Raymond W. Arritt

The North American Regional Climate Change Assessment Program: Overview of Phase I Results

The North American Regional Climate Change Assessment Program (NARCCAP) is an international effort designed to investigate the uncertainties in regional-scale projections of future climate and produce highresolution climate change scenarios using multiple regional climate models (RCMs) nested within atmosphere–ocean general circulation models (AOGCMs) forced with the Special Report on Emission Scenarios (SRES) A2 scenario, with a common domain covering the conterminous United States, northern Mexico, and most of Canada. The program also includes an evaluation component (phase I) wherein the participating RCMs, with a grid spacing of 50 km, are nested within 25 years of National Centers for Environmental Prediction–Department of Energy (NCEP–DOE) Reanalysis II. This paper provides an overview of evaluations of the phase I domain-wide simulations focusing on monthly and seasonal temperature and precipitation, as well as more detailed investigation of four subregions. The overall quality of the simulations i...

Nonclassical mesoscale circulations caused by surface sensible heat-flux gradients

Abstract Significant spatial heterogeneities of daytime surface sensible heat flux are common over land within mesoscale domains. Thermally induced circulations, similar to the sea/lake breeze [termed nonclassical mesoscale circulations (NCMSs)], are anticipated in these situations. Growing research interest in NCMSs has developed in the recent decade. In this article, general quantifications of NCMC characteristics are surveyed based on modeling and observational studies, along with further elaborations on specific NCMSs. The numerical modeling studies have indicated NCMSs with intensity comparable to the sea breeze in the ideal situations of sharp contrast between extended wet soil or crop and adjacent dry land areas. Similar results were obtained when contrast of cloud with clear sky and snow with snow-free areas were considered. For less ideal contrast, as well as for thermal contrast generated by some other types of forcing, weaker NCMCs were simulated. The limited observational studies have suggeste...

Hydrological responses to dynamically and statistically downscaled climate model output

Daily rainfall and surface temperature series were simulated for the Animas River basin, Colorado using dynamically and statistically downscaled output from the National Center for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR) re-analysis. A distributed hydrological model was then applied to the downscaled data. Relative to raw NCEP output, downscaled climate variables provided more realistic simulations of basin scale hydrology. However, the results highlight the sensitivity of modeled processes to the choice of downscaling technique, and point to the need for caution when interpreting future hydrological scenarios.

Project to Intercompare Regional Climate Simulations (PIRCS): Description and initial results

The first simulation experiment and output archives of the Project to Intercompare Regional Climate Simulations (PIRCS) is described. Initial results from simulations of the summer 1988 drought over the central United States indicate that limited-area models forced by large-scale information at the lateral boundaries reproduce bulk temporal and spatial characteristics of meteorological fields. In particular, the 500 hPa height field time average and temporal variability are generally well simulated by all participating models. Model simulations of precipitation episodes vary depending on the scale of the dynamical forcing. Organized synoptic-scale precipitation systems are simulated deterministically in that precipitation occurs at close to the same time and location as observed (although amounts may vary from observations). Episodes of mesoscale and convective precipitation are represented in a more stochastic sense, with less precise agreement in temporal and spatial patterns. Simulated surface energy fluxes show broad similarity with the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) observations in their temporal evolution and time average diurnal cycle. Intermodel differences in midday Bowen ratio tend to be closely associated with precipitation differences. Differences in daily maximum temperatures also are linked to Bowen ratio differences, indicating strong local, surface influence on this field. Although some models have bias with respect to FIFE observations, all tend to reproduce the synoptic variability of observed daily maximum and minimum temperatures. Results also reveal the advantage of an intercomparison in exposing common tendencies of models despite their differences in convective and surface parameterizations and different methods of assimilating lateral boundary conditions.

Altered hydrologic feedback in a warming climate introduces a ''warming hole''

[2] Changes in forcing of the climate system can trigger new or altered feedback processes. We have found evidence of such a feedback in the hydrological cycle of the central U.S. that creates a regional minimum within the continentalscale pattern of warming in an enhanced greenhouse-gas climate. The effect of this particular feedback is amplified because a change is introduced into a slowly varying component of the hydrologic cycle (soil moisture) thereby extending the impact of increased summer precipitation to later months in the annual cycle. We investigated these processes using a regional climate model (RCM) to downscale contemporary and future scenario climates from a global climate model (GCM) [Johns et al., 1997] in order to project resolution-enhanced patterns of climate change for the continental U.S. Previous work has shown that the downscaled climate from this approach provides a reasonable representation of the atmosphere-hydrology linkage in this region [Pan et al., 2001a; Gutowski et al., 2003]. [3] The most notable feature in the projected climate is a local minimum of warming (hereinafter called a ‘‘warming hole’’) in the central U.S. during summer (June, July and August) (Figure 1a). The increase in daily maximum surface air temperature (dTmax) in summer at the center of the warming hole is less than 0.5 K, which is substantially less than the mean increase of about 3 K over the continental U.S. The ground temperature has an even stronger warming hole with 0.5 K cooling, rather than warming, in the center. The warming hole starts to develop in June, reaches its maximum value in September, and gradually diminishes through October and November (Figure 1b). The purpose of this paper is to analyze the processes underlying the reduced warming and to show the hole’s links to observed climate trends. 2. Methods

Use of regional climate model output for hydrologic simulations

Abstract Daily precipitation and maximum and minimum temperature time series from a regional climate model (RegCM2) configured using the continental United States as a domain and run on a 52-km (approximately) spatial resolution were used as input to a distributed hydrologic model for one rainfall-dominated basin (Alapaha River at Statenville, Georgia) and three snowmelt-dominated basins (Animas River at Durango, Colorado; east fork of the Carson River near Gardnerville, Nevada; and Cle Elum River near Roslyn, Washington). For comparison purposes, spatially averaged daily datasets of precipitation and maximum and minimum temperature were developed from measured data for each basin. These datasets included precipitation and temperature data for all stations (hereafter, All-Sta) located within the area of the RegCM2 output used for each basin, but excluded station data used to calibrate the hydrologic model. Both the RegCM2 output and All-Sta data capture the gross aspects of the seasonal cycles of precipit...

The July 1995 Heat Wave in the Midwest: A Climatic Perspective and Critical Weather Factors

A brief but intense heat wave developed in the central and eastern United States in mid-July 1995, causing hundreds of fatalities. The most notable feature of this event was the development of very high dewpoint temperature (Td) over the southern Great Lakes region and the Upper Mississippi River Basin. At many locations, hourly values of Td set new records. The combination of high air and dewpoint temperatures resulted in daily average apparent temperatures exceeding 36°C over a large area on some days. A comparison with past heat waves shows that this was the most intense short-duration heat wave in at least the last 48 years at some locations in the southern Great Lakes region and Upper Mississippi River Basin. An analysis of historical data for Chicago, where the majority of fatalities occurred, indicates the intensity of this heat wave was exceeded only by a few periods in the 1910s and 1930s. Impacts in the Chicago urban center were exacerbated by an urban heat island that raised nocturnal temperatu...

Numerical Simulations of the Effect of Soil Moisture and Vegetation Cover on the Development of Deep Convection

Abstract A one-dimensional (column) version of a primitive equations model has been used to study the impact of soil moisture and vegetation cover on the development of deep cumulus convection in the absence of dynamical forcing. The model includes parameterizations of radiation, turbulent exchange, deep convection, shallow boundary layer convective clouds, vegetation, and soil temperature and moisture. Multiple one-dimensional experiments were performed using the average July sounding for Topeka, Kansas, as the initial condition. A range of volumetric soil moisture from one-half of the wilting point to saturation and vegetation cover ranging from bare soil to full cover were considered. Vegetation cover was found to promote convection, both by extraction of soil moisture and by shading the soil so that conduction of heat into the soil was reduced (thereby increasing the available energy). The larger values of initial soil moisture were found to delay the onset of precipitation and to increase the precipi...

A Climatology of the Warm Season Great Plains Low-Level Jet Using Wind Profiler Observations

Abstract Hourly observations from the Wind Profiler Demonstration Network during the warm season months of 1991 and 1992 were used to develop a climatology of the low-level jet (LLJ) over the Great Plains of the central United States. The maximum overall frequency of LLJ occurrence was in the southern part of the Great Plains, while the maximum frequency of the stronger LLJs extended farther north and east (Kansas through Nebraska). The frequency of occurrence for the weakest LLJs exhibited little diurnal variation. In contrast, the strongest jets were about six times more likely to occur within a few hours of local midnight than during the day. Southerly wind events that did not satisfy the criteria for low-level jets (i.e., those that did not include the low-level local maximum of the wind speed profile) showed comparatively little diurnal variability regardless of speed. Analysis of the synoptic patterns associated with LLJ occurrence showed that the LLJ was promoted by the warm sector of an extratropi...

Climate change projections of the North American Regional Climate Change Assessment Program (NARCCAP)

We investigate major results of the NARCCAP multiple regional climate model (RCM) experiments driven by multiple global climate models (GCMs) regarding climate change for seasonal temperature and precipitation over North America. We focus on two major questions: How do the RCM simulated climate changes differ from those of the parent GCMs and thus affect our perception of climate change over North America, and how important are the relative contributions of RCMs and GCMs to the uncertainty (variance explained) for different seasons and variables? The RCMs tend to produce stronger climate changes for precipitation: larger increases in the northern part of the domain in winter and greater decreases across a swath of the central part in summer, compared to the four GCMs driving the regional models as well as to the full set of CMIP3 GCM results. We pose some possible process-level mechanisms for the difference in intensity of change, particularly for summer. Detailed process-level studies will be necessary to establish mechanisms and credibility of these results. The GCMs explain more variance for winter temperature and the RCMs for summer temperature. The same is true for precipitation patterns. Thus, we recommend that future RCM-GCM experiments over this region include a balanced number of GCMs and RCMs.