On surface versus atmospheric forcing in regional climate simulations

This dissertation, a collection of three papers, performed a number of numerical experiments to contrast surface and atmospheric forcing in regional climate simulations. Results of the first paper showed that a transient increase in soil moisture enhanced domain total rainfall. 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;It was found in the second paper that as integrations continued without reinitialization, locations of weather systems drifted downstream because of the positive model bias on wind speeds, implying the necessity of periodically reinitializing the model. The simulated domain-averaged variables were not sensitive to model reinitialization, suggesting the feasibility of dividing long regional climate simulations into a set of shorter segments that could be run in parallel;The results of the third paper suggested that the current landuse increased domain-averaged rainfall by less than 1% over the pre-settlement landuse, and the hypothetical future uniform cropland increased total rainfall by 3% under the normal condition. The current and future landuse tended to reduce rainfall over the east-central U.S. and to increase rainfall over the western U.S., especially in the flood year;The following conclusions may be drawn as to surface versus atmospheric forcing: (i) The mean properties over the continental scale are determined mainly by large-scale atmospheric forcing, and surface forcing has little impacts on them. (ii) With given large-scale atmospheric forcing, surface forcing has noticeable roles to redistribute atmospheric variables by either shifting specific patterns around or modulating them. (iii) The displacement of weather patterns can result in large differences in climate variables on local scales, especially for rainfall

Soybean rust update and outlook

During the last two seasons, we projected that the risk of soybean rust (SBR) was no longer a concern when June ended. This season is different from last year because we received more rain early in the growing season, and the disease progressed much faster in the South than during the last two years. Below is an update on the movement of SBR in the two northward pathways and an outlook for the rest of the growing season

Soybean rust update and outlook, June 2007

For the past two years, we used a computer model to predict the risk of seasonal outbreaks for soybean rust during a growing season. This year, we improved the model and are making predictions using this model together with information from sentinel plot data. Below is an update of the occurrence of soybean rust and our seasonal outlook

How to read spore forecasting maps

In the coming season, forecasting or modeling of rust spore movement has been proposed to provide producers with a tool for rust management. There is a possibility of spore maps being used to guide our scouts if the model is implemented. This article addresses two related aspects: (1) how soybean rust spreads from southern overwintering regions to northern soybean production regions; (2) how to read spore forecasting maps that will be posted on the Web to help us scout soybean rust during the coming growing season

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\u27s 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

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

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

Long Simulation of Regional Climate as a Sequence of Short Segments

Regional climate simulations are long time integrations over an open system where the atmosphere over part of the model domain (boundary zones) is updated periodically. Model reinitialization after a long period of integration can allow several segments of a long simulation to be run in parallel and also minimize possible drift caused by accumulated model errors. However, the spinup problems introduced by each additional restart must be addressed. The necessity and feasibility of subdividing long integrations is investigated by means of a series of experiments in which the authors examine the effects of reinitialization frequency and the relative importance of surface forcing and atmospheric forcing. It is found that for integrations that continued without reinitialization, locations of specific meteorological features drifted downstream because simulated winds were too strong, implying the need for periodic reinitialization of the model. The results indicate also that when the model reinitialization interval is relatively long, simulated domain-averaged variables, including rainfall, were not very sensitive to model reinitialization since they are largely constrained by transient boundary conditions, suggesting the feasibility of dividing long regional climate simulations into a set of shorter ones that could be run in parallel