24 Jan 2013: Major upgrade of the BASCOE NRT assimilation system
The BASCOE assimilation system used in NRT has been upgraded to version 03 on January 01, 2013.
- The previous operational version (q02.05) suffered from aliasing errors in the input wind fields, leading to erroneous noise in the horizontal distribution of chemical tracers (e.g. ozone at 50 hPa ). The new version generates input wind fields from the operational analyses of vorticity and divergence, retrieved in spectral space at the correct truncation (T31).
- This new pre-processing of input wind fields is based on the algorithm of Segers ( Bregman et al., 2003) : the horizontal fluxes are adjusted to ensure mass conservation despite the temporal variations of surface pressure in the meteorological analyses.
- The vertical grid of the model is improved and extended, from 37 levels (model top at 0.1 hPa) to 91 levels (model top at 0.01 hPa). This new vertical grid comes from the IFS configuration used operationally for meteorological deterministic forecasts (since 1 February 2006). It has a much finer resolution in the Upper Troposphere-Lower Stratosphere (UTLS) region.
- The new version nudges tropospheric ozone towards a climatology based on ozone soundings for the period 1966-1996 ( Logan, JGR, 1999). Note that ozone at tropospheric levels should still not be used: its distribution is irrealistic due to the absence of any surface emissions, tropospheric processes and NMVOC species. Nudging to a climatology is only meant to improve the quality of total ozone columns.
- Photochemical reaction rates were updated from JPL-2006 to JPL-2009, except for the photolysis cross-section of Cl2O2 which was updated to JPL-2011. This last change has an important impact on chlorine radicals (ClOx, not assimilated) in ozone hole conditions (Papanastasiou et al., 2009).
- Format standardization for the dataset distributed in NRT: the BASCOE analyses were written in HDF-4 format using ad-hoc conventions. The new version writes the output in netCDF-3 format compliant with CF-1.0 conventions.