Unphysical behavior in the altitude correction for precipitation in the aws2cosipy module

[simsegoet]

Hi,

I have encountered an issue in the aws2cosipy module on how precipitaion (both liquid and solid) is corrected for different elevations.

Currently, the correction is done in the set_bias function (aws2cosipy line 174) using the folowing formula:

biased_data = data + altitude * _cfg.lapse[lapse_type]

with _cfg.lapse[lapse_type] being a lapse rate.

At locations above the input AWS this leads to an behavior that feels a bit unphysical since it just adds a value to every single timestep resulting in precipitation events at every single timestep, you can see the results in the plot below where the orange line is the data from an aws and the blue line is calculated using aws2cosipy at a location above the aws.

Maybe switching to a multiplicative scheme for precipitation would be a good idea, for example:

biased_data = data + data * altitude * _cfg.lapse[lapse_type]

or something more elaborate.

Thank you!

[Richteny]

Hi Simon,

Good find! I am still not 100% sure what the best approach is to this. In some way, having precipitation at higher elevations is "physical", and I can imagine real-life scenarios where that would be the case. However, I do agree that this is, probably more often than not, not desired and in combination with missing processes such as snowdrift creates quite a strong overestimation at higher altitudes.

Maybe both options for the lapse rate application would be a good idea, and raising a warning about the user's choice?
And then for simplicity's sake, we could apply the lapse rate conditionally, e.g.:

biased_data = np.where(data > 0, data + altitude * lapse_rate, 0).