until we figure out #18