# Spatio-Temporal Distribution of Deep Convection Observed along the Trans-Mexican Volcanic Belt
## 4 Results and Discussion
### Inter-Annual Variability  
Changes in wind regimens, climatic oscillations and tropical cyclone activity can play a significant role in the spatio-temporal distribution of precipitation [64; 65]. In the study area, where convective precipitation is relevant, research to improve the understanding of the reasons behind spatio-temporal variations of deep convective activity, also on larger temporal scales, can further benefit from estimates of inter-annual variabilities of deep convective cloud frequencies obtained from satellite observations. To this end, the annual anomaly of the total number of observed DCC is presented for the 10 year time period; see Figure 9. The annual DCC anomaly results from the difference between the annual mean (2008-2017) and the value of the year of analysis. From this, the presence of relatively dry (e.g., 2011, 2017) and wet (2010, 2013, 2015) years can be identified within this 10 year time period. The transition between wet or dry years can be divided into an explosive stage--i.e., the change comes from one year to another; for example, 2010-2011--or a progressive stage, where the change comes with an intermediate year; for example, 2011-2012-2013. Interestingly, the transitions identified through the anomalies in Figure 9 are more distinct over the coastal regions, at the east and west edges of the domain, suggesting that these regions are more sensitive to large-scale dynamic changes. It is important to mention that the increase or decrease of observed DCC frequency can be concentrated in only a couple of months within the 10 year time period. For example, from Figure 8a it is evident that the Transition sub-region shows a significant variability mainly in the summer months. Figure 8: Monthly means (lines) and one standard deviation (shaded areas) of **(a)** observed deep convective cloud (DCC) frequency, **(b)** cloud fraction (CF), **(c)** precipitation threshold values and **(d)** number of days with precipitation exceeding 10 mm per Nearctic, Neotropical, and Transition sub-regions. Moreover, for the coastal regions, especially in the east, the annual mean DCC frequency difference between two subsequent years (2010 and 2011) can be about twice as large as the largest difference between monthly means inferable from Figure 8a (Neotropical sub-region), indicating the relevant impact of (changes in) large-scale dynamics on observed annual DCC distribution in the study area. A one-sample, two-sided student t-test was performed to identify where external events (such as climatic oscillations) could be mainly responsible for the modification of the annual statistical values of the number of convective events in the Trans-Mexican Volcanic Belt and where local events predominate. The hypothesis was applied that the mean value of the number of convective events of a specific year did not vary significantly concerning the mean value of the data set of the entire 10 year period considered. Figure 9 includes the results for the period 2008-2017 for a confidence level of 95%. In the year 2011, it is observed that there is a large region in the northeast where the hypothesis is rejected. This indicates that external processes might influence and modify the number of convective events. There are other small regions, predominantly in the southern part, where the hypothesis is also rejected. Changes in weather regimes derived from the influence of climatic oscillations could have an impact on the study area and the DCC frequency. These preliminary findings  
Figure 9: Annual anomaly of the total number of observed deep convective clouds (2008–2017) based on MODIS cloud data, with a horizontal resolution of 0.1 degrees. The black lines indicate the grid boxes with a significant anomaly. indicate the necessity to perform more in-depth analyses on observational datasets that extend beyond 10 years to further investigate which dynamics cause these modifications.