Time stepping of particle tracking about speedyweather.jl

[milankl]

I just printed the time step and time for dynamics and particle advection (PA)

julia> run!(simulation, period=Day(1));
[ Info: (0, DateTime("2000-01-01T00:00:00"))
[ Info: ("PA:", 0, DateTime("2000-01-01T00:00:00"))
[ Info: (0, DateTime("2000-01-01T00:15:00"))
[ Info: (1, DateTime("2000-01-01T00:30:00"))
[ Info: (2, DateTime("2000-01-01T01:00:00"))
[ Info: (3, DateTime("2000-01-01T01:30:00"))
[ Info: (4, DateTime("2000-01-01T02:00:00"))
[ Info: (5, DateTime("2000-01-01T02:30:00"))
[ Info: (6, DateTime("2000-01-01T03:00:00"))
[ Info: (7, DateTime("2000-01-01T03:30:00"))
[ Info: (8, DateTime("2000-01-01T04:00:00"))
[ Info: ("PA:", 8, DateTime("2000-01-01T04:00:00"))
[ Info: (9, DateTime("2000-01-01T04:30:00"))
[ Info: (10, DateTime("2000-01-01T05:00:00"))
[ Info: (11, DateTime("2000-01-01T05:30:00"))
[ Info: (12, DateTime("2000-01-01T06:00:00"))
[ Info: (13, DateTime("2000-01-01T06:30:00"))
[ Info: (14, DateTime("2000-01-01T07:00:00"))
[ Info: (15, DateTime("2000-01-01T07:30:00"))
[ Info: (16, DateTime("2000-01-01T08:00:00"))
[ Info: ("PA:", 16, DateTime("2000-01-01T08:00:00"))
[ Info: (17, DateTime("2000-01-01T08:30:00"))
[ Info: (18, DateTime("2000-01-01T09:00:00"))
[ Info: (19, DateTime("2000-01-01T09:30:00"))
[ Info: (20, DateTime("2000-01-01T10:00:00"))
[ Info: (21, DateTime("2000-01-01T10:30:00"))
[ Info: (22, DateTime("2000-01-01T11:00:00"))
[ Info: (23, DateTime("2000-01-01T11:30:00"))
[ Info: (24, DateTime("2000-01-01T12:00:00"))
[ Info: ("PA:", 24, DateTime("2000-01-01T12:00:00"))
[ Info: (25, DateTime("2000-01-01T12:30:00"))
[ Info: (26, DateTime("2000-01-01T13:00:00"))
[ Info: (27, DateTime("2000-01-01T13:30:00"))
[ Info: (28, DateTime("2000-01-01T14:00:00"))
[ Info: (29, DateTime("2000-01-01T14:30:00"))
[ Info: (30, DateTime("2000-01-01T15:00:00"))
[ Info: (31, DateTime("2000-01-01T15:30:00"))
[ Info: (32, DateTime("2000-01-01T16:00:00"))
[ Info: ("PA:", 32, DateTime("2000-01-01T16:00:00"))
[ Info: (33, DateTime("2000-01-01T16:30:00"))
[ Info: (34, DateTime("2000-01-01T17:00:00"))
[ Info: (35, DateTime("2000-01-01T17:30:00"))
[ Info: (36, DateTime("2000-01-01T18:00:00"))
[ Info: (37, DateTime("2000-01-01T18:30:00"))
[ Info: (38, DateTime("2000-01-01T19:00:00"))
[ Info: (39, DateTime("2000-01-01T19:30:00"))
[ Info: (40, DateTime("2000-01-01T20:00:00"))
[ Info: ("PA:", 40, DateTime("2000-01-01T20:00:00"))
[ Info: (41, DateTime("2000-01-01T20:30:00"))
[ Info: (42, DateTime("2000-01-01T21:00:00"))
[ Info: (43, DateTime("2000-01-01T21:30:00"))
[ Info: (44, DateTime("2000-01-01T22:00:00"))
[ Info: (45, DateTime("2000-01-01T22:30:00"))
[ Info: (46, DateTime("2000-01-01T23:00:00"))
[ Info: (47, DateTime("2000-01-01T23:30:00"))

As you can see the first Euler step starts with 0, the "2nd" time step (the first Leapfrog step) isn't counted, so that after time step 1 30min, time step 2 1h, timestep 3 1h30min, etc. The particle advection is then done after each 8 (by default but that's a parameter you can choose) timesteps correctly skipping the not counted first time steps.

The 48th time step that would step from 24h to 24h30 is not executed as the simulation period is set to Day(1). So I am indeed wondering whether we should do the particle advection after time step -1, 7, 15, 23, 31, 39, 47, instead of after 0, 8, 16, 24, 32, 40, 48 (which is right now not executed). I also realised that we're technically using the velocities at n+1 (i.e. 1, 9, 17, 25, ...) because particle_advection! is currently called after gridded! which transforms the spectral state at the end of a time step (i.e. the new time step) to grid-point space. Maybe that should be changed too. The decision when to execute the particle advection is made here

SpeedyWeather.jl/src/dynamics/particle_advection.jl

Lines 72 to 76 in 0f6d9be

	# escape immediately if advection not on this timestep
	clock.timestep_counter % particle_advection.every_n_timesteps == 0 || return nothing
	# don't do particle advection on the 2nd time step (which is half a leapfrog step that isn't counted)
	clock.timestep_counter == 0 && lf == 2 && return nothing

and particle_advection! is called here

SpeedyWeather.jl/src/dynamics/time_integration.jl

Line 266 in 0f6d9be

particle_advection!(progn, diagn, lf2, model.particle_advection)

Feel free to put some more thought into when to call particle advection and createa pull request for it!