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doi: 10.1002/qj.49709439902pmid: N/A
An experimental study has been made of the external and internal structure of artificial hailstones grown freely supported in a vertical wind tunnel. In addition the nature of ice deposits formed on fixed stationary objects and objects rotating about a horizontal axis has been investigated at airspeeds of 35 to 40 m s−1. Lobe‐like growth similar to that found in natural hailstones has been reproduced. The lobes are most pronounced when the accreted droplets are small and when growth takes place near the wet limit. With large droplets and when the growth is very spongy the surface irregularities are far less marked. It is inferred that the lobe‐like growth is due to a collection efficiency effect.
Benwell, G. R. R.; Timpson, Margaret S.
doi: 10.1002/qj.49709439903pmid: N/A
Vertical smoothing of the initial height fields proved to be a crucial step in obtaining 24 hr forecasts using the 10‐level primitive equation model (Bushby and Timpson 1967) on any set of data : the particular operator used and the effects on the forecast fields are described. The latent heat effect is examined quantitatively by comparing two 24 hr forecasts, one of which included and the other excluded the contribution of the latent heat of condensation. The forecast fields obtained when a larger grid length is used, in conjunction with an appropriately lengthened time‐step, are compared with forecasts obtained from the basic grid length and time‐step and the implications arising from the increase of scale are discussed.
doi: 10.1002/qj.49709439904pmid: N/A
Laboratory studies using a microscope cold stage with controlled humidity environment have been carried out on the ice nucleation and preactivation properties of several naturally occurring particulate substances. For initial nucleation water saturation was necessary, but below a critical temperature a constant ice supersaturation (water subsaturation), was sufficient. The values of both temperature and supersaturation depended on the substance. In preactivation the behaviour was similar but with a higher initial and critical temperature. Typical values are : for kaolinite, initial nucleation − 10·5, − 19°C, 20 per cent; preactivation − 6, − 11·5°C, 12 per cent: montmorillonite, − 25, below − 27°C; preactivation − 4, − 13·5°C, 14 per cent. The first temperature gives one ice crystal in 104 particles; the second is critical for nucleation at water subsaturation. The low values of supersaturation required for nucleation suggest that preactivation is caused by the freezing of a supercooled, adsorbed, liquid‐like layer, rather than by the retention of ice embryos in cavities. A necessary condition for the measurement of the ice nucleating ability of particles from an air sample is that they must not be heated to a temperature above at least − 5°C or subjected to a relative humidity over ice of less than about 35 per cent before the measurement is made.
doi: 10.1002/qj.49709439905pmid: N/A
A technique to measure the concentration of freezing nuclei in the atmosphere is described. It is a development of the Millipore filter method described by Bigg, Mossop and Heffernan (1961), from which many of the difficulties experienced by them and other workers in the field have been eliminated.
doi: 10.1002/qj.49709439906pmid: N/A
Investigation of a proposed relationship between the skin friction part of the drag on a natural surface and a simultaneous transfer of mass or (sensible) heat led to measurements of the dimensionless transfer coefficients Cd, Cv and Ch for the exchange of (stream‐wise) momentum, mass, and heat, between a single artificial leaf and the airflow in a wind‐tunnel. It was shown that Cv, h = C0(D/v, k/v)2/3 where D, k and v are the molecular diffusivities in air of gas or vapour, heat, and momentum, and where C0 is a generalized mass or heat transfer coefficient almost independent of ϕ, the angle of incidence between the leaf and the airflow, Cd, however, made up of a bluff‐body or pressure part Cb, in addition to a molecular skin friction part Cf, depended strongly on ϕ. C0 was close to the theoretical skin friction drag coefficient at ϕ = 0 of a thin flat plate with dimensions similar to the leaf. As a general relation between the corresponding coefficients of a natural rough surface CF = βC0, where β lies between 0·1 and unity, depending on the form and inclination (ϕ) of the roughness elements, and on wind speed.
doi: 10.1002/qj.49709439907pmid: N/A
The results of an analysis of the chemical composition of precipitation and air over the British Isles and Eire are presented. Tables are given showing the median concentrations of S, Cl, Na, K, Ca and nitrogen in ammonia and nitrate, and maps are presented to indicate the variations in sources for different chemicals. The errors in the sampling technique are discussed.
doi: 10.1002/qj.49709439908pmid: N/A
A region surrounding a cumulus cloud is defined in such a way that (when virtual temperature effects are neglected) there is no sensible heat transfer across the boundaries of the region. This simplifies the discussion of the interaction between the cumulus and its environment. A model of the region, which is called the white box, is developed and an estimate is made of the net vertical mass transport within it as a function of height. Rain and cloud radiation have the effect of modifying this transport and may be described in terms of it. If the requirements of continuity are satisfied by means of an alteration of the wind profile of the environment downstream, the wind near the base of the cloud downstream is increased. It is shown that cumulus convection can destabilize as well as stabilize its environment.
doi: 10.1002/qj.49709439909pmid: N/A
This paper discusses the reference equilibrium atmosphere for the approximate equations of shallow layer thermal convection.
doi: 10.1002/qj.49709439910pmid: N/A
The rate of melting of realistically rough artificial hailstones in an airstream has been measured and the dependence of the rate of heat transfer on Reynolds number determined. The rates of heat transfer show a marked divergence from the values for smooth spheres as the Reynolds number increases from 104 to 2 × 105. The results indicate that the effective critical liquid water concentrations of large hailstones (diameters greater than 6 cm) are 1·2 to 3 times those previously computed.
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