摘要 :
The relative contributions of locally evapotranspired (i.e., recycled) moisture versus externally advected water vapor for the growing-season precipitation of the U.S. Corn Belt and surrounding areas (1.23 X 10~6 km~2) are estimat...
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The relative contributions of locally evapotranspired (i.e., recycled) moisture versus externally advected water vapor for the growing-season precipitation of the U.S. Corn Belt and surrounding areas (1.23 X 10~6 km~2) are estimated in this paper. Four May-August seasons with highly contrasting precipitation and crop yields (1975, 1976, 1979, and 1988) are investigated. A simple recycling equation--developed from the traditional atmospheric moisture budget and involving regional evapotranspiration and atmospheric water vapor inflow--is applied on daily, monthly, and seasonal time scales. Several atmospheric moisture budget components {moisture flux divergence [MFD], storage change [or change in precipitable water (dPW)], and inflow [IF]} are evaluated for 24-h periods using standard finite difference and line integral methods applied to objectively analyzed U.S. and Canadian rawinsonde data (50-hPa vertical resolution, surface to 300 hPa) for 0000 and 1200 UTC. Daily area-averaged precipitation (P)totals are derived from approximately 600 evenly distributed (but ungridded) recording rain gauges. Evapotranspiration (E) is estimated as the residual of the moisture budget equation for 24-h periods; values compare favorably with the few existing observations. Traditional budget results show the following: E is weakly related to P on monthly and seasonal time scales; there is surprising interannual constancy of seasonal E cycles and averages given the large variation in resulting crop yields; and monthly and seasonal variability of the export of the E -- P surplus is determined largely by the horizontal velocity divergence component of MFD. New recycling analyses suggest that the contribution of local to P (i.e., P_EIP) is relatively small and remarkably consistent (largely 0.19-0.24) for monthly and seasonal periods, despite large P and crop yield variations. However, the monthly/seasonal averaging process is found to completely mask a striking decrease of daily P_E/P (from approximately 0.30 to0.15) with increasing P from 0 to 8 mm day~(-1). Unique and detailed analyses off-stratified daily moisture budget results provide key insights into apparent contradictions between daily and monthly/seasonal recycling and related results and concomitantinterannual variability, especially for the very dry 1988 season. Interpretation is facilitated by the use of modeled daily global radiation values, measured (instantaneous) and modeled (monthly) soil moisture, United States Department of Agriculture (USDA) crop yield estimates, and satellite normalized difference vegetation index (NDVI) imagery. This paper shows that land-atmosphere interactions are intimately involved in pronounced seasonal climate anomalies for the world's richest agricultural region, but apparently with considerable complexity that includes plant behavior, solar radiation forcing, and challenging time-scale interrelations.
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摘要 :
Atmospheric moisture budget components are evaluated for a large area (1.23 X 10~6 km~2) in the midwestern United States for all 12-h (1200-0000, 0000-1200 UTC) and 24-h (1200-1200 UTC) periods during the contrasting summers (May-...
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Atmospheric moisture budget components are evaluated for a large area (1.23 X 10~6 km~2) in the midwestern United States for all 12-h (1200-0000, 0000-1200 UTC) and 24-h (1200-1200 UTC) periods during the contrasting summers (May-August) of 1975, 1976. 1979, and 1988. The atmospheric moisture flux divergence (MFD, separated into horizontal and vertical advection components, HA and VA) and storage change (dPW) are estimated using a standard finite-difference method applied to objectively analyzed U.S. and Canadian rawinsonde data (50-hPa vertical resolution, surface-300 hPa) for 0000 and 1200 UTC. Area-averaged precipitation (P) totals are derived from approximately 600 relatively evenly distributed (but ungridded) recording rain gauges. Evapotranspiration (E) is estimated as a residual of the moisture budget equation and compares favorably with the few existing observations, especially when totaled for periods of 1 month or longer. Relationships between the budget components are established for the daily, monthly, and seasonal timescales using stratification, correlation, and cross-spectral analyses. On monthly and seasonal timescales, the surface is a net source of water vapor (positive E-P) and the bulk of this surplus is exported from the region, largely through HA. For the daily budget, a threshold P rate (approx 4 mm day~(-1)) separates surplus E-P budgets from deficit budgets. On all timescales, most of the P variance is reflected in the VA component of MFD, while HA explains approx 80 percent of the variation in dPW. For the monthly and (especially) daily budgets, E has bimodal distributions with P where the minimum E occurs at P approx 2.6 mm day~(-1) (monthly) and P approx 4-5 mm day~(-1) (daily). For drier daily P regimes, relativelyhigh E is associated with increased (decreased) dry VA (HA). The correlation of E with P becomes substantially more positive from the daily-to-monthly timescale, confirming the importance of land-atmosphere interactions over longer periods. The above stratification and correlation results are complemented by cross-spectral analyses that identify strong associations between P-HA and P-dPW previously masked by phase differences. The cross-spectral results also prompt the development of a conceptual modelthat describes the temporal relationships among the budget components for eastward-moving large-scale, "wavelike" disturbances with 3-10-day timescales. The suggested sequence of interactions--moist HA is accompanied by a pronounced PW increase and thenfollowed by a moist VA maximum; this horizontal and then vertical moisture redistribution is first associated with an E minimum and then culminates in a P maximum; after the P event, atmospheric drying occurs through increased (diminished) dry HA (moistVA), which leads to an E maximum and then P minimum.
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Typical mid-winter anthropogenic air pollution episodes are caused when pollutants are trapped in the lower atmospheric boundary layer due to the generation of surface inversion favored by synoptic conditions. We analyzed the opti...
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Typical mid-winter anthropogenic air pollution episodes are caused when pollutants are trapped in the lower atmospheric boundary layer due to the generation of surface inversion favored by synoptic conditions. We analyzed the optical properties of atmospheric aerosol particles obtained during one such episode using a sun/sky radiometer at two measurement sites: one located in the densely populated and industrialized central part of Israel, and the other in a reference site, about 150 km away. Aerosol optical thickness and volume size distributions showed an increased burden of fine aerosol particles in the central part of Israel. In order to verify the local origin and anthropogenic nature of the effect, the analysis was accompanied by examinations of the synoptic conditions, air mass backward trajectories, and conventional in situ air pollution measurements made by a ground-based sampling station. This case study shows the ability of optical measurements to track urban and industrial atmospheric air pollution expressed by high concentration of fine aerosol particles. In addition, it emphasizes the role of local Israeli air pollution sources and may explain the difference in the properties of long-term aerosol optical observations between the two sites. The advantages of the optical method presented are speed (almost instantaneous), automated measurement, and sensitivity to aerosol particle concentration as well as aerosol size fraction. The drawback is that the optical measurements discussed deal only with aerosol particles and cannot distinguish between different types of pollutant gases.
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This study introduces a new conceptual model to explain the recently observed changes in winter precipitation over Israel. The model is based on our earlier published work (where a connection was reported between the occurrence of...
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This study introduces a new conceptual model to explain the recently observed changes in winter precipitation over Israel. The model is based on our earlier published work (where a connection was reported between the occurrence of major rain days (MRDs) in different parts of the country and three prototypes, A, B and C, of the 500 hPa trough axis orientation prevailing on MRDs) and on additional results obtained by an extension of that work in the present paper. The first part of the present study is devoted to the extension of our early work. Composite techniques have been used on National Center for Atmospheric Research 9NCAR)-National Meteoreological Center grid-point data for the rain seasons 1981-82 to 1985-86 to identify the sea-level pressure (SLP) distribution associated with each of the three 500 hPa prototypes. Prototype A (trough axis oriented from northwest to southeast, earlier shown to be associated with MRDs in northern Israel) was found in the present work to be associated with a surface low in the vicinity of Antalya, southern Turkey. Prototype B (trough axis oriented from north to south, earlier shown to be associated with MRDs in central Israel) was found in the present work to be associated with a surface low over southeastern turkey. Prototype C (trough axis oriented from northeast to southwest, earlier shown to be associated with MRDs in southern Israel) was found to be associated with elevated surface pressure over northwestern Turkey and a trough over eastern Turkey. In the second part of the study we used our results to construct a conceptual model of the mechanism responsible for the relative increases in seasonal (winter) rainfall over the southern part of the country and the decrease over the north. Using National Centers for Environmental Prediction-NCAR reanalysis data for the period 1982-2000, we demonstrated that the direct atmospheric agent responsible for this change in the spatial rainfall distribution is an increased frequency of occurrence of 500 hPa troughs oriented from northeast to southwest (Prototype C) accompanied by prominent positive SLP anomalies centred over turkey. Our analysis further shows that these atmospheric systems are consistent with the persistence of a positive phase of the North Atlantic Oscillation on the one hand and with latest IPCC predictions of precipitation patterns over the eastern Mediterranean basin on the other hand.
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The turn of the millennium, just a few short years ago, brought much anticipation for the future and a renewed commitment to ensure its strength. Our Society, spurred by that fervor, embarked on the AMS 21st Century Campaign to pr...
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The turn of the millennium, just a few short years ago, brought much anticipation for the future and a renewed commitment to ensure its strength. Our Society, spurred by that fervor, embarked on the AMS 21st Century Campaign to provide a focused institutional mechanism for the advancement of the atmospheric and related sciences and their services to society. Through this campaign, AMS members--in increasing numbers--are making voluntary contributions (large and small) that are paving the way for the continued advancement of our sciences and their ability to address 21st century societal needs. Society members, young and older, have benefitted greatly over the years from the solid foundation of basic and advanced technical education opportunities that abound in our disciplines and the broad range of career options and experiences that our professions provide for us. These education and experience foci provide the cornerstones for the 21st Century Campaign, with specific opportunities for targetedgiving in the following areas: Public Awareness; Education of Future Scientists (K-12 through graduate studies); Historical Documentation of the Atmospheric and Related Sciences; Atmospheric Policy Program. By contributing to the Campaign in one of thesespecific areas, now and in the future, you ensure the continued vitality and growth of our sciences. Beyond traditional "cash" contributions, the AMS earlier had established additional giving mechanisms that allow donors to make substantial and sustaining gifts to the Society.
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