Fog forecasting in HARMONIE: a case study to current issues with the overestimation of fog in HARMONIE

Abstract

A recurring issue in HARMONIE-AROME is the overestimation of the occurrence of dense fog over the Netherlands. In this thesis, the performance of Harmonie on simulating radiation fog is studied by comparison of simulated fog to observed fog at the Cabauw observatory. By evaluating the representation of physical processes related to radiation fog formation in HARMONIE-AROME, acute issues are addressed that may lead to erroneous forecasting. The main focus lies on the representation of fog in the longwave radiation scheme and its dependence on the amount of cloud condensation nuclei. After analysis of a fog case study in which the fog simulated by HARMONIE-AROME is compared to Cabauw observations, the 1D model MUSC is used to evaluate the fog evolution in more detail for testing the performance of new parametrizations. From these experiments appears that the overestimation of fog is correlated to too strong longwave cooling by the fog. Based on the outcomes of many MUSC experiments, the HARMONIE-AROME case study is rerun in experiments with adaptations based on cloud longwave emissivity (0.096 instead of 0.144), longwave cloud inhomogeneity (0.7 instead of 1) and the number of cloud condensation nuclei (10-50 cm−3 instead of 300 cm−3 over land and 100 cm−3 over sea). The results of this study show that a reduction of the longwave emissivity combined with reduced CCN leads to a large improvement in the behavior of simulated fog.