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William H. Quinn and Wayne V. Burt

Abstract

Canton Island (2°49′S, 171°41′W) was selected to study equatorial trough weather conditions and their effects on incoming solar radiation. The chosen study period, July 1957–June 1958, was quite appropriate since the mean position of the trough for this year was about 4° south of its mean annual position and it frequently affected Canton.

Cloud distribution, moist layer thickness, precipitable water content and weather conditions in the equatorial trough zone (out to 10° on either side of the trough axis) are discussed. Incoming solar radiation is considered in relation to opaque sky cover since the total sky cover often included an excessively large thin cirriform contribution which did not significantly reduce incoming solar radiation.

In the vicinity (within 3°) of the trough axis only one out of five days was affected by disturbance type weather (extensive cloud cover and precipitation). In these disturbance cases, for an overall average, 34.6% of the solar radiation on a horizontal surface at the top of the atmosphere penetrated through to the earth's surface. For fair weather cases 61.3% penetrated through to the earth's surface.

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William H. Quinn and Wayne V. Burt

Abstract

Information and data recently received from Chilean meteorological authorities provide an improved atmospheric pressure record for Easter Island for 1912–13 and 1942–67. This study indicates that variations in the difference of mean monthly sea level pressure between Easter Island and Darwin, Australia, provide a much more effective aid to forecasting extended periods of abnormally heavy rainfall over the central and western equatorial Pacific than our earlier approach which used just the Darwin pressure trend. The principal advantages of using this forecasting aid over the earlier method are that. 1) it provides a physically significant index with regard to atmospheric and hydrospheric circulation over the Indo-Pacific region, and more clearly reflects fluctuations associated with the Southern Oscillation; 2) it provides a 1–2 month increase in lead time for forecasts of abnormally heavy equatorial Pacific rainfall; and 3) it is much more effective over the 1943–61 period (when the Darwin pressure trend method broke down). Considering these advantages, the equally successful performance for comparable periods prior to and following the break-down interval for the earlier method, plus the fact that a large part of the pressure difference is controlled by the Darwin pressure input, it appears that the use of this aid would also be at least as effective as the earlier method over the rest of its 79-year evaluation period.

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William H. Quinn and Wayne V. Burt

Abstract

This paper discusses findings in regard to cloud distributions over the equatorial Pacific and the development of a method, compatible with actual cloud distributions, for using certain existing formulas to compute incoming solar radiation from meteorological data.

U. S. Air Force Uniform Summaries of Surface Weather Observations for 10 central and western equatorial Pacific sites indicate that the very large cirriform cloud contribution to total sky cover, noted in the July 1957–June 1958 Canton Island weather data, is a general characteristic of the cloud distribution over a large part of the western equatorial Pacific. Since most formulas for computing incoming solar radiation depend on a mean total cloud cover entry, a large bias toward thin cirriform cloud could lead to sizeable underestimations of incoming solar radiation and cause significant errors in heat budget calculations for this region.

In order to determine a suitable cloud cover term, monthly mean total sky cover, low cloud cover and total opaque sky cover values were obtained from the July 1957-June 1958 Canton Island weather data and tested in six commonly used formulas for computing incoming solar radiation. On comparing computations with recorded values, the mean total sky cover entry gave large underestimations of incoming solar radiation with all formulas. Test results suggested the use of a monthly mean sky cover term obtained by using mean low cloud cover values for the daylight hours of fair weather days and mean total opaque sky cover for the daylight hours of days dominated by disturbance weather. The Kimball, Black, Budyko and Berliand formulas gave very satisfactory results when using the suggested term. Computations from the latter three formulas were within 10% of recorded values for all months. The use of this approach with Black's formula is particularly appropriate, since the computations are then based on the incoming solar radiation on a horizontal surface at the top of the atmosphere, rather than on the radiation received at the earth's surface under clear sky conditions.

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William H. Quinn, Wayne V. Burt, and Walter M. Pawley

Abstract

The goals of this study were to investigate the potential of various meteorological factors for representing the more variable insolation reductions in the tropical atmosphere, and to develop curves and formulas for those relationships which appeared to be effective. Emphasis was placed on developing approaches that depend on simple meteorological entries.

The following were studied in relation to the average daily atmospheric transmission ratio (insolation recorded at the earth's surface divided by the computed insolation on a horizontal surface at the top of theatmosphere) on a monthly basis using four years of Canton Island data and considering only hours between sunrise and sunset: average total opaque sky cover, average total sky cover, average daily duration (in minutes) of precipitation, average daily amount (in inches) of precipitation and fraction of days with precipitation. Also, the average daily amount of precipitation and fraction of days with precipitation were studied on an annual basis in the above manner considering all hours of the day and seven years of Canton Island data.

For monthly insolation estimates, the method using average total opaque sky cover during daylight hours should give the best results (perhaps within 8% of measured values). The method using average daily duration of precipitation during daylight hours should also give good results (perhaps within 12% of measured values). For annual insolation estimates, the approach using average daily amounts of precipitation appears to show considerable promise.

The more suitable monthly approaches should be applicable at least to that part of the equatorial Pacific which experiences weather affecting the dry zone area in its more extensive state. They may also be useful over the greater part of the equatorial Pacific and that part of the tropics extending into the summer hemisphere of the western Pacific. The methods using annual averages could only be useful in the equatorial region. In the future it would be desirable to consider sites which record much larger monthly and annual amounts of precipitation.

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John H. Seinfeld, Gregory R. Carmichael, Richard Arimoto, William C. Conant, Frederick J. Brechtel, Timothy S. Bates, Thomas A. Cahill, Antony D. Clarke, Sarah J. Doherty, Piotr J. Flatau, Barry J. Huebert, Jiyoung Kim, Krzysztof M. Markowicz, Patricia K. Quinn, Lynn M. Russell, Philip B. Russell, Atsushi Shimizu, Yohei Shinozuka, Chul H. Song, Youhua Tang, Itsushi Uno, Andrew M. Vogelmann, Rodney J. Weber, Jung-Hun Woo, and Xiao Y. Zhang

Although continental-scale plumes of Asian dust and pollution reduce the amount of solar radiation reaching the earth's surface and perturb the chemistry of the atmosphere, our ability to quantify these effects has been limited by a lack of critical observations, particularly of layers above the surface. Comprehensive surface, airborne, shipboard, and satellite measurements of Asian aerosol chemical composition, size, optical properties, and radiative impacts were performed during the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) study. Measurements within a massive Chinese dust storm at numerous widely spaced sampling locations revealed the highly complex structure of the atmosphere, in which layers of dust, urban pollution, and biomass- burning smoke may be transported long distances as distinct entities or mixed together. The data allow a first-time assessment of the regional climatic and atmospheric chemical effects of a continental-scale mixture of dust and pollution. Our results show that radiative flux reductions during such episodes are sufficient to cause regional climate change.

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