# Spatio-Temporal Distribution of Deep Convection Observed along the Trans-Mexican Volcanic Belt
## 4 Results and Discussion
### Spatio-Temporal Distribution of Deep Convective Clouds and Severe Weather Events  
The monthly distribution of observed DCC frequencies on a grid with a spatial resolution of 0.1 degrees in the study area is presented in Figure 3. Note, the numbers are considered to represent single convective events for each grid box. As for the cloud  
Figure 2: Monthly distribution of mean cloud fraction based on Moderate-Resolution Imaging Spectroradiometer (MODIS) cloud data (2008–2017) with a horizontal resolution of 0.1 degrees. fraction, wet, dry and transition phases are noticed. In the first two winter months, the higher frequencies are located over the Gulf of Mexico and can be related to the passage of mid-latitude frontal systems enhanced by orographic effects [55], which may also consist of high COTs and low CTPs. In the next two months, in the dry season, not more than 30 DCCs are observed. May is the first transition phase, and it is when the terrain features start to play a significant role in the increase of observed DCC frequency. Between June and September, the wet phase is established. In this period, the highest observed DCC frequencies are extended along the mountains of the TMVB. Some portions of the domain show total values of over 100 DCC events per month; i.e., about 10 events per month on average or one-third of total days with convective activity. The spatial distribution of observed DCC frequency, especially in this active phase, is consistent with the spatial distribution of cloud-to-ground lightning activity described by [PERSON] et al. [26], mainly in the western part of the domain. In this wet season, the interaction of the moisture fluxes from the Gulf of Mexico and then from the Pacific Ocean (in late summer) with the terrain features seems to modulate the deep convection activity. The spatio-temporal behavior of the convection shown here shares some main characteristics with the results of [PERSON] and [PERSON] [56] who described the convection and rainfall in Mexico and their relationship with the Southern Oscillation using highly reflective cloud data, upper air soundings and surface ship observations; for example, the peak activity in September along the east coast of the domain and the highest DCC frequency in the summer months in central Mexico. Another remarkable aspect is the spatial transition from east to west of the observed DCC frequency; possible reasons for this are discussed later. October is identified as a second transition phase. Although the observed DCC frequencies are low, the most active zone is over the western portion of the TMVB. The last months, November and December, follow the same pattern as January and February, but with a general decrease, even in the Gulf of Mexico. A possible explanation comes from the change of atmospheric circulation patterns and moisture availability. At this stage, dry and cold air masses from the north begin to have some influence on the study area [56; 57]. These air masses work as convective inhibition elements, mainly in the northern portion of the complex terrain of the TMVB. To assess the spatio-temporal relationships between the observed DCC frequency and extreme precipitation events, the monthly distribution of the number of days with daily  
Figure 3: Monthly distribution of the total number of observed deep convective clouds based on MODIS cloud data (2008–2017) with a horizontal resolution of 0.1 degrees. precipitation exceeding 10 mm (Figure 4) and the monthly distribution of precipitation threshold for the 90 th percentile (Figure 5) were computed using the CHIRPS dataset. The 10 mm threshold of daily precipitation represents heavy precipitation days and is useful to identify the regions in which these events are common (adopted from the European Climate Assessment (ECA) indices of extremes). On the other hand, the 90 th percentile helps to characterize the behavior of the extreme precipitation events within the study area.