Climate & Radiation Laboratory Seminar Series
FRIDAY, FEBRUARY 28, 2020
Bldg. 33 Room H114
Institute for Climate and Atmospheric Science
University of Leeds
Empirical constraints on midlatitude cloud feedbacks and aerosol-cloud interactions
Constraining how much the Earth’s climate will warm in response to greenhouse gas emissions is one of the primary goals of climate science. Our understanding of clouds and their interactions with their environment represent a central uncertainty in constraining climate sensitivity. I will present research seeking empirical constraints on two features of clouds that substantially impact our ability to constrain climate sensitivity: shortwave cloud feedback and aerosol-cloud interactions (aci). I will focus on the midlatitude regime because it has been shown in recent research to contribute strongly to uncertainty in both effective radiative forcing due to aci (ERFaci), and global-mean cloud feedback in global climate models (GCMs).
ERFaci represents the largest uncertainty in understanding what climate sensitivity can be reconciled with the observational record. In liquid clouds this forcing is partitioned into (1) changes in radiative forcing due to changes in cloud droplet number concentration (Nd) in response to increasing availability of cloud condensation nuclei, and (2) changes in cloud macrophysical properties induced by these changes in Nd. These are referred to as the first indirect effect, and aerosol-cloud adjustments. While the first indirect effect has proven to be somewhat amenable to constraint, the radiative forcing due to aerosol-cloud adjustments remains uncertain in strength and even sign. I will present a constraint on aerosol-cloud adjustments across the anthropogenically-perturbed midlatitudes. Anthropogenic aerosol emissions in this region are strong, but meteorological behavior is complex. This makes separating aerosol-driven changes in cloud properties from meteorologically-driven changes challenging. Synoptic regime compositing is used to examine aerosol-cloud adjustments inside and outside midlatitude cyclones. Observations are interpreted using simulations at convection-permitting resolution within the MetOffice Unified Model as well as the CMIP6 version of the HadGEM model.
Climate sensitivity can also be constrained by constraining the cloud feedback in GCMs. Uncertainty in the shortwave cloud feedback explains the majority of uncertainty in overall cloud feedback in GCMs. While shortwave cloud feedback varies dramatically between GCMs, one feature that is consistent is a positive shortwave cloud feedback in the subtropics and a negative shortwave cloud feedback across the extratropics. Confidence has grown in the mechanisms that lead to, and the strength of, the subtropical shortwave cloud feedback, but significant uncertainty remains in constraining and understanding the extratropical negative shortwave cloud feedback. In keeping with my analysis of aerosol-cloud adjustments I separate the midlatitudes by synoptic regime. Remote-sensing observations are used to provide empirical constraints on cloud feedback in each synoptic regime. Models from CMIP5 are compared to sensitivity studies in the HadGEM model which was contributed to CMIP6. It is found that the extratropical cloud feedback is robustly negative and that remote-sensing can discard the cloud feedback in many GCMs- narrowing the possible range of overall cloud feedback.
613 Seminar Series Coordinators