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B. F. Ryan

Abstract

A model is presented which simulates the glaciation of a cloud. In this model both vapor transfer and accretion processes are computed, the latter being treated stochastically. Two model clouds have been examined, one a modified continental cloud and the other a maritime cloud. These clouds have been seeded with ice crystal concentrations of 0.5, 10 and 500 liter−1. In all numerical experiments the resulting shape of the ice particle spectrum was a function of the initial water drop spectrum. At the lowest ice crystal concentration the rate of formation of ice in the clouds is relatively slow.

The importance of the vapor transfer processes in the calculation is discussed and a comparison is made between this model and one that uses a continuous formulation for the accretion processes.

From the point of view of the production of precipitation-size particles it appears that seeding a cloud with 10 ice crystals per liter would be the most productive of the three ice crystal concentrations tested. For the assumed cloud liquid water content of ∼1 gm m−3 it is unlikely that even the largest seedings used in the experiments would initiate marked dynamical changes in the cloud by the glaciation process.

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B. F. Ryan

The aim of the Global Energy and Water Cycle Experiment Cloud System Study (GCSS) is to promote the description and understanding of key cloud system processes, with the aim of developing and improving the representation of cloud processes in general circulation models. The GCSS Science Panel identified a need to document important observational gaps in the structure of cloud systems inhibiting the development of cloud-resolving models as a tool for parameterizing cloud systems in general circulation models.

The nature of precipitating layer clouds around the world is not well documented. To better quantify this, a synthesis of observations of these types of clouds made during field experiments conducted around the world has been developed. The synthesis draws on observations made in Australia, Canada, China, Israel, Japan, Russia, the Ukraine, the United States, and several European countries.

The survey examines the global variation of the horizontal scales of cloud and precipitation, embedded phenomena such as rainbands, conveyor belt characteristics, ice crystal and water droplet concentrations, and raindrop and ice crystal size distributions.

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B. F. Ryan

Abstract

It is shown that the recent theoretical collection efficiencies derived by Klett and Davis imply a more effective coalescence process than those previously used. The effect of vertical shear on coalescence is examined and it is shown that this can improve the rate of production of “large” drops.

When the water drops are also growing by condensation there is an acceleration of the rate of coalescence which is due to the increasing collection efficiency of the growing drops.

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B. F. Ryan and K. J. Wilson

Abstract

Observations of the Australian summertime cool change have ranged over both the subsynoptic scale (200–2000 km) and the mesoscale (20–200 km). In this paper, the final in a series of three, a conceptual subsynoptic and mesoscale model is developed. The model suggests that such features as

(i) the speed of movement of the surface cold front,

(ii) the inflow of moisture into the frontal transition zone, and

(iii) the midtropospheric wind field associated with the frontal transition zone

are determined largely by the synoptic scale flow. Mesoscale controlled processes occur within the frontal transition zone.

The model is consistent with models developed for the Northern Hemisphere. The significant difference between the subsynoptic and mesoscale models appropriate to the United Kingdom and the Pacific Northwest of the United States and the summertime cool change is the lack of low-level moisture in the Australian situation. The model highlights the influence of the hot, dry Australian continent on the development of fronts.

The application of the model to forecasting in the Australian region is explored. In particular, it provides a systematic framework for analyzing the weather lines associated with the passage of the frontal transition zone. Further, the conceptual model suggests that the speed of the frontal transition zone should be able to be forecast with some skill by current operational numerical models. Finally, the model predicts the type of surface weather associated with the passage of the frontal transition zone.

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B. F. Ryan and W. D. Scott

Abstract

No abstract available.

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B. F. Ryan and J. C. Carstens

Abstract

A simple parcel model has been used to examine the possibility that observed profiles of θe following the passage of a squall line are produced by air originating in middle-level clouds and descending in steady rain. The model predicts realistic profiles of θ and q.

The calculations suggest that in meteorological situations such as squall lines the thermodynamic properties of air associated with evaporating downdrafts can be parameterized by this simple model.

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J. J. Katzfey and B. F. Ryan

Abstract

The importance of subcloud evaporation to the thermodynamics and movement of cold fronts is investigated through inclusion of an explicit cloud scheme within a 30-km resolution limited-area model. Two cases are examined: 18 November 1984 and 26 February 1995. The effect of the subcloud evaporation is deduced by comparing simulations with and without the evaporation for these two cases. The implications of these results for weather prediction and climate models are discussed.

The first case occurred during the Australian Cold Fronts Research Programme with mesoscale data available to verify the model simulation. The results indicate that the movement of the synoptic cold front was retarded when a prefrontal cool change developed in response to the evaporation of the grid-resolved precipitation. However, the cooling ahead of the front effectively accelerated the cool change, more in line with observations.

The second case involved the prognoses of a cool change crossing Victoria in southeast Australia during a potential bushfire day. In this case, most of the precipitation occurred along and behind the cold front. With precipitation evaporation, the cool change accelerated several hundred kilometers farther in 36 h than in the simulation without precipitation evaporation.

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J. J. Katzfey and B. F. Ryan

Abstract

The importance of subgrid-scale processes for the simulation of midlatitude frontal clouds by global models is investigated. The case chosen is a frontal cloud associated with a cool change crossing the southern Australian coastline between 17 and 19 November 1984. The Commonwealth Scientific and Industrial Research Organisation limited-area model, Division of Atmospheric Research Limited-Area Model, was run at horizontal resolutions of 30 and 300 km, and the results of the 30-km simulation were then averaged to 300-km resolution. Comparisons and evaluations of the simulations showed that the 300-km simulation failed to develop the frontal clouds. Comparison with the 30-km simulation averaged to 300 km showed the importance of the subgrid-scale vertical motions for this cloud development. In particular, it is found that the covariance of the subgrid-scale terms, although of smaller magnitude when compared with the larger-scale terms, needs to be parameterized to capture correctly the frontal cloud development. It is suggested that parameterization of the subgrid-scale dynamical forcing is important for the correct cloud development in general circulation models.

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W. C. Macklin and B. F. Ryan

Abstract

The structure of ice grown in water supercooled to temperatures between −2C and −7.5C has been studied and recorded by flash photography. The ice structures formed below −3C are not co-planar with the basal plants of the seed crystals but are split into two, and occasionally more, segments. At temperatures below −5.5C secondary splitting occurs on the major growth segments, the complexity of the structure increasing with increased supercooling. A stepped growth mechanism has been suggested to explain thew observations. The three-dimensional structures so formed are sufficiently, complex to retain unfrozen liquid and so give rise to spongy ice.

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B. F. Ryan, E. R. Wishart, and D. E. Shaw

Abstract

Ice crystals were grown in a supercooled cloud at temperatures ranging from −3°C to −21°C for periods from 30–40 s to 150–180 s. When the axial dimensions at a given time were examined as a function of temperature, there was a marked maximum along the a axis at −15°C and a secondary broader maximum along the c axis at −6°C. The growth of the axial dimensions can he adequately represented by a linear function of time.

A power function of time was fitted to the crystal mass growth measurements; these show a sharp maximum at −15°C and a secondary broader maximum at −7°C.

Crystal bulk densities estimated from the masses and axial dimensions vary with temperature in a complicated way, with a minimum of about 0.4 Mg m−2 at −5 and − 17°C, and a maximum of 0.92 Mg m−2(pure rice) at and appear to he independent of time.

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