OTREC: Convective Heating Profiles and the Transition from Shallow to Deep Convection over the Tropical East Pacific and Southwest Caribbean

点击翻译按钮获取中文摘要

This award supports work on the Organization of Tropical East Pacific Convection (OTREC) field campaign. The campaign seeks to understand the formation and development of tropical convective clouds and associated heavy rainfall in the adjacent but distinct regions of the eastern equatorial Pacific and the southwest Caribbean, along with their evolution over the intervening portions of Central America and Colombia. The campaign also examines the genesis and evolution of easterly waves, large westward-moving atmospheric disturbances (wavelengths of about 2,000 km) which occur on a weekly basis in the Caribbean and eastern Pacific. The full set of campaign awards is discoverable on the nsf.gov/awardsearch webpage by a keyword search on "OTREC".Tropical convection plays a critical role in earth's climate system and is also responsible for extreme precipitation and hurricane formation. Easterly waves are of particular interest for weather forecasting given their tendency to spawn hurricanes. More generally, tropical convection is an engine of weather and climate worldwide yet poorly understood and difficult to simulate. Work leading to better representation of tropical convection in weather and climate models can thus lead to better weather forecasts and impact assessments, both for the public and decision makers. Aside from the societal relevance of the science, the campaign includes several education and outreach activities including support for undergraduates to participate in fieldwork and production of short documentary videos for use in classroom teaching and informal science education. Work performed under this award fosters international scientific collaboration with scientists in Colombia and Costa Rica and helps to develop local infrastructure for rainwater collection and isotopic analysis.The eight week OTREC deployment consists primarily of a set of 20 flights in the Gulfstream V (GV) research aircraft maintained by the National Center for Atmospheric Research (NCAR). The campaign uses an airport in Costa Rica for easy access to the equatorial Pacific and southwest Caribbean study regions, and conditions in these regions are sampled using dropsondes and a wing-mounted W-band radar. Dropsondes contain the same instrument package as standard weather balloons, only dropped from an aircraft (in this case from about 40,000 feet) with a small parachute. Work performed here augments the aircraft observations through the collection and analysis of rainwater samples from ground sites in Costa Rica and Colombia.Despite intensive study we lack a satisfactory theory for how environmental conditions determine when and where tropical convection will form, how long it will last, how much it will expand and organize, and how much rain it will produce. Vertical structure is a key property of tropical convection that determines its evolution and its interactions with the circulation and thermodynamics of the surrounding atmosphere. The vertical structure can be simply characterized as top- or bottom-heavy, according to the relative amounts of rising motion that occur in the upper and lower troposphere.Work conducted here seeks to determine the vertical structure of tropical convection by measuring the ratios of stable isotopes in rainwater produced by it. Rainwater is almost entirely composed of ordinary H2O, but it contains trace amounts of water in which deuterium (D) replaces hydrogen (H) to form HDO, or oxygen-18 replaces oxygen-16. These heavier isotopes condense more readily than ordinary H2O and are thus more common in raindrops formed lower rather than higher in the troposphere. The relative abundance of heavier isotopes in raindrops should thus provide a means to determine if they formed in top-heavy or bottom-heavy convection. Rainwater collection for isotopic analysis is planned at three sites in Colombia: Nuqui, on the west coast; Quibdo, further inland; and Isla San Andres, in the southwest Caribbean. Estimates of convective vertical profiles from isotopic composition are compared with estimates from aircraft measurements to determine if the method works. Further examination of the isotopic data would be conducted using numerical simulations from a variety of atmospheric models including some in which tracer "particles" representing water vapor are tracked as they rise through cloud systems and ultimately condense.A further goal of the research is to understand the transition from shallow to deep convection, a critical component of the lifecycle of convective clouds. Three contributing factors in the transition are examined: the gradual moistening of the atmospheric boundary layer; organization of the subcloud layer, which allows cloud roots to form in air which has greater moisture than the large-scale mean; and cold pools formed by convective downdrafts, which organize the subcloud layer to produce larger cloud roots which are in turn less susceptible to the diluting effect of dry air entrainment. The third of these factors may lead to a positive feedback, in which convective downdrafts organize the subcloud layer in a way that further promotes convection. Better characterization of entrainment is of particular interest over the southwest Caribbean, as midtropospheric air in this region is quite dry yet convection still occurs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.