The Quarterly Journal of the Royal Meteorological Society

doi: 10.1002/qj.49711247101pmid: N/A

Woods, J. D.; Barkmann, W.

doi: 10.1002/qj.49711247102pmid: N/A

The results of two earlier papers on convection in the mixed layer and on the solar heating profile are here introduced into a one‐dimensional model in order to investigate the following consequences of the daily cycle of solar heating in the upper ocean:

Woods, J. D.; Strass, V.

doi: 10.1002/qj.49711247103pmid: N/A

The current profile generated by a steady wind stress is disturbed by the diurnal variation of mixed layer depth forced by solar heating. Momentum diffused deep at night is abandoned to rotate inertially during the day when the mixed layer is shallow and then re‐entrained next night when it deepens. The resulting variation of current profile has been calculated with a one‐dimensional model in which power supply to turbulence determines the profile of eddy viscosity. The resulting variations of current velocity at fixed depths are so complicated that it is not surprising that current meter measurements have seldom yielded the classical Ekman solution. However, the progressive vector diagrams do exhibit an Ekman‐like response (albeit with superimposed inertial disturbances) suggesting that the model might be tested by tracking drifters designed to follow the flow at fixed depths. The inertial rotation of the current in the diurnal thermocline leads to a diurnal jet, the dynamical equivalent of the nocturnal jet in the atmospheric boundary layer over land. The role of inertial currents in deepening the mixed layer is clarified, leading to proposals for improving the turbulence parametrizations used in models of the upper ocean. The model predicts that the diurnal thermocline contains two layers of persistent vigorous turbulence separated by a thicker band of patchy turbulence in otherwise laminar flow.

Reverdin, Gilles; Cadet, Daniel L.; Gutzler, David

doi: 10.1002/qj.49711247104pmid: N/A

The interannual variability of surface observations in the equatorial Indian Ocean is investigated from 23 years of ship reports (1954‐1976). In October and November, during certain years, the monthly analyses show strong wind anomalies in the eastern and central equatorial Indian Ocean. Simultaneously, cloud cover anomalies of opposite signs are observed in the eastern Indian Ocean and in the central Indian Ocean. A linear analysis of heat‐induced circulation suggests that the surface wind anomalies are forced by anomalies of rainfall. This circulation is probably dissipative with time scales in the range two to five days.

Gill, A. E.; Phlips, P. J.

doi: 10.1002/qj.49711247105pmid: N/A

A nonlinear model of the atmospheric response to an isolated region of heating centred on the equator is constructed by expanding in a small amplitude parameter. In order to emphasize nonlinear effects relative to frictional ones, the asymptotics of the small‐friction limit are first derived, and this limiting solution is used as the first term in the expansion with respect to the amplitude parameter. Compared with the linear solution. the nonlinear solution has more inflow from the west at the surface and a reduced easterly outflow at the tropopause. Moreover the pressure is reduced when velocities are large because of the Bernoulli effect. The conclusions are in good general agreement with the numerical results of Nobre and the observational study of Sardeshmukh and Hoskins.

Chen, Shou‐Jun; Dell'Osso, Lorenzo

doi: 10.1002/qj.49711247106pmid: N/A

Some numerical simulations from real data were carried out to examine the response of the monsoon circulation to moist processes. Two cases were selected: one for the monsoon onset and the other for the active (fully established) period. The results show that latent heat is the main feed‐back process in the monsoon circulation. The south‐west monsoon current from the Arabian Sea to the South China Sea in the lower troposphere and the easterly jet stream over southern Asia in the upper troposphere are greatly enhanced by this process both in the onset and active periods. The surface latent heat flux (mostly over the ocean) is important in the maintenance of the circulation during the active period but is less important for the onset.

Carruthers, D. J.; Choularton, T. W.

doi: 10.1002/qj.49711247107pmid: N/A

A cloud microphysical model and a model of airflow over hills are used to examine the microstructure of hill cap clouds. The lifetime of the clouds and the distance travelled by the clouds are both assumed to be sufficiently small so that condensation alone is considered to be important; the coalescence of drops is neglected. The effects of changing (i) the height of cloud base, (ii) humidity fluctuations below cloud base and (iii) incloud turbulence, are described. Process (i) is found to be important, its effect on the cloud microstructure being as great as the effect of changes in the CCN distribution or in the mean wind. Droplet loss to ground, radiative cooling at cloud top and entrainment of dry air are also discussed; these processes may all significantly affect the cloud droplet distribution.

Choularton, T. W.; Consterdine, I. E.; Gardiner, B. A.; Gay, M. J.; Hill, M. K.; Latham, J.; Stromberg, I. M.

doi: 10.1002/qj.49711247108pmid: N/A

In an effort to study aspects of their evolution, simultaneous measurements have been made, in two separate studies, of the microphysical properties of cap clouds over Great Dun Fell, in Cumbria, at two vertically displaced sites along the line of the wind. Dynamical, optical (10.6 γm) and acoustic sounder measurements were also made.

Griggs, D. J.; Choularton, T. W.

doi: 10.1002/qj.49711247109pmid: N/A

Laboratory experiments have been performed to study the strength of rime and vapour‐grown ice crystals to investigate the possibility that fragmentation may occur following collisions in cloud. The results show that fragmentation of rime is very unlikely to occur in natural clouds. Vapour‐grown crystals are found to be considerably more fragile, with dendrites the weakest crystal habit. In the experiments one or two microscopic fragments were produced. Collisions between dendritic crystals and graupel particles may be able substantially to enhance ice crystal concentrations in some clouds.

De Baas, Anne F.; Van Dop, Han; Nieuwstadt, Frans T. M.

doi: 10.1002/qj.49711247110pmid: N/A

Dispersion in one dimension is simulated by the Langevin equation dW = −(W/TL)dt + dμ, where W is the velocity of the particle (hypothetical fluid element), TL the Lagrangian time scale and dμ the random velocity increment induced by forces exerted by the turbulence on the particle during dt. The moments of dμ in the Langevin equation in inhomogeneous conditions can be determined, by requiring that for large times the density distribution of the particles is the same as that of the air. In our numerical experiment the Langevin equation with the above‐defined moments is applied to diffusion in the convective boundary layer. Profiles of the moments of the vertical turbulence velocities, U 3n(z), n = 1, 2, 3, are based on measurements and scaled by convective scaling; TL is assumed constant. Particles are released at several heights, with an initial velocity distribution that has the same moments as the Eulerian turbulence velocity distribution at that height. At the boundaries reflection conditions are imposed. Our results are extensively compared with water‐tank experiments of Willis and Deardorff, wind‐tunnel experiments of Poreh and Cermak, field experiments by Briggs and a model of Baerentsen and Berkowicz. The mean height and variance of the particles, the concentration field as a function of down‐wind distance and height, and ground level concentrations are presented. They agree very well with observations of dispersion in the convective boundary layer.