Abstract

Sub-Saharan West Africa (10–20°N) receives moisture from the tropical Atlantic via low-level south-westerly flow across the southwestern coast of West Africa. This paper utilizes a 1arge data set to identify the tropical Atlantic (30°N–30°S) surface atmospheric and oceanic patterns for two years when sub-Saharan West Africa experienced anomalous weather. Comparison is made with 60-year (1911–70) average fields.

The following tropical Atlantic surface features were located/centered 300–500 km further south in the deficient sub-Saharan rainy season (July-September) of 1968 than the more abundant 1967 rainy season— the kinematic axis between the Northern and Southern Hemisphere trades, the near-equational convergence tune, the near-equatorial pressure trough, the zone of maximum sea surface temperature (SST), the mid-Atlantic maxima of precipitation frequency and total cloudiness, and the center of the North Atlantic subtropical high. Sixty-year mean positions of these features were generally intermediate between the 1967 and 1968 locations. Rainfall was more frequent immediately south of the Gulf of Guinea coast and more abundant along this coast, during the 1968 sub-Saharan drought than in 1967. During the dry July-September 1968, positive SST departures occurred south of 10°N and east of 35°W, with a southwest-northwest oriented negative SST anomaly immediately to the northwest. The opposite SST departure pattern characterized July-September 1967.

The July-September 1968 departures from 60-year average patterns were largely characteristic of April-June 1968. In contrast, the July-September 1967 anomalies showed little evidence of evolving during preceding

A review on the development of climatic scenarios related to policy-oriented assessment of the impact of climatic variations is presented. It seeks to provide background information needed to evaluate the extent to which existing regional scenarios have utility in the above context, and whether and how such utility could be increased in the future. An appraisal of alternative approaches (both GCM and empirically based) that have been used to develop scenarios from the standpoints of their respective motivations, the methods employed, the acknowledged strengths (both present and potential) and weaknesses, the results obtained, and the credibility of those results is given. Types of research needed to make regional climatic scenarios of greater utility for the policy-oriented assessment of the impact of climatic variations are suggested.

This paper is derived from an address at an Illinois agricultural conference on the specified topic, “Are weather patterns changing?” It examines three contrasting perspectives on the weather and climate of the recent past and immediate future evident in the contemporary literature. One standpoint has interpreted recent weather extremes and climatic fluctuations as evidence that the earth is undergoing a larger-scale climatic change towards a cooler and more variable regime, while an alternative view considers these extremes and fluctuations to be part of normal climate. The third perspective results from the recent pronounced increase in atmospheric CO₂, which may induce a warm climate change. The request to provide this address offered the additional opportunity of drawing agriculturalists' attention to the research their scientists, economists, and sociologists must perform before the adverse socioeconomic effects of weather and climate variability can be minimized. It was also stressed, however, that the atmospheric sciences will have to demonstrate credibility to win and retain this vital support required from other specialists.

Abstract

Changes in visibility and the occurrence of smoke or haze during the last three decades are identified for eight locations in and around Illinois. The analyses utilize individual daily data and are performed on both seasonal and annual bases. Visibility variation is investigated using cumulative percentiles and mean ridits.

Summer is the season that experienced the greatest 1950–80 visibility change. Except at Chicago, this was dominated by a pronounced overall decline that coincided with a marked increase in the frequency of smoke/haze. Superimposed on these trends are 1) a strong early-1960s visibility maximum and smoke/haze minimum for Indianapolis and the northern half of Illinois and 2) particularly pronounced visibility degradation and increased smoke/haze occurrence during the late 1960s at most stations. The 1950–80 summer visibility decline at Chicago was much smaller than elsewhere and coincided with a marked downward smoke/hue frequency trend.

The extra-Chicago visibility results for spring are less pronounced versions of their summer counterparts; those for autumn contain the same overall decline, but not the foregoing smaller-scale variations. The spring and autumn occurrence of smoke/haze outside of Chicago exhibits little spatially coherent trend for the study period. Chicago's spring visibility improved slightly during 1950–80 and was accompanied by a stronger decrease in the number of smoke/haze days than occurred for summer. Autumn is the season in which Chicago visibility has degraded most in the last three decades, even though the concurrent reduction in the frequency of smoke/haze has exceeded that of summer and spring.

The winter results differ substantially from those for the other seasons. The 1950–80 winter visibility trends for individual stations range between a moderate decrease and a noticeable improvement, and are associated with strong reductions in smoke/haze frequency. These favorable changes are greatest at Chicago. Superimposed on them are 1) strong visibility maxima and smoke/haze minima during the mid-1950s and mid-1960s at most stations, 2) marked visibility degradation and increased smoke/haze occurrence outside of Chicago (especially in northwestern Illinois) in the late 1960s, and 3) some improvement in that situation in the early 1970s, followed by renewed deterioration.

The extra-Chicago annual results are determined by the similar patterns for summer (especially), spring and autumn. Their Chicago counterparts are the product of larger season-to-season variation, and accordingly reflect winter results to a greater extent.

Abstract

This study investigates the relation between tropospheric static stability and central North American growing season (May–August) rainfall for the highly contrasting years of 1975. 1976, and 1979. It uses two extensive sets of meteorological data (individual rawinsonde soundings for 38 stations; hourly rainfall totals for 854–944 locations) for the region extending from the Rocky to the Appalachian Mountains and from the Gulf Coast to approximately 55°N in Canada. The major objectives are to: (i) ascertain which of the many available methods of parameterizing static stability are most strongly related to the above (predominantly convective) rainfall; and (ii) quantify the rainfall variance fraction explained by static stability alone, as opposed to other atmospheric processes/conditions. Forty static stability indices and related thermodynamic parameters (SSITPs) are treated.

The results pertaining to objective (i) are definitive and those concerning (ii) are encouraging. The SSITPs that correlate most strongly with rainfall amount consistently include the lifting condensation level (LCL) (near-regionwide) and the convective condensation level (CCL) (western U.S. Great Plains) for the afternoon half-day, and K-type and SWEAT indices (eastern United States) and the CCL and convective temperature (U.S. Great Plains) for the morning half-day. In contrast, the SSITPs developed for forecasting severe thunderstorms and tornadoes correlate poorly with rainfall amount. Except on the U.S. Great Plains, the maximum SSITP-rainfall amount correlation magnitudes tend to be larger for the afternoon half-day (average of 0.47–0.49) than the morning half-day (0.37–0.39). Particularly high maximum afternoon SSITP-rainfall amount correlation magnitudes were obtained for the eastern United States (0.50–0.70); earlier work of this type seldom yielded correlation magnitudes above 0.32. For the SSITPs that correlate most strongly with rainfall amount on a regionwide basis (ICL variant for afternoon; modified-K index for morning), we also document the considerable spatial and intraseasonal variability of the thresholds beyond which the probability of rainfall exceeds that of no rainfall.

Abstract

We investigate the method-dependence of large-scale vertical motion (LSVM) estimates given by four variants of the kinematic approach (Endlich-Clark triangle; Chien-Smith pentagon; Objective analysis; Kung optimization) and the Limited-area Fine-Mesh II (LFM-II) model of the U.S. National Weather Service. The treatment spans 54 rawinsonde sounding times from three contrasting periods during the 1979 summer in the central United States that each included both widespread and abundant rainfall and intervening dry spells. Quantitative LSVM intercomparisons and evaluations for 500 and 700 mb are are with respect to cloud cover and rainfall data for 138 locations.

The Kung (especially) and LFM-II estimates have the narrowest frequency distributions. A particularly broad distribution is evident for the Endlich-Clark method. The Kung and LFM-II approaches yield significantly more frequent estimates of upward LSVM for overcast and rainy conditions (64–74 percent, depending on level and weather category) than the other methods (57–66 percent). However, the LFM-II also gives significantly more frequent estimates of upward LSVM for the fair weather condition of “0–5/10 cloud cover” (44–47 percent, depending on level) than all of the other techniques (36–41 percent). The Kung method gives very low such frequencies (38–39 percent). The above fair weather result, together with more detailed frequency distribution information, suggest that the LFM-II may be biased toward giving upward LSVM at 500 and 700 mb.

The foregoing findings are supported by additional analyses that intercompare the LSVM methods with respect to (i) their full frequency distributions when the more extreme cloud/rainfall conditions prevail and (ii) the frequency of occurrence of various cloud/rainfall categories when strong LSVM (defined separately for each method) is estimated. The results collectively suggest the Kung method to be superior to the other kinemtic approaches, in some cases substantially so, and also to the LFM-II. They offer guidance for the treatment of LSVM in meso- and synoptic-scale studies and climate dynamics.

Abstract

This paper presents the results of climatic pattern analyses of three- and seven-day summer (May–August) rainfall totals for the central United States. A range of eigenvectorial methods is applied to 1949–80 data for a regularly spaced network of 402 stations that extends from the Rocky to the Appalachian Mountains and from the Gulf Coast to the Canadian border. The major objectives are to quantitatively assess the sensitivity of eigenvectorial results to several parameters that have hitherto been the subject of considerable qualitative concern, and to identify the potential applications of those results.

The entire domain variance fractions cumulatively explained by a) the first 10 correlation-based unrotated Principal Components (PCs) and b) the 10 orthogonally rotated (VARIMAX criterion) PCs derived from them are identical for the same data. They vary between 35–47 percent depending on the data time scale and form, being higher for seven- than three-day totals and further enhanced when those totals are square-root (especially) and log₁₀ transformed. The (highly contrasting) sets of unrotated and VARIMAX PC spatial loading patterns are invariant with respect to data time scale and form. They receive strong statistical support from analyses performed on subsets of the data, their covariance- and cross-products-based equivalents, counterpart common factor patterns, and (for VARIMAX) an obliquely rotated (Hanis–Kaiser Case II B′B criterion) PC analysis. The unrotated PC loading patterns very closely resemble the set that Buell claimed would tend to characterize a domain of the present rectangular shape, irrespective of the meteorological parameter treated. They receive little physical support from analyses performed separately for subareas of the domain or from comparison with the interstation correlation matrix from which they are derived. The VARIMAX PC loading patterns, in contrast, derive strong physical support from those verifications. Each of these patterns emphasizes a relatively strong anomaly in a different part of the domain; they collectively yield a regionalization of the domain into 10 subareas within which three- and seven-day summer rainfall tends to be spatially coherent. The regionalization is suggested to be of considerable potential utility for crop-yield modeling, short-range weather prediction, and research into climatic variation and change.

Abstract

The increased U.S. natural gas price volatility since the mid-to-late-1980s deregulation generally is attributed to the deregulated market being more sensitive to temperature-related residential demand. This study therefore quantifies relations between winter (November–February; December–February) temperature and residential gas consumption for the United States east of the Rocky Mountains for 1989–2000, by region and on monthly and seasonal time scales. State-level monthly gas consumption data are aggregated for nine multistate subregions of three Petroleum Administration for Defense Districts of the U.S. Department of Energy. Two temperature indices [days below percentile (DBP) and heating degree-days (HDD)] are developed using the Richman–Lamb fine-resolution (∼1° latitude–longitude) set of daily maximum and minimum temperatures for 1949–2000. Temperature parameters/values that maximize DBP/HDD correlations with gas consumption are identified. Maximum DBP and HDD correlations with gas consumption consistently are largest in the Great Lakes–Ohio Valley region on both monthly (from +0.89 to +0.91) and seasonal (from +0.93 to +0.97) time scales, for which they are based on daily maximum temperature. Such correlations are markedly lower on both time scales (from +0.62 to +0.80) in New England, where gas is less important than heating oil, and on the monthly scale (from +0.55 to +0.75) across the South because of low January correlations. For the South, maximum correlations are for daily DBP and HDD indices based on mean or minimum temperature. The percentiles having the highest DBP index correlations with gas consumption are slightly higher for northern regions than across the South. This is because lower (higher) relative (absolute) temperature thresholds are reached in warmer regions before home heating occurs. However, these optimum percentiles for all regions are bordered broadly by surrounding percentiles for which the correlations are almost as high as the maximum. This consistency establishes the robustness of the temperature–gas consumption relations obtained. The reference temperatures giving the highest HDD correlations with gas consumption are lower for the colder northern regions than farther south where the temperature range is truncated. However, all HDD reference temperatures greater than +10°C (+15°C) yield similar such correlations for northern (southern) regions, further confirming the robustness of the findings. This robustness, coupled with the very high correlation magnitudes obtained, suggests that potentially strong gas consumption predictability would follow from accurate seasonal temperature forecasts.

Abstract

Since the late 1960s, the West African Sudan–Sahel zone (10°–18°N) has experienced persistent and often severe drought, which is among the most undisputed and largest regional climate changes in the last half-century. Previous documentation of the drought generally has used monthly, seasonal, and annual rainfall totals and departures, in a standard “climate” approach that overlooks the underlying weather system variability. Most Sudan–Sahel rainfall occurs during June–September and is delivered by westward-propagating, linear-type, mesoscale convective systems [disturbance lines (DLs)] that typically have much longer north–south (10²–10³ km) than east–west (10–10² km) dimensions. Here, a large set of daily rainfall data is analyzed to relate DL and regional climate variability on intraseasonal-to-multidecadal time scales for 1951–98. Rain gauge–based indices of DL frequency, size, and intensity are evaluated on a daily basis for four 440-km square “catchments” that extend across most of the West African Sudan–Sahel (18°W–4°E) and are then distilled into 1951–98 time series of 10-day and seasonal frequency/magnitude summary statistics. This approach is validated using Tropical Applications of Meteorology Using Satellite Data (TAMSAT) satellite IR cold cloud duration statistics for the same 1995–98 DLs.

Results obtained for all four catchments are remarkably similar on each time scale. Long-term (1951–98) average DL size/organization increases monotonically from early June to late August and then decreases strongly during September. In contrast, average DL intensity maximizes 10–30 days earlier than DL size/organization and is distributed more symmetrically within the rainy season for all catchments except the westernmost, where DL intensity tracks DL size/organization very closely. Intraseasonal and interannual DL variability is documented using sets of very deficient (8) and much more abundant (7) rainy seasons during 1951–98. The predominant mode of rainfall extremes involves near-season-long suppression or enhancement of the seasonal cycles of DL size/organization and intensity, especially during the late July–late August rainy season peak. Other extreme seasons result solely from peak season anomalies. On the multidecadal scale, the dramatic decline in seasonal rainfall totals from the early 1950s to the mid-1980s is shown to result from pronounced downtrends in DL size/organization and intensity. Surprisingly, this DL shrinking–fragmentation–weakening is not accompanied by increases in catchment rainless days (i.e., total DL absence). Like the seasonal rainfall totals, DL size/organization and intensity increase slightly after the mid-1980s.