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Walker, Julia; Rowntree, P. R.
doi: 10.1002/qj.49710343503pmid: N/A
The sensitivity of a tropical model to soil moisture content has been investigated using a simplified version of the UK Meteorological Office 11‐layer model. the model area represented a zonally‐symmetric version of West Africa. the results of one experiment (D) with a desert in the same latitudes as the Sahara were compared with those of another experiment (W) in which the desert was replaced by moist land. the initial fields contained an idealized easterly wave which developed during the experiments to form depressions whose movement, structure and associated rainfall were strongly influenced by the dryness of the underlying surface. the energetics of these disturbances were analysed and two distinct mechanisms for maintaining their eddy kinetic energy are proposed, depending on the soil moisture.
Simpson, J. E.; Mansfield, D. A.; Milford, J. R.
doi: 10.1002/qj.49710343504pmid: N/A
The advance inland of the sea‐breeze past Lasham, 45 km from the south coast of England, has been measured over a period of twelve years. In this time 76 sea‐breeze fronts passed Lasham, 36 passed Reading and 12 reached Harwell, 85km from the coast. the rate of advance from Lasham to Harwell in calm or light offshore winds was close to 3 ms−1, and the latter station was passed after sunset at a mean time of 2105 GMT. With onshore winds fronts only appear 30 or 40 km inland, and some fronts may be modified by effects due to the east coast. In some cases detailed measurements were made using pilot balloons, an instrumented motor‐glider and radar. the fronts were shown to have raised heads about twice the depth of the following flow and within the heads dry patches occur. Variable secondary flows are inferred and comparison with water‐tank models shows that several dynamical features in the tank and in the atmosphere are similar. Measurements support the picture of the development of the front during the day as a gravity current, and theoretical rates of advance are deduced. the observed acceleration of the front in the late afternoon is explained in terms of the increased temperature contrast at the front due to the decrease in solar heating of the sea‐air. Shortly before sunset, a sea‐breeze vortex may be detached from the rest of the flow. In a case study, it is shown that friction at the upper surface is at least five times that at the ground, and heat and humidity budgets are given for the detached vortex.
Hastenrath, Stefan; Heller, Leon
doi: 10.1002/qj.49710343505pmid: N/A
Departure characteristics of the large‐scale circulation are studied in relation to extreme drought and flood years in northeastern Brazil identified from a collective of long‐term rainfall stations and series of river discharge. Ship observations during 1911–72 compiled with a one degree square resolution and extending between 30°N and 30°S from the African coast to the eastern Pacific form a major observational basis.
doi: 10.1002/qj.49710343506pmid: N/A
Green's parameterization of the latitudinal flux of zonal momentum by large‐scale eddies is investigated. In a steady‐state, β‐plane model of the zonal mean troposphere a simple basis for estimating transfer coefficients (eddy diffusivities) leads to a satisfactory representation of this large‐scale momentum flux, as indicated by the surface zonal flow. the simple basis appears inappropriate for systems in which the β‐effect is not dominant. In a spherical polar, quasi‐geostrophic model of the zonal mean troposphere an adaptation of the method leads to less satisfactory results. It is clear that the spatial variation of the transfer coefficients is more important than in the β‐plane case; and a more precise way of estimating this variation is desirable. Barotropic/baroclinic stability analyses seem likely to provide the most reliable basis, but their use would amount to an essentially different parameterization technique: less elaborate methods may be adequate.
doi: 10.1002/qj.49710343507pmid: N/A
Analysis of profiles measured over irrigated cotton in the Sudan Gezira showed that the crop boundary layer remained stable throughout most of the day. The shape of wind profiles measured within the canopy suggested that momentum was absorbed mostly in the upper layers of the canopy, with the lower regions remaining isolated from the microclimate above. A similarity analysis based on dynamic scaling factors yielded a generalized wind profile from which momentum diffusivities and mixing lengths were calculated. The analysis showed that aerodynamic features of the upper layers of the canopy were characterized by the friction velocity and the height of the zero‐plane displacement.
doi: 10.1002/qj.49710343508pmid: N/A
The separate effects of fluid inertia and friction at the lower boundary on the flow of a rotating, stably stratified, incompressible fluid over a three‐dimensional shallow isolated obstacle are investigated.
Tsui, K. S.; Bell, G. J.; Fung, P. C. W.
doi: 10.1002/qj.49710343509pmid: N/A
Satellite and aircraft data indicate an approximate 0–64 probability that the intensity of a typhoon will have increased during the 24 hours prior to the time when a plume of middle and high cloud streaming from the centre of the storm towards the northeast is first seen on satellite photograph.
doi: 10.1002/qj.49710343510pmid: N/A
Frequent and detailed upper air observations at Laverton (38°S) for 30 days during September/October 1966 are analysed to determine the local horizontal eddy flux of heat and the mean horizontal potential temperature gradient. Components in the meridional and zonal directions are studied, but a more meaningful relation emerges when the vector eddy flux is considered in relation to the vector temperature gradient. the angle, δ, between these two vectors, and the down‐gradient and cross‐gradient exchange coefficients, K and R, are derived. A 3‐tiered structure between the surface and 30 km emerges, with down‐gradient flux (K positive) in most of the troposphere and mid‐stratosphere, and counter‐gradient flux (K negative) in the lower stratosphere. the cross‐gradient flux is in the direction of the thermal wind (R positive) at nearly all heights except in the lowest layers and between 20 and 24 km. δ exhibits a fairly steady and systematic change throughout the height range analysed (2–28 km). Computations using standard upper air observations at selected Australian stations for 1970 show a mean annual pattern which is remarkably coherent with latitude (10–55°) and height (0–22km), except in the tropical troposphere. Typical derived mid‐latitude values for 1970 are: \documentclass{article}\pagestyle{empty}\begin{document}$$ \begin{array}{*{20}c} {} & {K(10^9 {\rm cm}^{\rm 2} {\rm s}^{{\rm ‐ 1}})} & {R(10^9 {\rm cm}^{\rm 2} {\rm s}^{{\rm ‐ 1}} } & {\delta ^0 } \\ {{\rm Lowerstratosphere}} & {{\rm ‐ 10}} & {{\rm 10}} & {{\rm 45}} \\ {{\rm Uppertroposphere}} & {{\rm 10}} & {{\rm 20}} & {{\rm 116}} \\ {{\rm Lower troposphere}} & {{\rm 20}} & {{\rm ‐ 10}} & {{\rm 206}} \\ \end{array} $$ \end{document}
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