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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
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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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
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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Harry T. Ochs III
and
C. S. Yao

Abstract

The numerical methods necessary for the application of moment-conserving techniques to the study of warm rain microphysical processes in an Eulerian mass coordinate are described. When this technique is applied to simulations of condensation, collection and breakup the total drop distribution is divided into a number of Eulerian categories and three quantities pertaining to the drops within each category are retained between integration time steps. These three numbers are related to the drop concentration, the mean drop size and the standard deviation about this mean size. These techniques serve to minimize numerical spreading which would otherwise lead to the premature development of precipitation sized particles in a detailed microphysical simulation.

A picewise linear mass coordinate in conjunction with the moment-conserving techniques. allows conservation of cloud condensation nuclei and water mass to within computer truncation error. Methods for tracing the nuclei mass through the condensation, collection and breakup processes in saturated and sub-saturated air are developed.

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Kenneth V. Beard
and
Harry T. Ochs III

Abstract

Collisions between small precipitation drops in free fall were analyzed for sizes applicable to self-collection, the process that controls the spreading of precipitation drops to larger sizes. Results from 45 laboratory experiments were generalized using dimensionless parameters to scale the coalescence efficiency, for the temporary coalescence probability, and the satellite occurrence frequency. The coalescence efficiency for uncharged drops (ε0) was found to be highly correlated (ρ = 0.99) with a simple combination of factors that scale the tendency for colliding drops to bounce apart as a function of the Weber number (We) and size ratio (p). Charge-induced coalescence was scaled by the electric field between the drops, assuming charged conducting spheres. The coalescence efficiency was obtained as a function of the normalized charge using a semiempirical formula (ρ = 0.95) for the amount of charge required to eliminate bounce and temporary coalescence.

The occurrence of temporary coalescence is predicted by p We > 4 with a lower limit of p We > 1 for charge-induced coalescence. The fraction of collisions resulting in temporary coalescences increased with (1 – ε0)p We, whereas the fraction of collisions producing satellites increased with (1 – ε0) We2. Both fractions were highly correlated with their respective scaling parameters (ρ = 0.99). Satellite drop radii were found to increase linearly with the geometric mean radius of the parent drops. Mass transfer in collisions involving temporary coalescence and satellite generation was estimated for use in modeling studies.

Contour diagrams are provided for coalescence efficiency, temporary coalescence probability, and satellite occurrence frequency over a wide range of drop sizes for comparison with formulas based on previous laboratory results in the accretion and breakup regimes. Recommendations are given for applying present formulas to self-collection, as well as extending our findings to accretion and breakup.

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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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