Solar Penetration Schemes

[fobryan3]

One hypothesis for the warm bias over the SE Pacific stratus region in MOM6 that did not appear in POP is that the solar penetration scheme we are using (Manizza) has larger opacity for a given Chlorophyll concentration. It transmits less energy through the topmost layers, trapping the heating nearer to the surface. This is illustrated in the following figure comparing 4 schemes:Paulson & Simpson, Manizzza, Morel&Antoine and Ohlmann. The Ohlmann scheme has higher transmission in the upper 50m or so, especially for oligotrophic waters as found in the subtropical gyre of the SE Pacific.

This shows it a different way with the transmission through 2.5m (bottom of layer 1) and 10m (bottom of layer 4) as a function of Chlorophyll concentration.

We can see the effect of this for a hypothetical MOM6 65 layer column with incident SW flux of 200W/m^2. The heating rate (the derivative of the transmission curve) is considerably lower at the surface for Ohlmann compared to the other schemes. The heating rate from Ohlmann actually becomes slightly higher than the others below the first layer, but integrated over the top 10m (the vertical bars) it is still somewhat lower. This suggest that we may be getting more heating both directly from the solar penetration depth and indirectly from the impact of this on stratification shallowing the mixed layer.

It seems worthwhile testing the sensitivity of the simulations to the Ohlmann scheme. My initial assessment is that this is a relatively straightforward coding task. I have a first cut of a modified MOM_opacity.F90 ready to try. As an aside, in the process of scoping this out I found a number of questionable chioces in the implementation of the Morel and Manizza schemes. It looks like they were imlemented with the idea that the first layer would be O(10m) thick, ignoring certain terms in the published parameterizations.