Experimental evaluation of fire weather conditions during periods of wildfires in Ukraine in 2020

Introduction. Almost every year, the territory of Ukraine is affected by drought of different intensity and duration, what is a favorable condition for wildfires. In the forest areas of the country arose about 106.8 thousand fires with a total area of 139.2 thousand hectares during 1990-2017. An analysis of the long-term dynamics shows that wildfires in Ukraine are a sustainable phenomenon. In 2020, under the severe drought conditions, 209 forest fires occurred, the most significant of them were in the Chernobyl zone and in the Lugansk region. Formulation of the problem. There is still no universally accepted method to describe all drought related processes due to complexity of drought phenomena. Although drought indices and fire weather indices usually have different time scales, their combination can be a basis in the prediction scheme in which the drought index acts as a background indicator, which enhances or decreases the current fire weather index. The purpose of this study is evaluate informativeness of a new fire weather index HDW (Hot-Dry-Windy Index) for the territory of Ukraine, which takes into account the main meteorological parameters necessary to describe the degree of wildfire danger due to weather conditions, so this index may become a component of the drought and fire monitoring system. Data and methods. The HDW index has a physical justification as the atmosphere affects the fire, namely, the combination of three atmospheric parameters characterizes the current conditions for the ignition and distribution of fire: wind, temperature, humidity. For the calculation of the HDW index, the data of GFS global numerical model with grid step 0.25 degrees is used. The calculation of the daily fields of the HDW index was carried out for the territory of Ukraine and for local areas. To localize areas of wildfires, in the study were used daily hotspots data provided by the web-resource FIRMS (https://firms2.modaps.eosdis.nasa.gov/), which are determined using satellite instruments MODIS (AQUA and Terra Satellites) and VIIRS (S-NPP and NOAA-20 satellites). Research results. Comparison of the calculated HDW fields in Ukraine with the position of hotspots showed that the maximum values of the HDW index are generally concentrated in fire areas. Comparison with the synoptic situation showed that the maximum values of the HDW are located in the zones of advection of warm and dry air in the lower troposphere and zones of strong surface winds, which usually correspond to the periphery of anticyclone or the warm sector of cyclone. The analysis of the time courses of the HDW index showed the typical feature, which expressed in the fact that in all cases there was an increase in the index values in the period before the wildfire, maximum of the HDW is fixed at the time of ignition and after that the index values sharply decreases. Scientific novelty and practical significance. The fields of new HDW index reflects the weather conditions typical for wildfires periods in the territory of Ukraine. The presence of specific features and extreme values in the index time course can be used to track the process of increase of fire danger under changing weather conditions to make short- and medium-range fire danger forecasts with evaluation of fire danger classes.