Penetrating radiation temperature update

[MarcusGastaldello]

I'm not sure that lines 77 - 82 in 'penetratingRadiation.py' are accurate:

T_rad = float( GRID.get_node_temperature(idxNode) + (E[idxNode] / (GRID.get_node_density(idxNode) * spec_heat_ice)) * (dt / GRID.get_node_height(idxNode)))

I think there are possiblity two issues with this equation:

1. The module runs for each model timestep and is supplied with the net shortwave radiation; 10 - 20% of which is apportioned as energy ' E ' to heat subsurface layers. This energy is a fixed quantity not 'power' therefore there is no need for ' dt ' - there should be no time element in this equation.

2. Should the combined node specific heat not be used?

Therefore I think the equation should be:

T_rad = float(GRID.get_node_temperature(idxNode) + (E[idxNode] / (GRID.get_node_density(idxNode) * GRID.get_node_specific_heat(idxNode) * GRID.get_node_height(idxNode))))

This has the effect of massively reducing subsurface melt quantities - if you're using hourly timesteps it reduces it by 3600.

Would be great to discuss if anyone has a chance.

Marcus

[Richteny]

Hey Marcus,

Interesting questions, and I am not 100% sure I have the correct answer to both, but I'll try anyway:
Re 1: The equation is basically old temperature + change in temperature. Since SWnet is provided in W/m² (J $s^{-1}$ $m^{-2}$), it is indeed 'power' (a rate), and to get the total amount of energy that has been delivered over the hourly timestep we need the dt. Or why do you think it is a fixed quantity?

Re 2: I think we assume a subfreezing, dry snowpack that does not contain liquid water and thus can use the constant. Maybe that is a remnant from the Bintanja and van den Broeke (1995) paper from Antarctica? But I have no actual clue about what would be best here, sorry!

Cheers,
Niklas

[MarcusGastaldello]

Hi Niklas,

1. Yes I've just realised I've got confused with regard to the first point - it is indeed a power / rate. The 'dt' value is correct!

2. I'm not too sure either but I presume it should consider warming the mass of the entire node:

GRID.get_node_density(idxNode) * GRID.get_node_specific_heat(idxNode) * GRID.get_node_height(idxNode)

If typically a near surface node has a low ice fraction, it would also have a much lower combined specific heat capacity of around 500 J kg-1 K-1 because of the high air content and therefore have a 4x greater increase in temperature?

I might have a look into this and investigate.

Cheers,

Marcus