NRL Scientists Improve Ozone and UV Index Forecasts

11/20/2006 - 62-06r
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CHEM2D-OPP, a fast new computer code for predicting stratospheric ozone photochemistry developed by scientists in the Naval Research Laboratory's (NRL's) Space Science Division, was recently implemented in the National Centers for Environmental Prediction Global Forecast System (NCEP GFS). The GFS computer model produces weather forecasts for the National Weather Service (NWS). Since CHEM2D-OPP became operational in the GFS on August 22, it has substantially improved the GFS ozone forecasts and related products, such as Ultraviolet (UV) Index forecasts for the continental United States issued by the Environmental Protection Agency (EPA) at

We know that prolonged exposure to the Sun causes sunburn, and repeated overexposure can lead to chronic health problems, such as skin cancers and cataracts. To help the general public monitor these risks, the NWS and EPA now issue forecasts of the expected levels of the most biologically harmful UV solar radiation using the so-called UV Index. When the local UV Index is forecast to be high (6 or higher), eyes and exposed skin should be protected when outdoors. Since ozone molecules in the stratosphere absorb these harmful UV wavelengths from the Sun, UV Index forecasts rely on accurate forecasts of stratospheric ozone levels. In addition to improving UV forecasts, accurate stratospheric ozone forecasts provide important additional benefits for computer weather forecasting. Ozone forecasts improve forecasts of stratospheric heating rates, help to more accurately incorporate satellite weather measurements into models though improved understanding of how ozone absorption affects these measurements, and can be used to track and help forecast wind patterns in the lower stratosphere.

A team in the Space Science Division's Upper Atmospheric Physics Branch, led by Dr. John McCormack, developed this new ozone photochemistry algorithm. The starting point for this research was CHEM2D, a global two-dimensional computer model of the Earth's atmosphere from 0-122 km altitude, developed over the past decade in the the Space Science Division's Upper Atmospheric Physics Branch. CHEM2D contains state-of-the-art descriptions of stratospheric radiative transfer, transport, and photochemistry. Unfortunately, the detailed ozone photochemistry algorithms in CHEM2D are far too demanding computationally to run with acceptable speed in weather forecasting models. To solve this problem, Dr. McCormack performed an exhaustive series of perturbation experiments with the CHEM2D model to quantify the net photochemical response of ozone to changes in local ozone concentrations, atmospheric temperatures, and incoming solar UV radiation. These ozone responses were characterized using photochemical coefficients that were tabulated globally as functions of month, altitude and latitude. They cover the globe and range from the ground to the edge of space at ~80 km altitude.

The coefficients from these CHEM2D perturbation experiments form the basis of the new fast ozone photochemical parameterization (OPP) known as CHEM2D-OPP. Dr. McCormack and his team extensively tested the new algorithm in detailed global forecasting experiments using an advanced level physics, high-altitude (ALPHA) version of the Navy Operational Global Atmospheric Prediction System (NOGAPS), the Navy's global weather forecasting model. The NOGAPS-ALPHA prototype is being developed at NRL as a formal collaboration among scientists in NRL's Space Science, Marine Meteorology and Remote Sensing Divisions. Among its many new capabilities, NOGAPS-ALPHA incorporates initialization and transport capabilities for a range of atmospheric trace chemicals that, when coupled with the new CHEM2D-OPP ozone photochemistry package, provide state-of-the-art forecasting capabilities for stratospheric ozone.

In addition to this recent transition of CHEM2D-OPP to the operational GFS, a preliminary prognostic ozone capability for NOGAPS using CHEM2D-OPP and elements of NOGAPS-ALPHA was transitioned recently to the Fleet Numerical Meteorology and Oceanography Center. Furthermore, the international weather forecasting community has expressed interest in adopting CHEM2D-OPP in their own systems. For example, preliminary comparisons between CHEM2D-OPP and other ozone photochemistry algorithms in the United Kingdom Meteorological Office's ozone forecasting and assimilation system have shown that the NRL scheme exhibits the best overall performance.

Dr. McCormack's development and implementation of CHEM2D-OPP for operational weather centers in the US was sponsored by the Joint Center for Satellite Data Assimilation and the National Polar-orbiting Environmental Satellite System (NPOESS) Integrated Program Office. CHEM2D-OPP represents another spin-off application from NRL's investment in atmospheric modeling research through the 6.1 and 6.2 base programs. Current 6.2-sponsored research involving scientists in NRL's Remote Sensing, Marine Meteorology and Space Science Divisions is focused on developing state-of-the-art operational ozone assimilation and forecasting capabilities for NOGAPS in preparation for future ozone sensors slated to fly on NPOESS. CHEM2D-OPP is playing a pivotal role in this ongoing research.

Total ozone as forecast by the GFS on 21 September 2006 at 0000 UTC, showing the large "ozone hole" over Antarctica and higher total ozone levels near Chile. These improved GFS ozone forecasts use NRL's new CHEM2D-OPP photochemistry code.

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