Christine A. Shields

Quantifying Climate Feedbacks Using Radiative Kernels

Abstract The extent to which the climate will change due to an external forcing depends largely on radiative feedbacks, which act to amplify or damp the surface temperature response. There are a variety of issues that complicate the analysis of radiative feedbacks in global climate models, resulting in some confusion regarding their strengths and distributions. In this paper, the authors present a method for quantifying climate feedbacks based on “radiative kernels” that describe the differential response of the top-of-atmosphere radiative fluxes to incremental changes in the feedback variables. The use of radiative kernels enables one to decompose the feedback into one factor that depends on the radiative transfer algorithm and the unperturbed climate state and a second factor that arises from the climate response of the feedback variables. Such decomposition facilitates an understanding of the spatial characteristics of the feedbacks and the causes of intermodel differences. This technique provides a si...

The low resolution CCSM3

The low-resolution fully coupled configuration of the Community Climate System Model version 3 (CCSM3) is described and evaluated. In this most economical configuration, an ocean at nominal 3° resolution is coupled to an atmosphere model at T31 resolution. There are climate biases associated with the relatively coarse grids, yet the coupled solution remains comparable to higher-resolution CCSM3 results. There are marked improvements in the new solution compared to the low-resolution configuration of CCSM2. In particular, the CCSM3 simulation maintains a robust meridional overturning circulation in the ocean, and it generates more realistic El Nino variability. The improved ocean solution was achieved with no increase in computational cost by redistributing deep ocean and midlatitude resolution into the upper ocean and the key water formation regions of the North Atlantic, respectively. Given its significantly lower resource demands compared to higher resolutions, this configuration shows promise for studies of paleoclimate and other applications requiring long, equilibrated solutions.

Climate simulation of the latest Permian: Implications for mass extinction

Life at the Permian-Triassic boundary (ca. 251 Ma) underwent the largest disruption in Earths history. Paleoclimatic data indicate that Earth was significantly warmer than present and that much of the ocean was anoxic or euxinic for an extended period of time. We present results from the first fully coupled comprehensive climate model using paleo- geography for this time period. The coupled climate system model simulates warm high- latitude surface air temperatures related to elevated carbon dioxide levels and a stagnate global ocean circulation in concert with paleodata indicating low oxygen levels at ocean depth. This is the first climate simulation that captures these observed features of this time period.

Analysis of daily variability of precipitation in a nested regional climate model: comparison with observations and doubled CO2 results

Abstract We analyze daily mean, variability, and frequency of precipitation in two continuous 3 1 2 year long climate simulations over the continental U.S., one for present conditions and one for conditions under doubled carbon dioxide concentration, conducted with a regional climate model (RegCM) nested in a general circulation model (GCM). The purpose of the work is to analyze model errors and limitations in greater detail than previously done and to calculate quantities that eventually will be used to form climate change scenarios that account for changes in daily variability of precipitation. The models used are a version of the NCAR Community Climate Model (CCM) and the climate version (RegCM) of the NCAR/Penn State mesoscale model (MM4) at 60 km horizontal grid point spacing. Model output is compared with a 30-year daily observational data set for mainly two regions of the U.S.: the Northwest, and the central Great Plains. Statistics compared include mean daily precipitation, mean daily intensity, frequency, transition probabilities, quantiles of precipitation intensity, and interquartile ranges. We discuss how different measures of daily precipitation lead to different conclusions about the quality of the control run. For example, good agreement between model and observed data regarding mean daily precipitation usually results from compensating errors in the intensity and frequency fields (too high frequency and too low intensity). We analyze how detailed topographic features of the RegCM enhance the simulation of daily precipitation compared to the CCM simulation. In general, errors in all measures are smallest at the Northwest grid points, and the damping of the seasonal cycle of mean daily precipitation from the coast to inland Oregon is basically well reproduced. However, some errors in the frequency and intensity fields can be traced to inadequate representation of topography, even with a horizontal resolution of 60 km. Differences in the control and doubled CO2 runs (for both RegCM and CCM) for these regions are also presented. The most significant changes for the RegCM grid points is increased variability of daily precipitation under doubled CO2 conditions. Areas with significant changes (both increases and decreases) of precipitation frequency and intensity are found. There are some areas where frequency decreases, but precipitation mean daily amounts increase. Such changes, which would be masked by more traditional analyses of precipitation change, are important from a climate impacts point of view. The limitations on the analyses posed by small sample sizes are discussed.

Using the Radiative Kernel Technique to Calculate Climate Feedbacks in NCAR’s Community Atmospheric Model

Abstract Climate models differ in their responses to imposed forcings, such as increased greenhouse gas concentrations, due to different climate feedback strengths. Feedbacks in NCAR’s Community Atmospheric Model (CAM) are separated into two components: the change in climate components in response to an imposed forcing and the “radiative kernel,” the effect that climate changes have on the top-of-the-atmosphere (TOA) radiative budget. This technique’s usefulness depends on the linearity of the feedback processes. For the case of CO2 doubling, the sum of the effects of water vapor, temperature, and surface albedo changes on the TOA clear-sky flux is similar to the clear-sky flux changes directly calculated by CAM. When monthly averages are used rather than values from every time step, the global-average TOA shortwave change is underestimated by a quarter, partially as a result of intramonth correlations of surface albedo with the radiative kernel. The TOA longwave flux changes do not depend on the averagin...

The Climate Sensitivity of the Community Climate System Model Version 3 (CCSM3)

Abstract The climate sensitivity of the Community Climate System Model (CCSM) is described in terms of the equilibrium change in surface temperature due to a doubling of carbon dioxide in a slab ocean version of the Community Atmosphere Model (CAM) and the transient climate response, which is the surface temperature change at the point of doubling of carbon dioxide in a 1% yr−1 CO2 simulation with the fully coupled CCSM. For a fixed atmospheric horizontal resolution across model versions, we show that the equilibrium sensitivity has monotonically increased across CSM1.4, CCSM2, to CCSM3 from 2.01° to 2.27° to 2.47°C, respectively. The transient climate response for these versions is 1.44° to 1.09° to 1.48°C, respectively. Using climate feedback analysis, it is shown that both clear-sky and cloudy-sky processes have contributed to the changes in transient climate response. The dependence of these sensitivities on horizontal resolution is also explored. The equilibrium sensitivity of the high-resolution (T8...

Tests of precipitation parameterizations available in latest version of NCAR regional climate model (RegCM) over continental United States

This paper presents a series of tests of resolvable scale precipitation and cumulus convection schemes available in the latest version of the NCAR regional climate model (RegCM). For resolvable scale precipitation we use a new simplified explicit moisture (SIMEX) scheme, while for cumulus convection, we test the mass-flux scheme of Grell [1993], the Kuo-type scheme of Anthes [1977], and the newly implemented scheme of Zhang and McFarlane [1995]. The period of simulation is 3 years long, from March 1993 to February 1996, and the domain covers the continental United States at 60 km resolution. When using the SIMEX and Grell schemes, the model shows the best performance, with generally good simulation of precipitation, surface air temperature, and large-scale circulations throughout the United States. Both averages and anomalies associated with individual seasons are well reproduced at the regional scale. Main model deficiences are an underestimate of cold season precipitation over the eastern United States and a displacement of the summer precipitation maximum over the central plains. The Kuo scheme has a performance more similar to that of the Grell scheme than that of the Zhang and McFarlane (ZMF) scheme. Use of the ZMF scheme yields a reasonably good simulation of regionally averaged precipitation, but deteriorates some aspect of the simulation such as the intensity of the summer low-level jet over the central plains. We also present sensitivity tests to various parameters in the schemes which can be of guidance to the model user for optimizing the model performance over different regions.

A Regional Model Study of the Importance of Local versus Remote Controls of the 1988 Drought and the 1993 Flood over the Central United States

Abstract Regional model experiments for the drought period of May-June-July (MJJ) 1988 and the flood period of MJJ 1993 over the Central Plains of the United States are conducted to study the contribution of local versus nonlocal processes to the maintenance and/or enhancement of the conditions. It is found that the effect of local recycling of evaporated water is not important for the overall development of these two extreme climatic regimes as compared to the effect of large-scale moisture fluxes and synoptic cyclonic activity. In fact, sensitivity experiments indicate that in the Upper Mississippi Basin (UMB) the main effect of decreased evaporation associated with dry soil conditions at the beginning of the simulated periods is to increase buoyancy, dynamically sustain convection, and increase precipitation, thereby providing a negative feedback mechanism for the drought-flood conditions. Overall, the model shows a reasonably good performance in simulating various characteristics of surface climatolog...

Regional Nested Model Simulations of Present Day and 2 × CO2 Climate over the Central Plains of the U.S.

A nested regional climate model is used to generate a scenario of climate change over the MINK region (Missouri, Iowa, Nebraska, Kansas) due to doubling of carbon dioxide concentration (2 × CO2) for use in agricultural impact assessment studies. Five-year long present day (control) and 2 × CO2 simulations are completed at a horizontal grid point spacing of 50 km. Monthly and seasonal precipitation and surface air temperature over the MINK region are reproduced well by the model in the control run, except for an underestimation of both variables during the spring months. The performance of the nested model in the control run is greatly improved compared to a similar experiment performed with a previous version of the nested modeling system by Giorgi et al. (1994). The nested model generally improves the simulation of spatial precipitation patterns compared to the driving general circulation model (GCM), especially during the summer. Seasonal surface warming of 4 to 6 K and seasonal precipitation increases of 6 to 24% are simulated in 2 × CO2 conditions. The control run temperature biases are smaller than the simulated changes in all seasons, while the precipitation biases are of the same order of magnitude as the simulated changes. Although the large scale patterns of change in the driving GCM and nested RegCM model are similar, significant differences between the models, and substantial spatial variability, occur within the MINK region.

Climate Scenarios for the Southeastern U.S. Based on GCM and Regional Model Simulations

We analyze the control runs and 2 × CO2 projections (5-yearlengths) of the CSIRO Mk 2 GCM and the RegCM2 regional climate model, which was nested in the CSIRO GCM, over the Southeastern U.S.; and we present the development of climate scenarios for use in an integrated assessment of agriculture. The RegCM exhibits smaller biases in both maximum and minimum temperature compared to the CSIRO. Domain average precipitation biases are generally negative and relatively small in winter, spring, and fall, but both models produce large positive biases in summer, that of the RegCM being the larger. Spatial pattern correlations of the model control runs and observations show that the RegCM reproduces better than the CSIRO the spatial patterns of precipitation, minimum and maximum temperature in all seasons. Under climate change conditions, the most salient feature from the point of view of scenarios for agriculture is the large decreases in summer precipitation, about 20% in the CSIRO and 30% in the RegCM. Increases in springprecipitation are found in both models, about 35% in the CSIRO and 25% in theRegCM. Precipitation decreases of about 20% dominate in winter in the CSIRO,while a more complex pattern of increases and decreases is exhibited by the regional model. Temperature increases by 3 to 5 °C in the CSIRO, the higher values dominating in winter and spring. In the RegCM, temperature increases are much more spatially and temporally variable, ranging from 1 to 7 °C acrossall months and grids. In summer large increases (up to 7 °C) in maximum temperature are found in the northeastern part of the domain where maximum drying occurs.