skip to main content


The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 5:00 PM ET until 11:00 PM ET on Friday, June 21 due to maintenance. We apologize for the inconvenience.

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

Tropical ascent area (Aa) and high cloud fraction (HCF) are projected to decrease with surface warming in most Coupled Model Intercomparison Project Phase 5 (CMIP5) models. Perturbing deep convective parameters in the Community Atmosphere Model (CAM5) results in a similar spread and correlation between HCF andAaresponses to interannual warming compared to the CMIP5 ensemble, with a narrowerAacorresponding to greater HCF reduction. Perturbing cloud physics parameters produces a comparatively smaller range ofAaresponses to warming and a dissimilar HCF‐Aarelation to that in CMIP5; a narrowerAacorresponds to less HCF reduction, likely due to cloud radiative effects. A narrowing ofAacorresponds to a regime shift toward stronger precipitation in both experiments. We infer that model differences in deep convection parameterization likely play a greater role than differing cloud physics in determining the diverse responses ofAaand HCF to warming in CMIP5.

more » « less
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geophysical Research Letters
Page Range / eLocation ID:
p. 10112-10121
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Shifts in Southern Ocean (SO, 40–85°S) shortwave cloud feedback (SWFB) toward more positive values are the dominant contributor to higher effective climate sensitivity (ECS) in Coupled Model Intercomparison Project Phase 6 (CMIP6) models. To provide an observational constraint on the SOSWFB, we use a simplified physical model to connect SOSWFBwith the response of column‐integrated liquid water mass (LWP) to warming and the susceptibility of albedo to LWP in 50 CMIP5 and CMIP6 GCMs. In turn, we predict the responses of SO LWP using a cloud‐controlling factor (CCF) model. The combination of the CCF model and radiative susceptibility explains about 50% of the variance in the GCM‐simulatedSWFBin the SO. Observations of SW radiation fluxes, LWP, and CCFs from reanalysis are used to constrain the SOSWFB. Observations suggest a SO LWP increase in response to warming and albedo susceptibility to LWP that is on the lower end relative to GCMs. The overall constraint on the contribution of SO to global meanSWFBis −0.168 to +0.051 W m−2 K−1, relative to −0.277 to +0.270 Wm−2 K−1. In summary, observations suggest SOSWFBis less likely to be as extremely positive as predicted by some CMIP6 GCMs, but more likely to range from moderately negative to weakly positive.

    more » « less
  2. Abstract Tropical areas with mean upward motion—and as such the zonal-mean intertropical convergence zone (ITCZ)—are projected to contract under global warming. To understand this process, a simple model based on dry static energy and moisture equations is introduced for zonally symmetric overturning driven by sea surface temperature (SST). Processes governing ascent area fraction and zonal mean precipitation are examined for insight into Atmospheric Model Intercomparison Project (AMIP) simulations. Bulk parameters governing radiative feedbacks and moist static energy transport in the simple model are estimated from the AMIP ensemble. Uniform warming in the simple model produces ascent area contraction and precipitation intensification—similar to observations and climate models. Contributing effects include stronger water vapor radiative feedbacks, weaker cloud-radiative feedbacks, stronger convection-circulation feedbacks, and greater poleward moisture export. The simple model identifies parameters consequential for the inter-AMIP-model spread; an ensemble generated by perturbing parameters governing shortwave water vapor feedbacks and gross moist stability changes under warming tracks inter-AMIP-model variations with a correlation coefficient ∼0.46. The simple model also predicts the multimodel mean changes in tropical ascent area and precipitation with reasonable accuracy. Furthermore, the simple model reproduces relationships among ascent area precipitation, ascent strength, and ascent area fraction observed in AMIP models. A substantial portion of the inter-AMIP-model spread is traced to the spread in how moist static energy and vertical velocity profiles change under warming, which in turn impact the gross moist stability in deep convective regions—highlighting the need for observational constraints on these quantities. Significance Statement A large rainband straddles Earth’s tropics. Most, but not all, climate models predict that this rainband will shrink under global warming; a few models predict an expansion of the rainband. To mitigate some of this uncertainty among climate models, we build a simpler model that only contains the essential physics of rainband narrowing. We find several interconnected processes that are important. For climate models, the most important process is the efficiency with which clouds move heat and humidity out of rainy regions. This efficiency varies among climate models and appears to be a primary reason for why climate models do not agree on the rate of rainband narrowing. 
    more » « less
  3. Abstract

    Severe convective storms (SCS) and their associated hazards present significant societal risk. Understanding of how these hazards, such as hailfall, may change due to anthropogenic climate change is in its infancy. Previous methods used to investigate possible changes in SCS and their hail used climate model output and were limited by their coarse spatiotemporal resolution and less detailed representations of hail. This study instead uses an event-level pseudo–global warming (PGW) approach to simulate seven different hailstorms in their historical environments, and again in five different end-of-century PGW environments obtained from the worst-case scenario increases in CO2of five different CMIP5 members. Changes in large-scale environmental parameters were generally found to be consistent with prior studies, showing mostly increases in CAPE, CIN, and precipitable water, with minor changes in vertical wind shear. Nearly all simulated events had moderately stronger updrafts in the PGW environments. Only cold-season events showed an increase in hail sizes both within the storms and at the surface, whereas warm-season events exhibited a decrease in hail sizes at the surface and aloft. Changes in the event-total hailfall area at the ground also showed a seasonal trend, with increases in cold-season events and decreases in warm-season events. Melting depths increased for all PGW environments, and these increases likely contributed to greater rainfall area for warm-season events, where an increase in smaller hail aloft would be more prone to melting. The differences in PGW simulation hail sizes in cold-season and warm-season events found here are likely related to differences in microphysical processes and warrant future study.

    Significance Statement

    It is uncertain how severe thunderstorm hazards (such as hail, tornadoes, and damaging winds) may change due to human-induced climate change. Given the significant societal risk these hazards pose, this study seeks to better understand how hailstorms may change in the future. Simulated end-of-century storms in winter months showed larger hail sizes and a larger area of event-total hailfall than in the historical simulations, whereas simulated future storms in spring and summer months showed smaller hail sizes and a reduction in the area where hail fell. An analysis of traditional environmental and storm-scale properties did not reveal a clear distinction between cold-season and warm-season hailstorms, suggesting that changes in small-scale precipitation processes may be responsible.

    more » « less
  4. Abstract

    This study evaluates a hypothesis for the role of vertical wind shear in deep convection initiation (DCI) that was introduced in Part I by examining behavior of a series of numerical simulations. The hypothesis states, “Initial moist updrafts that exceed a width and shear threshold will ‘root’ within a progressively deeper steering current with time, increase their low-level cloud-relative flow and inflow, widen, and subsequently reduce their susceptibility to entrainment-driven dilution, evolving toward a quasi-steady self-sustaining state.” A theoretical model that embodied key elements of the hypothesis was developed in Part I, and the behavior of this model was explored within a multidimensional environmental parameter space. Remarkably similar behavior is evident in the simulations studied here to that of the theoretical model, both in terms of the temporal evolution of DCI and in the sensitivity of DCI to environmental parameters. Notably, both the simulations and theoretical model experience a bifurcation in outcomes, whereby nascent clouds that are narrower than a given initial radiusR0threshold quickly decay and those above theR0threshold undergo DCI. An important assumption in the theoretical model, which states that the cloud-relative flow of the background environmentVCRdetermines cloud radiusR, is scrutinized in the simulations. It is shown that storm-induced inflow is small relative toVCRbeyond a few kilometers from the updraft edge, andVCRtherefore plays a predominant role in transporting conditionally unstable air to the updraft. Thus, the critical role ofVCRin determiningRis validated.

    more » « less
  5. Abstract Aim

    This study examines how climate shapedMicrotus californicus(Rodentia: Arvicolinae) ecomorphology throughout the Quaternary. It tests three hypotheses: (a) climate corresponds with consistent shape variation inM. californicusdentition; (b) Quaternary warming and drying trends causedM. californicusmorphotypes to predictably shift in range through time and (c) Quaternary warming and drying led to predictable changes in tooth morphological variation. Finally, we discuss how shifts in climate‐linked morphological variation may affect the potential ofM. californicusto react to future climate change.


    Western United States.


    Microtus californicus(Peale, 1848).


    Geometric morphometrics and partial least squares analyses were used to discern how climate contributes to consistent variation in the shapes of theM. californicuslower first molar (m1), validated for the full toothrow. We further corroborate this relationship, reconstructing precipitation at fossil localities using m1 morphology and comparing those values to palaeoclimate‐model‐derived precipitations. Disparity analyses and a MANOVA were performed to examine changes in variation and whether a shift in tooth shape occurred through time.


    Microtus californicusm1 and toothrow shapes are narrower and more curved in cooler, wetter climates. Morphology‐based palaeoclimate reconstructions align with model‐based palaeoclimate estimations. When time averaging is accounted for,M. californicusdemonstrates a 12% reduction in variation from fossil to present‐day specimens, and these changes in tooth shape correspond with climate‐related morphotypes.

    Main conclusions

    As California became drier and hotter since the late Pleistocene,M. californicusdental morphology generally tracked these changes by adapting to the consumption of rougher vegetation in drier environments. This resulted in the loss of some high‐precipitation morphotypes, indicating that ecomorphology, often observed at the species and community levels, translates to intraspecific variation and dynamically changes in response to changing climates. The loss of climate‐linked morphological variation since the late Pleistocene may limit the ability ofM. californicusto respond to future changes in climate. These findings portend that other species may have experienced similar losses in adaptability.

    more » « less