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

[摘要] The purpose of this work is determine the extent of closure betweenmeasurements and models of UV irradiances at diverse sites using state ofthe art instruments, models, and the best available data as inputs to themodels. These include information about aerosol optical depth (unfortunatelynot extending down as far into the UVB region as desirable because suchinformation is not generally available), ozone column amounts, as well asvertical profiles of temperature. We concentrate on clear-sky irradiances,and report the results in terms of UV Index (UVI).

Clear-sky data from one year of measurements at each of four diverse sites(Lauder – New Zealand, Mauna Loa Observatory – Hawaii, Boulder – Colorado,and Melbourne – Australia) have been analysed in detail, also taking accountof different measurements of ozone, including satellite-derived values, aswell as ground measured values, both from Dobson instruments and asretrieved from the UV spectra under study. Previous studies have generallyfocussed on data from a single site, and for shorter periods. As such, it isthe most comprehensive study of its kind to date.

At Lauder, which is the cleanest low altitude site, we obtained agreementbetween measurement and model at 5% level, which is consistent with thebest agreement found previously. At Mauna Loa Observatory, similar agreementwas achieved, but model calculations need to allow for reflections fromcloud that are present below the observatory. At this site, there areoccasional problems with using satellite-derived ozone. At Boulder, meanagreements were similar but the dispersion around the mean was slightlylarger, corresponding to larger uncertainties in the aerosol inputs to themodel. However, at Melbourne, which is the only non-NDACC (Network for theDetection of Atmospheric Composition Change) site, there remain unexplaineddiscrepancies. The measured values are significantly lower than thecalculated values. We investigate the extent to which this discrepancy canbe explained by incomplete knowledge of aerosol extinctions in the UV atthis site. We conclude that further information about aerosol optical depthand single scattering albedo in the UVB region is needed to resolve theissues. For more polluted sites (the four considered locations show ingeneral small aerosol load), the uncertainties in the aerosol inputparameters would lead to less confidence on the modelling approach. At thethree NDACC sites, the closure provided by the study gives confidence inboth the measurements and our ability to model them. This study revealed alimitation in the use of PTFE diffusers when temperatures are lower thanapproximately 20°C. It also documents the range of clear sky UVI valuesexpected at these diverse sites.

[学科分类] 大气科学