Prognostic Ozone For ICON: Enabling UV Forecasts

Stratospheric ozone (O3) absorbs biologically harmful solar ultraviolet (UV) radiation, mainly in the UV-B and UV-C spectral range. At the surface, enhanced UV radiation poses a well documented hazard to human health. In order to quantify the amount of UV radiation and to make warnings easily understandable, the World Health Organization (WHO) has defined an UV Index[1]. It is calculated using a weighted integral of the incoming solar irradiance at surface level between 250 and 400 nanometers and scaling the result to values that generally range between 1 to 10, surpassing 10 for excessive UV exposure.

Implementing UV Index forecasts in numerical weather prediction (NWP) models allows to alert the public in time if special care for sun protection needs to be taken. The German Weather Service (DWD) uses its NWP model ICON (ICOsahedral Nonhydrostatic Model)[2] to offer such a forecast for Germany[3] using external data such as ozone forecasts by the Royal Netherlands Meteorological Institute (KNMI) and radiation lookup tables[4].

Here, we extend the capability of ICON such as to provide a self-consistent UV Index forecasts that do not require external/auxiliary data. For this, we use ICON-ART[5],[6] with a linearized prognostic (stratospheric) ozone scheme (LINOZ)[7] and couple the prognostic ozone (and other model variables) to the atmospheric radiation scheme Solar-J[8]. To validate this new ICON-ART setup, we study the model performance in a selected reference time frame in comparison to CERES[9] satellite data and find generally a good agreement. This is an indication for the suitability of the model system to forecast the UV Index.

References:

[1] World Health Organization. Global solar uv index : a practical guide,2002.

[2] Günther Zängl et al.. The icon (icosahedral non-hydrostatic) modelling framework of dwd and mpi-m: Description of the non-hydrostatic dynamical core. Quarterly Journal of the Royal Meteorological Society, 2015.

[3] https://kunden.dwd.de/uvi/index.jsp.

[4] Henning Staiger and Peter Koepke. Uv index forecasting on a global scale. Meteorologische Zeitschrift, 2005.

[5] D. Rieger et al.. Icon–art 1.0 – a new online-coupled model system from the global to regional scale. Geoscientific Model Development, 2015.

[6] J. Schröter et al.. Icon-art 2.1: a flexible tracer framework and its application for composition studies in numerical weather forecasting and climate simulations. Geoscientific Model Development, 2018.

[7] C. A. McLinden et al. Stratospheric ozone in 3-d models: A simple chemistry and the cross-tropopause flux. Journal of Geophysical Research: Atmospheres, 2000

[8] J. Hsu, M. J. Prather et al.. A radiative transfer module for calculating photolysis rates and solar heating in climate models: Solar-j v7.5. Geoscientific Model Development, 2017.

[9] NASA/LARC/SD/ASDC. (2017). CERES and GEO-Enhanced TOA, Clouds and Aerosols 1-Hourly Terra-Aqua Edition4A [Data set]. NASA Langley Atmospheric Science Data Center DAAC. https://doi.org/10.5067/TERRA+AQUA/CERES/SYN1DEG-1HOUR_L3.004A