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Charles F. Chappell
and
Floyd L. Johnson

Abstract

A simple numerical model is formulated for a mixed-phase cold cloud which simulates the interaction among the updraft, crystal growth, droplet growth or evaporation, and ice crystal concentration. The model accounts for temperature and pressure changes due to vertical displacements, and crystal concentrations are specified as a function of cloud-top temperature. Cloud temperature and updraft speed were varied in the model and equations integrated for a time period of up to 1 hr. Results indicate that a cloud-top temperature of about −25C will demarcate cold orographic cloud systems with respect to a potential for snowfall augmentation. There is also a suggestion that the concentrations of artificial ice nuclei, one would add to maximize snowfall, probably need not be considered single-valued. Rather, it appears that crystal concentrations may exceed a critical concentration by an order of magnitude without noticeable adverse effects on precipitation efficiency, even in the absence of agglomeration mechanisms.

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Robert A. Maddox
,
L. Ray Hoxit
, and
Charles F. Chappell

Abstract

It has been frequently observed that thunderstorms which interact with a warm front, or an old thunder-storm outflow boundary, are likely to increase in severity and become tornadic. The physical mechanisms responsible for this observed characteristic of severe storm evolution are not well understood. A physical model of subcloud wind profiles near thermal boundaries has been developed and a number of cases have been analyzed. Within a hot, moist and conditionally unstable air mass, warm thermal advection and surface friction cause the winds to veer and increase with height. Whereas within a cool, Moist air mass (such as a thunderstorm outflow region) cool thermal advection and friction combine to produce a wind profile that has maximum speeds near the surface and veers little with height. The spatial distribution of differing vertical wind profiles and moisture contents within the boundary layer may act in concert to maximize mesoscale moisture contents, convergence and cyclonic vorticity within a narrow mixing zone along the thermal boundary. These characteristics may explain, in part, why storms often reach maximum intensity within the environment attending thermal boundaries.

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Lee R. Hoxit
,
J. Michael Fritsch
, and
Charles F. Chappell

Abstract

No abstract available.

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Lee R. Hoxit
,
Charles F. Chappell
, and
J. Michael Fritsch

Abstract

Correction to Volume 104, Issue 11, Article 1408.

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J. Michael Fritsch
,
Charles F. Chappell
, and
Lee Ray Hoxit

Abstract

Synoptic-scale mass and moisture budgets are objectively computed by kinematic techniques and compared to the mass and moisture budgets of a tornado-producing Oklahoma squall line. The rate of consumption of water vapor by the squall line is found to be significantly larger than the synoptic-scale moisture convergence; similarly, the mass processed by the squall line exceeds the synoptic-scale budget. This implies that additional vertical circulations must exist on a scale smaller than synoptic, probably a reflection of organized convective lifting and compensating downdrafts. Consequently, parameterization of mid-latitude organized convection, using synoptic-scale mass or moisture convergence to estimate cloud fluxes, will likely underestimate the amount of convective precipitation, the total vertical mass exchange and, therefore, the magnitudes of the thermodynamic stabilization and vertical momentum exchange as well.

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Lee Ray Hoxit
,
Charles F. Chappell
, and
J. Michael Fritsch

Abstract

Mesoscale lows or pressure troughs have been observed downwind of many mid-latitude cumulonimbus cloud systems, especially those that subsequently produce severe weather such as tornadoes, large hail, or damaging wind gusts. Case studies are presented which link the formation and existence of mesolows or troughs to subsidence warming in the upper troposphere and lower stratosphere. This subsidence warming, which is believed to result from the interaction of the convective cloud with a sheared environment, leads to an instability between the meso-γ and meso-β scales by hydrostatically reducing surface pressures, thereby organizing and increasing the low-level convergence ahead of existing convection.

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Charles F. Chappell
,
Lewis O. Grant
, and
Paul W. Mielke Jr.

Abstract

The nature of precipitation changes resulting from seeding cold orographic clouds is examined by separating the observed total precipitation change into duration and intensity change components. The total precipitation change and its two components are then evaluated as functions of cloud temperature using precipitation data recorded in the primary target area during the cloud seeding experiment conducted near Climax, Colo. The results show that the total change in observed precipitation is mainly controlled by changes in precipitation duration, rather than intensity. The main effects of seeding appear to be the initiation of a precipitation release for the warmer clouds during many hours when it would not have occurred naturally, and the suppression of precipitation for the coldest clouds during some hours when it would have occurred naturally. These results are consistent with the concepts of cloud microstability and cloud over-seeding.

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Paul W. Mielke Jr.
,
Lewis O. Grant
, and
Charles F. Chappell

Abstract

An orographic cloud seeding experiment conducted in the vicinity of Climax, Colo., has been continued for five additional wintertime periods from 1965–70. A comparison of this new independent information is made with previously discussed wintertime operations of the experiment from 1960–65. As a whole, agreement between these independent data sets is good. In particular, the agreement in temperature and wind partitions is consistent with a previously reported model which describes seeding effects under various physically defined conditions. These comparisons have been made using pooled groups of precipitation sensors having similar elevations and locations.

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Charles F. Chappell
,
E. L. Magaziner
, and
J. Michael Fritsch

Abstract

Non-iterative methods for computing isobaric wet-bulb temperature and the temperature at the lifted sublimation level are presented. These quantities are then computed by these direct techniques and compared to values derived through iterative methods. In the case of the isobaric wet-bulb temperature, differences in the two methods are less than 1C over normal ranges of atmospheric temperature and pressure for dewpoint depressions <30C. Differences in the two methods are less than 0.03C for the temperature at the lifted sublimation level over the same range of dewpoint depression.

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Paul W. Mielke Jr.
,
Lewis O. Grant
, and
Charles F. Chappell

Abstract

This study is concerned with the elevation and spatial variation effects of wintertime orographic cloud seeding over an area encompassing Fremont, Hoosier and Vail mountain passes in the central Colorado mountains during a period from 1960 to 1965. The observation network consisted of 65 precipitation stations distributed over the three passes. Depending on the grouping of precipitation stations used to represent the prime target area of the study, the average daily precipitation for all 120 seeded days was from 6 to 11% greater than the average daily precipitation for all 131 non-seeded days. There is a high probability that these differences could have occurred by chance alone.

Analyses have also been made according to physically defined stratifications based on a model which describes the seeding effects ascribed to the various strata. Statistically significant increases (decreases) were observed over much of the area for the seeded periods in comparison with the non-seeded periods when 500 mb temperatures were −20C and warmer (−27C and colder). Little or no effects were noted in the intermediate temperature range. When 500-mb wind velocities were from 22–28 m sec−1, statistically significant increases were observed during the seeded period in comparison with the non-seeded period throughout the area.

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