MCS Initiation and Precipitation over Offshore Northwestern South America: New Observational Evidence on the Role of Diurnal Gravity Waves

Solar heating in the northern Andes and environmental easterly winds lead to strong westward-propagating diurnal gravity waves over the offshore region of northwestern South America. Previous studies have suggested that when the cold phase of diurnal gravity waves propagates to the offshore regions, it can reduce local atmospheric stability and lead to enhanced rainfall propagation in local early morning time. While offshore rainfall propagation has been widely used as evidence for diurnal gravity waves, the role of gravity waves in convective initiation and rainfall over offshore northwestern South America has not been well quantified.

The goal of this study is to improve our understanding of the diurnal gravity wave triggered mesoscale convective system (MCS) and rainfall over the offshore region of northwestern South America. Using 20-year satellite observations, MCS tracking, and ERA5 reanalysis, we will show that the offshore propagating MCS initiation frequency signal has a faster speed (~25-30 m/s) than the offshore rainfall propagation (~15 m/s). Analysis of the temperature tendency fields shows that there are distinct downward propagating cooling signals just before the offshore MCS initiations. The vertical structure of the temperature anomalies is consistent with the vertical structures of diurnal gravity waves in the theoretical linear model. It provides direct evidence for the role of diurnal gravity waves in triggering MCSs over the offshore region of northwestern South America.

However, further analysis of MCS precipitation suggests that the offshore precipitation is only partially contributed by the gravity wave triggered MCSs. In addition, the offshore rainfall is also significantly contributed by: (1) MCSs initiated along the west coasts of Costa Rica and Panama; (2) MCSs initiated over the inland region of northwestern Colombia; (3) MCSs initiated over the offshore side of the west coasts of Colombia and Ecuador; (4) Diurnal MCSs over the open ocean region. The result suggests that several physical processes can contribute to the observed slow offshore propagation of diurnal precipitation over the region. The large-scale environments favorable for MCS initiation over different subregions will also be discussed.