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Measurements of the basic meteorological parameters and ultraviolet (UV) erythemalradiation (UVER) were performed in Didcot, England, on 6 and 7 April 2017. The UVERdata was collected by dosimeters, which consist of two sensors (named UVA and UVBC)collecting the erythemally weighted UV-A (320 - 400 nm) and UVBC (UV-B and UV-C, 250- 320 nm) radiation parts, respectively. Both measurement days were characterized by clearconditions during the morning and afternoon hours. Around noon of both days,however, development of shallow stratocumulus clouds (SSC) occurred. In addition, alow-ozone event (ozone mini-hole) took place on 7 April being characterized bya 34 DU (Dobson Unit) drop in total stratospheric ozone content. Compared to6 April, the UVER increase as a result of the low-ozone event amounts to 2.67standard erythema dose (SED) for diffuse and 4.32 SED for global radiation. Thecorresponding radiation amplification factors (RAF) amount to 1.62 and 1.52 fordiffuse and global UVER, respectively. The SSC coverage caused reductions forthe total global UVER doses amounting to 2.33 SED (6 April) and 2.81 SED(7 April). As innovation, we present a decomposition of the RAF into two parts, called cloud ozonefactor (COF) and radiation amplification factor based on measured data (RAFm). The goalof this decomposition is to quantify the low-ozone event’s effect and the SSC influence inindependently modifying the UVER doses. Hereby the newly introduced COFexpresses the weight of each of these two effects acting during the same low-ozoneevent. For the case presented, the COF values range between -0.13 and -0.11 fordiffuse UVER and -0.03 to -0.07 for the global UV and UV-B parts. A positive COFvalue (0.18) results for the global UV-A range. Negative (positive) COF valuescorrespond to a decreased (increased) cloud influence on the second of both referencedays. [ABSTRACT FROM AUTHOR]

Fog and low stratus clouds are a typical feature in coastal deserts. In the hyper-arid Namib Desert at the southwestern African coast, fog is an important source of water for ecosystems. The knowledge of the spatial and temporal patterns of fog in the Namib-region contributes to a deeper understanding of fog processes and fog-related ecosystems and thus is of great ecological and socio-economic interest.The central aim of our study is to understand processes controlling the spatial and temporal development of coastal desert fog in the Namib by means of numerical simulations. Low stratus clouds form at the top of the marine boundary layer over the cold Benguela Current. These low clouds are advected overland by meso-scale circulations. The interaction of turbulent mixing with microphysical and advection processes in the formation, maintenance and dissipation of fog and low clouds in the Namib Desert imposes high requirements on the corresponding parametrizations. Numerical simulations are performed with an extension of the regional weather prediction model COSMO (Consortium for Small-scale Modeling) which is adapted for the application in the Namib region. To account for microphysical processes involved in fog formation, the microphysical parametrization of the one-dimensional fog forecast model PAFOG (PArameterized FOG) has been implemented into COSMO. The resulting fog forecast model COSMO-PAFOG is run with kilometer-scale horizontal resolution.In our study, five case studies are analyzed and the model results are compared to satellite and ground observations obtained during the field campaign within the framework of the NaFoLiCA (Namib Fog Life Cycle Analysis) project. Tuning of turbulence-related model parameters improves the agreement of model simulations with observations. Especially the cloud cover compared to satellite observations and the diurnal cycle of temperature and humidity are better reproduced in the model simulations with adjusted turbulence parameters. The analysis of the thermodynamical processes yielding fog formation reveals that cooling in the atmospheric boundary layer is the main process leading to saturation while moisture changes play a minor part. [ABSTRACT FROM AUTHOR]

The stratocumulus deck over the South Atlantic is regularly advected into the hyper-arid Namib where it appears as fog when it intercepts with the ascending terrain. This fog is a major source of water for plants and animals. The project Namib Fog Life Cycle Analysis (NaFoLiCA) aims at improving the knowledge about the vertical and spatial patterns of this fog through three subprojects using field measurements, remote sensing and numerical modelling.For this purpose, an intensive operation period (IOP) was conducted between 10th September and 5th October 2017 in the central Namib. The measurements were linked to the FogNet network with main activities taking place at the Gobabeb research and training centre. During the IOP, micrometeorological data including cloud base height, fog deposition, liquid water path and vertical profiles of wind speed and direction were measured continuously. Additionally, profiles of temperature and relative humidity were sampled during five selected nights with stratus/fog using tethered balloon soundings, drone profiling and radiosondes. Measurements took place at both coastal and inland sites as well as on different elevation levels in order to gather data on the spatial dynamics of the fog events. Fog droplet distribution was measured with a cloud droplet probe and liquid water content/liquid water flux estimated with a co-located sonic anemometer. The fog events exhibited considerable spatial and temporal variation between single events and within the network. Selected results from the IOP will be presented with the focus on the development of one fog event. [ABSTRACT FROM AUTHOR]