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STRONG SURFACE WINDS AT BIG DELTA, ALASKA

An Example of Orographic Influence on Local Weather

J. MURRAY MITCHELL Jr.

Abstract

The remarkably high frequency of strong surface winds in the region of Big Delta, Alaska, is studied with respect to its cause, characteristics, and local effects. During the winter, the winds are predominantly east-southeast and, unlike glacier or valley winds, are caused by a topographically induced convergence of the flow of air down the Tanana Valley which occurs at times of southeast gradient winds aloft. Strong south winds are also experienced the year round. A noteworthy characteristic of the east-southeast winds is their persistence; an extreme case is described in which gusts in excess of 40 m. p. h. endured for 7½ days (January 20–28, 1952). Another characteristic of these winds is the marked diurnal variation in the frequency of their commencement, by which a strong control by atmospheric tides is inferred. An important effect of the winds is to interrupt periods of very low temperature, but sometimes to create severe “wind chill.” The paper concludes with a brief account of the forecast problem.

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J. Murray Mitchell Jr.

Hourly temperature data for eight first-order stations in the United States have been used to determine the effect of observation time on mean temperature derived in the usual way from 24-hr maximum and minimum values. Results, presented in detail for exemplary stations and observation times, show that the greatest possible effect on temperature of arbitrary time changes varies with place and season between less than 1/2F and more than 3F at the stations investigated. A means of estimating both this maximum effect, and the effect of any specific time change, at an arbitrary location is presented. It is concluded that historical temperature data based on evening observations are apt to be more homogeneous than those based on morning observations. An example of the effect of typical observation time changes on a secular temperature series is presented, and stresses the value of thorough documentation of observation time by each cooperative observer.

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J. Murray Mitchell Jr.
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J. Murray Mitchell Jr.

Abstract

No Abstract available.

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J. Murray Mitchell Jr

Abstract

A general survey is made of the feasibility of using trends in long-period temperature records as representations of climatic change. A breakdown of all probable significant sources of influence on such records, together with quantitative estimates of instrumental, observational, and local environmental factors, points up the generally critical influence of non-climatic factors, which for the most part induce apparent secular temperature rises.

Three independent studies of city influence are presented. In the first, recent overlapping observations between the New Haven city and airport stations are used to estimate the local city influence which in turn is used to revise the secular station trend. In the second, evidence of negligible city influence but of real climatic change at Blue Hill Observatory since 1890 is discussed. In the third, a statistical study involving 77 stations in the United States, whose temperature records were observationally homogeneous between 1900 and 1940, bears out the prevalence of important city influence in this country.

Except in the period of rapid climatic temperature change occurring since about 1890, observed temperature records, with few individual exceptions, are concluded to be very misleading as direct measures of macroclimatic change over periods longer than a few decades. With their use in climatic studies, particularly those extending back of 1900, isolation of the effects of widespread urban development and frequent thermometer relocation is imperative. At average stations in the United States, urban development has contributed local temperature rises at the rate of more than 1F in a century. The influence of very large cities has not been in proportion.

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J. Murray Mitchell Jr.

Abstract

A generalized model of the effect of an optically thin atmospheric aerosol on the terrestrial heat budget is proposed, and applied to the problem of estimating the impact of the aerosol on temperatures near the earth's surface. The distinction between warming and cooling near the surface attributable to the aerosol is found on the basis of this model to depend on whether the ratio of absorption a to backscatter b of incoming solar radiation by the aerosol is greater or less than the critical ratio

            (a/b)O = C(1−A)(1−Ak)/[D(1+A)−C(1−A)],

where A is the surface albedo, C the fraction of sensible to total (sensible plus latent) solar heating of the surface, D the fraction of aerosol that is in convective contact with the surface, and k a multiple of b that measures the relative aerosol backscattering efficiency with respect to solar radiation reflected upward from the surface.

A distinction is drawn between a stratospheric aerosol (D=0) which generally cools the atmosphere near the surface, and a tropospheric aerosol (D→1) which may either cool or warm the atmosphere near the surface depending on various properties of the aerosol and of the surface itself. Over moist surfaces, such as vegetated areas and oceans, the critical ratio (a/b)o is of order 0.1. Over drier surfaces, such as deserts and urban areas, (a/b)o is of order unity. If the actual ratio a/b of most tropospheric aerosols is of order unity, as inferred by previous authors, then the dominant effect of such aerosols is warming except over deserts and urban arms where the effect is somewhat marginal between warming and cooling.

Further aerosol climatic effects are found likely to include a slight decrease of cloudiness and precipitation, and an increase of “planetary” albedo above the oceans, although not necessarily above the continents. Suggestions by several previous authors to the effect that the apparent worldwide cooling of climate in recent decades is attributable to large-scale increases of particulate pollution of the atmosphere by human activities are not supported by this analysis.

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