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Abstract:The Caucasus Region has been affected by an increasing number of heat waves during the last decades, which have had serious impacts on human health, agriculture and natural ecosystems. A dataset of 22 homogenized, daily maximum (Tmax) and minimum (Tmin) air temperature series is developed to quantify climatology and summer heat wave changes for Georgia and Tbilisi station between 1961 and 2010 using the extreme heat factor (EHF) as heat wave index. The EHF is studied with respect to eight heat wave aspects: event number, duration, participating heat wave days, peak and mean magnitude, number of heat wave days, severe and extreme heat wave days. A severity threshold for each station was determined by the climatological distribution of heat wave intensity. Moreover, heat wave series of two indices focusing on the 90th percentile of daily minimum temperature (CTN90p) and the 90th percentile of daily maximum temperature (CTX90p) were compared. The spatial distribution of heat wave characteristics over Georgia showed a concentration of high heat wave amplitudes and mean magnitudes in the Southwest. The longest and most frequently occurring heat wave events were observed in the Southeast of Georgia. Most severe heat wave events were found in both regions. Regarding the monthly distribution of heat waves, the largest proportion of severe events and highest intensities are measured during May. Trends for all Georgia-averaged heat wave aspects demonstrate significant increases in the number, intensity and duration of low-and high-intensity heat waves. However, for the heat wave mean magnitude no change was observed. Heat wave trend OPEN ACCESSClimate 2015, 3 309 magnitudes for Tbilisi mainly exceed the Georgia-averages and its surrounding stations, implying urban heat island (UHI) effects and synergistic interactions between heat waves and UHIs. Comparing heat wave aspects for CTN90p and CTX90p, all trend magnitudes for CTN90p were larger, while the correlation between the annual time-series was very high among all heat wave indices analyzed. This finding reflects the importance of integrating the most suitable heat wave index into a sector-specific impact analysis.

<p>In this study, the latest version of the Abdus Salam International Center for Theoretical Physics (ICTP) Regional Climate Model RegCM4.7.0 is used to simulate climate of Georgia for the period 1986-2005.</p><p>Georgia is the mountainous country located in the south-western part of the Greater Caucasus. Its area is 69.875 km<sup>2</sup>. Mountains cover significant part of the territory 54% of them is located at 1,000 m elevation. From the west Georgia is washed by the Black Sea, from the south it borders with Turkey and Armenia, from the south-east – with Azerbaijan and from the north – with the Russian Federation.</p><p>Georgia displays diverse climate and vegetation types: there are almost all climate types from high mountains eternal snow and glaciers to steppe continental climate of eastern Georgia and the Black Sea coastal subtropical humid climate.</p><p>To simulate climate with high horizontal resolution and represent more special details for the complex terrain of Georgia the double-nested dynamic downscaling method has been used. First, RegCM was driven by ERA-Interim data at a grid spacing of 50 km. For 50 km resolution simulation, we defined central latitude and central longitude of model domain clat=42.27, clon=42.70 degrees as well as 30 number of points in the N/S direction and 60 number of points in the E/W direction. The 12-km resolution RegCM simulation was nested in the simulation at 50 km resolution. For 12 km resolution simulation, we chose central latitude and central longitude of model domain clat=42, clon =43 degrees as well as 48 N/S 100 E/W points. We selected domain size to be large enough to account for the relevant large-scale processes (such as the large-scale flow modulations due to orographic features and water bodies)<em> </em>but at the same time small enough in size to minimize the use of computational resources.  </p><p>We have used the default BATS (Biosphere-atmosphere transfer scheme) land surface parameterization scheme, Emanuel cumulus convective parameterization scheme, SUBEX (Sub-grid Explicit Moisture Scheme) moisture scheme and Holstlag planetary boundary layer scheme for the simulations.</p><p>The simulated surface annual and seasonal air temperature and precipitation as well as extreme climate events are compared with Climatic Research Unit (CRU), ERA5 reanalysis, GPCP data sets. For extreme events analyzes, we chose and used some indices, defined by the Expert Team on Climate Change Detection and Indices, recommended by the World Meteorological Organization.</p><p>This work was supported by Shota Rustaveli National Science Foundation of Georgia (SRNSFG) № FR-19-8110.</p><p> </p>

Sixteen temperature minimum and maximum series are used to quantify annual and seasonal changes in temperature means and extremes over Georgia (Southern Caucasus) during the period 1961 and 2010. Along with trends in mean minimum and maximum temperature, eight indices are selected from the list of climate extreme indices as defined by the Expert Team on Climate Change Detection and Indices (ETCCDI) for studying trends in temperature extremes. Between the analysis periods 1961-2010, 1971-2010 and 1981-2010 pronounced warming trends are determined for all Georgia-averaged trends in temperature means and extremes, while all magnitudes of trends increase towards the most recent period. During 1981 and 2010, significant warming trends for annual minimum and maximum temperature at a rate of 0.39°C (0.47°C) days/decade and particularly for the warm temperature extremes, summer days, warm days and nights and the warm spell duration index are evident, whereas warm extremes show larger trends than cold extremes. The most pronounced trends are determined for summer days 6.2 days/decade, while the warm spell duration index indicates an increase in the occurrence of warm spells by 5.4 days/decade during 1981 and 2010. In the comparison of seasonal changes in temperature means and extremes, the largest magnitudes of warming trends can be observed for temperature maximum in summer and temperature minimum in fall. Between 1981 and 2010, summer maximum temperature shows a significant warming at a rate of 0.84 °C/decade, increasing almost twice as fast as its annual trend (0.47 °C/decade). The Georgia-averaged trends for temperature minimum in fall increase by 0.59 °C/decade. Strongest significant trends in temperature extremes are identified during 1981 and 2010 for warm nights (4.6 days/decade) in summer and fall as well as for warm days (5.6 days/decade) in summer. Analyses demonstrate that there have been increasing warming trends since the 1960s, particularly for warm extremes during summer and fall season, accompanied by a constant warming of temperature means in Georgia.

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