Aerospace Engineering

OrgUnit Logo
Date established

Publication Search Results

Now showing 1 - 3 of 3
No Thumbnail Available

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.

No Thumbnail Available

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.

No Thumbnail Available

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.