Investigation of the dynamic cloud structures and meteorological analysis of heavy rain event that caused flash flood in Hopa on August 24, 2015

Eastern Black Sea Region receives the most rainfall among the regions in Turkey. Precipitation in this region can reach to 2350 mm per year. The rainfall is about four times the average annual precipitation of Turkey in the region of Hopa, Artvin province, which is located in the region where rainfall occurs every month of the year. Because of the heavy rainfall and the interventions on the stream beds, Hopa is a region where sudden flood disaster is often experienced. Being able to form even in the driest and most humid areas, flood can be defined in many different ways. Flood can be defined as overflowing of the areas that are dry under normal conditions due to rising water bed by natural or non-natural causes. Nature has a self-preservation mechanism. Within this process, flood is accepted as a hydrometeorological incident, but as a result of the disruption of the balance of nature, floods can turn into a disaster. Formation of a flood can take several minutes to several days. The short floods are called sudden floods. Sudden floods usually occur as a result of heavy rainfalls that take shorter than six hours. Despite the fact that the main factors of sudden floods are the duration and amount of precipitation, the moisture status, permeability and geographical conditions of the previous soil, that are other causes of flooding, are also of great importance. As a result of the sudden flood disaster that took place in Hopa District of Eastern Black Sea Region on August 24, 2015, eight citizens lost their lives and a great amount of physical damage occured. In this study, sudden flood disaster which occurred in Artvin-Hopa district on August 24, 2015 was investigated in terms of synoptic and hydrometeorological aspects, and the prediction capability of IFS, ALR and WRF models and Flood Early Warning System results which are frequently used in rainfall forecasts were examined. In addition, cloud systems causing excessive rainfall have been identified and cloud dynamics have been investigated. Within the scope of the study, the synoptic conditions causing sudden flood on August 24, 2015 were investigated. As a result of this analysis; the humid air coming from the Black Sea, the surface temperatures being higher than the averages of August, the surface and the dew point temperatures being very close to each other. At 850 hPa level, cold air inflow and low contour values were observed. At 700 hPa level; a high level groove and relative humidity close to 100% take attention in the Aegean and the Black Sea region. At 500 hPa; presence of cold cathode and high levels of moisture as at the and 700 hPa were detected. At 300 hPa, it was determined that there was a strong jet flow over Turkey. As a result, it has been determined that the atmospheric conditions at the investigated levels create ideal conditions for convective and orographic cloud and precipitation formation. In addition to this, topographical effects have been shown to support the convective systems of Hopa and its surroundings. Although no atmospheric sounding measurements were made in the Hopa town of Artvin province, in the analysis of instability using the Dzugba (Russia) station nearest to the region, it was observed that all indecision indices entered a severe thunderstrom potential scale. In the analysis of the instability, the CAPE predictions of the IFS model show that the atmosphere over Hopa is unstable since the evening of August 23rd. This explains the severe convective movement that brings extreme rainfall to the scene. It was determined that extreme precipitation caused by the sudden flood disaster in Hopa started early in the morning of August 24, and reached maximum between 09:00 - 10:00, causing sudden flood disaster in this area. According to the precipitation recurrence analysis, it was determined that the sums of 60, 120, 180, 240 and 300 minutes of the 287.2 mm daily total rainfall in Hopa had a recurrence interval of more than 100 years. The dBZ value obtained from the meteorological radar at these times appears to be the values that enter the Severe-Extreme precipitation scale. For this region, it is seen that WRF model gave the best estimates of this convective and orographic rainfall area in Hopa when the precipitation outputs of the IFS, ALR and WRF models were compared to the ground observations. It was observed that although the rainfall was local, the results of the Flash Flood Early Warning System (FFGS) were very successful in localizing the rainfall zone, but not in estimating the amount of rainfall in the Hopa region. Using NOAA's HYSPLIT model, it was determined that air parcels of 50 m (1000 hPa), (850 hPa) and 3000 m (700 hPa) in standard atmospheric levels would reach Hopa at low levels over the Black Sea for a long time when traversing the air parcel delivering maximum rainfall to maximum, thus increasing the moisture levels in all levels. After the use of Doppler radars in the field of meteorology, the reflectivity (dBZ) values obtained in the scanning mode and precipitation estimation have been replaced by Doppler radar products. Although this approach is still a good approximation for cumuliform clouds, that for stratiform clouds can make up to 2-fold mistakes based on the amount of rainfall measured. As Hopa is in the average distance of 160 km to the Trabzon radar, Doppler radar products (maximum distance 120 km) can not be used there. For this reason, although the reflectivity values obtained from the scanning mode radar data do not give information on the amount of precipitation directly, the reflectivity values for cumuliform clouds can predict the amount of precipitation. The hourly mean reflectivity values are on the Severe/Extreme precipitation scale during the hours when morning rainfall reached maximum levels on August 24. The scan mode dBZ values also showed a good correlation with precipitation. These correlations indicate that using reflectivity values for cumuliform clouds is still a valid method. In addition, distribution graphs are drawn and regression statistics are given in order to determine the significance of precipitation amount and reflectivity values. Rainfall amount and reflexivity values were statistically significant according to the ANOVA significance test. It can be concluded that 48.5% of the measured reflectivity values can be explained by the amount of precipitation when 70% of the data are collected. In this study, 2nd Generation Meteosat Satellites (MSG) products were used to determine the cloud dynamic structures. From these products, RGB images led to the observation that Cumulonimbus clouds around Hopa and its vicinity causing disaster are formed by convective and oreographic effects of the cumuliform clouds. The active clouds during the precipitation were founnd to be Cumulonimbus (CB) clouds. It has been determined that the ascending movements composed of ice crystals are active in the CB clouds that have begun to develop from the 23rd of August night onwards and that the ascending and descending movements have completed the maturity of the CB clouds after reaching an equilibrium in the afternoon. It was observed that the precipitation capacity reached the maximum level during the developmental phase of the CB clouds and stayed at intermittently mild level after reaching the maturity phase. During the rain that caused the sudden flood in Hopa, no frontal effect was found. Despite the presence of high vorticity anomalies on the Black Sea during the day on August 24, 2015, hot air masses on Hopa were found to be effective. It was also determined that the level of tropopause was high throughout the day due to the severity of convective activity and the hot air mass. In addition, cloud peak temperatures, elevations, and convective precipitation rates were determined using the NWC-SAF / MSG Cloud Topping Height and Elevation Product (CTTH) and Convective Rain Rate Product (CRR) of MSG satellites. It has been observed that there is a very high degree of collocation between the cloud top temperature and the height due to the convective and orographic effects of active clouds during disasters. In general, when the cloud top temperatures are compared with the ground precipitation observations, it is observed that the amount of precipitation is high in cases where low cloud top temperatures are observed. Despite the fact that rainfall is completely convective and with orographic effects, rainfall data obtained with the SAF-NWC / MSG convective precipitation product (CRR) show a reasonable distribution with ground measurements and cloud peak temperatures, but precipitation estimates are lower than measured rainfall. Regression analyzes were performed and distribution plots were drawn to determine the significance of the data obtained from the SAF-NWC / MSG cloud products. According to the ANOVA significance test results, cloud peak temperature - rainfall amount, convective rainfall estimate - hourly observed rainfall and cloud peak temperature - convective rainfall correlations were statistically significant (<0.05). The correlation rates were high; being 50% between the cloud peak temperature and precipitation rate, 75% between convective precipitation estimates and hourly total precipitation based on observed ground measurements, and 74% between hourly average cloud peak temperature and convective precipitation estimates.