wet radius initialisation (with examples from 1D kinematic example)

[jbarr444]

Hi Sylwester,

Working with the Shipway/Hill example. I find strange behavior with the particle size distribution at time 0, where the wet and dry spectra instantaneously differ dramatically at a certain height corresponding to the maximum initial relative humidity. I think this growth is far beyond equilibrium, so I'm puzzled. Can you explain what's going on under the hood?

wet spectrum: https://github.com/open-atmos/PySDM/assets/166780711/b306d37f-4e0b-471e-abe4-6d2616b5336b RH vs z: https://github.com/open-atmos/PySDM/assets/166780711/7688e95e-56f1-4fbd-9f0f-39c72bb9afa7

Thanks, Jason

[slayoo]

Hi Jason,

Thanks for reporting it. Could you elaborate on how the "dry spectra instantaneously differ"?

Trying with a much lower kappa could help to check if this is not a matter of high hygroscopicity?

The way the RH profile is plotted might be a bit misleading here, as we do not spatially interpolate RH for super particles; plotting a stepwise constant profile could help to connect the two plots.

HTH, Sylwester

[jbarr444]

Hi Sylwester, thanks for the reply,

Here's the dry spectrum: https://github.com/open-atmos/PySDM/assets/166780711/bb6f3520-62f0-4291-8f24-afab0d212049

Notice how there's no 'kink' at 740 m. I'm wondering how the initial wet spectrum is calculated from the initial dry spectrum, as clearly it is not assumed that they are the same to begin with. I would think perhaps then you start at equilibrium with respect to condensation/evaporation, but this also does not seem to be the case, though I could be wrong about that.

Lowering the hygroscopicity from 0.9 to 0.1 shifts the wet sizes closer to the dry spectrum, but the 'kink' remains: https://github.com/open-atmos/PySDM/assets/166780711/1a51676e-368a-4135-a731-9a8019d67367

[slayoo]

The dry spectrum is set to constant with altitude in this example, so the only variability will originate from random sampling - consistent with your plot.

The "kink" in the wet spectrum comes from exactly what you indicate - the equilibrium wrt ambient RH, which is solved for at initialization here:

• https://github.com/open-atmos/PySDM/blob/4de8327e57ccbe1035d531de0156c50a9895e236/examples/PySDM_examples/Shipway_and_Hill_2012/simulation.py#L95-L103
• https://github.com/open-atmos/PySDM/blob/4de8327e57ccbe1035d531de0156c50a9895e236/PySDM/environments/kinematic_1d.py#L73-L78
• https://github.com/open-atmos/PySDM/blob/4de8327e57ccbe1035d531de0156c50a9895e236/PySDM/initialisation/equilibrate_wet_radii.py#L16-L25

HTH, S.