[Liwg-core] A possible unintended (and bad!) consequence of the P1 modification

Bill Sacks sacks at ucar.edu
Wed Dec 5 10:09:37 MST 2018

Hi all,

While writing my commit message documenting the P1 modification (in cold 
temperatures, make rain run off rather than converting it to snow), I 
realized a possible unintended consequence: The rain-snow repartitioning 
has two purposes:

(1) Downscaling to elevation classes: changing the balance between rain 
and snow for different elevation classes

(2) Correcting problems in CAM

We've been focused on (2), but I don't remember hearing any discussion 
of the implications for (1), and I'm wondering if that is an unintended 
consequence of this change.

For example, imagine a grid cell where CAM is generating rain at a grid 
cell mean near-surface temperature of 3°C; CAM's topographic height here 
is 1000 m. On the CLM side, we have two elevation classes, one at 0 m 
(50% of grid cell) and one at 2000 m (50% of grid cell). In the 2000 m 
elevation class, the downscaled temperature will be –3°C. This will 
result in all rain being converted to snow.

However, with the new formulation, in the higher elevation class, rather 
than receiving precipitation in the form of snow, it would instead 
receive no precipitation! To me, this feels really bad, and really hard 
to justify in a paper.

A possible way around this would be to separate pieces (1) and (2), with 
an algorithm like this:

(a) Repartition grid cell mean precipitation using grid cell mean 
atmospheric temperature. If this would result in rain-to-snow 
conversion, instead make that amount run off.

(b) Downscale precipitation to columns, repartitioning using downscaled 
column atmospheric temperatures. At this stage, any rain-to-snow 
conversion would remain snow (as it has in the past).

I think it wouldn't be too hard to implement that algorithm: it would 
probably take an extra day or two for me to implement and test. However, 
there are a couple of issues that immediately come to mind:

(i) We use different parameters for the precipitation repartitioning 
over glacier vs. non-glacier columns. In step (a), we are working on the 
grid cell mean, so which parameters should we use?

(ii) This has the potential to give fairly different results from what 
was tested for P1. In some instances (like the above example), we'll end 
up with more snow; in other instances we could end up with less snow 
than with the original P1 implementation.

Please let me know your thoughts on this.

Bill S
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