Calculation of precipitation during period of catastrophic flood 21-27 july 2008 in Ukrainian Carpathians
Authors: Udovenko O.I. , Kovalets I.V. 
Published: November 2018

Intense precipitation event happened in Ukraine on 21-27 July 2008 leading to extreme flash floods in the Ukrainian Carpathians which are characterized by the return period of approximately 50 years. Besides favourable synoptic conditions leading to quasi-stationary low situated over Balkans and South-West part of Ukraine precipitation during this event was intensified by mountains. This lead to formation of the specific precipitation patterns in the Ukrainian Carpathians which was not adequately resolved by the existing measurement network.

The purpose of this publication is application of the mesoscale meteorological model WRF with high resolution (1 km) for reconstruction of precipitation during flood in July, 2008 in the Ukrainian Carpathians and evaluation of the simulated results against measurements.

Calculation of precipitation during the period of catastrophic summer flood in 2008 at the territory of Ukrainian Carpathians had been carried out with the use of meteorological model WRF. The precipitation field was well simulated for the period of maximum floods (July, 25-th) as compared to precipitation data measured at meteorological stations. However the second and lower peak of precipitation which happened during July, 27-th was underestimated by WRF. The reason of such underestimation is possibly the influence of boundary conditions on simulated results. The precipitation field formed during  July, 25-th is elongated along the main ridge of the Ukrainian Carpathians and precipitation maximums on that date, reaching up to 70 mm/3 hours are situated above foothills of the Carpathians. On July, 27-th the precipitation maximums are shifted south-eastward, close to the boundary of the computational domain. The vertical crossections of the calculated meteorological fields demonstrate characteristic system of gravitational waves occurring in flow above mountains and sequence of convective cells situated mostly above the windward (north-eastern) hills and coinciding with the precipitation maximums. The depth of some of the convective cells reaches 10 km, which is supported by satellite data. The indirect evaluation of the calculated precipitation field is performed using the measured water discharge data at the watersheds of the rivers in Carpathians. For the period of maximum flood the relative precipitation amount at each watershed (over total precipitation amount at all water-sheds) appears to be approximately equal to relative water discharge of the corresponding watershed.

In conclusion we could state that the precipitation field was well simulated by WRF for the period of maximum floods (25 July). High quality of simulated results is supported by comparison of the calculated and measured precipitation as well as with indirect juxtaposition of the calculated integral precipitation at the watersheds of Carpathian rivers and water discharges in the corresponding river outlets. The calculated results demonstratre that existing measurement network is too sparse to capture the details of  the orographically enhanced precipitation field. The scarcity of the measurement network makes it difficult to evaluate integral and maximum characteristics of precipitation happening in the Ukrainian Carpathians.

Directions for further research should include usage of the coupled mesoscale meteorological-distributed hydrological model chain for calculation and forecasting of meteorological and hydrological characteristics of floods.