# Spatio-Temporal Distribution of Deep Convection Observed along the Trans-Mexican Volcanic Belt
## 4 Results and Discussion
### Sub-Region Analysis  
The study area has been divided into three sub-regions (see Figure 1b). The Neotropical region includes the coastal plains of the Pacific Ocean and the Gulf of Mexico. The lowlands of the Balsas Basin are considered to be part of this sub-region as well. This sub-region is under the immediate influence of moisture fluxes from the oceans [30]. The Nearctic region is dominated by high and drylands affected by frontal systems from the north [61]. This sub-region covers the southern portion of the Chihuahua Desert. On the other hand, the Transition region holds the main topographic features from the TMVB, the Western Sierra Madre, the Eastern Sierra Madre and the Southern Sierra Madre. This sub-region is influenced by moisture fluxes from both the Pacific Ocean and the Gulf of  
Figure 7: Cross-section at constant latitudes (**a**) 20.5, (**b**) 19.5, and (**c**) 19.0 degrees north of deep convective cloud frequency. The red line represents the total number of observed DCCs (2008–2017) and the blue line the corresponding values for the wet phase only (June, July, August, and September) based on MODIS data with a horizontal resolution of 0.1 degrees. The grey area shows the terrain height in meters. Mexico. Furthermore, the position of these terrain features represents a natural barrier perpendicular to the coastal lines [31]. The geographical location of each area represents unique land surface, meteorological and climatic characteristics. Monthly means of observed DCC frequencies and CF for each sub-region are shown in Figure 8. The values shown here represent a sub-region average per month of the total number of the observed DCCs and are calculated depending on the extension of each sub-region. There is a clear dominance of the Transition sub-region with the highest values (and with the highest variability as well) of observed DCC frequencies throughout almost the entire annual cycle. In this sub-region, two key elements are present: the moisture advection from both the Gulf of Mexico and the Pacific Ocean and the mountainous areas located over the TMVB and the Sierras Madres, enabling orographic convection. Such elements seem to modulate the high frequency of DCCs, mainly over the highest elevations. In the Neotropical sub-region, the presence of a sea breeze over the coastal plains with large mountainous areas perpendicular to the fluxes spread is considered to be the main element in the second-highest observed DCC frequencies. The peak shown in September in this sub-region is related to the passage of tropical cyclones (Figure 8a), and the generation of stratiform clouds (by sea breeze effect) is the cause of the high values of CF (Figure 8b). The Nearctic sub-region holds the minimum frequency of DCCs and the lowest values of CF. This part of Mexico is characterized as the southernmost zone of the dry portion of the country (Chihuahua Dessert) [62]. In general terms, in the Transition sub-region, all the necessary key elements for deep convection activity converge, in the Neotropical sub-region, here is a significant influence of sea breeze and the pass of tropical cyclones, while the Nearctic sub-region holds the dryer characteristics. A decrease in frequency is detected in August for both CF and DCC. Such behavior has been identified in all the sub-regions, but is most evident in the Transition sub-region. This period can be related to the midsummer drought--i.e., canicula--in Mexico. Such a signal of the midsummer drought was also observed in a daily precipitation dataset based on a combination of station and satellite data for Mexico [63]. However, an in-depth and more detailed analysis of the midsummer drought event and how it relates to our observations is necessary for future studies. To the best of our knowledge, the identification of a drought marker in the summertime, the \"canicula\", in the deep convective cloud frequencies obtained from satellite measurements (in this case MODIS) in this part of the country is new.