The ChocoJet Experiment (CHOCO-JEX) is an interinstitutional research program developed by the Universidad Nacional de Colombia, the General Maritime Directorate of the Ministry of National Defense of Colombia, the Colombian Air Force, and the Desert Research Institute. The main goal of CHOCO-JEX is to characterize the vertical structure of the low-level Chocó jet (ChocoJet) through observations and modeling. Thus, four 7-day intensive observation periods (IOPs) took place during different seasons in 2016, two over land and two over the far eastern Pacific off the coast of Colombia, including the deployment of upper-air soundings four times per day to monitor the predominant diurnal cycle and the synoptic and seasonal variability. Preliminary results show deeper westerly moisture flow and a stronger diurnal cycle over land than over ocean. IOP4 provides the first observational evidence of the southwesterly ChocoJet with mean winds of 5 m s–1. Diurnal cycles of zonal wind are coherent with mountain–valley and sea–land breezes at low levels and the easterly flow is predominant at midlevels. Potential temperature anomalies appear to be related to gravity waves that modulate the diurnal cycle of precipitation in the region.
According to Tropical Rainfall Measuring Mission (TRMM) and Global Precipitation Measurement (GPM) satellite precipitation composites, a broad maritime area over the far eastern tropical Pacific and western Colombia houses one of the rainiest spots on Earth. This study aims to present a suite of mechanistic drivers that help create such a world‐record‐breaking rainy spot. Previous research has shown that this oceanic and nearly continental precipitation maximum has a strong early morning precipitation peak and a high density of mesoscale convective systems. We examined new and unique observational evidence highlighting the role of both dynamical and thermodynamical drivers in the activation and duration of organized convection. Results showed the existence of a rather large combination of mechanisms, including: (1) dynamics of the Choco (ChocoJet) and Caribbean Low‐Level Jets along their confluence zone, including the Panama semi‐permanent low; (2) ChocoJet deceleration offshore is favored by land breeze, enhancing the nighttime and early morning low‐level convergence; (3) a wind sheared environment that conforms to the long‐lived squall line theory; (4) action of mid‐level gravity waves, which further support the strong diurnal variability; and (5) mesoscale convective vortices related to subsidence in the stratiform region and top‐heavy mass flux profiles. This study emphasizes the multiscale circulation and thermodynamics mechanisms associated with the formation of one of the rainiest spots on Earth and showcases new observations gathered during the Organization of Tropical East Pacific Convection field campaign (OTREC; August–September, 2019) that support the outlined mechanisms.
The diurnal cycle of precipitation and thermodynamic profiles over western Colombia are examined in new GPM satellite rainfall products, first-ever research balloon launches during 2016 over both sea and land, and numerical simulations with the Weather Research and Forecasting Model (WRF). This paper evaluates the Mapes et al. (2003) mechanism for midnight-early morning coastal convection that propagates offshore: reduction of inhibition in the crests of lower-tropospheric internal waves. Shipborne balloon launches confirm the evening development of such inhibition by a warm overhang in saturation moist static energy (SMSE) near 700-800 hPa. This feature relaxes overnight, consistent with the dis-inhibition hypothesis for early morning rains. Over the coastal plain, soundings also show late afternoon increases in near-surface MSE large enough to predominate over the overhang's inhibition effect, driving a second peak in the rainfall diurnal cycle. Parameterized convection simulations fail to simulate the observed coastal rainfall. Still, during a November 2016 wet spell, a cloud-permitting one-way nested 4 km simulation performs better, simulating morning coastal rainfall. In that simulation, however, early morning cooling in the 700-800 hPa layer appears mainly as a standing signal resembling the local radiative effect rather than as a propagating wave. We consider the additional hypothesis that the offshore propagation of that morning convection could involve advection or wind shear effects on organized convective systems. Strong easterlies at mountaintop level were indeed simulated, but that is one of the model’s strongest biases, so the mechanisms of the model’s partial success in simulating diurnal rainfall remain ambiguous.
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