Identifying sensitive ranges in global warming precipitation change dependence on convective parameters

Abstract

A branch‐run perturbed‐physics ensemble in the Community Earth System Model estimates impacts of parameters in the deep convection scheme on current hydroclimate and on end‐of‐century precipitation change projections under global warming. Regional precipitation change patterns prove highly sensitive to these parameters, especially in the tropics with local changes exceeding 3 mm/d, comparable to the magnitude of the predicted change and to differences in global warming predictions among the Coupled Model Intercomparison Project phase 5 models. This sensitivity is distributed nonlinearly across the feasible parameter range, notably in the low‐entrainment range of the parameter for turbulent entrainment in the deep convection scheme. This suggests that a useful target for parameter sensitivity studies is to identify such disproportionately sensitive “dangerous ranges.” The low‐entrainment range is used to illustrate the reduction in global warming regional precipitation sensitivity that could occur if this dangerous range can be excluded based on evidence from current climate.

Relationships Between Tropical Ascent and High Cloud Fraction Changes With Warming Revealed by Perturbation Physics Experiments in CAM5

“…The convection experiment (CONV) perturbs four parameters in the modified Zhang-McFarlane deep convection scheme (Neale et al, 2008;Zhang & McFarlane, 1995). Each parameter in CONV is perturbed up to five times (see also Bernstein & Neelin, 2016;Langenbrunner & Neelin, 2017). The cloud physics experiment (CLOUD) perturbs ten macrophysical and microphysical parameters.…”

Tropical Convective Transition Statistics and Causality in the Water Vapor–Precipitation Relation

“…The present paper focuses on the dominant direction of causality in the fast-time-scale precipitation-CWV relationship and addresses the impacts of entrainment on the two-way interaction between deep convection and environmental humidity. Our methodology is based on analysis of parameter sensitivity experiments (Bernstein and Neelin 2016) in the Community Earth System Model (CESM), which is able to simulate the sharp precipitation pickup with the default setting (Sahany et al 2012(Sahany et al , 2014. We show that the set of statistics associated with the transition to deep convection (or ''convective transition statistics'') in the CESM can be radically altered if different values of entrainment are prescribed in the deep convective scheme.…”

Multiobjective constraints for climate model parameter choices: Pragmatic Pareto fronts in CESM1

“…The first 10 years are discarded to allow for model equilibration, so climatologies represent a 1985–2005 average. Bernstein and Neelin [] show that the hydrological cycle tends to adjust quickly to parameter changes (on time scales less than 10 years). Small remaining imbalances in top‐of‐atmosphere (TOA) radiation associated with adjustment of the deep ocean could be relevant to some climate quantities but are not a strong effect for those examined here.…”

“…Figure shows model RMSE as a function of parameter value for precipitation, column water vapor, and skin temperature fields in the tropical Pacific domain. A notable theme here is that all parameters show some degree of nonlinearity as a function of parameter value for a given field—even when the parameter dependence appears fairly linear in others—and these results parallel those of Bernstein and Neelin []. For example, the parameter dependence across α appears linear for column water vapor but more notably nonlinear at low α values for both precipitation and skin temperature.…”

“…Parameter descriptions are listed in the fourth column. More information on this ensemble can be found in Bernstein [2014] and Bernstein and Neelin [2016]. For more information on CESM1 or the deep convection scheme, see the community atmosphere model version 4 (CAM4) documentation [Neale et al, 2010].…”