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

1999-08-27 , Gutowski, William , Gutowski, William , Arritt, Raymond , Pan, Zaitao , Anderson, Christoper , Ramos da Silva, Renato , Takle, Eugene , Chen, Shyh-Chin , Giorgi, F. , Christensen, Jens , Hong, Song-You , Juang, Hann-Ming , Katzfey, Jack , Lapenta, William , Laprise, Rene , Liston, Glen , Lopez, Philippe , McGregor, John , Pielke, Roger , Roads, John , Aerospace Engineering , Ames Laboratory , Agronomy , Geological and Atmospheric Sciences

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.

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Influences of Model Parameterization Schemes on the Response of Rainfall to Soil Moisture in the Central United States

1996-08-01 , Pan, Zaitao , Takle, Eugene , Segal, Moti , Turner, Richard , Takle, Eugene , Aerospace Engineering , Ames Laboratory , Agronomy , Geological and Atmospheric Sciences

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.

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Model simulation of impacts of transient surface wetness on summer rainfall in the US Midwest during drought and flood years

1995-05-01 , Pan, Zaitao , Segal, Moti , Turner, Richard , Takle, Eugene , Takle, Eugene , Aerospace Engineering , Ames Laboratory , Agronomy , Geological and Atmospheric Sciences

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.

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On the Potential Impact of Irrigated Areas in North America on Summer Rainfall Caused by Large-Scale Systems

1998-03-01 , Segal, Moti , Pan, Zaitao , Turner, R. , Takle, Eugene , Takle, Eugene , Aerospace Engineering , Ames Laboratory , Agronomy , Geological and Atmospheric Sciences

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.

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Simulation of potential impacts of man-made land use changes on U.S. summer climate under various synoptic regimes

1999-03-27 , Pan, Zaitao , Takle, Eugene , Segal, Moti , Arritt, Raymond , Takle, Eugene , Aerospace Engineering , Ames Laboratory , Agronomy , Geological and Atmospheric Sciences

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.