Improving the Prognostic Ozone Parameterization in the NCEP GFS and CFS for Climate Reanalysis and Operational Forecasts

The objective of this proposal is to improve the representation of stratospheric ozone (O3) and water vapor in NOAA’s climate reanalyses. This work will also improve NOAA’s simulation, analysis, and forecasting of weather and climate variability, including forecasts of UV radiation to protect public health. A complete treatment of O3 photochemistry is too computationally intensive for current models. Therefore, a parameterization is included in the current NCEP Global Forecast System (GFS) atmosphere/land model used in the 20th Century Reanalysis and operational forecasts, and also used in the coupled Climate Forecast System (CFS) Reanalysis and operational CFSv2. The GFS parameterization for the time tendency of O3 is based on parts of NRL’s CHEM2D Ozone Photochemistry Parameterization (CHEM2D-OPP). It includes terms representing net production and loss and a dependency on the ozone mixing ratio itself. It is based on gas-phase chemistry of the late-20th century, which includes the depletion of ozone by chlorofluorocarbons (CFCs). For climate reanalyses and climate modeling extending back to the early 20th century or earlier, before large quantities of CFCs began to be released into the atmosphere, a new version of this parameterization is needed to represent pre- CFC stratospheric O3 chemistry. To understand, analyze, and predict atmospheric variability in the 21st century, the parameterization should utilize additional interactions included in CHEM2D-OPP that affect stratospheric O3. Stratospheric water vapor is also an important radiative constituent. Its representation in the GFS will also be improved, paving the way for improved assimilation of satellite radiances and for interactive chemistry.

In this proposal, a more advanced O3 parameterization using the full CHEM2D-OPP and an improved treatment of stratospheric water vapor will be implemented for use in new versions of the GFS, CFS, and next generation NOAA climate reanalysis systems. The O3 parameterization will include the effect of changes in temperature, changes in the vertical distribution of O3, and the time-variation of CFCs. As a first step, the parameterization will be tested with two modes, one for times before CFCs and one for times after CFCs began to be released in large quantities. The team will also implement a new stratospheric H2O climatology as a necessary first step toward future implementation of parameterized H2O photochemistry. The upgraded parameterization and new climatology will be tested in climate reanalyses and weather and climate simulations. The impact of the new O3 and water vapor treatments on reanalysis, GFS medium-range forecast skill, and CFS climate simulations will be evaluated using comparisons with both historical and modern O3 and temperature observations throughout the troposphere and stratosphere as well as with UV radiation observations.

This project is directly related to both foci of Priority 1 of the MAPP call for proposals. It is also directly relevant to NOAA’s Next-Generation Strategic Plan goals for climate adaption and mitigation. As noted by the WMO Global Framework on Climate Services, reanalyses are a key component of the climate information needed for informed decision making for climate change mitigation and adaption. This proposal directly addresses the MAPP call to pursue research on “Outstanding issues in atmospheric reanalysis”, in particular by attempting to “overcome the impact of model bias” and “exploit new data”. Improved stratospheric ozone and water vapor representations will both reduce model error in the first guess fields and permit more effective assimilation of satellite radiances affected by these constituents. The improved stratospheric O3 and water vapor will also be an important contribution to foci 2 as explicit chemistry begins to be included in Earth System analyses.