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J. Rothermel
and
E. M. Agee

Abstract

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E. M. Agee
and
K. E. Dowell

Abstract

An observational study of mesoscale cellular convection occurring over vast regions of the North Atlantic and North Pacific has been done for the period 1 January 1969 through 30 June 1970. Satellite cloud photography from the ESSA 7, ESSA 9 and ATS 3 satellites and conventional rawinsonde data have been analyzed for a total of 38 cases, consisting of 25 open and 13 closed convective patterns. Computations have shown that: 1) the average diameter for open cells is 30 km and for closed cells 32 km; 2) the average convective depth for open cells is 2.3 km, greater than the 1.3 km average for closed cells; 3) the average aspect (diameter-to-depth) ratio for open cells, 15:1, is less than that for closed cells, 28:1; 4) the aspect ratio is inversely proportional to increasing convective depth; 5) sea surface temperature exceeds the air temperature on the average by 2.1C in open cells but is 0.4C less in closed cells; 6) directional and magnitude shear (in the vertical) of the horizontal wind is small, less than 7° km−1 and 2 m sec−1 km−1, respectively, but indicative of backing or cold air advection in open cells and veering or warm air advection in closed cells; 7) a characteristic lapse for the convecting layer of 8.2C km−1; and 8) a strong total heat flux of 218 1y(4 hr) −1 from the sea to the atmosphere in regions of open cell and a weaker total heat flux of 65 1y(4 hr) −1 from the air to the sea in regions of closed cells.

Open cellular patterns which preferably occur in cyclonic synoptic-scale flow portray the oceans as a major energy source for driving the atmosphere's circulation. Closed cellular patterns, on the other hand, usually occurring under conditions of anticyclonic synoptic-scale flow, portray the oceans as a weak sink for the atmosphere's energy.

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E. M. Agee
and
T. S. Chen

Abstract

A mathematical model consisting of a complete set of the Boussinesq equations governing Rayleigh convection in the atmosphere has been solved for the marginal stability case in order to study the geometry and circulation patterns of mesoscale cellular convection. The significant new feature of the model is the inclusion of eddy viscosity variation with height through the convecting layer. Results obtained show that the direction of convection circulations are controlled by the sign of the vertical gradient of eddy viscosity. It is also concluded that variable convective depth has a significant but small effect on the geometry of the convection, with the degree of cell flatness being principally controlled by the degree of anisotropy.

Using an assumed periodic form of the perturbation solutions, the problem reduced to solving a sixth-order differential stability equation with variable coefficients and accompanying boundary conditions. By applying the two-variable, small-perturbation technique, the stability equation was solved yielding a marginal stability diagram relating an “atmospheric” Rayleigh number to the wavenumber of a given disturbance. Further, a family of neutral stability curves was generated for different convective depths. Marginal stability solutions obtained for different eddy viscosity changes with height show the dependency of the circulation reversal on changing the sign of the eddy viscosity gradient. In order to obtain the direction of the vertical motion it was necessary to impose Stommel's argument of minimum frictional loss. Therefore, convection cells with descending motions in the center, occurring in an environment of decreasing eddy viscosity with height, are seen to change their circulation direction from an “open” cell to a “closed” cell by simply changing the eddy viscosity to increase with height (and conversely).

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E. M. Agee
and
R. P. Howley

Abstract

Computations of latent and sensible heat flux at the air-sea interface using the bulk-aerodynamic method are presented for the period of the 1974 Air Mass Transformation Experiment (AMTEX) conducted over the East China Sea. Results show quantitatively the impact of the outbreak of cold and dry polar air over the warm Kuroshio Current as sensible and latent heat flux increased, respectively, from minima of 49 and 182 cal cm−2 day−1 to maxima of 485 and 1099 cal cm−2 day−1. Correspondingly, the total heat flux (sensible and latent) from the sea to the atmosphere from the warm to cold periods increased from 231 cal cm−2 day−1 to 1584 cal cm−2 day−1 which should be a representative range of extreme values for weak and strong heat flux associated with air-sea interaction. Some discussion is given as to the effect of this strong increase in heat flux during air mass modification on the evolution of cyclones over the Kuroshio Current and the Gulf Stream.

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D. H. Lenschow
and
E. M. Agee

The field phases of AMTEX, a GARP subprogram on air-sea interaction implemented by Japan, were conducted over the East China Sea in the environs of Okinawa, Japan, during the last two weeks of February in 1974 and 1975. Investigators from Australia, Canada, and the United States also participated in this experiment. The weather was generally very favorable for this study of air mass transformation processes in 1975 because of an extensive cold air outbreak during most of the experimental period. A basic synoptic data set was obtained from 6 h soundings from an array of aerological stations centered at Okinawa. In addition, satellite, hourly surface and surface marine, oceanographic, boundary layer, radiation, radar, cloud physics, and aircraft data were obtained and have been or will be available in published data reports or on magnetic tape.

Preliminary results from 1974 and 1975 reported at the Fourth AMTEX Study Conference and joint United States–Japan Cooperative Science Program Seminar, “Air Mass Transformation Processes over the Kuroshio in Winter,” held in Tokyo, 26–30 September 1975, are presented and discussed.

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Ernest M. Agee
and
Francis E. Lomax

Abstract

Aerological data from the Air Mass Transformation Experiment (AMTEX) conducted during the period 14 February–1 March 1975 have been analyzed to study the mixed layer and capping inversion layer associated with mesoscale cellular convection (MCC) created in a cold continental air mass heated from below by a warm ocean surface.

Six horizontal analyses of the depth of the mixed layer were superimposed on their respective satellite images which contained MCC events in the AMTEX region. In all six cases, open cells tended to lie in the troughs and closed cells in the ridges of the convective depth contour pattern. Vertical cross sections revealed that the actual transition from a coexisting region of open cells to an adjacent region of closed cells may actually occur at a point of inflection where the mixed layer depth contour pattern changes from a plano-concave shape to a plano-convex shape.

Mean convective depths for open and closed cells were 1529 and 2066 m, respectively, based on 15 soundings in regions of open cells and 16 soundings in regions of closed cells. Also, vertical profiles of potential temperature in regions of open and closed cells were obtained, with the convectively mixed layer characterized by a superadiabatic lapse rate near the surface, adiabatic through the subcloud layer and moist adiabatic through the cloudy regions. Mean vertical potential temperature gradients and geometric thicknesses of the inversion layer for open and closed cells were, respectively, 21.9 K km−1 (437 m) and 25.5 K km−1 (480 m).

The time cross sections of potential temperature indicated two types of undulations in the depth of the mixed layer due to 1) the passage of regions of coexisting open and closed cells of synoptic-scale proportions, and 2) the elevations and depressions of the inversion base associated with the passage of individual MCC. Three cases using the potential temperature field and the relative humidity field were used to construct observational “thermodynamic” models of an open cell. Features were (i) the inversion base and top were higher in the wall (cloudy) region than in the central (clear) region, (ii) the potential temperature pattern contained in the inversion layer followed the shape of the inversion base, (iii) in each case, the potential temperature pattern was reversed above the inversion layer, and (iv) the relative humidity pattern was in phase with the inversion base at all levels.

Quantitative estimates were made for all terms in Arakawa’s formulation for the time rate of change in the depth of the planetary boundary layer (PBL). Local changes observed in the depth of the PBL were ±2.8,μb s−1. Values for the advective term were in the range of ±5 μb s−1. Large-scale vertical motion was always downward leading to a decreasing thickness in the PBL depth with time. Average values for open cells and closed cells were, respectively, 3.6 and 2.5 μb s−1. The entrainment term was always positive and tended to counteract the effects of the large-scale vertical motion. Mean values for the open and closed cell regions were 1.7 and 1.1 μb s−1, respectively. The penetrative convection term was assumed negligible based on photographic evidence. The net effect of the entrainment and vertical motion terms is to support greater thickness of the PBL in regions of closed cells compared to regions of open cells, which was consistent with the observations. Implications of a complete budget analysis, including radiation and latent heat released, are also discussed.

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D. H. Lenschow
and
E. M. Agee

The first field phase of AMTEX was conducted during 14–28 February 1974 in the vicinity of the Southwest Islands of Japan, with the operational control center at Okinawa. Investigators from Japan, Australia, and the United States participated in the experiment. The measurements and synoptic situation during the field program as well as some preliminary results are presented. The weather was characterized by a warm period from 14 to 23 February, followed by a cold period from 24 to 28 February when extensive modification of the continental air took place.

Plans for the 1975 field program, scheduled for 16 February to 3 March, are discussed. No major changes from the 1974 program are expected in 1975.

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E. M. Agee
,
T. S. Chen
, and
K. E. Dowell

A brief review of the classical theory for convective instability is presented, with primary emphasis on the more recent theoretical and experimental findings that pertain to mesoscale cellular convection in the atmosphere. Important physical and geometrical features of three-dimensional patterns of free convection commonly observed over the oceans are discussed, especially those features that point to distinct differences when compared to laboratory convection. Some suggestions for future study are offered with particular reference to the AMTEX program being planned by the Japanese Committee for GARP.

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C. R. Church
,
J. T. Snow
, and
E. M. Agee

A 4 m wide and 7 m tall tornado vortex generator (including exhaust fan and duct work) has been constructed at Purdue University that operates on a principle similar to that of the earlier machine modeled by Ward (1972). Characteristics of the Purdue simulator are described, as well as the corresponding modifications and improvements that have been made to Ward's machine. Selected photographs of vortex configurations obtained in the simulator demonstrate the ability of the machine to achieve vortex breakdown and multiple vortex configuration. A radial-axial profile of velocity magnitudes (using hot-film anemometry) has been obtained for the state of vortex breakdown characterized by two interlocking helical spiral vortices. This preliminary result shows the potential that the experimental system offers for obtaining quantitative information about the flow field of selected vortex configurations. Multiple vortex phenomena in the thunderstorm-tornado system are examined in light of the laboratory simulation and the similarity concept.

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E. M. Agee
,
J. T. Snow
, and
P. R. Clare

Abstract

Analyses are presented of the tornado tracks for four tornado families affecting Indiana and neighboring states on 3 April 1974. The study by Agee et al.(1975)on the occurrence of multiple suction vortices in the tornado has been used by further extending Ward's (1972) multiple vortex phenomenon to the scale of the tornado cyclone. This has allowed an interpretation of multiple tornado events by means of which consecutive tornado damage paths may be frequently recognized as segments of cycloidal tracks for multiple vortices occurring within the larger tornado cyclone.

A tornado cyclone, most likely within the right rear quadrant of the severe thunderstorm, may contain two, three, or more smaller scale vortices (mini tornado cylones) that revolve cyclonically about the center of the. parent tornado cyclone as the entire system translates along with the thunderstorm. It is shown that these centers of action have cycloidal tracks that can be matched with the damage paths of tornado families. Tornadoes are observed to be left-turners or right-turners, depending on what portion along the cycloidal track touchdown and demise occur. A tornado family may he associated with a single vortex parent tornado cyclone which tends to produce longer tornado tracts that do not undergo appreciable turning. Tornado families, however, may be generated by multiple vortex tornado cyclone systems, and in this case tornado tracks follow a curtate cycloid, eventually turning into the cold air and dissipating. Another tornado can reappear, however, as the parent vortex caster returns cyclonically (along the cycloidal path) to more favorable environmental conditions for redevelopment. Finally, a summary is presented depicting the scales of possible multiple vortex systems associated with the tornado producing thunderstorm.

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