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

Bill Sacks sacks at ucar.edu
Wed Dec 5 13:09:46 MST 2018


Hi all,

Okay, I just talked with Bill Lipscomb about this, and his understanding 
is that the current behavior is correct. i.e., the fact that we might 
have this:

> 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.
is actually intentional and desired. (Though I should clarify that 
"receive no precipitation" really means, "precipitation immediately runs 
off".) His justification is: If CAM is generating rain at a given 
elevation / temperature, that doesn't necessarily imply that an equal 
water equivalent of snow would be generated at a higher elevation / 
lower temperature: indeed, in reality, there might not be any 
precipitation falling at that higher elevation / lower temperature.

So as long as others agree, I will stick with what I have now. I want to 
do some more testing, then will send out the branch for others to try out.

Bill S

Bill Sacks wrote on 12/5/18 10:46 AM:
> 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 
>> 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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