OTREC: Organization of Tropical East Pacific Convection

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This award funds planning and organizational activity for a field campaign titled Organization of Tropical East Pacific Convection (OTREC). By making the award NSF encourages the PI and his collaborators to submit a set of proposals requesting support for each of the various campaign activities, which are reviewed collectively at a later date. This second round of reviews determines whether the campaign goes to the field and what elements of the campaign are supported by NSF.The goal of OTREC is to understand the formation and development of tropical convective clouds and their associated heavy rainfall in the adjacent but distinct regions of the eastern equatorial Pacific and the southwest Caribbean. The sea surface temperature (SST) in the eastern equatorial Pacific has a strong north-south temperature contrast, with temperatures increasing northward from the "cold tongue" produced by the upwelling of deep water along the equator. Winds near the surface generally blow in the direction from colder to warmer SST until they reach the intertropical convergence zone (ITCZ), roughly between four and twelve degrees north of the equator. The ITCZ is a narrow region of cloudiness and convection which is generally regarded as a consequence of wind convergence near the surface. But the PI hypothesizes that other factors are required to produce deep, organized convection and heavy precipitation found in the eastern Pacific ITCZ. These other factors include atmospheric stability, mid-tropospheric vorticity (a term which combines rotation and horizontal shear of the wind), and moist thermodynamics, combined in a theory developed by the PI and others (see for example AGS-1546698).In contrast, the southwest Caribbean is a region of uniform SSTs which do not favor low-level wind convergence, and air at mid-tropospheric levels tends to be quite dry. These conditions are unfavorable for convection yet convection forms regardless, and the campaign seeks to understand the mechanisms through which the obstacles to convection are surmounted. One possible mechanism involves atmospheric motions forced by the adjacent Central American landmass, either by the diurnal cycle of solar heating or the blocking effects of topography.A further issue to be addressed is the formation mechanisms of easterly waves, which are the most frequent weather disturbances in the tropical east Pacific and in the SW Caribbean. Easterly waves are westward-moving atmospheric disturbances with wavelengths of roughly 2,000km which appear on a weekly basis and sometimes develop into hurricanes. Easterly waves over the Atlantic and Caribbean generally originate over Africa (see for example AGS-1433764), but the formation mechanisms for easterly waves in the eastern equatorial Pacific are unclear. OTREC campaign data would be used to determine the relative contributions of mechanical and thermodynamic mechanisms to the formation and intensification of these waves.OTREC would use the Gulfstream V (GV) research aircraft maintained by the Earth Observing Laboratory of the National Center for Atmospheric Research (NCAR) as its primary observing platform. The GV would be based in Costa Rica, allowing easy access to the east Pacific and southwest Caribbean. Observations of winds, pressure, temperature and humidity would be taken using dropsondes, which use essentially the same instrument package as standard weather balloons only dropped from the aircraft with a small parachute. Additional cloud measurements would be taken with a W-band radar mounted in a wing pod, and ground-based observations are also proposed for the campaign. A key objective of the campaign is to determine the convective mass flux associated with tropical convection, as this quantity plays a central role in the theory of convective cloud development which the campaign seeks to test.Tropical convection plays an important role in weather and climate, as it can produce extreme precipitation and develop into hurricanes with damaging impacts. It can also perturb atmospheric circulation in ways that affect weather and climate over long distances, as can be seen for example in the effects of El Nino events on the continental US. Thus research on tropical convection is of societal as well as scientific interest, as better understanding of tropical convection can lead to improvements in models used to forecast daily weather and anticipate climate variability and change. The campaign would also have broader impacts through the development of international scientific partnerships with researchers in Central America, and would provide a valuable educational opportunity for graduate students in atmospheric science.