# [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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