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Harry T. Ochs III

Abstract

A numerical cloud modeling project has been initiated as part of METROMEX. A two-dimensional time-dependent model of a vertical slice of the atmosphere has been applied to investigations of cumulus initiation in the St. Louis area on 7 and 10 August 1973. The results indicate that on these days the urban-related surface temperature distribution was a significant factor in determining the location of cumulus initiation. This result is supported by field observations from METROMEX.

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Harry T. Ochs III

Abstract

A model of an ascending parcel of air is used to evaluate and demonstrate the accuracy and utility of moment-conserving numerical techniques in cloud microphysical simulations. The sensitivity of the results to parameters of the computer code which might affect accuracy is evaluated. Discrepancies in the evolution of radar reflectivity are found to be satisfactorily small since the vertical displacement of equivalent radar reflectivities is shown to be within typical cloud model grid spacings. The collection calculation results are compared to a known solution and the condensation algorithm is tested by a comparison with a Lagrangian simulation. Simulations employing a maritime and two continental CCN distributions illuminate various features of the techniques employed. One of the continental cases is used to show the sensitivity of the evolution of large drops to the values of linear collision efficiency.

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Harry T. Ochs III
and
Richard G. Semonin

Abstract

Observations showed increased concentrations of cloud condensation nuclei (CCN) in air samples collected over and downwind of St. Louis when compared to upwind samples. Aircraft observations of urban clouds showed corresponding increased concentrations of cloud base droplets. In addition, observations indicated higher cloud bases and decreased elevations of average first echo base heights in the St. Louis/East St. Louis area as compared with similar clouds over rural areas.

The purpose of this paper is to examine the possible role of CCN chemical composition and number concentration in producing the observed phenomena. A closed parcel model of condensation and collection was employed for this purpose. The results suggest that the observed differences of depth from cloud base to first echo height between urban and rural clouds do not result from concentration differences in any CCN size range. Results of model calculations also suggest that variations in chemical composition of the largest CCN (≥1.0 μm radius) were not responsible for the observed urban/rural differences. A hypothesis based on observations and model results is presented for explaining the observed differences in cloud base to first echo depth in terms of differences between the evolution and strength of updrafts in urban and rural clouds.

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Harry T. Ochs III
and
David B. Johnson

Abstract

The properties of 3 cm radar first echoes were used to study the effects of the St. Louis, Missouri metropolitan area on precipitation initiation during the summer months of Project METROMEX. Good statistical support was found for a ∼150 m lowering of urban first echo tops and a ∼250 m lowering of urban first echo bases when compared to rural echoes observed on the same day. Urban echoes were found to be thicker than their rural counterparts; however, there is no statistical support for this difference. This result is most easily interpreted as suggesting a slight weakening of the updraft in urban clouds.

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Robert R. Czys
and
Harry T. Ochs III

Abstract

The influence of charge on coalescence was determined in the laboratory for isolated pairs of 340 and 190 μm water drops failing freely at terminal velocity. A microcomputer-controlled apparatus was used to produce collisions. Drop charges were independently controlled and collisions occurred in a humidified chamber at laboratory temperature and pressure. Two cameras were used to record the interactions from orthogonal directions and the photographs used to determine the impact angle for each collision. A single charge-independent critical impact angle of 43° ±1° was observed that distinguished an inner collision cross section for coalescence from an outer annular cross section for noncoalescence. Collisions occurring for impact angles less than the critical value always resulted in coalescence regardless of charge. Collisions having impact angles greater than the critical value resulted in either bounce, temporary coalescence or coalescence depending on charge. A satellite drop was produced with temporary coalescence at the intermediate charges of this experiment. Electrohydrodynamic theory in plane geometry applied to the conditions for deformable spheres gave an electric field strength between the approaching drop surfaces sufficient to cause a surface instability that may aid in drop coalescence.

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Kenneth V. Beard
and
Harry T. Ochs III

Abstract

Rain triggering mechanisms are evaluated in three microphysical steps: droplet activation on cloud condensation nuclei, droplet growth by condensation, and droplet growth by coalescence. Although considerable progress has been made since the pioneering work of Squires, crucial questions in each of the above steps remain unresolved: Under what conditions do giant particles trigger rain by acting as coalescence nuclei? What is the contribution of stochastic condensation to the growth of large droplets in regions of entrainment? What are the collection efficiencies for droplet sizes critical to the onset of colaescence growth? Such questions cannot be answered without better observations. Aircraft instruments are becoming available with the potential to measure the very largest particles and cloud droplets at the concentration of raindrops. Recent advances in sampling and analysis techniques have extended observations of cloud microstructure to smaller scales, providing new insight on the growth of droplets by mixing. Continued progress in laboratory research should furnish collection efficiencies for the droplets sizes critical to warm-rain initiation. With such improved observations, careful evaluations using available microphysical and dynamical models should provide answers to key questions about warm-rain initiation.

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Kenneth V. Beard
and
Harry T. Ochs III

Abstract

No abstract available

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Harry T. Ochs III
and
Kenneth V. Beard

Abstract

A closed parcel model which simulates condensation, collection and breakup was used to evaluate the effects of recently measured collection efficiencies on precipitation development. Computations were made using theoretical collision efficiencies assuming coalescence efficiencies of 100% and, for comparison, with semiempirical coalescence efficiencies of 50–100%. The model indicated that the development of a detectable radar echo was (retarded in time) by 250 to 500 meters from the effects of limited coalescence and that the concentration of precipitation drops was generally reduced by several orders of magnitude.

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Harry T. Ochs III
and
K. V. Beard

Abstract

Collection efficiencies for accretion were measured for six pairs of nearly unchanged drops. Cloud droplets of 11 and 17 μm and collector drops between 100 and 400 μm radius were used. The resulting efficiencies were in the 51–70% range and all values were significantly below computed collision efficiencies for rigid spheres. Inferred coalescence efficiencies between 54 and 82% were found to decrease with increasing collector drop and cloud droplet sizes. Drop separation was attributed to the grazing bounce mechanism whereby an air film nullifies the relative closure velocity allowing the tangential velocity of the cloud droplet to carry it past the collector drop.

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K. V. Beard
and
Harry T. Ochs III

Abstract

The collection efficiency has been measured for 15 size pairs of relatively uncharged drops in over 400 experimental runs. The results indicate that collection efficiencies fall in a narrow range of 0.60 to 0.70 even though the collector drop was varied between 63 and 100 μm radius and the collected drop from 11 to 26 μm radius. The measured values of collection efficiencies were consistently below collision efficiencies based on calculations using rigid sphere hydrodynamics. The coalescence efficiencies computed from the ratio of the theoretical collision efficiencies to the measured collection efficiencies were between 0.6 and 0.8. Our data show fair agreement with one previous coalescence model result, with an existing semi-empirical formula for the coalescence efficiency, and with the predicted trend of the coalescence efficiency to decrease with the size of the collected drop. A plausible interpretation of our experimental findings, consistent with previous coalescence studies, is that contact is prevented during a grazing trajectory by a hydrodynamic deflection which is enhanced by a slight deformation of the collector drop.

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