SUMMARYA 10-level primitive equation model suitable for studying the dynamics of fronts and frontal rainfall is described. The atmosphere is assumed to be hydrostatic and inviscid and the effects of friction and topography are ignored. Latent heat due to evaporation and condensation is incorporated in the thermodynamic equation.No distinction is made between the ice and water stage and the atmosphere is assumed to be dry above 300 mb.The horizontal grid length is 40 km. The results of one 24-hr integration are described in detail.
SUMMARYVertical 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. VERTICAL SMOOTHING OF THE INITIAL HEIGHT FIELDS(a) The problem Bushby and Timpson (1967) described a 10-level primitive equation model of the atmosphere and discussed the results of one 24 hr forecast. Considerable difficulty was experienced when attempts were made to obtain another 24 hr forecast owing to the smallscale roughness which developed during the integration, especially in the vertical motion fields. Some roughness had also been noted in the first 24 hr forecast but in that case the roughness had not prevented the computations from proceeding to the full 24 hours.The procedure followed in preparing the initial height data on which the first 24 hr forecast was based was as follows. The basic height data consisted of grid point values extracted from upper air contour charts at 1,000, 850, 700, 500, 400, 300, 200 and 100 mb. These height fields were then horizontally smoothed using a three point smoothing operator of the form where Ht", H$ are the smoothed and unsmoothed heights respectively at grid point i and Ht-1, H~+ I are the unsmoothed heights at the adjacent grid points in the direction along which the smoothing operation is being carried out. The smoothing operation was applied along each of the x , y directions using a = 4 and the process was then repeated using a = -+ ; a s explained by Wallington (1962) such a procedure decreases the amplitudes of the smaller scale features which it is desired to eliminate, leaving the larger scale features not greatly altered. No other smoothing was carried out on the initial height data for the first 24 hr forecast, the 900, 800 and 600 mb height fields being obtained by interpolation from those at 1,000,850,700 and 500 mb. The above procedure proved to be unsuccessful when attempts were made to obtain 24 hr forecasts for other occasions and a close examination of the lapse rates derived from the height fields showed that there were on all occasions some parts of the chart where the derived lapse rates were super dry adiabatic. For the initial data for 0000 GMT 1 December 1961, the occasion of the first forecast, such parts were rather small and the lapse rates did not greatly exceed the dry adiabatic lapse rate,
It is interesting to observe that below the critical temperature a constant super-saturation with respect to ice is required both for nucleation on large crystals (Bryant, Hallett and Mason 1959) and also in these experiments. I find this result rather surprising since, if it were simply a question of nucleating new layers, I should have thought a constant vapour pressure would have been more likely than a constant relative humidity. REFERENCE' The epitaxial growth of ice on single crystalline substrates,'
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