Fog and low-level stratus characteristics at the airport of Odesa from surface observations

Abstract

Introduction. Fog that limits visibility and low-level stratus represent a significant hazard to aviation especially during takeoff and landing, and also low-level flying of aircrafts, because accidents often occur in reduced visibility conditions and low clouds. Therefore, forecasting fog and low ceilings is one of the most important, but at the same time the most difficult issue, because both phenomena strongly depend on local conditions and unsteady in both time and space. The aim of the study is to obtain the statistical characteristics of low-level stratus and fog at the airport of Odessa and determine local dependencies that would enable to improve aviation weather forecasts related to low-level stratus and fog physics. Scientific novelty of results obtained. In this study for the Ukraine for the first time for Odesa airport frequency distribution of low-level stratus and fogs is obtained as a function of the time of the day and the month of the year and also as a function of the temperature and relative humidity near the surface. Practical importance of results obtained. The results obtained could be used for providing weather forecast model with historical data and improving forecast of fogs and low-level stratus. Materials and method. To study fog and low-level stratus characteristics occurring at the airport of Odesa, Ukraine, half hourly observations in the period of 2010-2021 are used. Applying a statistical approach annual, seasonal and diurnal distribution of fog and low stratus and their frequency distribution associated with various meteorological parameters are obtained. Results and discussion. The monthly distributions of low-level stratus reveal maximum occurrence frequencies in November and January, and fog most frequently occurs in December. No significant diurnal cycle of stratiform cloud occurrence is discovered, as opposed to fog for which the highest frequency is observed in the hours before sunrise, while when the day sets in, frequencies are declining and increasing at night. Fog and low-level stratus have the same distribution in duration and the mean event duration is 4.5 h while 55% of the events lasted 2 h or less. The most long-lived fog and stratiform clouds can last about 4 days during the December-January period. Occurrence of fog and stratiform clouds as function of temperature and relative humidity reveals a close statistical relationship, especially for fog events. More than 33% of all fogs are observed at temperatures of 0°C to 6°C and 96-100% relative humidity, the most frequencies of low-level clouds (13%) occur in the same temperature interval, but at lower values of relative humidity (91‑95%). Regarding fog density 75% of the events have minimum visibility lower than 400 m, which indicates the severity of the problem, because, despite the season and type of fog, they are usually quite intense and dense. In all seasons of the year, the highest frequency of low-level stratiform clouds is in interval of 3...4 m/s, excluding summer, when most often such cloud is registered at higher speeds. The wind directions associated with low-level stratiform clouds are, as a rule, northern and eastern ones. Fogs, on the contrary, most often in all seasons, except winter, are formed at calm, meaning that radiation fogs are the most common type in the Odesa airport. In winter fogs are most commonly associated with northern and easterly winds; in all other seasons the southern wind is the most frequent.