Sara A. Rauscher

Regional Climate Modeling for the Developing World: The ICTP RegCM3 and RegCNET

Regional climate models are important research tools available to scientists around the world, including in economically developing nations (EDNs). The Earth Systems Physics (ESP) group of the Abdus Salam International Centre for Theoretical Physics (ICTP) maintains and distributes a state-of-the-science regional climate model called the ICTP Regional Climate Model version 3 (RegCM3), which is currently being used by a large research community for a diverse range of climate-related studies. The RegCM3 is the central, but not only, tool of the ICTP-maintained Regional Climate Research Network (RegCNET) aimed at creating south–south and north–south scientific interactions on the topic of climate and associated impacts research and modeling. In this paper, RegCNET, RegCM3, and illustrative results from RegCM3 benchmark simulations applied over south Asia, Africa, and South America are presented. It is shown that RegCM3 performs reasonably well over these regions and is therefore useful for climate studies in...

Future changes in snowmelt-driven runoff timing over the western US

We use a high-resolution nested climate model to investigate future changes in snowmelt-driven runoff (SDR) over the western US. Comparison of modeled and observed daily runoff data reveals that the regional model captures the present-day timing and trends of SDR. Results from an A2 scenario simulation indicate that increases in seasonal temperature of approximately 3 degrees to 5 degrees C resulting from increasing greenhouse gas concentrations could cause SDR to occur as much as two months earlier than present. These large changes result from an amplified snow-albedo feedback driven by the topographic complexity of the region, which is more accurately resolved in a high-resolution nested climate model. Earlier SDR could affect water storage in reservoirs and hydroelectric generation, with serious consequences for land use, agriculture, and water management in the American West.

Recent summer precipitation trends in the Greater Horn of Africa and the emerging role of Indian Ocean sea surface temperature

We utilize a variety of climate datasets to examine impacts of two mechanisms on precipitation in the Greater Horn of Africa (GHA) during northern-hemisphere summer. First, surface-pressure gradients draw moist air toward the GHA from the tropical Atlantic Ocean and Congo Basin. Variability of the strength of these gradients strongly influences GHA precipitation totals and accounts for important phenomena such as the 1960s–1980s rainfall decline and devastating 1984 drought. Following the 1980s, precipitation variability became increasingly influenced by the southern tropical Indian Ocean (STIO) region. Within this region, increases in sea-surface temperature, evaporation, and precipitation are linked with increased exports of dry mid-tropospheric air from the STIO region toward the GHA. Convergence of dry air above the GHA reduces local convection and precipitation. It also produces a clockwise circulation response near the ground that reduces moisture transports from the Congo Basin. Because precipitation originating in the Congo Basin has a unique isotopic signature, records of moisture transports from the Congo Basin may be preserved in the isotopic composition of annual tree rings in the Ethiopian Highlands. A negative trend in tree-ring oxygen-18 during the past half century suggests a decline in the proportion of precipitation originating from the Congo Basin. This trend may not be part of a natural cycle that will soon rebound because climate models characterize Indian Ocean warming as a principal signature of greenhouse-gas induced climate change. We therefore expect surface warming in the STIO region to continue to negatively impact GHA precipitation during northern-hemisphere summer.

CMIP5 Projected Changes in the Annual Cycle of Precipitation in Monsoon Regions

AbstractAnalyses of phase 5 of the Coupled Model Intercomparison Project (CMIP5) experiments show that the global monsoon is expected to increase in area, precipitation, and intensity as the climate system responds to anthropogenic forcing. Concurrently, detailed analyses for several individual monsoons indicate a redistribution of rainfall from early to late in the rainy season. This analysis examines CMIP5 projected changes in the annual cycle of precipitation in monsoon regions, using a moist static energy framework to evaluate competing mechanisms identified to be important in precipitation changes over land. In the presence of sufficient surface moisture, the local response to the increase in downwelling energy is characterized by increased evaporation, increased low-level moist static energy, and decreased stability with consequent increases in precipitation. A remote mechanism begins with warmer oceans and operates on land regions via a warmer tropical troposphere, increased stability, and decrease...

Projected Future Changes in Vegetation in Western North America in the Twenty-First Century

Rapid and broad-scale forest mortality associated with recent droughts, rising temperature, and insect outbreaks has been observed over western North America (NA). Climate models project additional future warming and increasing drought and water stress for this region. To assess future potential changes in vegetation distributions in western NA, the Community Earth System Model (CESM) coupled with its Dynamic Global Vegetation Model (DGVM) was used under the future A2 emissions scenario. To better span uncertainties in future climate, eight sea surface temperature (SST) projections provided by phase 3 of the Coupled Model Intercomparison Project (CMIP3) were employed as boundary conditions. There is a broad consensus among the simulations, despite differences in the simulated climate trajectories across the ensemble, that about half of the needleleaf evergreen tree coverage (from 24% to 11%) will disappear, coincident with a 14% (from 11% to 25%) increase in shrubs and grasses by the end of the twenty-first century in western NA, with most of the change occurring over the latter half of the twenty-first century. The net impact is a ;6 GtC or about 50% decrease in projected ecosystem carbon storage in this region. The findings suggest a potential for a widespread shift from tree-dominated landscapes to shrub and grass-dominated landscapesin westernNA becauseof future warmingandconsequentincreases in waterdeficits.These results highlight the need for improved process-based understanding of vegetation dynamics, particularly including mortality and the subsequent incorporation of these mechanisms into earth system models to better quantify the vulnerability of western NA forests under climate change.

Exploring a Global Multiresolution Modeling Approach Using Aquaplanet Simulations

AbstractResults from aquaplanet experiments performed using the Model for Prediction across Scales (MPAS) hydrostatic dynamical core implemented within the Department of Energy (DOE)–NCAR Community Atmosphere Model (CAM) are presented. MPAS is an unstructured-grid approach to climate system modeling that supports both quasi-uniform and variable-resolution meshing of the sphere based on conforming grids. Using quasi-uniform simulations at resolutions of 30, 60, 120, and 240 km, the authors evaluate the performance of CAM-MPAS via its kinetic energy spectra, general circulation, and precipitation characteristics. By analyzing an additional variable-resolution simulation with grid spacing that varies from 30 km in a spherical, continental-sized equatorial region to 240 km elsewhere, the CAM-MPAS’s potential for use as a regional climate simulation tool is explored.Similar to other quasi-uniform aquaplanet simulations, tropical precipitation increases with resolution, indicating the resolution sensitivity of ...

The Regional Climate Change Hyper‐Matrix Framework

The accurate assessment of the potential impacts of climate change on societies and ecosystems requires regional and local-scale climate change information. This assessment is critical for the development of local, national, and international policies to mitigate and adapt to the threat of climate change. Characterizing uncertainties in regional climate change projections (RCCPs) is therefore crucial for making informed decisions based on quantitative risk analysis. However, information about fine-scale climate change and associated uncertainties is lacking due to the absence of a coordinating framework to improve the characterization of such uncertainties. Here we propose the inception of such a framework.

The Role of Regional SST Warming Variations in the Drying of Meso-America in Future Climate Projections

AbstractThis paper addresses several hypotheses designed to explain why AOGCM simulations of future climate in the third phase of the Coupled Model Intercomparison Project (CMIP3) feature an intensified reduction of precipitation over the Meso-America (MA) region. While the drying is consistent with an amplification of the subtropical high pressure cells and an equatorward contraction of convective regions due to the “upped ante” for convection in a warmer atmosphere, the physical mechanisms behind the intensity and robustness of the MA drying signal have not been fully explored. Regional variations in sea surface temperature (SST) warming may play a role. First, SSTs over the tropical North Atlantic (TNA) do not warm as much as the surrounding ocean. The troposphere senses a TNA that is cooler than the tropical Pacific, potentially exciting a Gill-type response, increasing the strength of the North Atlantic subtropical high. Second, the warm ENSO-like state simulated in the eastern tropical Pacific could...

Empirical and process-based approaches to climate-induced forest mortality models

Globally, forests store ∼45% of car-bon sequestered terrestrially, contributemore to the terrestrial sink per areathan any other land cover type, andassimilate an important portion ofanthropogenic emissions (Bonan, 2008).Forests exert strong biophysical controlon climate via surface energy balance(Bonan, 2008; Rotenberg and Yakir, 2010;Houspanossian et al., 2013), and thehydrological cycle (Zhang et al., 2001;Brown et al., 2005). Widespread for-est mortality in response to drought,increased temperatures, and infestationof tree pests has been observed globally,potentially threatening forests’ regula-tion of climate (Kurz et al., 2008; Adamset al., 2010; Allen et al., 2010; Anderegget al., 2013a). This threat has promptedgreat interest in understanding and pre-dicting tree mortality due to climatevariability and change, especially drought.Initial tests of hydraulic failure (mortal-ity caused by irreversible loss of xylemconductivity from air embolism), carbonstarvation (mortality due to carbohy-drate limitation), insect attacks, wildfire,and their interdependence (Allen, 2007;McDowell et al., 2008, 2011, 2013a), sug-gest proximal causes of mortalityare likelycomplex, co-occurring, interrelated, andvariable with tree species (supported byAdams et al., 2009, 2013; Sala et al.,2010; Piper, 2011; Zeppel et al., 2011;Anderegg et al., 2012a, 2013b; Adamsetal.,2013;AndereggandAnderegg,2013;Galvez et al., 2013; Gaylord et al., 2013;Hartmann et al., 2013a,b; Mitchell et al.,2013; Quirk et al., 2013; Williams et al.,in review). While the interdependent rolesof carbon and water in plant mortalityare consistently observed, this work iscontinuously prompting new questions(Sala et al., 2010; McDowell et al., 2013b;O’Grady et al., 2013).The justification for physiologicalresearch on drought-induced tree mortal-ity is often stated as a need to improve thepredictive capability of vegetation mod-els through incorporation of mortalitymechanisms(Fisheretal.,2010;McDowellet al., 2011, 2013a; Powell et al., 2013). Yetif mortality is particularly complicatedand associated with failure of multiplephysiological processes (Manion, 1981;McDowell et al., 2011; Anderegg et al.,2012b), then a key question emerges: is amechanistic approach necessary for accu-rate prediction of future mortality? Theanswertothisquestionultimatelydependson the application and goal of the model.At issue is whether increasing modelcomplexitywillimproveprediction,whichis influenced in part by the modelingapproach employed. Two endpoints ona theoretical continuum of approachto mechanism are process-based andempirical model types. The process-based approach focuses on simulatingdetailed physical or biological processesthat explicitly describe system behav-ior, while the empirical approach relieson correlative relationships in line withmechanistic understanding, but with-out fully describing system behaviorsand interactions (Korzukhin et al., 1996;

Regional Climate Model–Simulated Timing and Character of Seasonal Rains in South America

Abstract The potential of an experimental nested prediction system to improve the simulation of subseasonal rainfall statistics including daily precipitation intensity, rainy season onset and withdrawal, and the frequency and duration of dry spells is evaluated by examining a four-member ensemble of regional climate model simulations performed for the period 1982–2002 over South America. The study employs the International Centre for Theoretical Physics (ICTP) regional climate model, version 3 (RegCM3), driven with the NCEP–NCAR reanalysis and the European Centre–Hamburg GCM, version 4.5. Statistics were examined for five regions: the northern Amazon, southern Amazon, the monsoon region, Northeast Brazil, and southeastern South America. RegCM3 and the GCM are able to replicate the distribution of daily rainfall intensity in most regions. The analysis of the rainy season timing shows the observed onset occurring first over the monsoon region and then spreading northward into the southern Amazon, in contras...