# Is spin-up really required to run the WRF-Chem model and if so, what determines the time period of the spin-up?

For example, I am interested in making a simulation for some chemical species for 2019. I am planning to provide NCEP/GDAS FNL meteorological data at 0.25 degree and CAM-Chem chemical boundary layer. I want to know what time period of meteorological data and chemical boundary layer should I provide for my simulation?

• Depending on the initial conditions you use and the meteorological event involved, I've found it takes at least 2-3 days of spin-up for the chemical conditions to stabilize in regional CTMs.
– f.thorpe
Jan 10 at 1:39
• I am trying to do an year-long simulation. I do not think 2-3 days of spin-up would be enough for the chemical conditions to stabilize in regional CTMs. What I am confused about is that if I am trying to do a simulation for 2019, do I have to provide meteorological data and chemical boundary layer for 2019 and/or let us say, December 2018? Jan 10 at 7:55
• Meteorology usually takes longer to spin up than chemistry. Once your chemical boundary conditions wash over your entire domain... it's spun. If winds are stagnant it could take more than 3 days but it doesn't really matter how long your simulation is. Since you are using WRF-CHEM and they are coupled, maybe you take 5 days (e.g. start at Dec 27) but that's more for the meteorology than the chemistry.
– f.thorpe
Jan 10 at 8:06
• If you are looking at typical air quality products like ozone and aerosols, etc. then the chemistry is quick and transport dominates. If you are modeling GHGs and other long-lived pollutants, then you would need a much longer chemical spin-up (or really good initial conditions). If you have a large domain (e.g. continental) you might need more days so the transport can flush out the initial conditions from the entire domain. forum.mmm.ucar.edu/phpBB3/viewtopic.php?t=9894 wiki.harvard.edu/confluence/pages/…
– f.thorpe
Jan 10 at 22:32
• If you really want to be conservative you could do a full week of Met/Chem spinup. Then noone can question the first dew days of your 2019 results.
– f.thorpe
Jan 10 at 22:35

When I was using WRF-Chem to simulate $$\ce{O3}$$, I used a couple days for the emission of $$\ce{NO_x}$$ to be somewhat representative.