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PICTURE OF THE MONTH

Observations of Isolated Mesoscale Cellular Convection Cells

Wayne V. Burt

Abstract

What appeared to he isolated mesoscale convection cells embedded in an otherwise solid deck of stratocumulus clouds were observed from a commercial air liner as it approached the coast of the state of Washington on a flight from Tokyo. The presence of cells of the correct dimensions was confirmed by a satellite photograph of the area.

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Wayne V. Burt
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Wayne V. Burt

Abstract

The observed albedo over a natural water surface, as a function of solar elevation and cloud cover, is reproduced semi-theoretically by considering the distribution of slope on the sea surface. The relation of the albedo to the wind velocity and the turbidity of the atmosphere is also considered.

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Wayne V. Burt

Spanish merchant ships, called Manila Galleons, made the roundtrip between the Philippine Islands and the ports on the west coast of what is now Mexico, almost every year for the 250-yr period between 1565 and 1815. The log books of these galleons contained weather information which was thought to be of value in the study of the historical climatology of the tropical and midlatitude areas of the Pacific Ocean. A search was made for the log books of the Manila Galleons to determine how many are still extant and the nature of the weather data that they contained. The search was made with the assistance of expert historians and paleographers who were experienced in searching archives where the log books were thought to be on file. The following archives were searched: the Museo Naval in Madrid, Spain; the Archivo General de Simancas in Simancas, Spain; Archivo De Indias in Seville, Spain; Archivo General de la Nación in Mexico City, Mexico; and the National Archives in Manila, The Philippine Islands. Only sixteen log books were on file in the archives in Madrid and Seville. All of the logs that were found were from the period between 1766 and 1808. The inevitable conclusions that were reached were that log books for some of the earlier voyages may never have been written and that most of the log books that were written were either physically lost or were lost due to the ravages of time, moisture, insects or war.

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Wayne V. Burt

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In experiments with identical cup anemometers, one mounted on a spar buoy and one mounted on a toroidbuoy that were anchored close together, the anemometer mounted on the toroid indicated consistently higher10-min integrated averages of wind speed than the cup anemometer mounted on the spar buoy. The differ-ence was 7% of the mean wind speed and was linear over a range of mean wind speeds from 1½ to 9½ m s-1.

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William H. Quinn and Wayne V. Burt

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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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Clifford L. Trump and Wayne V. Burt

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An array of three current meters were anchored on the continental shelf of the East China Sea during the last half of February 1975 as part of the Japanese Air Mass Transformation Experiment, AMTEX-75. The results indicate that the currents are dominated by the rotational semidiurnal M 2 component superimposed on a slow mean drift to the northeast. Differences in direction of several days duration between two of the current meters suggest the presence of transient mesoscale eddies or meanders in the flow regime.

Rotary spectra indicated that the tidal currents are characterized by a clockwise ellipse of eccentricity ≈0.6 and that there was some inertial energy present. The current rose for bandpassed velocity fluctuations (0.2–2 eph) was elongated in a direction 25–30° to the right of the wind direction indicative of Ekman veering.

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Willlam H. Quinn and Wayne V. Burt

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A simple objective method for using the barometric pressure changes associated with the Southern Oscillation to predict abnormally heavy rainfall over the Pacific equatorial dry zone was applied to 79 years of Darwin sea-level pressure data and precipitation data for a large number of small islands in the equatorial Pacific. It appears that this method, based on a consideration of pressure changes from one year to the next, could provide a prediction one to eight months in advance of heavy precipitation over most of the central and western part of the zone with about 76% effectiveness. There also appears to be some degree of correlation between the amount of pressure departure and the extensiveness (in time and space) of this heavy rainfall.

A check into those cases where an associated El Niño occurred indicated that the El Niño usually sets in a few to several months earlier than the heavy precipitation over the central and western equatorial Pacific dry zone and prior to the obvious Darwin pressure deviation. The El Niño development degree does not appear to correlate with the magnitude of the Darwin pressure deviation. The early appearance of El Niño indicates the possibility that it and the variable magnitude of the pressure shift associated with the Southern Oscillation may both be influenced to some extent by other factors such as variations in the antarctic circumpolar atmospheric and oceanic circulation and interhemispheric interaction in the eastern tropical Pacific.

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William H. Quinn and Wayne V. Burt

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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 and Wayne V. Burt

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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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