the Longwave radiation of Bulkflux

[Rocky_Wang]

Hi everyone,

I am using the ROMS in the COAWST model to simulate the temperature of some parts of the South China Sea, but I find that the temperature of the model is dropping rapidly. Figure 1 is the sea surface temperature of 195 days in a year of the HYCOM model, and Figure 2 is the simulation result of the model. It can be seen that the difference is about 10 degrees after only 195 days of simulation. So I'm checking to see if the thermal radiation in the forceing field is correct. Here is the CPP scheme I used:

# define MIX_GEO_TS
# define BULK_FLUXES
# define ANA_BTFLUX
# define ANA_BSFLUX
# define ATM_PRESS
# define EMINUSP
# define SOLAR_SOURCE

In my understanding, I did not turn on the LONGWAVE_OUT switch, which means that net longwave radiation flux and net shortwave radiation flux need to be provided. Figure 3 shows the shortwave radiation at a certain moment in the ROMS atmospheric forcing file, which is positive during the day and close to zero at night. FIG. 4 shows the net longwave radiation in the forcing file of the ROMS model, which is negative most time. Figure 5 is the original data (Surface net Thermal Radiation) downloaded from ERA5. The ERA5 downward fluxes are positive, and net flux refers to the downward flux minus the upward flux that is negative most time. It is the same moment as Figure 4, but the unit is different. I divide this by 3600s to change the cumulative flux, to the instantaneous flux.

So in my model, net shortwave radiation is positive and net longwave radiation is negative. I think it is reasonable, but my model temperature decreases rapidly. My friend told me that the net shortwave radiation in his model is positive, but the net longwave radiation is also positive, and the temperature he simulated is not abnormal. So I want to know whether the net long-wave radiation should be positive or negative.

In addition, I tried to change WTYPE from MIX_S_TS to MIX_GEO_TS, but none of them improved much. Besides these reasons, what else might cause the temperature to decrease rapidly?

Thank you

[wilkin]

The ROMS sign convention is that positive heat flux warms the ocean. This is so for swrad, lwrad and lwrad_down.

In your region you would typically have 400 W/m^2 of downwelling longwave radiation https://link.springer.com/article/10.10 ... /figures/4 so the fact you have negative values for net longwave is consistent with very strong heat loss from outgoing longwave emission due to the very warm SST (radiation goes like SST^4) overcoming the incoming value.

Be careful working with the ERA5 data that are given as accumulated fluxes. I have seen the data reported with accumulation intervals that exceed the output time interval, so unraveling the fluxes to W/m^2 can be tricky.

Look beyond the temperature trend in your simulation to understand the actual drivers of the heat content. You have presumably output the net heat flux calculated by Bulk Fluxes, so from this you can compute the rate of SST change assuming some typical mixed layer depth. If that is not enough to explain the SST change then you can start looking at other dynamics. You can also output the individual flux components (latent, sensible, net longwave) to see if anything there catches your eye as being odd.

[APJ]

@Rocky_Wang did you find out the reason? I am also stuck with a similar problem. But I am providing monthly mean forcings. Could this be a reason?