Cloud Droplet Evolution and Thermal Radiation

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

Water in the atmosphere plays a crucial role in Earth's radiation budget in all three phases. Water vapor is Earth's primary greenhouse gas; clouds play a major role in Earth's radiative energy balance. Yet there remain significant uncertainties in the understanding of water's radiative roles in the atmosphere. This uncertainty shows up prominently in cloud-radiation interactions. Some researchers have suggested there are significant missing pieces of fundamental physics regarding clouds and thermal radiation. The need to understand better the effect of increasing amounts of water in the atmosphere and the implications of missing pieces of cloud-radiation physics are the primary motivations for this project. The objective of this project is to establish a better understanding of the role of thermal radiation in warm cloud droplet evolution: stability, evaporation, and condensation growth. Lab measurements will focus on radiation-augmented cloud droplet condensation growth and evaporation. Two separate, complementary laboratory devices are planned: a steady, isobaric flow process with radiative cooling/condensation and an unsteady, batch process with adiabatic expansion cooling followed by radiative heating/evaporation. Theoretical development and validation will be done for both droplet radiative properties and radiation-augmented mass transfer to obtain mathematical models for interpreting lab data and for microphysical and larger scale cloud simulations. Both measurements and modeling will be used to provide validation for and better understanding of the role of thermal radiation in cloud droplet evolution. Intellectual Merit: This study has intellectual merits at levels ranging from fundamental, scientifically oriented to product oriented. New knowledge will be generated about radiative- and phase-transitions for the most important thermal regulating substance on the planet. The findings will impact fundamental questions in cloud physics and atmospheric radiation: uncertain cloud albedos, uncertain aerosol properties, "anomalous" shortwave radiation absorption, anomalous longwave emission, water-vapor continuum absorption and emission, and the problem of cloud droplet stability in radiative and thermodynamic environments that are constantly changing. Remote sensing products and data retrieval will be impacted if this project can exploit spectral changes in condensing IR emission induced by phase-transition radiation to sense the onset of drizzle earlier and more reliably. Broader Impacts: This study will also have broader impacts, ranging from individuals to society at-large. At the individual level this study will most immediately affect the education and career of a PhD graduate student whose aspirations are to become a professor and as part of his graduate training wants to return to his HBCU masters-degree institution and make presentations on research findings and experience in graduate school. Locally, workshops will be conducted for K-12 students on water properties, phase-change heat transfer, and global climate change in which inexpensive atmospheric radiation and cloud-formation monitors will be built and tested. This study will also have a broader educational impact by being incorporated into the graduate curriculum at the University of Illinois as a design project. At a larger, societal level this study will also have far-reaching impact. The radiative phase-transition findings obtained herein will allow a better fundamental understanding of cloud microphysics and may open a door to improved global climate understanding as well as numerical weather prediction. By exploring the connections between "anomalous" infrared water radiation, the water-vapor continuum, and vapor-condensed phase transition, this study is potentially transformative in changing the way atmospheric radiation is modeled. The ultimate broad impact of this project will be a more informed society with a more accurate understanding of the fundamental thermophysical properties of Earth's primary thermal regulating fluid and principal greenhouse substance: water.