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Robert G. Read

Abstract

Microclimate data collection and analyses were made in support of an ecological study of changes in transmission cycles of insect-borne disease during a three-year period of construction of a hydroelectric dam. The dam will cause impoundment of some 300 km2 of tropical forest in the Bayano River Basin, Panama. The pre-impoundment microclimate appeared to be one of strong seasonality with appreciable changes in the wind flow, sensible and latent heat flux, evaporation, rainfall and humidity. Average daily net radiation balance was 106 W m−2 above the forest canopy and 46 W m−2 on the floor of the forest. The moisture balance of the forest indicates an annual rainfall of 2 m of which 1 m reaches the floor of the forest. An appreciable amount of rain is intercepted in the forest canopy. The evaporation and runoff on the forest floor are both about 0.5 m. Average daily wind speeds are light, varying from 8 km day−1 in the rainy season to 126 km day−1 in the dry season. Ecological significance was found in the nocturnal unstable vertical temperature profile of the forest and the associated small updrafts and downdrafts which may permit easier vertical migration of forest insects at night. Rainfall and light wind speeds during 24 h periods of collection seem to appreciably affect the activity of the most abundant man-biting species of insects.

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Robert G. Read

Abstract

The physical processes that affect evapotranspiration and the technique of measuring the drying influence of the air in the tropical forest of the Panama Canal Zone during the rainy season were examined. The time rate of evaporation from the surfaces of white spherical porcelain atmometers was the measurement used to determine the evapotranspiration power of this environment.

Evaporation, temperature, vapor pressure deficit and relative humidity profiles are shown from the forest floor through 28.5 m to the top of the canopy. The mean hourly rate of evaporation at the top of the canopy is 4 times that near the forest floor but may vary from as little as 2.8 times to as much as 12 times the rate of evaporation near the forest floor. Mean hourly rates of evaporation at the top of the canopy and near the forest floor change dramatically from day to night. The average daytime rate near the surface was about 2.5 times the nighttime rate, while at the top of the canopy it was about 4 times the nighttime rate.

The saturation vapor pressure deficit is closely related to the evapotranspiration power, but the slight air movements in the moist stagnant spaces below the forest canopy and the active convection of relatively dry air from above the canopy into the trunk spaces below appear to be the dominant factors.

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William J. Thompson, Russell L. Elsberry, and Robert G. Read

Abstract

A statistical analysis is performed of the tropical cyclone forecast advisories and bulletins issued by the Eastern Pacific Hurricane Center, Redwood City, California, during the 1971–78 seasons. Each forecast is normalized by comparison with the performance of an objective model (EPCLPR) that is based on climatology and persistence. The normalized official forecasts show an improvement in skill during the period. This improvement is attributed to the availability of satellite data for determining the storm positions and to the introduction of objective forecast techniques. Forecast errors are related to a number of storm-related variables, such as initial latitude and longitude and deviations from the climatological track. Stepwise discriminant analysis is used to classify the forecasts into groups of above or below average errors.

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Steven C. Sherwood, E. Robert Kursinski, and William G. Read

Abstract

The probability distribution of local relative humidity R in the free troposphere is explored by comparing a simple theoretical calculation with observations from the global positioning system (GPS) and the Microwave Limb Sounder (MLS). The calculation is based on a parcel of air that conserves its composition during diabatic subsidence, until it is resaturated by randomly entering a convective system. This simple “advection–condensation” model of relative humidity predicts a probability density for R proportional to R r−1, where r is the ratio of time scales associated with subsidence drying and random moistening. The observations obey this distribution remarkably well from 600 to 200 hPa in the Tropics and midlatitudes; possible reasons for this are discussed. The lowest values of R are predicted, and observed, to be the most probable. The observed vertical variation of R is well explained by that of the subsidence time scale, which is set by large-scale dynamics and radiation. These results imply that cloud microphysics exerts little control on water vapor’s greenhouse effect, but that relatively subtle dynamical changes have the potential to alter the strength of its feedback on climate change.

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Lucien Froidevaux, Joe W. Waters, William G. Read, Lee S. Elson, Dennis A. Flower, and Robert F. Jarnot

Abstract

Global ozone observations from the Microwave Limb Sounder (MLS) aboard the Upper Atmosphere Research Satellite (UARS) are presented, in both vertically resolved and column abundance formats. The authors review the zonal-mean ozone variations measured over the two and a half years since launch in September 1991. Well-known features such as the annual and semiannual variations are ubiquitous. In the equatorial regions, longerterm changes are believed to be related to the quasi-biennial oscillation (QBO), with a strong semiannual signal above 20 hPa. Ozone values near 50 hPa exhibit an equatorial low from October 1991 to June 1992, after which the low ozone pattern splits into two subtropical lows (possibly in connection with residual circulation changes tied to the QBO) and returns to an equatorial low in September 1993. The ozone hole development at high southern latitudes is apparent in MLS column data integrated down to 100 hPa, with a pattern generally consistent with Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) measurements of total column; the MLS data reinforce current knowledge of this lower-stratospheric phenomenon by providing a height-dependent view of the variations. The region from 30°S to 30°N (an area equal to half the global area) shows very little change in the ozone column from year to year and within each year.

The most striking ozone changes have occurred at northern midlatitudes, with the October 1992 to July 1993 column values significantly lower than during the prior year. The zonal-mean changes manifest themselves as a slower rate of increase during the 1992/93 winter, and there is some evidence for a lower fall minimum. A recovery occurs during late summer of 1993; early 1994 values are significantly larger than during the two previous winters. These results are in general agreement with variations measured by the Nimbus-7 TOMS and Meteor-3 TOMS instruments at midlatitudes. However, the southern midlatitudes exhibit less of a column ozone decrease (relative to the north) in the MLS data (down to 100 hPa) than in the TOMS column results. The timing and latitudinal extent of the northern midlatitude decreases appear to rule out observed CIO enhancements in the Arctic vortex, with related chemical processing and ozone dilution effects, as a unique cause. Local depletion from CIO-related chemical mechanisms alone is also not sufficient, based on MLS CIO data. The puzzling asymmetric nature of the changes probably requires a dynamical component as an explanation. A combination of effects (including chemical destruction via heterogeneous processes and QBO phasing) apparently needs to be invoked. This dataset will place constraints on future modeling studies, which are required to better understand the source of the observed changes.

Finally, residual ozone values extracted from TOMS-minus-MLS column data are briefly presented as a preliminary view into the potential usefulness of such studies, with information on tropospheric ozone as an ultimate goal.

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