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    Defining Holdridge's life zones over Turkey
    (Wiley, 2016) Tatlı, Hasan; Dalfes, H. Nüzhet
    This study is proposing a classification for the Holdridge's life zones associated with the surface climatic variables over Turkey. Holdridge's scheme is aiming to group meteorological and climatic quantities at the regional scale according to some pre-defined rules. On the other hand, without rules or pre-defined restrictions, the process of combination of similar stations in groups is recognized as clustering. A spectral clustering method was suggested for identifying the characteristics of the local climate over the country. To increase the proxy value of the estimated life zones, the missing values in the data sets of temperature and precipitation series were filled by fitting appropriate theoretical variogram. A programme coded in Fortran 95 was developed for determining the life zones. The probabilistic features of the life zones were estimated by using empirical cumulative frequencies. In addition, a new spectral clustering method based on Sampson correlation ratio was developed for clustering high-dimensional data. From these results, four major categorical life zones were obtained, namely montane, lower montane, cool temperate and warm temperate. Additionally, 14 types of the Holdridge's vegetation classes were estimated, the frequent zones are cool temperate steppe and warm temperate dry forest; some moist forests are seen particularly in the eastern Black Sea region, the rainiest region of the country. The suggested spectral clustering method is capable of capturing the principal life zones. The tendency of life zones shifting towards the climate conditions of lower latitudes indicates that the long-term meteorological normals changed; this is exactly the point of climate change. A better understanding of life zones of Holdridge hopefully can contribute to sustainability plans of the policy makers.
  • Küçük Resim
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    Long-Time Memory in Drought via Detrended Fluctuation Analysis
    (Springer, 2020) Tatlı, Hasan; Dalfes, H. Nüzhet
    The persistence of drought events largely determines the severity of socioeconomic and ecological impacts, unfortunately the performance of current weather forecasting models (WFM) to simulate such events is subject to great uncertainties. This study is investigating time-domain characteristics of drought persistence over Turkey by applying the detrended fluctuation analysis (DFA) method to the Palmer drought severity index (PDSI). The existence of long-range power-law correlation in PDSI fluctuations is demonstrated for time scales ranging from monthly to decadal. Understanding of such statistical patterns in PDSI values can definitely be a step forward in drought predictability. From a climatological point of view, it is found that the areas with high level DFA scaling exponent (generalized Hurst) indicate the areas of higher sensitivity to droughts and associated risks. Furthermore, the characteristics of the persistence of the PDSI in climate zones have also been examined by applying the Holdridge Life Zones (HLZ) classification. HLZ classification over Turkey leads to two climate-zones: cool-temperate and warm-temperate. In addition, when topography is taken in account, montane (cool-temperate) and lower-montane (warm-temperate) climate zones can be treated as two different zones. It has been observed that the predictable index (PI) of the PDSI derived from the DFA Hurst exponent is relatively high in the cool-temperate and montane climate zones compared to others. In fact, very different PI values were also obtained in a few HLZ climate classes within the same climate zone and with same vegetation index (i.e. steppe, dry-forest, warm-forest etc.).