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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.
Model simulation of impacts of transient surface wetness on summer rainfall in the US Midwest during drought and flood years
Surface moisture availability has been hypothesized by various investigators to provide additional negative (positive) feedback on rainfall during summer drought (flood) conditions in the Midwest. In this note, we report on a preliminary numerical modeling effort in which the impact of transient changes in surface wetness an summer rainfall events in the midwestern United States during two recent drought and flood years is assessed. It was found that during the drought of 1988, hypothetical temporary extreme moistening of the surface resulted in large relative increases in simulated rainfall, often by as much as a factor of 2. However, from an agricultural perspective these large relative changes in rainfall might not necessarily have translated into meaningful increases since the original absolute rainfall amounts were quite small. In the flood year of 1993, an assumed transient drying of the surface resulted in relative decreases in simulated rainfall by as much as 30%–40%. This relative decrease in rainfall did, however, translate into a discernible drop in the absolute rainfall.
Review of: Boundary Elements IX, Vols. 1–3
This three-volume edition contains edited versions of 128 papers presented at the 9th International Conference on Boundary Elements held at the University of Stuttgart, Germany, from 31 August to 4 September 1987. The conference series is devoted to a review of the latest developments in the technique and theory of the boundary element methods (BEM) with emphasis on new advances and trends. This particular meeting was to be devoted to ''the engineering aspects versus mathematical formulations, in an effort to consolidate the basis of many new applications.'' Whether this goal was achieved is necessarily going to be determined by whether the reader has the mathematical ability to digest and interpret the several papers that address the mathematical bases of some of the newer applications. Each volume is divided into six to eight sections of related papers. One or two invited papers lead off most of the sections, followed by three to five contributed papers touching on both mathematical theory and applications of boundary elements. The references at the end of each contribution, when summed together, represent a substantial compilation of related literature representing a diversity of languages.
Numerical Simulation of the Nocturnal Turbulence Characteristics over Rattlesnake Mountain
A two-dimensional second-order turbulence-closure model based on Mellor-Yamada level 3 is used to examine the nocturnal turbulence characteristics over Rattlesnake Mountain in Washington. Simulations of mean horizontal velocities and potential temperatures agree well with data. The equations for the components of the turbulent kinetic energy (TKE) show that anisotropy contributes in ways that are counter to our intuition developed from mean flow considerations: shear production under stable conditions forces the suppression of the vertical component proportion of total TKE, while potential-temperature variance under stable conditions leads to a positive (countergradient) contribution to the heat flux that increases the vertical component proportion of total TKE. This paper provides a qualitative analysis of simulated turbulence fields, which indicates significant variation over the windward and leeward slopes. From the simulation results, turbulence anisotropy is seen to develop in the katabatic flow region where vertical wind shears and atmospheric stability are large. An enhancement of the vertical component proportion of the total TKE takes place over the leeward slope as the downslope distance increases. The countergradient portion of the turbulent heat flux plays an important role in producing regions of anisotropy.
Simulation of potential impacts of man-made land use changes on U.S. summer climate under various synoptic regimes
This study evaluates impacts of land use changes due to human settlement on regional summer climate over the central and western United States by performing 30-day simulations during normal, drought, and flood years. Under current land use the simulated evapotranspiration increased noticeably over the central United States where grassland has been replaced by crops. Simulated evapotranspiration decreased slightly in the western United States. These changes produced wetter and cooler surface air over the central United States and slightly drier and warmer air over the western United States. Responses of surface fluxes and thus screen height variables to land use changes were consistent from year to year, whereas rainfall showed strong interannual variations because of the combination of various dynamic processes involved in precipitation. For normal year conditions, average evapotranspiration and rainfall under current land use increased by 18% and 8%, respectively, over the central United States, whereas they slightly decreased in the western United States. In both flood and drought years, current land use exhibited a rainfall increase in the western United States and a decrease over the central United States. The decrease of rainfall with increased evapotranspiration in the central United States was likely associated with weakening of the dynamic forcing needed to produce precipitation.
On the Potential Impact of Irrigated Areas in North America on Summer Rainfall Caused by Large-Scale Systems
The potential impact of the increase in irrigated areas in North America during the past 100 years on summer rainfall associated with medium- to large-scale precipitation systems is evaluated conceptually and by several illustrative numerical model simulations. The model results for the simulated cases suggest a tendency toward some increase in the continental-average rainfall for the present irrigation conditions compared with those of past irrigation. The maximum increase obtained for several studied cases of 6-day duration each was 1.7%. Rainfall increases typically occur in the location of existing rainfall areas, and the main effect of irrigation is to redistribute rainfall in those preexisting precipitation regions.
A modeling study of the effects of baroclinicity on the structure of the interacting atmospheric and oceanic boundary layers
The effects of baroclinicity on the air and ocean boundary layers under conditions for strong dynamical (compared to thermodynamic) forcing are studied by use of a numerical model of air-sea interaction, which consists of a closed system of equations including equations of motion, turbulent kinetic energy, turbulent exchange coefficient, local turbulent length scale, and assumptions of fixed stratification and aroclinicity in both the atmosphere and ocean. Baroclinicity is incorporated into the equations of motion by specifying horizontal gradients of air temperature in the atmosphere and seawater density in the ocean. Experiments were conducted to determine the effects of different magnitudes and directions of baroclinicity and of atmospheric stratification on the dynamical and turbulent structure of the interacting boundary layers. The results of the simulations demonstrate that certain levels of baroclinicity produce double maxima in the K profiles in the atmosphere and ocean. Baroclinic effects change the dominant components of the turbulent kinetic energy in both air and sea boundary layers from shear production and dissipation for dimensionless heights and depths of less than 0.1 (about 20% of the height or depth of the boundary layer at zero surface heat flux) to shear production and buoyant destruction for dimensionless heights and depths greater than 0.1. The results show that the most significant effects of baroclinicity in the air and sea boundary layers are the increases in turbulent exchange coefficient, turbulent kinetic energy budget, shear stresses, and dimensionless wind and wind-induced current in the regions of the boundary layers far from the interface. The results of the simulations also show that for fixed stratification and baroclinicity, surface quantities (e.g., friction velocity, drag coefficient, and geostrophic drag coefficient) are affected more by surface heat flux than by baroclinicity, whereas the opposite is true for characteristics of the whole boundary layer (e.g., boundary layer height and angle between the geostrophic wind and surface stress). Our results show good agreement with the few observations that have been taken where baroclinicity has been reported.
Influences of Model Parameterization Schemes on the Response of Rainfall to Soil Moisture in the Central United States
The sensitivities of soil moisture impacts on summer rainfall in the central United States to different commonly used cumulus parameterization and surface flux schemes are examined using the PSU-NCAR MMS under different atmospheric and soil moisture conditions. The cumulus convection schemes used are the Kuo and Grell parameterization schemes, while the surface-moisture flux schemes used are the aerodynamic formulation and the Simple Biosphere (SiB) Model. Results show that a transient increase in soil moisture enhanced total rainfall over the simulation domain. The increase in soil moisture enhanced local rainfall when the lower atmosphere was thermally unstable and relatively dry, but it decreased the rainfall when the atmosphere was humid and lacked sufficient thermal forcing to initiate deep convection. Soil moisture impacts were noticeably stronger for the Kuo scheme, which simulated lighter peak rainfall, than those for the Grell scheme, which simulated heavier peak rainfall. The greater sensitivity to soil moisture exhibited by the Kuo scheme than either the Grell or explicit scheme implies that it exaggerated the role of soil moisture. This difference was related to how each scheme partitioned rainfall between convective and stable forms, and possibly to each scheme's closure assumptions. Adding details to the surface-moisture flux schemes had a secondary influence on soil moisture impacts on rainfall within a 24-h period.
A Synoptic Climatology for Forest-Fires in the NE US and Future Implications From GCM Simulations
We studied surface-pressure patterns corresponding to reduced precipitation, high evaporation potential, and enhanced forest-fire danger for West Virginia, which experienced extensive forest-fire damage in November 1987. From five years of daily weather maps we identified eight weather patterns that describe distinctive flow situations throughout the year. Map patterns labeled extended-high, back-of-high, and pre-high were the most frequently occurring patterns that accompany forest fires in West Virginia and the nearby four-stare region. Of these, back-of-high accounted for a disproportionately large amount of fire-related damage. Examination of evaporation acid precipitation data showed that these three patterns and high-to-the-south patterns ail led to drying conditions and all other patterns led to moistening conditions. Surface-pressure fields generated by the Canadian Climate Centre global circulation model for simulations of the present (1xCO2) climate and 2xCO2 climate were studied to determine whether forest-fire potential would change under increased atmospheric CO2. The analysis showed a tendency for increased frequency of drying in the NE US, but the results were not statistically significant.