Ian N. Williams

Global warming, convective threshold and false thermostats

[1] We demonstrate a theoretically expected behavior of the tropical sea surface temperature probability density function (PDF) in future and past (Eocene) greenhouse climate simulations. To first order this consists of a shift to warmer temperatures as climate warms, without change of shape of the PDF. The behavior is tied to a shift of the temperature for deep convection onset. Consequently, the threshold for appearance of high clouds and associated radiative forcing shifts along with temperature. An excess entropy coordinate provides a reference to which the onset of deep convection is invariant, and gives a compact description of SST changes and cloud feedbacks suitable for diagnostics and as a basis for simplified climate models. The results underscore that the typically skewed appearance of tropical SST histograms, with a sharp drop-off above some threshold value, should not be taken as evidence for tropical thermostats. Citation: Williams, I. N., R. T. Pierrehumbert, and M. Huber (2009), Global warming, convective threshold and false thermostats, Geophys. Res. Lett., 36, L21805, doi:10.1029/2009GL039849.

Vegetation controls on surface heat flux partitioning, and land‐atmosphere coupling

We provide observational evidence that land-atmosphere coupling is underestimated by a conventional metric defined by the correlation between soil moisture and surface evaporative fraction (latent heat flux normalized by the sum of sensible and latent heat flux). Land-atmosphere coupling is 3 times stronger when using leaf area index as a correlate of evaporative fraction instead of soil moisture, in the Southern Great Plains. The role of vegetation was confirmed using adjacent flux measurement sites having identical atmospheric forcing but different vegetation phenology. Transpiration makes the relationship between evaporative fraction and soil moisture nonlinear and gives the appearance of weak coupling when using linear soil moisture metrics. Regions of substantial coupling extend to semiarid and humid continental climates across the United States, in terms of correlations between vegetation metrics and evaporative fraction. The hydrological cycle is more tightly constrained by the land surface than previously inferred from soil moisture.

Impacts of climate extremes on gross primary production under global warming

The impacts of historical droughts and heat-waves on ecosystems are often considered indicative of future global warming impacts, under the assumption that water stress sets in above a fixed high temperature threshold. Historical and future (RCP8.5) Earth system model (ESM) climate projections were analyzed in this study to illustrate changes in the temperatures for onset of water stress under global warming. The ESMs examined here predict sharp declines in gross primary production (GPP) at warm temperature extremes in historical climates, similar to the observed correlations between GPP and temperature during historical heat-waves and droughts. However, soil moisture increases at the warm end of the temperature range, and the temperature at which soil moisture declines with temperature shifts to a higher temperature. The temperature for onset of water stress thus increases under global warming and is associated with a shift in the temperature for maximum GPP to warmer temperatures. Despite the shift in this local temperature optimum, the impacts of warm extremes on GPP are approximately invariant when extremes are defined relative to the optimal temperature within each climate period. The GPP sensitivity to these relative temperature extremes therefore remains similar between future and present climates, suggesting that the heat- and drought-induced GPP reductions seen recently can be expected to be similar in the future, and may be underestimates of future impacts given model projections of increased frequency and persistence of heat-waves and droughts. The local temperature optimum can be understood as the temperature at which the combination of water stress and light limitations is minimized, and this concept gives insights into how GPP responds to climate extremes in both historical and future climate periods. Both cold (temperature and light-limited) and warm (water-limited) relative temperature extremes become more persistent in future climate projections, and the time taken to return to locally optimal climates for GPP following climate extremes increases by more than 25% over many land regions.

The influence of land cover on surface energy partitioning and evaporative fraction regimes in the U.S. Southern Great Plains

Author(s): Bagley, JE; Kueppers, LM; Billesbach, DP; Williams, IN; Biraud, SC; Torn, MS | Abstract:

Separating the effects of phenology and diffuse radiation on gross primary productivity in winter wheat

Author(s): Williams, IN; Riley, WJ; Kueppers, LM; Biraud, SC; Torn, MS | Abstract: ©2016. American Geophysical Union. All Rights Reserved. Gross primary productivity (GPP) has been reported to increase with the fraction of diffuse solar radiation, for a given total irradiance. The correlation between GPP and diffuse radiation suggests effects of diffuse radiation on canopy light-use efficiency, but potentially confounding effects of vegetation phenology have not been fully explored. We applied several approaches to control for phenology, using 8 years of eddy-covariance measurements of winter wheat in the U.S. Southern Great Plains. The apparent enhancement of daily GPP due to diffuse radiation was reduced from 260% to 75%, after subsampling over the peak growing season or by subtracting a 15 day moving average of GPP, suggesting a role of phenology. The diffuse radiation effect was further reduced to 22% after normalizing GPP by a spectral reflectance index to account for phenological variations in leaf area index LAI and canopy photosynthetic capacity. Canopy photosynthetic capacity covaries with diffuse fraction at a given solar irradiance at this site because both factors are dependent on day of year or solar zenith angle. Using a two-leaf Sun-shaded canopy radiative transfer model, we confirmed that the effects of phenological variations in photosynthetic capacity can appear qualitatively similar to the effects of diffuse radiation on GPP and therefore can be difficult to distinguish using observations. The importance of controlling for phenology when inferring diffuse radiation effects on GPP raises new challenges and opportunities for using radiation measurements to improve carbon cycle models.

Observational evidence against strongly stabilizing tropical cloud feedbacks

We present a method to attribute cloud radiative feedbacks to convective processes, using sub-cloud layer buoyancy as a diagnostic of stable and deep convective regimes. Applying this approach to tropical remote-sensing measurements over years 2000-2016 shows that an inferred negative short-term cloud feedback from deep convection was nearly offset by a positive cloud feedback from stable regimes. The net cloud feedback was within statistical uncertainty of the NCAR Community Atmosphere Model (CAM5) with historical forcings, with discrepancies in the partitioning of the cloud feedback into convective regimes. Compensation between high-cloud responses to tropics-wide warming in stable and unstable regimes resulted in smaller net changes in high-cloud fraction with warming. In addition, deep convection and associated high clouds set in at warmer temperatures in response to warming, as a consequence of nearly invariant sub-cloud buoyancy. This invariance further constrained the magnitude of cloud radiative feedbacks, and is consistent with climate model projections.

Using ARM Observations to Evaluate Climate Model Simulations of Land‐Atmosphere Coupling on the U.S. Southern Great Plains

Author(s): Phillips, TJ; Klein, SA; Ma, HY; Tang, Q; Xie, S; Williams, IN; Santanello, JA; Cook, DR; Torn, MS | Abstract: ©2017. American Geophysical Union. All Rights Reserved. Several independent measurements of warm-season soil moisture and surface atmospheric variables recorded at the ARM Southern Great Plains (SGP) research facility are used to estimate the terrestrial component of land-atmosphere coupling (LAC) strength and its regional uncertainty. The observations reveal substantial variation in coupling strength, as estimated from three soil moisture measurements at a single site, as well as across six other sites having varied soil and land cover types. The observational estimates then serve as references for evaluating SGP terrestrial coupling strength in the Community Atmospheric Model coupled to the Community Land Model. These coupled model components are operated in both a free-running mode and in a controlled configuration, where the atmospheric and land states are reinitialized daily, so that they do not drift very far from observations. Although the controlled simulation deviates less from the observed surface climate than its free-running counterpart, the terrestrial LAC in both configurations is much stronger and displays less spatial variability than the SGP observational estimates. Preliminary investigation of vegetation leaf area index (LAI) substituted for soil moisture suggests that the overly strong coupling between model soil moisture and surface atmospheric variables is associated with too much evaporation from bare ground and too little from the vegetation cover. These results imply that model surface characteristics such as LAI, as well as the physical parameterizations involved in the coupling of the land and atmospheric components, are likely to be important sources of the problematical LAC behaviors.