Using COSP2 within GCM (technical)

[dustinswales]

For inline use of COSP2, the relevant code in the repository can be categorized into two parts:
*) Example interstitial (or coupling) code to translate the model state into optical quantities (optical-depth, LIDAR backscatter coefficients, radar reflectivity, etc...), which are then used by the instrument simulators.
*) Instrument simulators.

The idea here is to keep all of the "cosp-to-model" coupling code independent, and the "instrument-simulator" code agnostic to the overlaying host model. Organizationally this allows for all the the model dependent code to be under the host-models version control, while keeping the true COSP simulator code (src/) independent.

BEWARE: In the host-model where COSP2 is being implemented, there is a strong likelihood that different cloud-microphysical and cloud-overlap assumptions are being used when coupling the clouds to the radiation scheme. Single/Double-moment microphysical schemes, for example, provide different amounts of cloud information to the radiation, and subsequently the coupling to COSP should reflect this.

An example of this in CESM2(CAM5), where there is radiatively active snow (i.e. The cloud-MP scheme makes snow and is accounted for in RRTMG). In the "example" code provided with COSP there are only two cloud-hydrometeor types, liquid and ice. So to preserve this same cloud-2-radiation coupling in COSP, we needed to add the computation of the snow optical properties.

In this case, the example code cosp_optics.F90 was extended to include cloud snow in the optical computations, https://github.com/ESCOMP/CAM/blob/cam_cesm2_2_rel/src/physics/cosp2/optics/cosp_optics.F90
Since this is a "CAM thing", it is under CAMs version control.

If you look at CAMs COSP2 code,

you will see that all of the subsample_and_optics_example/ was copied here, and built upon for CAMs MP needs.

The instrument simulator code, src/, is imported directly to the CAM code as a submodule. (You can't see this directly on github.com, but if you "cloned" the CESM2 repository, it would check out the COSP2 submodule src/ into CAM/src/physics/cosp2)