From model intercomparison toward benchmark UV spectra for six real atmospheric cases

Abstract

Abstract. The validity of a radiative transfer model can be checked either by comparing its results with measurements or with solutions for artificial cases. Unfortunately, neither type of comparison can guarantee that the spectral UV surface irradiance is accurately calculated for real atmospheric cases. There is a need therefore for benchmarks, i.e., standard results that can be used as a validation tool for UV radiation models. In this paper we give such benchmarks for six cloud-free situations. The chosen cases are characterized by different values of solar zenith angle, ozone column, aerosol loading, and surface albedo. Observations are also available for these cases to allow a further comparison between model results and measurements. An intercomparison of 12 numerical models is used to construct the benchmarks. Each model is supplied with identical input data, and a distinction is made between models that assume a planeparallel geometry and those that use a pseudospherical approximation. Differences remain between the model results, because of different treatments of the input data set. Calculations of direct and global transmission and direct and global irradiance are within 3% for wavelengths longer than 320 nm. For the low-Sun cases the calculations are within 10% for wavelengths longer than 300 nm. On the basis of these calculations, six benchmark UV spectra (295-400 nm) are established with a standard deviation of 2%. Relative standard deviations are higher for the lowest absolute intensities at low Sun (5% at 300 nm). The variation between models is typically less than the variation seen between model and measurement. Differences between the benchmarks and the observed spectra are mainly due to the uncertainty in the input parameters. In four of the six cases the benchmarks agree with the observed spectra within 13% over the whole UV spectral region. IntroductionIn this paper we compare 12 radiative transfer models that are used in various institutes throughout Europe to calculate the surface solar spectral UV irradiance. For this study all Several groups involved in SUVDAMA have a radiation code, which they use for analyzing their spectral irradiance measurements. The objective of this paper is to intercompare these codes, as they are usually run, using the best (and always limited) ancillary data as input parameters, and to compare the model results with actual irradiance measurements. The paper shows that the interpretation of ancillary measurements such as aerosol optical depth and total ozone, which are often performed simultaneously with irradiance measurements, can lead to differences in the calculated spectra. 4915

Technical note: The libRadtran software package for radiative transfer calculations - description and examples of use

“…The ratio of the modelled spectrum over the measured spectrum is shown in blue. Similar cloudless comparisions between measurement and model results have been presented by for example Forster et al (1995); Mayer et al (1997); Kylling et al (1998);Van Weele et al (2000) and Meloni et al (2003). The agreement between model simulation and measurement found here is comparable to that reported by those authors.…”

“…Some of the above measurement versus model comparison papers also include model versus model comparisons (Van Weele et al, 2000;Bais et al, 2003;Hofzumahaus et al, 2004;Shetter et al, 2003). In addition to these the uvspec model has also successfully participated in a model comparison of UV-indices and the Intercomparison of 3-D Radiation Codes (Cahalan et al, 2005).…”

“…The very first uvspec model versus measurement comparison involved stratospheric balloon measurements of direct and scattered solar radiation in the UV (Kylling et al, 1993). Since then the uvspec model has been compared against surface spectral UV irradiance measurements (Mayer et al, 1997;Kylling et al, 1998;Van Weele et al, 2000); surface spectral UV actinic flux measurements ; airborne spectral UV actinic flux data (Hofzumahaus et al, 2002); surface integrated UV irradiance (DeBacker et al, 2001); surface photolysis frequency measurements (Hofzumahaus et al, 2004;Shetter et al, 2003); stratospheric balloon measurements of the actinic flux at large solar zenith angles (Kylling et al, 2003a); airborne visible spectral irradiance measurements (Wendisch and Mayer, 2003); and both surface and airborne measurements of the spectral UV irradiance and actinic flux (Kylling et al, 2005).…”

Towards closure between measured and modelled UV under clear skies at four diverse sites

“…Differences up to 14% in modelled UV were found between runs with estimated and climatic values of single scattering albedo (SSA), respectively. In four of the six benchmark cases considered by van Weele et al (2000) they found agreements against observed spectra within ±13% over the whole UV spectral region. These studies showed that model uncertainties are comparable with, but slightly larger than, measurement uncertainties.…”

“…Several intercomparisons of UV models have been performed. Van Weele et al (2000) compared global and direct spectral UV calculations from 12 numerical models for six benchmark spectra corresponding to diverse conditions. They found agreements within ±3% for wavelengths longer than 320 nm and for high sun conditions.…”