Why Bottom-Heaviness of Vertical Motion Profiles Varies during Organization of Tropical East Pacific Convection (OTREC)

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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 features several education and outreach activities. These including support for undergraduate participation and production of short documentary videos for use in classroom teaching and informal science education. In addition, the PI performs science outreach through the Expanding Your Horizons program, which conducts a one-day conference for about 300 middle school girls, giving them the opportunity to explore careers that use math and science. The project also provides support and training for two graduate students, thereby providing for the future workforce in this research area.The eight week OTREC deployment consists primarily of a set of 20 flights in the Gulfstream V research aircraft maintained by the National Center for Atmospheric Research. 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. The radar determines cloud properties such as the relative abundance of ice particles versus liquid droplets, and uses Doppler shift to measure updraft and downdraft speeds in clouds. The aircraft sampling is augmented by observations collected at ground sites in Costa Rica and Colombia.Despite intensive study we still 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. Over the eastern Pacific tropical convection occurs in a narrow east-west strip a few degrees north of the equator, in a region where near-surface winds come together in an intertropical convergence zone (ITCZ). There are convincing arguments that rising motions associated with heavy rainfall in the ITCZ should have a bottom-heavy vertical profile, meaning that the bulk of the rising motion in clouds occurs in the lower and middle troposphere. But the idea of a bottom-heavy profile is contradicted by satellite observations (from the NASA TRMM mission) which appear to show a large fraction of stratiform precipitation. Such precipitation typically falls from upper-level clouds in regions of widespread convection characterized by a top-heavy profile of rising motion. In this project OTREC campaign data is used to resolve the vertical profile contradiction and test an alternative interpretation of the satellite data.A further issue is the extent to which convection in the ITCZ is driven by the near-surface wind convergence in the region. Some argue (see for example AGS-1758603) that convection in the eastern Pacific ITCZ is largely due to thermodynamic factors associated with stability and energy balance, with near-surface convergence playing a secondary role. Here the PI hypothesizes that the thermodynamic arguments are not sufficient and the wind convergence plays a dominant role.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.