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Peter J. Lamb

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

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Peter J. Lamb

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.

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Peter J. Lamb

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.

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Aondover Tarhule and Peter J. Lamb

Beginning in response to the disastrous drought of 1968–73, considerable research and monitoring have focused on the characteristics, causes, predictability, and impacts of West African Soudano–Sahel (10°–18°N) rainfall variability and drought. While these efforts have generated substantial information on a range of these topics, very little is known of the extent to which communities, activities at risk, and policy makers are aware of, have access to, or use such information. This situation has prevailed despite Glantz's provocative BAMS paper on the use and value of seasonal forecasts for the Sahel more than a quarter century ago. We now provide a systematic reevaluation of these issues based on questionnaire responses of 566 participants (in 13 communities) and 26 organizations in Burkina Faso, Mali, Niger, and Nigeria. The results reveal that rural inhabitants have limited access to climate information, with nongovernmental organizations (NGOs) being the most important source. Moreover, the pathways for information flow are generally weakly connected and informal. As a result, utilization of the results of climate research is very low to nonexistent, even by organizations responsible for managing the effects of climate variability. Similarly, few people have access to seasonal climate forecasts, although the vast majority expressed a willingness to use such information when it becomes available. Those respondents with access expressed great enthusiasm and satisfaction with seasonal forecasts. The results suggest that inhabitants of the Soudano–Sahel savanna are keen for changes that improve their ability to cope with climate variability, but the lack of information on alternative courses of action is a major constraint. Our study, thus, essentially leaves unchanged both Glantz's negative “tentative conclusion” and more positive “preliminary assessment” of 25 years ago. Specifically, while many of the infrastructural deficiencies and socioeconomic impediments remain, the great yearning for climate information by Soudano–Sahalians suggests that the time is finally ripe for fostering increased use. Therefore, a simple model for improved dissemination of climate research and seasonal climate forecast information is proposed. The tragedy is that a quarter century has passed since Glantz's clarion call.

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Stefan Hastenrath and Peter J. Lamb

Abstract

The heat budget of the atmosphere-ocean-land system in the Indian Ocean region (30°N–30°S, 30–120°E) is studied on the basis of ocean surface heat flux calculations from long-term ship observations and satellite-derived estimates of net radiation at the top of the atmosphere.

The hydrosphere to the north of the equator exports heat at rates of 5 × 1014 W for the year as a whole, and more than 8 × 1014 W during the northern summer (May–October) half-year, respectively. In contrast, the heat budget of the Southern Hemisphere water is dominated by the seasonal storage/depletion of heat transferred through the ocean surface. Oceanic heat export/import is small for this region during both the November–April and May–October half-years, and near zero for the year as a whole. The mean annual net meridional oceanic heat transport is directed southward throughout the study area, reaching a maximum of 8 × 1014 W at 10–15°S. From heat balance considerations, the annual average upwelling north of the equator is calculated to be ∼6 × 10−7 m s−1. Most of the compensatory down-welling must occur outside the tropical Indian Ocean.

Residually determined heat export by the atmosphere north of the equator averages 18 and 4 × 10−14 W during the northern summer and winter half-years, respectively. South of the equator the atmosphere exports heat at a mean annual rate of 19 × 10−14 W, with little seasonal variation. During northern summer, the atmospheric energy export from the southern tropical Indian Ocean is largely in the form of latent heat and is directed northward across the equator. The southern tropical Indian Ocean is the major source of the atmospheric water vapor carried across the coastline of southern Asia during the northern summer southwest monsoon. The larger water vapor flux divergence south of the equator at this time is fed by strong evaporation. This is supported by a combination of the seasonal depletion of the local oceanic heat content and oceanic heat import from north of the equator, in addition to the surface net radiation.

South of about 10°S, the atmosphere must dispose of both the net radiative heat input at the top of the system and the heat imported within the oceanic water body. In contrast, to the north the atmosphere and hydrosphere make similar contributions to the lateral energy export.

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Peter G. Vinzani and Peter J. Lamb

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.

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Diane H. Portis and Peter J. Lamb

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.

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Randy A. Peppler and Peter J. Lamb

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.

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Peter J. Lamb and Randy A. Peppler

An outline of the concept of the North Atlantic Oscillation (NAO), along with some of its history is presented. This is followed by a brief presentation of the results and implications of an encouraging new application of the NAO to a regional climate problem—the interannual variation of Moroccan winter-semester precipitation. That precipitation is shown to be inversely related to the concurrent state of the NAO, and the relationship is relatively strong by the standards of recent research into the mechanisms of tropical and subtropical precipitation fluctuations. It is suggested that the NAO is of particular significance for the important issue of the long-range prediction of Moroccan (and probably also Spanish, Portuguese, and Algerian) winter precipitation, and that further research on this subject is warranted. Several specific recommendations in the latter regard are made.

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Peter J. Lamb and Stanley A. Changnon Jr.

Abstract

Historical (1901–79) temperature and precipitation data for four Illinois stations were used to determine the frequency with which summer and winter averages for periods of various length (i.e., different climatic normals) are closest to the value for the next year, and hence its best predictor. The normal achieving the highest frequency in this regard is considered the best for characterizing the recent climate for a given point in time and assessing the abnormality of the following year.

Normals for 5, 10, 15, 20 and 25 years were investigated, along with the 30-year ones generally used. Five-year normals most frequently provided the closest estimate of the next year's value for both parameters in both seasons. Ten-year normals also have a high probability of being the best predictors, whereas 20-year normals have a particularly low probability of such success. The standard 30-year normals also perform poorly in this regard. These results contrast strongly with earlier suggestions that 15–25 year normals are “optimum” for prediction because they possess the minimum extrapolation variance when normals are employed as predictors. This difference between the two sets of results indicated that 5-year normals tend to possess larger prediction errors when they are not the best predictors, than do other normals on the greater number of occasions they are not the best predictors. The present findings were used by the Illinois Commerce Commission in evaluating weather normalization rate adjustments proposed by utility companies in 1979–80.

An investigation also is made into the nature of the climatic variation occurring when each normal is the best predictor. Five-year normals tend to attain this position for precipitation when the difference from the preceding year and the departures from longer-term averages are all moderate-to-small. When 5-year normals are the best temperature predictors, in contrast, the departures from this normal (and hence prediction errors) are very large. The frequency with which various normals were the best predictors shows no marked temporal variation during the study period.

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