Analysis of Possible Triggering Mechanisms of Severe Thunderstorms in the Tropical Central Andes of Peru, Mantaro Valley

Abstract

The aim of the present study is to analyze the triggering mechanisms of three thunderstorms (TSs) associated with severe rainfall, hail and lightening in the tropical central Andes of Peru, specifically above the Huancayo observatory (12.04 ∘ S, 75.32 ∘ W, 3313 m a.s.l.) located in the Mantaro valley during the spring-summer season (2015–2016). For this purpose, we used a set of in-situ pluviometric observations, satellite remote sensing data, the Compact Meteorological Ka-Band Cloud Radar (MIRA-35C), the Boundary Layer Tropospheric Radar and downscaling model simulations with the Weather Research and Forecasting (WRF) Model (resolutions: 18 km, 6 km and 2 km), and the Advance Regional Prediction System (ARPS) (resolution: 0.5 km) models in order to analyze the dynamic of the atmosphere in the synoptic, meso and local scales processes that control the occurrence of the three TS events. The results show that at synoptic scale, the TSs are characterized by the southern displacement of the South-east Pacific Subtropical Anticyclone up to latitudes higher than 35 ∘ S, by the weakening and south-eastern displacement of the Bolivian high–North east low system and by the intrusion of westerly winds along the west side of the central Andes at upper and medium levels of the atmosphere. At meso-scale, apparently, two important moisture fluxes from opposite directions are filtered through the passes along the Andes: one from the north-west and the other from the south-east directions converge and trigger the deep convection into the Mantaro valley. These moisture fluxes are generated by the intrusion of the sea-breeze from the Pacific ocean along the west of the Andes coupling with upper and middle westerly winds and by the thermally induced moisture fluxes coming from the South American low level jet at the east side of the Andes. At the local scale, there is a low-level conditional instability in the previous hours as well as during the occurrence of the TSs above the Huancayo observatory. In addition, the simulation results indicated the possibility of generation of inertial gravity waves in the Amazon basin, associated with geostrophic adjustment which transports energy and moisture into the central Andes plateau and consequently intensifies the thunderstorms above the Mantaro valley.

Diurnal Cycle of Raindrops Size Distribution in a Valley of the Peruvian Central Andes

“…This spatial pattern shows the dynamics of the cloudiness that presents a large amount of cloudiness along the Andes cordillera (AC) and on the opposite side the North-East cloudiness (CC). This configuration with cloudiness regions CC and AC is present during the daytime (Figure 13) when the flows of the Pacific Ocean pass over the mountain range and interact with the moist flows from the Amazon over the Mantaro valley [22,25,44]. Therefore, the convective rains mentioned in Table 3 depend mostly on the configuration of the AC and CC regions.…”

“…In general, the rains that are registered in the Mantaro valley are fed mainly by those two sources; one on the north side of the valley and the other on the south side (Figure (Figure 13e,f). Probably, the interaction between moisture fluxes coming from the west side (Pacific Ocean) and the east side (Amazon basin) of the Mantaro valley generates intense convective activity associated with the occurrence of thunderstorms and heavy rains as in the case studies discussed by [22,25]. This dynamic process coincides with the decay of the direct effect of solar radiation.…”

“…It is clearly observed that the RSD spectrum is different for the three types of rainfall events. The general dependence of the form of the spectra and the spectral values with rainfall rate class is independent of time of day, as the for all the intervals the slope of the spectrum decreases with rainfall rate, but it can be noticed that the largest diameters are reached in the time interval (15)(16)(17)(18)(19)(20), corresponding to the time of maximum development of convection, where hailstorms occur in the valley [22,23,40]. In the morning and early afternoon (09-14 LST) also occur spectra with large raindrops in the high rainfall rate class, corresponding to early developing storms that can also produce hail in some cases.…”

“…In the diurnal cycle, the maximum rainfall occurs in the afternoon when there is simultaneous interaction between the flows (Amazon and Pacific Ocean). Such as the case studies of convective events that was shown by [25] and Flores-Rojas et al (2019) [22]. These events developed under the influence of low and medium flows from the Amazon, at the same time that the high-level flows from the Pacific Ocean ascending or detouring the mountain barriers, causing the formation of windward orographic ascent and cloud formation or the onset of convection on the valley, as discussed in in [25].…”

“…Flores-Rojas et al (2019) [22] analyzed the dynamical mechanisms of the formation and moisture sources at different atmospheric scales of three convective storms in the Mantaro basin. At the west side of the Andes the westerly circulations at high and mid-levels are coupled with the flow from the Pacific Ocean, carrying moisture into the Central Andes and generating orographic convection, while at the east side of the Andes, which is a transition region limiting with the Amazon, there is a significant moisture flux associated with the South American low level jet.…”