## 3 Results
### Comparative Analyses of Soil Moisture Anomalies Versus Land Surface Temperature Anomalies
#### 3.3.3 Spatial Variability of Correlation between SMAI and LST Anomalies  
To characterize the spatial variability of LST values to indicate drought over Bulgaria, monthly mean correlations of SMAI and LST anomalies from March to October were calculated for the reference period 2007-2018. Their spatial distribution over Bulgaria is visualized in Figure 8; weak correlation coefficients are in reddish colors, strong correlation coefficients in greenish colors. The following dependences in spatial consistency between SMAI and LSASAF LST anomalies during the different stages of vegetation activity were revealed.  
On a mean annual bases (including the whole growing season) the increasing dry anomalies are related to the increase of land surface temperature anomalies. The correlation varies mostly between 0.50 and 0.60 (maximum-0.77) for the whole territory. The color map on Figure 8a indicates that correlations are above moderate, r = \(-\)0.45 on mean annual bases.  
A strong correlation between the anomalies of the two indices in July and August (coefficients mostly around 0.8, maximum 0.84) was obtained (Figure 8d,e). The behavior of LST closely corresponds to the drought severity as indicated by SMAI for the whole country: the lower the SMAI, the higher the LST. This result is consistent with the findings of past studies [51] that the amplitude of the land surface temperature diurnal cycle was related to soil moisture in regions controlled by thermal inertia, not in transpiration-driven regions [52]. The validation study presented in [PERSON] et al. [53] exhibits the same conclusions: results from the comparison show an overall good performance over semiarid regions and degradation toward more wet and vegetated areas. Proceeding from a diurnal cycle [37] to long-term seasonal consideration of correlation between soil moisture and land surface temperature, the results of our study confirm this effect. In the spring and autumn months, the land-atmospheric interaction is a transpiration-driven process that determines the specific seasonal behavior demonstrated in Figure 8b-f.  
Figure 7: Monthly mean anomalies distribution over Bulgaria, 2012 of SMAI (50 cm soil layer) for **(a)** July, (b) August, and (c) September, and of LSASAF LST 0900 UTC for **(d)** July, (e) August, and (f) September. Anomalies are calculated for the 2007–2018 period.  
In May (Figure 8b), the correlation is about 0.60, lower than in July/August for most of the country and even much lower for NUTS in the western mountain part and the mountain region in Southern Bulgaria. During that period of the year soil moisture is high and LST still not so high compared to summer months (see Figures 2a and 3b).  
For June (Figure 8c), the correlation tends to become worse. This behavior is due partly to the increased soil moisture as a result of convective precipitation and also to the maximum of solar insolation input, which is related to the increase of LST. The convective activity exhibits its maximum in June and especially near the predominantly mountain areas in the western part of Bulgaria, showing the lowest correlation between SMAI and LST.  
In contrast to the behavior in June, the correlation in the western part of Bulgaria is much better in October (Figure 8f), while it is weak over the eastern part of the country. Such a behavior can be explained by the progressive increase of precipitation and soil moisture over eastern Bulgaria due to the enhanced appearance of cyclogenesis over the Aegean Sea that occurs predominantly throughout the cold months (from October to May, [54]). The activation of cyclogenesis in the Eastern Mediterranean basin in October is also confirmed by [55]. The cyclone development often occurs over the Black Sea associated with enhanced latent heat release from warm sea water in October. The increased horizontal moisture flux and resulting increase in moisture flux convergence is consistent with the inland precipitation maximum over the region of eastern Bulgaria in this period [42].