Date of Award
Fall 2022
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Geology and Geophysics
First Advisor
Fedorov, Alexey
Abstract
This thesis examines precipitation efficiency, from its definition, measurements in observations and models, to its role in global climate change. Precipitation Efficiency (PE) is the ratio of surface precipitation to cloud condensation and an essential parameter in the atmospheric circulation and hydrological cycle. However, what sets its current range and whether this range will change in future climates is poorly understood. Recent developments of satellite observations and high-resolution models expand the horizon for PE research, enabling deeper exploration into this fundamental characteristic of the atmosphere. Although first studies of PE traces to the mid-20th century, the work that brought the research community’s attention to PE is one that put forth a hypothesis – the iris hypothesis – that PE may regulate Earth’s surface temperature through an infrared cloud feedback mechanism. The potential mechanisms underlying this hypothesis is investigated by imposing the convective cloud-rain conversion in the Community Earth System Model, a General Circulation Model (GCM) to varying degrees of temperature sensitivity. High cloud-rain conversion (i.e., high PE) experiments have better agreement with present-day satellite observations and, in abrupt carbon dioxide doubling experiments, exhibit higher climate sensitivity compared to default configurations. These results are contrary to a previous study performing the identical experiment in a different GCM. Analyzing cloud properties, we find that the thinning of deep tropical cumulonimbus clouds resulted in less reflection of sunlight and contributed to a positive climate feedback that amplified warming. This mechanism, where more rainout causes clouds to become optically thinner, is consistent across different microphysics schemes in the GCM and may already be occurring in nature. Several definitions of PE are used in existing studies, making it difficult to reconcile across different lines of PE research. To facilitate further exploration into PE’s climate impacts, we define a PE measure $\epsilon$ – the ratio of surface precipitation to column-integrated cloud condensate – that can be readily calculated in observations and models. We explore $\epsilon$’s observed climatology, temporal variation, and link to the large-scale tropical circulation and precipitation extremes. In particular, GCMs in Phase 6 of the Climate Model Intercomparison Project (CMIP6) separate into two groups based on the sign of PE change in climate warming scenarios. We find that PE decreases in GCMs where precipitation in deep convective updrafts is independent of vertical mass flux and that PE increases in GCMs with this dependency. Models with increasing PE with temperature robustly show more weakening of the Hadley and Walker circulations by 5-15\%, amplified global atmospheric warming by 1 K, and higher sensitivity of extreme precipitation events by 10\%. Due to these consequences of positive PE sensitivity, constraining the sensitivity of $\epsilon$ to temperature is critical for future climate projections. Employing the $\epsilon$ definition, PE’s role in climate is assessed by employing satellite observations, 36 cloud-resolving models (CRMs), and 35 CMIP6 GCMs. Observations show signs of increasing PE in the first two decades of the 21st century in the eastern equatorial Pacific. Collectively, CRMs from the Radiative-Convective Equilibrium Model Intercomparison Project imply higher PE at warmer temperatures. CMIP6 GCMs show that increasing PE is associated with tropical circulation slowdown and greater eastern equatorial Pacific warming. This PE increase in GCMs occurs simultaneously with pan-tropical positive cloud feedback through stratiform anvil cloud reduction and stratocumulus suppression, resulting in higher climate sensitivity. In 24 of 35 GCMs matching the CRMs in simulating increasing PE with greenhouse warming, their mean climate sensitivity is 1 K higher than in PE-decreasing GCMs. Hence, the link between PE, clouds, and circulation is key to better understanding climate change.
Recommended Citation
Li, Ryan, "Precipitation Efficiency in Climate Change" (2022). Yale Graduate School of Arts and Sciences Dissertations. 1087.
https://elischolar.library.yale.edu/gsas_dissertations/1087
