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Help with increasing vertical momentum diffusion?

minminfu

Member
Dear All,

I am interested in increasing the vertical eddy viscosity in CAM5.3 (using diag_TKE) to study Hadley cell dynamics. My goal is to introduce more viscosity, so as to weaken the subtropical jet. The best way I have found so far is through increasing the namelist variable "kv_freetrop_scale", which scales up the free tropospheric eddy diffusivity (in vertical_diffusion.F90). I feel this should be an effective way of increasing vertical momentum diffusion, and I can study the output variables "DUV" and "KVM" to understand how much the zonal velocity is being diffused and what the diffusivity is.

Although the KVM increases linearly with kv_freetrop_scale, as expected, actual momentum diffusion (output variable DUV) appears only very weakly sensitive to kv_freetrop_scale. I have found by default DUV is ~ -0.3 m/s/day near the midlatitude upper troposphere (acting to retard the jet stream). When I increase kvm tenfold, it becomes ~ -0.6, by 100x, ~ -1.3, and by 1000x, -2.3m/s/day.

I am confused why the velocity diffusion is not responding linearly (is it responding logarithmically?) to kv_freetrop_scale. I wonder if I am perhaps missing something simple here. Is anybody able to explain why momentum diffusion is behaving this way, or if there is a better way to achieve the goal I have in mind? Your help is much appreciated!

Minmin Fu
 
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