Dynamic and Thermodynamic Controls on Deep Convection in Organization of Tropical East Pacific Convection (OTREC)

Columbia University, New York NY

Investigators

Abstract

This grant 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 grants 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 Principal Investigators (PIs) maintain a blog during the period just before, during, and just after the field campaign, helping to communicate the science to a broader audience. The award also provides funding for the education of a graduate student and to bring three additional graduate students to the campaign, allowing them to gain valuable experience in fieldwork. The eight week campaign 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). The near-surface wind convergence is a consequence of a strong north-south sea surface temperature (SST) contrast between the "cold tongue" along the equator and the warmer waters to the north. Surface winds near the equator tend to blow across SST contrasts and converge over the warmer water, and surface convergence induces rising motions favorable to convection. But the PIs argue against the notion that convection in the eastern Pacific ITCZ is driven primarily by surface convergence, pointing out that surface convergence does not automatically result in strong convection and heavy rainfall. Instead they hypothesize that thermodynamic factors including convective available potential energy (CAPE), convective inhibition (CIN), and the influence of moisture through entrainment and downdrafts are key to understanding convection in the region. The project also considers convection over the southwest Caribbean, where convection occurs despite a lack of SST contrasts and a relatively dry atmosphere. The PIs suggest that convection in these unfavorable conditions constitutes a self-sustaining equilibrium state which can develop given the right initial state, although the same environmental conditions would equally favor suppressed convection. Such multiple equilibrium behavior is seen in idealized models and has been associated with self-aggregation in cloud resolving models, and the PIs hypothesize that it is implicated in the active and suppressed phases of easterly waves. The research is conducted using data collected during the campaign in combination with a variety of numerical models including a cloud resolving version of the Weather Research and Forecasting (WRF) model. The PIs have several strategies for representing the relevant dynamics and thermodynamics in an idealized form, often using a small doubly-periodic domain. Observational comparisons use data from OTREC as well as earlier field campaigns (EPIC and TEPPS). 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.