Ralf Meerkötter

Case study on the influence of inhomogeneous surface albedo on UV irradiance

The influence of inhomogeneous surface albedo on UV irradiance is investigated for cloud free conditions using a recently developed 3-dimensional radiative transfer model. From the calculations the relative enhancement of UV irradiance due to (partial) snow coverage as well as an 1-dimensional effective albedo is derived. This 1d-albedo is a quantity allowing to account for effects of inhomogeneous surface albedo in 1-dimensional radiative transfer models. The results indicate that the surface albedo must be known on an area with considerable size: even if surface albedo is specified on an area with 40km radius around the measuring site the influence of areas outside this range on UV irradiance can be still up to 3%. Although we already found encouraging agreement between modeled and measured spectral irradiance for clear skies and low surface albedo, the general evaluation of the 3-dimensional model remains an open issue.

Influence of inhomogeneous surface albedo on UV irradiance: Effect of a stratus cloud

The influence of inhomogeneous surface albedo on UV irradiance is investigated assuming the sky to be overcast by a horizontally homogeneous stratus cloud. From calculations with a three-dimensional (3-D) radiative transfer model the snow-induced relative UV irradiance enhancement as well as the 1-D albedo is derived for two different snow bidirectional reflectance distribution functions (Lambertian and anisotropic) and snow coverages varying between 0 and 100%. The 1-D albedo is a quantity defined to consider effects of inhomogeneous surface albedo even in 1-D radiative transfer codes. It turned out that anisotropy of snow reflectance is of minor importance for quantification of inhomogeneous surface albedo influence on UV irradiance. This holds for a cloudy as well as for a cloud-free atmosphere. Furthermore, the comparison of the new results with corresponding data obtained for cloudless sky indicates that a stratus cloud, on the one hand, enhances the surface albedo effect by about a factor 2–3, depending on wavelength. On the other hand, the area significantly influencing UV irradiance via its surface albedo is distinctly smaller. Looking, e.g., at wavelength 330 nm and clear sky, the maximum albedo effect of the surface surrounding a quadratic area of 6400 km2 on UV irradiance at area center is 3%. In the case of a stratus cloud the same effect is found for an area of only 1500 km2.

A Radiative Transfer Case Study for 3‐d cloud effects in the UV

Satellite UV mapping is usually based on the independent pixel approximation (IPA) which neglects horizontal photon transport between adjacent columns. Horizontal inhomogeneity of cloud fields therefore causes uncertainties in the derived UV radiation fields. While these effects are small for large pixel satellites, the broken-cloud errors increase as the pixel size decreases. By comparing results of 1-d and 3-d UV radiative transfer calculations for three selected cloud scenes that cover a rather broad range of cloud inhomogeneity the main 3-d cloud effects on the atmospheric UV transmission are identified and quantified in their order of magnitude. With respect to the different spatial resolutions of satellite instruments it is further shown how 3-d cloud effects average out with increasing spatial scale. It turns out that locally the IPA cause maximum uncertainties up to ±100% for a spatial resolution of about 1 × 1 km² (e.g., AVHRR), they are reduced to ±10% for a resolution of about 15 × 15 km² and below 5% for a resolution greater than 30 km (e.g., TOMS).

The UV service of the ESA-GSE Project PROMOTE

In Europe (EU25) about half a million skin cancer cases are occurring per year and this is strongly associated with personal habits in relation to sun exposure and its UV component. Within the frame of the European GMES-Program (GMES=Global Monitoring for Environment and Security) the ESA-GSE Project PROMOTE addresses this problem by developing and implementing a UV information service that aims to reach as many as possible citizens of Europe (EU25). The overall PROMOTE UV service contains forecast and monitoring products. The underlying methods, the use of satellite data, the various UV products including related user interfaces, as well as accuracy aspects are described. One central ambition of the PROMOTE project is the close interaction between providers and users. Experiences that have been made and will be made during the different stages of the PROMOTE project contribute significantly to the further up-grading of the services.