Verification of visibility forecasts from NWP model with satellite and surface observations

Abstract

Forecast of fog and visibility over most parts of Indo-Gangetic plains are becoming increasingly important in the winter season because of the high frequency of occurrence of dense fog and reduced visibility which has significant socio-economic impacts. The life cycle of fog is mainly controlled by different meteorological factors and the microphysical/chemical properties of the particulate matter in the atmosphere. The present day numerical weather prediction (NWP) models of high spatial resolution are able to forecast situations that are favorable for the occurrence of fog events with reasonable accuracy for few days in advance. NCMRWF has started producing visibility/fog forecasts using the Unified Model (NCUM), which has a diagnostic fog scheme. The visibility is computed in the model based on the extinction of light at visible ranges due to fog particles. The visibility/fog forecasts during the months of December, 2013 and January, 2014 obtained from NCUM over the Indo-Gangetic plains are verified using the surface as well as satellite observations in this study. Surface visibility observations from meteorological airport reports (METAR) and satellite based fog product from Moderate Resolution Imaging Spectroradiometer (MODIS) are used for the verification. NCUM short-range forecast shows good skill in indicating the occurrence of fog/no-fog events, based on two visibility categories defined in this study to represent the heavy and light fog events, over different locations over the Indo-Gangetic plains.