sensitivity of surface albedo decline with snow cover reduction in the Rockies: differences between high and low elevations

[Laurent_Terray]

Describe your problem or question:
Hello, I am using the MESACLIP simulations to assess elevation-dependent warming (EDW) in the Rocky Mountains (defined by a shapefile which goes from the Colorado Rockies up to the Canadian Rockies). I am looking at EDW for a climate at a global warming level of 4°C compared to pre-industrial. My question is about the differing sensitivities of the surface albedo decline to the reduction in snow cover fraction between the low-elevation (0-1500m) and high-elevation (2200-) regions of the Rockies. The attached figure shows that for winter and spring, there is s larger decline in albedo at high elevation (red line fit) than at low elevation (blue line fit) for the same reduction in snow cover fraction. My initial and very (too?) simple explanation is that the albedo of the soil beneath the snow has a smaller value at high-elevation (rocks, bare soil) than at low-elevation (vegetation). I would like to test if this assumption is a reasonable one or not by looking at the prescribed albedo of the different soil types. I know it is probably much more complicated (snow albedo depends on many things and can be different between low and high elevations, vegetation albedo depends on leaf area etc ...). Any advice or idea to test the hypothesis or other possible explanations would be great !
Thanks, Laurent

 

[oleson]

The prescribed soil albedo values can be found in Table 2.3.3 as shown in the technical note here:

2.3. Surface Albedos — ctsm release-clm5.0 documentation

The soil color number itself can be found on the surface dataset that you are using.
These also range between dry and saturated, so you might need to account for the top soil layer moisture as well as the prescribed values.
A description of the snow albedo parameterizations is here:

2.3. Surface Albedos — ctsm release-clm5.0 documentation

And yes, the gridcell albedo is also going to depend on the vegetation distribution, in particular, deciduous types are likely to not have much LAI in winter (more soil/snow exposed), but will probably be leafing out in spring.

 

[Laurent_Terray]

Thanks Keith, very useful. I have contacted Fred (Castruccio) to get the surface dataset that they have used to run the Mesaclip simulations but he is away until August 11th. I 'll post something on the HR forum in the mean time to see if someone else can provide the file. Also the clm version they have used is 4.5 I think, I am assuming that the prescribed soil albedo values haven't changed. Thanks again !