NCAR Climate System Model Research Articles

Numerical aspects of applying the fluctuation dissipation theorem to study climate system sensitivity to external forcings

Abstract The fluctuation dissipation theorem (FDT), a classical result coming from statistical mechanics, suggests that, under certain conditions, the system response to external forcing can be obtained using the statistics of natural fluctuation of the system. The application of the FDT to the most sophisticated climate models and the real climate system represents a difficult problem due to the huge dimensionality of these systems and the lack of the data available for proper sampling of the system natural variability. As a consequence, one has to use some regularization procedures constraining the form of permitted perturbations. Naturally, the skill of the FDT depends on the type and parameters of the regularization procedure. In the present paper we apply FDT to predict the response of a recent version of the NCAR climate system model (CCSM4) to salinity and temperature forcing anomalies in the North Atlantic. We study the sensitivity of our results to the amount of available data and to key parameters used in our numerical algorithm.

ENSO in the Mid-Holocene according to CSM and HadCM3

Abstract The offline linearized ocean–atmosphere model (LOAM), which was developed to quantify the impact of the climatological mean state on the variability of the El Niño–Southern Oscillation (ENSO), is used to illuminate why ENSO changed between the modern-day and early/mid-Holocene simulations in two climate modeling studies using the NCAR Climate System Model (CSM) and the Hadley Centre Coupled Model, version 3 (HadCM3). LOAM reproduces the spatiotemporal variability simulated by the climate models and shows both the reduction in the variance of ENSO and the changes in the spatial structure of the variance during the early/mid-Holocene. The mean state changes that are important in each model are different and, in both cases, are also different from those hypothesized to be important in the original papers describing these simulations. In the CSM simulations, the ENSO mode is stabilized by the mean cooling of the SST. This reduces atmospheric heating anomalies that in turn give smaller wind stress anomalies, thus weakening the Bjerknes feedback. Within the ocean, a change in the thermocline structure alters the spatial pattern of the variance, shifting the peak variance farther east, but does not reduce the overall amount of ENSO variance. In HadCM3, the ENSO mode is stabilized by a combination of a weaker thermocline and weakened horizontal surface currents. Both of these reduce the Bjerknes feedback by reducing the ocean’s SST response to wind stress forcing. This study demonstrates the importance of considering the combined effect of a mean state change on the coupled ocean–atmosphere system: conflicting and erroneous results are obtained for both models if only one model component is considered in isolation.

Understanding Changes in the Asian Summer Monsoon over the Past Millennium: Insights from a Long-Term Coupled Model Simulation*

Abstract The Asian summer monsoon (ASM) and its variability were investigated over the past millennium through the analysis of a long-term simulation of the NCAR Climate System Model, version 1.4 (CSM 1.4) coupled model driven with estimated natural and anthropogenic radiative forcing during the period 850–1999. Analysis of the simulation results indicates that certain previously proposed mechanisms, such as warmer large-scale temperatures favoring a stronger monsoon through their effect on Eurasian snow cover, appear inconsistent with the mechanisms active in the simulation. Forced changes in tropical Pacific sea surface temperatures play an apparent role in the long-term changes in the ASM. Analyses of the simulation results suggest that the direct radiative effect of solar forcing variations on the ASM is quite weak and that dynamical responses may be far more important. Volcanic radiative forcing leads to a clearly detectable short-term reduction in the strength of the ASM. Comparisons with long-term proxy reconstructions of the ASM are attempted but are limited by the divergent behavior among different reconstructions as well as the limitations in the model’s coupled dynamics.

Solar influence on climate during the past millennium: Results from transient simulations with the NCAR Climate System Model

The potential role of solar variations in modulating recent climate has been debated for many decades and recent papers suggest that solar forcing may be less than previously believed. Because solar variability before the satellite period must be scaled from proxy data, large uncertainty exists about phase and magnitude of the forcing. We used a coupled climate system model to determine whether proxy-based irradiance series are capable of inducing climatic variations that resemble variations found in climate reconstructions, and if part of the previously estimated large range of past solar irradiance changes could be excluded. Transient simulations, covering the published range of solar irradiance estimates, were integrated from 850 AD to the present. Solar forcing as well as volcanic and anthropogenic forcing are detectable in the model results despite internal variability. The resulting climates are generally consistent with temperature reconstructions. Smaller, rather than larger, long-term trends in solar irradiance appear more plausible and produced modeled climates in better agreement with the range of Northern Hemisphere temperature proxy records both with respect to phase and magnitude. Despite the direct response of the model to solar forcing, even large solar irradiance change combined with realistic volcanic forcing over past centuries could not explain the late 20th century warming without inclusion of greenhouse gas forcing. Although solar and volcanic effects appear to dominate most of the slow climate variations within the past thousand years, the impacts of greenhouse gases have dominated since the second half of the last century.

Local and Global Climate Feedbacks in Models with Differing Climate Sensitivities

AbstractThe climatic response to a 5% increase in solar constant is analyzed in three coupled global ocean–atmosphere general circulation models, the NCAR Climate System Model version 1 (CSM1), the Community Climate System Model version 2 (CCSM2), and the Canadian Centre for Climate Modelling and Analysis (CCCma) Coupled General Circulation Model version 3 (CGCM3). For this simple perturbation the quantitative values of the radiative climate forcing at the top of the atmosphere can be determined very accurately simply from a knowledge of the shortwave fluxes in the control run. The climate sensitivity and the geographical pattern of climate feedbacks, and of the shortwave, longwave, clear-sky, and cloud components in each model, are diagnosed as the climate evolves. After a period of adjustment of a few years, both the magnitude and pattern of the feedbacks become reasonably stable with time, implying that they may be accurately determined from relatively short integrations.The global-mean forcing at the top of the atmosphere due to the solar constant change is almost identical in the three models. The exact value of the forcing in each case is compared with that inferred by regressing annual-mean top-of-the-atmosphere radiative imbalance against mean surface temperature change. This regression approach yields a value close to the directly diagnosed forcing for the CCCma model, but a value only within about 25% of the directly diagnosed forcing for the two NCAR models. These results indicate that this regression approach may have some practical limitation in its application, at least for some models.The global climate sensitivities differ among the models by almost a factor of 2, and, despite an overall apparent similarity, the spatial patterns of the climate feedbacks are only modestly correlated among the three models. An exception is the clear-sky shortwave feedback, which agrees well in both magnitude and spatial pattern among the models. The biggest discrepancies are in the shortwave cloud feedback, particularly in the tropical and subtropical regions where it is strongly negative in the NCAR models but weakly positive in the CCCma model. Almost all of the difference in the global-mean total feedback (and climate sensitivity) among the models is attributable to the shortwave cloud feedback component.All three models exhibit a region of positive feedback in the equatorial Pacific, which is surrounded by broad areas of negative feedback. These positive feedback regions appear to be associated with a local maximum of the surface warming. However, the models differ in the zonal structure of this surface warming, which ranges from a mean El Niño–like warming in the eastern Pacific in the CCCma model to a far-western Pacific maximum of warming in the NCAR CCSM2 model. A separate simulation with the CCSM2 model, in which these tropical Pacific zonal gradients of surface warming are artificially suppressed, shows no region of positive radiative feedback in the tropical Pacific. However, the global-mean feedback is only modestly changed in this constrained run, suggesting that the processes that produce the positive feedback in the tropical Pacific region may not contribute importantly to global-mean feedback and climate sensitivity.

Transient simulations of Holocene atmospheric carbon dioxide and terrestrial carbon since the Last Glacial Maximum

Conflicting hypotheses are investigated for the observed atmospheric CO2 increase of 20 ppm between 8 ka BP and pre‐industrial time. The carbon component of the Bern Carbon Cycle Climate (Bern CC) model, which couples the Lund‐Potsdam‐Jena Dynamic Global Vegetation Model to an atmosphere‐ocean‐sediment component, is driven by climate fields from time‐slice simulations of the past 21 ka with the Hadley Centre Unified Model or the NCAR Climate System Model. The entire Holocene ice core record of CO2 is matched within a few ppm for the standard model setup, and results are broadly consistent with proxy data of atmospheric 13CO2, mean ocean δ13C, and pollen data, within their uncertainties. Our analysis suggests that a range of mechanisms, including calcite compensation in response to earlier terrestrial uptake, terrestrial carbon uptake and release, SST changes, and coral reef buildup, contributed to the 20 ppm rise. The deep sea δ13C record constrains the contribution of the calcite compensation mechanism to 4–10 ppm. Terrestrial carbon inventory changes related to climate and CO2 forcing, the greening of the Sahara, peat buildup, and land use have probably influenced atmospheric CO2 by a few ppm only. The early Holocene CO2 decrease is quantitatively explained by terrestrial uptake and calcite compensation in response to terrestrial uptake during the glacial‐interglacial transition. The recent hypothesis by Ruddiman [2003] that anthropogenic land use caused a 40 ppm CO2 anomaly over the past 8 ka, preventing the climate system from entering a new glacial, would imply an anthropogenic emission of 700 GtC and a decrease in atmospheric δ13C of 0.6 permil. This is not compatible with the ice core δ13C record and would require an upward revision of land use emission estimates by a factor of 3 to 4.

Modeling El Niño and its tropical teleconnections during the last glacial‐interglacial cycle

Simulations with the NCAR Climate System Model (CSM), a global, coupled ocean‐atmosphere‐sea ice model, for the last glacial‐interglacial cycle reproduce recent estimates, based on alkenones and Mg/Ca ratios, of sea surface temperature (SST) changes and gradients in the tropical Pacific and predict weaker El Niños/La Niñas compared to present for the Holocene and stronger El Niños/La Niñas for the Last Glacial Maximum (LGM). Changes for the LGM (Holocene) are traced to a weakening (strengthening) of the tropical Pacific zonal SST gradient, wind stresses, and upwelling and a sharpening (weakening) of the tropical thermocline. Results suggest that proxy evidence of weaker precipitation variability in New Guinea and Ecuador are explained not only by changes in El Niño/La Niña but also changes in the atmospheric circulation and hydrologic cycle.

Coupled Climate Simulation of the Evolution of Global Monsoons in the Holocene*

Abstract Evolution of global monsoons in the Holocene is simulated in a coupled climate model—the Fast Ocean Atmosphere Model—and is also compared with the simulations in another coupled climate model—the NCAR Climate System Model. Holocene climates are simulated under the insolation forcing at 3000, 6000, 8000, and 11 000 years before present. The evolution of six major regional summer monsoons is investigated: the Asian monsoon, the North African monsoon, the North American monsoon, the Australian monsoon, the South American monsoon, and the South African monsoon. Special attention has been paid to the relative roles of the direct insolation forcing and oceanic feedback. It is found that the responses of the monsoons to the insolation forcing and oceanic feedback differ substantially among regions, because of regional features of atmospheric and oceanic circulation and ocean–atmosphere interaction. In the Northern Hemisphere, the coupled models show a significant enhancement of all of the monsoons in th...

On the Radiative and Dynamical Feedbacks over the Equatorial Pacific Cold Tongue

Abstract An analysis of the climatic feedbacks in the NCAR Community Climate Model, version 3 (CCM3) over the equatorial Pacific cold tongue is presented. Using interannual signals in the underlying SST, the radiative and dynamical feedbacks have been calculated using both observations and outputs from the NCAR CCM3. The results show that the positive feedback from the greenhouse effect of water vapor in the model largely agrees with that from observations. The dynamical feedback from the atmospheric transport in the model is also comparable to that from observations. However, the negative feedback from the solar forcing of clouds in the model is significantly weaker than the observed, while the positive feedback from the greenhouse effect of clouds is significantly larger. Consequently, the net atmospheric feedback in the CCM3 over the equatorial cold tongue region is strongly positive (5.1 W m−2 K−1), while the net atmospheric feedback in the real atmosphere is strongly negative (−6.4 W m−2 K−1). A further analysis with the aid of the International Satellite Cloud Climatology Project (ISCCP) data suggests that cloud cover response to changes in the SST may be a significant error source for the cloud feedbacks. It is also noted that the surface heating over the cold tongue in CCM3 is considerably weaker than in observations. In light of results from a linear feedback system, as well as those from a more sophisticated coupled model, it is suggested that the discrepancy in the net atmospheric feedback may have contributed significantly to the cold bias in the equatorial Pacific in the NCAR Climate System Model (CSM).

Comparison of future climate change over North America simulated by two regional models

Climate versions of the Pennsylvania State University/National Center for Atmospheric Research (NCAR) MM5 and of the International Centre for Theoretical Physics/NCAR RegCM2 are intercompared with two 10‐year simulations over North America for present (1990–1999) and close to doubled CO2 (2090–2099), driven by the same general circulation model forcing from a 100‐year transient run of the NCAR Climate System Model. Some model options and modules are identical in MM5 and RegCM2, including radiation and the entire land surface model. MM5 and RegCM2 agree in their general patterns of surface temperatures. Details related to mesoscale effects are well represented in both models. Precipitation is more consistent in MM5 and RegCM2 in winter than in summer. Differences in temperature and precipitation between MM5 and RegCM2 are small in winter due to the strong large‐scale forcing but are large in summer because of the increasingly important contribution of local processes. Despite the general agreement between MM5 and RegCM2, significant differences exist over some areas due to the regional models' internal atmospheric physics and dynamics. The magnitude of these differences is large enough in some areas to cause serious uncertainties in any potential assessment of societal impacts of regional climate change.

South Atlantic Multidecadal Variability in the Climate System Model

Abstract Strong multidecadal variability is detected in a 300-yr integration of the NCAR Climate System Model in the South Atlantic region, through the application of two signal recognition techniques: the multitaper method and singular spectrum analysis. Significant oscillations of a 25–30-yr period are found in the sea surface temperature, sea level pressure, and barotropic transport fields. A similar-scale signal is also captured in about one century-long observational records. A composite analysis of several model variables is performed based on the extremes of the sea surface temperature oscillation. The proposed mechanism for this basin-scale multidecadal signal involves changes in the intensity of the westerlies, associated with variability in the southward extension of the subtropical anticyclone, which drives changes in the ocean mass transport. This results in variability in the intensity of the Malvinas western boundary current and in the position of the Brazil–Malvinas confluence zone. Anomalo...

Understanding Differences between the Equatorial Pacific as Simulated by Two Coupled GCMs

Abstract Numerical experiments are performed to isolate the cause of differences between the simulations of SST in the low-latitude Pacific of two coupled atmosphere–ocean general circulation models, the Center for Ocean–Land–Atmosphere (COLA) coupled model and the NCAR Climate System Model (CSM). The COLA model produces a more realistic simulation of the annual cycle of SST and interannual SST variability. The CSM has the more realistic annual mean wind stress and east–west SST gradient. The approach to finding the causes of these differences is to systematically eliminate differences in the physical parameterizations and numerics of the two models, and to examine the effects of these changes on the simulations. The results indicate that the atmospheric models rather than the ocean models are primarily responsible for differences in the simulations. There is no dominant process in the atmospheric models that explains the differences; both physical parameterizations (convection, surface flux formulation, ...

Tropical Pacific Variability in the NCAR Climate System Model

A 300-yr simulation with the NCAR Climate System Model (CSM), version 1, captured only ∼60% of the observed ENSO signal and exaggerated the interannual variability of SST in the western tropical Pacific. Here, a simulation with a new version of the CSM, which significantly improves the spatial and temporal patterns of tropical Pacific variability, is described. Maximum SST variability is shifted to the central and eastern Pacific. A better simulation of the equatorial Pacific thermocline structure results in Niño-3 and Niño-4 statistics comparable to the observed estimates for the last century. The evolution of SST and subsurface temperature anomalies is in excellent agreement with observed events. The majority of events evolve as a standing mode with weak SST anomalies occurring in the northern spring in the eastern tropical Pacific and maximum anomalies covering the eastern tropical Pacific Ocean to the date line by the following northern winter. At the same time, subsurface temperature anomalies spread eastward and upward along the tropical thermocline. The “delayed oscillator” and Wyrtki's “buildup” hypothesis are consistent with aspects of the CSM simulation. On the equator, a westerly wind stress anomaly in the central Pacific forces off-equatorial upwelling anomalies, which propagate westward, reaching the western boundary about one-half year later. This upwelling signal then propagates eastward along the equator, arriving 2 months before cooling in the eastern Pacific basin. The tropical Pacific atmospheric response to warm oceanic events also agrees with observational analyses with a negative Southern Oscillation pattern in sea level pressure, wind stress anomalies, and low-level convergence to the west of the maximum SST anomalies and enhanced deep convection and precipitation in the central and eastern tropical Pacific.

Climate Changes in the 21st Century over the Asia-Pacific Region Simulated by the NCAR CSM and PCM

The Climate System Model (CSM) and the Parallel Climate Model (PCM), two coupled global climate models without flux adjustments recently developed at NCAR, were used to simulate the 20th century climate using historical greenhouse gas and sulfate aerosol forcing. These simulations were extended through the 21st century under two newly developed scenarios, a business-as-usual case (BAU, CO2 ≈ 710 ppmv in 2100) and a CO2 stabilization case (STA550, CO2 ≈ 540 ppmv in 2100). The simulated changes in temperature, precipitation, and soil moisture over the Asia-Pacific region (10°–60°N, 55°–155°E) are analyzed, with a focus on the East Asian summer monsoon rainfall and climate changes over the upper reaches of the Yangtze River. Under the BAU scenario, both the models produce surface warming of about 3–5°C in winter and 2–3°C in summer over most Asia. Under the STA550 scenario, the warming is reduced by 0.5–I.0°C in winter and by 0.5°C in summer. The warming is fairly uniform at the low latitudes and does not induce significant changes in the zonal mean Hadley circulation over the Asia-Pacific domain. While the regional precipitation changes from single CSM integrations are noisy, the PCM ensemble mean precipitation shows 10%–30% increases north of ~ 30°N and ~ 10% decreases south of ~ 30°N over the Asia-Pacific region in winter and 10%–20% increases in summer precipitation over most of the region. Soil moisture changes are small over most Asia. The CSM single simulation suggests a 30% increase in river runoff into the Three Gorges Dam, but the PCM ensemble simulations show small changes in the runoff.

Cloud cover variations over the United States: An influence of cosmic rays or solar variability?

To investigate whether galactic cosmic rays (GCR) may influence cloud cover variations, we analyze cloud cover anomalies from 1900–1987 over the United States. Results of spectral analyses reveal a statistically significant cloud cover signal at the period of 11 years; the coherence between cloud cover and solar variability proxy is 0.7 and statistically significant with 95% confidence. In addition, cloud data derived from the NCAR Climate System Model (CSM) forced with solar irradiance variations show a strong signal at 11 years that is not apparent in cloud data from runs with constant solar input. The cloud cover variations are in phase with the solar cycle and not the GCR. Our results suggest that cloud variabilities may be affected by a modulation of the atmospheric circulation resulting from variations of the solar‐UV‐ozone‐induced heating of the atmosphere.

Climates of the Twentieth and Twenty-First Centuries Simulated by the NCAR Climate System Model

The Climate System Model, a coupled global climate model without “flux adjustments” recently developed at the National Center for Atmospheric Research, was used to simulate the twentieth-century climate using historical greenhouse gas and sulfate aerosol forcing. This simulation was extended through the twenty-first century under two newly developed scenarios, a business-as-usual case (ACACIA-BAU, CO2 ≈ 710 ppmv in 2100) and a CO2 stabilization case (STA550, CO2 ≈ 540 ppmv in 2100). Here we compare the simulated and observed twentieth-century climate, and then describe the simulated climates for the twenty-first century. The model simulates the spatial and temporal variations of the twentieth-century climate reasonably well. These include the rapid rise in global and zonal mean surface temperatures since the late 1970s, the precipitation increases over northern mid- and high-latitude land areas, ENSO-induced precipitation anomalies, and Pole–midlatitude oscillations (such as the North Atlantic oscillation) in sea level pressure fields. The model has a cold bias (2°–6°C) in surface air temperature over land, overestimates of cloudiness (by 10%–30%) over land, and underestimates of marine stratus clouds to the west of North and South America and Africa. The projected global surface warming from the 1990s to the 2090s is ∼1.9°C under the BAU scenario and ∼1.5°C under the STA550 scenario. In both cases, the midstratosphere cools due to the increase in CO2, whereas the lower stratosphere warms in response to recovery of the ozone layer. As in other coupled models, the surface warming is largest at winter high latitudes (≥5.0°C from the 1990s to the 2090s) and smallest (∼1.0°C) over the southern oceans, and is larger over land areas than ocean areas. Globally averaged precipitation increases by ∼3.5% (3.0%) from the 1990s to the 2090s in the BAU (STA550) case. In the BAU case, large precipitation increases (up to 50%) occur over northern mid- and high latitudes and over India and the Arabian Peninsula. Marked differences occur between the BAU and STA550 regional precipitation changes resulting from interdecadal variability. Surface evaporation increases at all latitudes except for 60°–90°S. Water vapor from increased tropical evaporation is transported into mid- and high latitudes and returned to the surface through increased precipitation there. Changes in soil moisture content are small (within ±3%). Total cloud cover changes little, although there is an upward shift of midlevel clouds. Surface diurnal temperature range decreases by about 0.2°–0.5°C over most land areas. The 2–8-day synoptic storm activity decreases (by up to 10%) at low latitudes and over midlatitude oceans, but increases over Eurasia and Canada. The cores of subtropical jets move slightly up- and equatorward. Associated with reduced latitudinal temperature gradients over mid- and high latitudes, the wintertime Ferrel cell weakens (by 10%–15%). The Hadley circulation also weakens (by ∼10%), partly due to the upward shift of cloudiness that produces enhanced warming in the upper troposphere.

Improvements to the NCAR CSM-1 for Transient Climate Simulations

Abstract Improvements to the NCAR Climate System Model, CSM-1, primarily for transient climate forcing simulations, are discussed. The impact of the individual changes is assessed through atmosphere–land or ocean–ice experiments, and through short coupled simulations. A 270-yr control simulation has been performed using the model with all of the changes, defined as CSM-1.3. Trace gas concentrations appropriate for 1870 were used and the model produced a stable surface climate with less deep ocean drift than CSM-1. Changing the aerodynamic roughness length of sea ice to a value appropriate for first year ice reduced the deep ocean salinity trend by a factor of 100 and error in the transport by the Antarctic circumpolar current by 50% (60 × 106 m3 s−1). Three new features were added to the Community Climate Model, version 3 (CCM3), which is the atmospheric component of CSM-1. These additions were N2O, CH4, CFC11, and CFC12 as prognostic tracers and the oxidation of CH4 to form water vapor; a prognostic clou...

Parallel climate model (PCM) control and transient simulations

The Department of Energy (DOE) supported Parallel Climate Model (PCM) makes use of the NCAR Community Climate Model (CCM3) and Land Surface Model (LSM) for the atmospheric and land surface components, respectively, the DOE Los Alamos National Laboratory Parallel Ocean Program (POP) for the ocean component, and the Naval Postgraduate School sea-ice model. The PCM executes on several distributed and shared memory computer systems. The coupling method is similar to that used in the NCAR Climate System Model (CSM) in that a flux coupler ties the components together, with interpolations between the different grids of the component models. Flux adjustments are not used in the PCM. The ocean component has 2/3° average horizontal grid spacing with 32 vertical levels and a free surface that allows calculation of sea level changes. Near the equator, the grid spacing is approximately 1/2° in latitude to better capture the ocean equatorial dynamics. The North Pole is rotated over northern North America thus producing resolution smaller than 2/3° in the North Atlantic where the sinking part of the world conveyor circulation largely takes place. Because this ocean model component does not have a computational point at the North Pole, the Arctic Ocean circulation systems are more realistic and similar to the observed. The elastic viscous plastic sea ice model has a grid spacing of 27 km to represent small-scale features such as ice transport through the Canadian Archipelago and the East Greenland current region. Results from a 300 year present-day coupled climate control simulation are presented, as well as for a transient 1% per year compound CO2 increase experiment which shows a global warming of 1.27 °C for a 10 year average at the doubling point of CO2 and 2.89 °C at the quadrupling point. There is a gradual warming beyond the doubling and quadrupling points with CO2 held constant. Globally averaged sea level rise at the time of CO2 doubling is approximately 7 cm and at the time of quadrupling it is 23 cm. Some of the regional sea level changes are larger and reflect the adjustments in the temperature, salinity, internal ocean dynamics, surface heat flux, and wind stress on the ocean. A 0.5% per year CO2 increase experiment also was performed showing a global warming of 1.5 °C around the time of CO2 doubling and a similar warming pattern to the 1% CO2 per year increase experiment. El Nino and La Nina events in the tropical Pacific show approximately the observed frequency distribution and amplitude, which leads to near observed levels of variability on interannual time scales.

Application of tree-structured regression for regional precipitation prediction using general circulation model output

This study presents a tree-structured regression (TSR) method to relate daily precipita- tion with a variety of free-atmosphere variables. Historical data were used to identify distinct weather patterns associated with differing types of precipitation events. Models were developed using 67% of the data for training and the remaining data for model validation. Seasonal models were built for each of 2 US sites: San Francisco, California, and San Antonio, Texas. The average correlation between observed and simulated daily precipitation data series is 0.75 for the training set and 0.68 for the validation set. Relative humidity was found to be the dominant variable in these TSR models. Out- put from an NCAR CSM (climate system model) transient simulation of climate change were then used to drive the TSR models in the prediction of precipitation characteristics under climate change. A preliminary screening of the GCM output variables for current climate, however, revealed signifi- cant problems for the San Antonio site. Specifically, the CSM missed the annual trends in humidity for the grid cell containing this site. CSM output for the San Francisco site was found to be much more reliable. Therefore, we present future precipitation estimates only for the San Francisco site.

Simulation of sea ice in the NCAR Climate System Model

Simulation of sea ice in the NCAR Climate System Model