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

Abstract

Abstract. The purpose of this work is determine the extent of closure between measurements and models of UV irradiances at diverse sites using state of the art instruments, models, and the best available data as inputs to the models. These include information about aerosol optical depth (unfortunately not extending down as far into the UVB region as desirable because such information is not generally available), ozone column amounts, as well as vertical 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 account of different measurements of ozone, including satellite-derived values, as well as ground measured values, both from Dobson instruments and as retrieved from the UV spectra under study. Previous studies have generally focussed on data from a single site, and for shorter periods. As such, it is the most comprehensive study of its kind to date.At Lauder, which is the cleanest low altitude site, we obtained agreement between measurement and model at 5% level, which is consistent with the best agreement found previously. At Mauna Loa Observatory, similar agreement was achieved, but model calculations need to allow for reflections from cloud that are present below the observatory. At this site, there are occasional problems with using satellitederived ozone. At Boulder, mean agreements were similar but the dispersion around the mean was slightly larger, corresponding to larger uncertainties in the aerosol inputs to the model. However, at Melbourne, which is the only nonCorrespondence to: J. Badosa (jordi.badosa@gmail.com) NDACC (Network for the Detection of Atmospheric Composition Change) site, there remain unexplained discrepancies.The measured values are significantly lower than the calculated values. We investigate the extent to which this discrepancy can be explained by incomplete knowledge of aerosol extinctions in the UV at this site. We conclude that further information about aerosol optical depth and single scattering albedo in the UVB region is needed to resolve the issues. For more polluted sites (the four considered locations show in general small aerosol load), the uncertainties in the aerosol input parameters would lead to less confidence on the modelling approach. At the three NDACC sites, the closure provided by the study gives confidence in both the measurements and our ability to model them. This study revealed a limitation in the use of PTFE diffusers when temperatures are lower than approximately 20 • C. It also documents the range of clear sky UVI values expected at these diverse sites.

How large and how long are UV and total radiation enhancements?

“…This station belongs to the National Institute of Water and Atmosphere Research (NIWA) and is a primary source of radiation data in the Southern Hemisphere; the quality of the data is guaranteed by the strict supervision from NIWA personnel and in compliance with BSRN (Baseline Surface Radiation Network) standards through a collaborative effort with the Australian Bureau of Meteorology [3].…”

“…The separation of cloudiness into "thin" and "opaque" is based on the amount of blue tint of the clouds in the ( 1 ) UVI and TR for clear sky conditions were estimated for all 1-minute data. For clear-sky UV, a previously available parameterized model [5] was considered, taking as inputs sun photometer aerosol optical depth (AOD) measurements at 412 nm (also available at the site), with the Angström exponent and single scattering albedo set at 1.4 and 0.92 respectively [3], as well as the total ozone column (TOZ) from sun photometer measurements, and the corresponding solar zenith angle (SZA) value. The TR clear-sky estimations were calculated using the simple formula [6]:…”

A new parameterization of the UV irradiance altitude dependence for clear-sky conditions and its application in the on-line UV tool over Northern Eurasia

“…The uncertainty of the RT method is less than 1 % (Liou, 2010). A good agreement between the experimental spectral data in different geographical regions and simulated results using this RT method if the input atmospheric parameters were known was also shown in Badosa et al (2007).…”

The influence of the spatial resolution of topographic input data on the accuracy of 3-D UV actinic flux and irradiance calculations

“…Discrete ordinate methods are frequently applied in the 1-dimensional case, and in particular the two stream methods (i.e., radiation is considered in two directions: upward and downward) are suitable for many applications, e.g., numerical weather prediction models or climate models. In general one can state that radiative transfer for clear-sky conditions and over flat terrain is well understood and that established solutions to deal with cloudiness are available (e.g., Smith et al, 1992;Krotkov et al, 1998;Badosa et al, 2007).…”