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Bernice Ackerman

Abstract

The rapid fluctuations in airspeed experienced by an airplane have been used to define the areal distribution of turbulence associated with a cumulus cloud. In the majority of clouds examined, the turbulence was not confined to the cloud but was distributed around it asymmetrically. This asymmetry appears to be related to cloud shear. A mechanism by which the observed cloud-turbulence pattern may form is described.

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Bernice Ackerman

Abstract

As in-cloud measurements have accumulated it has become increasingly apparent that clouds are markedly non-uniform in structure. This is demonstrated by sizable and rapid variations in the horizontal for almost all meteorological variables. A systematic study was made of the fluctuations in measurements obtained in hurricane clouds to establish whether they could be ascribed to a random process or whether they exhibited some spatial order.

Variance spectrum analysis of continuous measurements of temperature, liquid-water content and air-speed for 49 cloud areas indicated that, in most cases, the variations were not random and that, in addition to the cloud scale, there were non-random components of smaller size. With considerable consistency, particularly in turbulent convection, the spectra indicated the existence of two frequency “domains” within which one or more dominant modes of activity occurred. These domains extended a) from 1.8–4 cycle mile−1 with peaks most frequent around 2.5 cycle mile−1, and b) from 4.5–9 cycle mile−1 with peaks occurring most often between 5 and 6 cycle mile−1.

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Bernice Ackerman

Abstract

Liquid-water-content measurements made during the 1957 and 1958 flights by the National Hurricane Research Project airplanes have been utilized in statistical analyses to determine some of the gross features of the water distribution in hurricanes. The water contents were generally well below, and only rarely approached, the theoretical value for an adiabatic process. The analyses indicate that, in the layer from 9000 to 18,000 ft, the water contents of the convective developments decreased with height. The fraction of the storm area occupied by convective developments was found to vary with the age of the storm, being at a maximum when a storm was at its peak intensity (lowest pressure). When computed for ring areas centered on the storm center, this fraction, the “areas density” of convection, tended to decrease with increasing radius.

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Bernice Ackerman

Abstract

Observations made in and around tropical cumuli in the vicinity of Puerto Rico have been used to determine the presence of precipitation and to obtain estimates of cloud buoyancy. It is shown that the development of precipitation was related to the buoyancy characteristics of the cloud prior to the formation of rain and to the environmental conditions of humidity and vertical wind shear. In addition, the effects of treating clouds with a water spray have been evaluated through the use of the buoyancy-precipitation relationships observed in untreated clouds.

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Bernice Ackerman

Abstract

Continuous measurements of liquid-water content have been used to determine the magnitude of cloud water and its distribution in tropical cumuli. The average water content for a cloud penetration was always less than the amount computed for an air parcel ascending moist adiabatically from cloud base with no water loss. However, many clouds were found to have regions in which the measured liquid-water content exceeded the computed. The data show that the magnitude of the water content was correlated to the vertical displacement from cloud base and to the horizontal and vertical dimensions of the cloud.

Cloud water was found to be unevenly distributed in a horizontal plane through the cloud. The location of the area of greatest water concentration appears to be related to the external wind field.

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Bernice Ackerman

Abstract

Hourly measurements at an urban airport and at a rural laboratory have been used in a study of Chicago area urban-rural humidity differences. Although the relative humidity was usually lower in the city than in the country, largely a consequence of the urban heat island, this was not the case for parameters less sensitive to temperature. On average, urban vapor pressures and dew points were less than rural ones only in the forenoon and spring afternoons. Urban-rural differences in these two variables also varied with ambient wind speed, cloud cover and moisture stratification in the surface boundary layer.

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Bernice Ackerman

Abstract

Twenty years of records from Midway Airport, located within the City of Chicago, and Argonne National Laboratory, a rural site 23 km southwest of the airport, have been used to study the diurnal and seasonal variation in the Chicago urban heat island. As expected, there was an elevation in temperature within the city most of the time, averaging 1.85°C during nonprecipitating hours. The magnitude of the heat island was characterized by diurnal and seasonal cycles, which were modulated by cloud and wind conditions. The influence of Lake Michigan on the magnitude of the heat island and on the location of its center is discussed, based on measurements at downtown and residential sites adjacent to the lake.

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Robert W. Scott and Bernice Ackerman

Abstract

Sudden changes in surface meteorological parameters were observed to propagate across a densely-instrumented network in central Illinois during a summer night in 1979. The changes were due to the outflow from an eastward moving, organized storm system passing well north of the network. Although no precipitation was observed within 45 km of the area (i.e., its passage across the network was “dry"), the change from ambient to outflow air was seen in other surface weather indicators nearly 100 km south of the point at which the outflow is estimated to have been initiated and more than 3 h after the generating storm had dissipated.

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Bernice Ackerman and Rueen-Yuh Sun

Abstract

Two one-dimensional steady-state models of cumulus convection in common use in weather modification research, the NOAA Experimental Meteorology Branch model (EMB) and the Great Plains Cumulus Model (GPCM), differ in their formulations in several ways. Some of the differences arise from the conceptualization of the convective phenomenon which is modeled in each and some from the physical parameterizations utilized. Predictions of cloud top and dynamic modification potential (seedability) by the two models for 57 midday radiosondes in the Midwest, differed significantly, with the EMB values consistently higher. GPCM simulations provided a better overall estimate of observed radar echo tops, while EMB consistently overestimated, by largest amounts when tops were below 12 km. Study of the impact of temporal and/or spatial separation between sounding and cloud area emphasizes the need, in the Midwest, to consider factors other than thermodynamic stratification (e.g. forced lifting, convergence) on the synoptic and mesoscale when applying the model for prediction of cloud development. However, it was also found that the average difference in predictions by the two models, for the same sounding, was of the same order as the average difference in predictions arising from spatial or temporal separation.

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Peter H. Hildebrand and Bernice Ackerman

Abstract

The influence of an urban area on the turbulent convective boundary layer is studied using an instrumented aircraft. Data from three undisturbed days with low wind speeds are presented to describe the urban effects on turbulent profiles of heat, moisture, momentum and turbulent intensity. The profiles in the urban Area are observed to be considerably different than the rural. The urban area substantially enhances the surface heat flux, hence the vertical component of turbulence. The enhanced urban vertical turbulent intensifies extend through the urban boundary layer, resulting in stronger entrainment through the urban inversion than the rural. This, in turn, results in enhanced vertical fluxes of temperature, moisture and stress at the urban inversion. The effects of the urban roughness on boundary-layer structure are observed to be small in comparison with the effects of urban heating for the light wind cases studied.

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