Abstract
Abstract. This paper provides an overview of the HLRADIA shortwave (SW) and longwave (LW) broadband radiation schemes used in the HIRLAM numerical weather prediction (NWP) model and available in the HARMONIE-AROME mesoscale NWP model. The advantage of broadband, over spectral, schemes is that they can be called more frequently within the model, without compromising on computational efficiency. In mesoscale models fast interactions between clouds and radiation and the surface and radiation can be of greater importance than accounting for the spectral details of clear-sky radiation; thus calling the routines more frequently can be of greater benefit than the deterioration due to loss of spectral details. Fast but physically based radiation parametrizations are expected to be valuable for high-resolution ensemble forecasting, because as well as the speed of their execution, they may provide realistic physical perturbations. Results from single-column diagnostic experiments based on CIRC benchmark cases and an evaluation of 10 years of radiation output from the FMI operational archive of HIRLAM forecasts indicate that HLRADIA performs sufficiently well with respect to the clear-sky downwelling SW and longwave LW fluxes at the surface. In general, HLRADIA tends to overestimate surface fluxes, with the exception of LW fluxes under cold and dry conditions. The most obvious overestimation of the surface SW flux was seen in the cloudy cases in the 10-year comparison; this bias may be related to using a cloud inhomogeneity correction, which was too large. According to the CIRC comparisons, the outgoing LW and SW fluxes at the top of atmosphere are mostly overestimated by HLRADIA and the net LW flux is underestimated above clouds. The absorption of SW radiation by the atmosphere seems to be underestimated and LW absorption seems to be overestimated. Despite these issues, the overall results are satisfying and work on the improvement of HLRADIA for the use in HARMONIE-AROME NWP system is ongoing. In a HARMONIE-AROME 3-D forecast experiment we have shown that the frequency of the call for the radiation parametrization and choice of the parametrization scheme makes a difference to the surface radiation fluxes and changes the spatial distribution of the vertically integrated cloud cover and precipitation.
Highlights
The radiation parametrizations in a numerical weather prediction (NWP) model serve two purposes
This paper provides an overview of the HLRADIA shortwave (SW) and longwave (LW) broadband radiation schemes used in the HIRLAM numerical weather prediction (NWP) model and available in the HARMONIE-AROME mesoscale NWP model
NWP models can be validated by comparing forecast surface radiation fluxes to surface observations and forecasts of upward LW and SW fluxes (LWUT and SWUT) at the top of the atmosphere (TOA) to fluxes measured by satellites
Summary
The radiation parametrizations in a numerical weather prediction (NWP) model serve two purposes. The orographic radiation parametrizations developed for the HIRLAM NWP model have been prepared for use by any atmospheric radiation scheme available in HARMONIE-AROME (Rontu et al, 2016a). Nielsen et al (2014) pointed out that biases in radiation fluxes predicted by an NWP model can be due to factors other than the atmospheric radiative transfer calculations Incorrect input to these calculations, such as inaccurate cloudiness, poorly known cloud and surface properties or unaccounted interactions between simulated processes in the model can lead to errors. The authors found that HLRADIA tends to overestimate LWDS fluxes when the clouds are optically thick This evaluation is continued in the present study which includes results from a ten-year comparison between surface radiation observations and FMI operational HIRLAM forecasts for Sodankylä and Jokioinen (Southern Finland), see Fig. 4 for the station locations. The clear and cloudy parametrizations are described separately
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