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Raymond P. Motha, Shashi B. Verma, and Norman J. Rosenberg

Abstract

Spectra of vertical and horizontal velocity, air temperature and humidity fluctuations were analyzed from measurements made over a well-watered alfalfa crop under conditions of sensible heat advection. Vertical velocity and air temperature spectra as well as cospectra of momentum, sensible heat and water vapor were found to be dependent on atmospheric thermal stratification. These spectra and cospectra were shifted toward higher frequencies under advective as compared to lapse conditions. These results indicate that the predominant eddy sizes are relatively smaller under advective conditions. Humidity spectra, on the other hand, were independent of daytime thermal stratification conditions but were influenced by conditions of the crop. The humidity spectra over a transpiring crop were shifted toward lower frequencies as compared to those over a less well-developed crop canopy.

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Raymond P. Motha, Shashi B. Verma, and Norman J. Rosenberg

Abstract

Thermal inversions induced by regional advection dominate the daytime climate of the central Great Plains during much of the growing season. The influence of these inversions on the turbulent transfer of momentum, sensible heat and water vapor was investigated through detailed observations over an alfalfa crop. The standard deviations of fluctuations in vertical velocity, air temperature and vapor pressure as well as the correlation coefficients for momentum, sensible heat and water vapor were found to be similar under both advective and lapse conditions. Results indicate that turbulent mixing is effectively maintained under advective conditions to transport large quantities of sensible heat to the crop surface and water vapor away from it.

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Raymond P. Motha and Thomas R. Heddinghaus

The Joint Agricultural Weather Facility (JAWF), a cooperative effort between the Climate Analysis Center, NMC/NWS/NOAA (National Meteorological Center/National Weather Service/National Oceanic and Atmospheric Administration) and the World Agricultural Outlook Board, U.S. Department of Agriculture (USDA), focuses on weather anomalies and their effects on the crop-yield potential in major international crop areas. The basic mission is to provide an objective procedure for translating the flow of global weather information into timely and accurate assessments of growing-season conditions which ultimately impact on global agricultural production and trade. Daily monitoring of satellite weather images and meteorological data provides the framework for agricultural weather analysis. Daily, weekly, and seasonal summaries are processed and merged with historical weather and crop data for evaluation of the crop-yield potential. Information is disseminated at routine briefings, in written summaries, and through informal discussions.

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Raymond P. Motha, Sharon K. Leduc, Louis T. Steyaert, Clarence M. Sakamoto, and Norton D. Strommen

Abstract

A regional precipitation analysis from a total of 813 recording stations in 11 West African countries for the drought period 1968–75 is presented. Results illustrate the severity and extent of meteorological drought which prevailed throughout the region and reached greatest magnitude in 1973. In that year, the critical 300–400 mm zone of annual rainfall was at least 200 km south of its normal position resulting in major crop failures in several sub-Saharan countries.

In Nigeria, 50-year records of rainfall from 28 stations were examined to study both temporal and spatial distributions. In the northern Sahelian zone of Nigeria, two prolonged drought periods were observed (i.e., 1940’s and 1968–76). This detailed analysis further demonstrated the strong relationship between rainfall in the Sahelian region and the position of the Intertropical Convergence Zone (ITCZ). However, there were years during which widespread below-normal rainfall occur-red throughout most of Nigeria which supports previous studies linking large-scale tropical circulation features to rainfall in the Sahel.

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