Initial Conditions for stratospheric composition forecasts of polar winter-spring

This dataset contains seven netCDF files which were used as initial conditions for seven chemical forecast experiments with IFS-COMPO Cy49R1, as described in section 4.2 and evaluated in section 6 of
__Chabrillat, S. et al.: Modelling stratospheric composition for the Copernicus Atmosphere Monitoring Service: multi-species evaluation of IFS-COMPO Cy49, Geosci. Model Dev.,__ https://doi.org/10.5194/gmd-18-8973-2025, 2025

These initial conditions for 121 gas-phase species combine composition output from two different modelling and data assimilation systems:

* BRAM3, the BASCOE Reanalysis of Aura-MLS version 3 (58 gas-pahase chemical species with O3, N2O, H2O, HNO3, HCl, CO, CH3Cl and ClO contrained from Aura-MLS v4.2 and 50 other species as unconstrained CTM output). See companion dataset: https://doi.org/10.18758/8klj122q
* IFS-COMPO Cy48R1 (121 gas-phase chemical species as unconstrained composition output)

in the following manner:

* Use BRAM3 species at stratospheric levels
* Use IFS-COMPO at stratospheric levels for species which are in IFS-COMPO but not in BRAM3
(i.e. species of interest for tropospheric composition but not for stratospheric composition)
* Use IFS-COMPO at tropospheric levels for all 121 species

Filename pattern indicates the two combined data sources, the horizontal grid and the validity datetime:

* BRAM3_NL.[xxxx]_F48_yyyymmdd0000_0h.nc
* BRAM3: data source for stratospheric species
* NL.[xxxx]: data source for tropospheric species is earlier IFS-COMPO experiment run by Vincent Huijnen (KNMI, class NL) and labelled [xxxx]
* \_F48\_ : IFS-COMPO data source was first interpolated to horizontal grid F48
* yyyymmdd: validity date of the intial conditions
* 0000_0h: validity time is 0h UTC

As indicated in the netCDF files which comply with CF 1.7 metadata conventions:

* Horizontal grid is regular longitude-latitude 2°x2° grid
* Vertical grid comprise 137 hybrid sigma-pressure levels

To use as initial conditions for another model than IFS-COMPO:

* Vertical grid is hybrid sigma-pressure levels i.e. require surface pressures. These should be downloaded from ERA5 for the corresponding validity dates.
* Proper comparison requires the same dynamical constraints on temperature, winds and surface pressure i.e. those fields should also be downloaded and used as input for your chemical forecast experiments, e.g. every 6h (CTM experiments) or every 24h (SD-GCCM experiments).