stilt.transforms.particle_pwf

stilt.transforms.particle_pwf(particles, surface_pressure=None)

Derive each particle’s release pressure and pressure weight.

Follows X-STILT’s get.wgt.*.func: fit a hypsometric curve ln p = b + a·z to the particles’ first-step heights and pressures (this smooths the one-time-step offset from the true release state and yields a surface pressure when none is supplied), evaluate it at each particle’s release height, and turn the spacing between neighbouring release pressures into the air mass each particle represents.

HYSPLIT spreads column particles evenly over height, each one randomized within its own 1/numpar slab, so a particle stands for the slab centred on it: the cell edges sit midway between adjacent release pressures, the surface closes the bottom, and the topmost cell mirrors its lower half-width. (X-STILT instead gives each particle the layer below it, which shifts every weight down by half a cell and leaves the lowest particle with almost none.)

Returns (xpres, pwf) indexed by indx. pwf sums to the fraction of the atmosphere’s mass the column covers, (p_sfc - p_top) / p_sfc; the rest lies above the column top, where surface fluxes cannot reach the receptor within the back-trajectory.

Return type:

tuple[Series, Series]

Parameters:

• particles (DataFrame)

• surface_pressure (float | None)