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

Bill Sacks sacks at ucar.edu
Wed Dec 5 10:46:38 MST 2018

```Actually, I think I see a way around this one:

> (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?
so that we can continue to use consistent, landunit-specific parameters.

Bill

Bill Sacks wrote on 12/5/18 10:09 AM:
> 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
> 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.
>