REAL-Fog: A simple approach for calculating the fog in the atmosphere at ground level

Abstract

We developed the “REAL-Fog” (R‑Estimated spatiAL Fog) tool to calculate the ground-level fog by means of liquid water content (lwc) at a temporal resolution of 1 hour. The tool is based on a distributed and mainly physical model and requires a digital elevation model (DEM) and hourly observations of temperature and humidity. The tool spatially interpolates the air temperature and derived vapour pressure to determine whether the air at ground level is supersaturated. From the supersaturation result, the lwc is then calculated. Due to errors in the measured humidity time series, preprocessing is additionally recommended. Different sources of errors, such as sensor ageing, calibration and sensor uncertainty, are discussed, and a correction method is proposed. The “REAL-Fog” method was applied in a case study of Germany from 1949 to 2017. The daily updated results of this analysis can be found at www.nebelzentrale.de. Two validation strategies are applied. First, modelled lwc values are compared with measured lwc values at Mt. Brocken (1141 m a.s.l.), which is one of the longest time series of lwc in the world and the longest in Germany. The event-based (fog, no fog) Heidke skill score (HSS) reached 0.8. Second, the event-based performance was calculated for 306 visibility sites in Germany. HSS was found to be 0.54.

Keywords

• lwc
• liquid water content
• spatial interpolation
• thin plate spline
• tps
• relative humidity correction
• lwc profile
• visibility data