Regional Drivers of Anvil Cloud Fraction and Cloud Radiative Effects in the Tropical Eastern Pacific and Atlantic Intertropical Convergence Zones (ITCZs)

Oregon State University, Corvallis OR

Investigators

Abstract

The cloud radiative effect (CRE) is a measure of the impact of clouds on the earth’s net radiation budget, defined as the clear-sky minus all-sky (including clouds) radiative flux at the top of the atmosphere measured by satellites or estimated from models. Cloud feedbacks are one of the largest uncertainties in future projections of global surface temperature and thus there is strong motivation for constraining these feedbacks. This project is focused on advancing understanding of the observed negative CRE in the East Pacific and Atlantic Intertropical Convergence Zones (ITCZs), addressing a gap in the current theory applicable mainly to the deep convective regions of the West Pacific and Indian Ocean where sea surface temperature gradients are weak and the CRE is close to neutral. The focus on oceanic ITCZs is motivated by the fact that the CRE for these regions contributes about 5% to the global CRE and has a larger magnitude than the estimated global energy imbalance. The negative CRE for oceanic ITCZ regions results from smaller anvil cloud areas relative to other deep convective regions of the Tropics, however little is known about what controls anvil cloud area in these regions. Thus, this project will also explore why a negative CRE is associated with small anvil cloud fraction in these regions. The study will use a combination of satellite observations and idealized modeling studies to address these issues. The project includes training undergraduate, graduate and postdoctoral scholars, and developing a learning module for elementary and high school students, meeting the mandate of the U.S. National Science Foundation to prepare the next generation of scientists in service to society. Beginning with the parameterization of the top of the atmosphere radiative flux from Hartmann et al. (2001), the project will investigate the hypotheses that the negative CRE in the oceanic ITCZ regions is due in part to differential energy export between the convective and non-convective regions associated with ocean circulation, as well as to the high albedo in the ITCZ subsidence regions resulting from the extensive stratocumulus clouds of the eastern subtropical oceans. The small convective cloud fraction relative to the subsidence cloud fraction is also suggested as a mechanism to amplify the CRE in these regions. The project will also investigate if the smaller anvil cloud fraction associated with oceanic ITCZs results from reduced cloud detrainment. These hypotheses will be tested using reanalyses and Community Earth System Model v2 (CESM2) non-rotating, aquaplanet simulations with a slab ocean. Causal relationships regarding what controls the anvil cloud fraction will be tested using mechanism denial and forcing experiments with CESM2. An expected outcome of this project is an update to the Hartmann et al. (2001) parameterization more applicable throughout the Tropics, by including both the original “shortwave” pathway through changes in albedo as well as a “longwave” pathway dependent on cloud anvil fraction. 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.