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M. Garstang

Progress in hurricane and tropical meteorology is reviewed over the period 1960–1971. The recognition of the role of scale-interaction; the necessity to include energy sources and sinks in the general circulation models; the complexity of the problem of subgrid scale processes; the growth of a quantiative observational base in the tropics; the advent of the meteorological satellite; and the power of the computer have collectively led to great effort and considerable progress in the realm of tropical meteorology during this period. This progress and the areas of weakness are presented within a framework of spatial and temporal scales ranging from the planetary to the turbulent. Without minimizing the difficulties that lie ahead there is reason for a degree of optimisim. A coherent picture of the tropical atmosphere is emerging. Theory and observations are finding common ground. Critical tests of both lie ahead in the first major experiment of the Global Atmospheric Research Program: the GARP Atlantic Tropical Experiment.

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Ward R. Seguin and M. Garstang

Abstract

Profiles of temperature and specific humidity and derived thermodynamic variables are presented, based on aircraft soundings of the tropical oceanic subcloud layer during the 1968 Barbados Experiment. The profiles show the results of strong cloud-subcloud layer coupling. In the vicinity of intense cumulus convection, the subcloud layer is strongly stabilized and diluted by cumulus clouds and the areas extent of this modification is large. The implications are that in this region the turbulent vertical transport of latent and sensible heat energy and the small-scale turbulence responsible for the transport are suppressed.

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Gary M. Barnes and Michael Garstang

Abstract

The thermodynamic modification of the subcloud layer in the GATE area is shown to be a function of precipitating convection. A critical rate of 2 mm h−1, based on the Z – R relationship, in conjunction with 4 km × 4 km scale 15 min mean radar maps, distinguishes between evaporation of precipitation in the subcloud layer (no change in moist static energy h) and vertical mass transport associated with penetrative downdrafts (decreases in h) into this layer from near and above cloud base. The spatial extent of the outflow of the active downdrafts is limited to a convective-mesoscale area directly under and as much as 15 km downwind of the precipitation causing the change. A more extensive wake region occurs on the upwind side of the precipitating region.

The initial thermodynamic environment directly affects energy transport per unit mass by moist convection. Precipitating cells which operate upon an initially undisturbed atmosphere cause a net transfer of 60% more energy per unit mass than those convective clouds which operate upon regions previously modified by precipitation and downdrafts. Results suggest that large, linearly shaped, moving cloud lines are the centers of the most efficient energy transfer per unit mass.

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W. R. Seguin and M. Garstang

Large scale experiments planned for the future as part of the Global Atmospheric Research Project (GARP) will call upon measurements taken from all categories of ocean vessels.

During the 1968 Barbados Experiment meteorological measurements were made aboard the USCGSS Discoverer using a variety of sensors and methods typical of many ocean vessels. A comparison of the Discoverer data systems provides an estimate of the inherent error in some shipboard measurements. It is demonstrated that under unstable stratification, wind speed differences from anemometers mounted at separate locations and heights are small while under neutral conditions large differences appear. Unless special precautions are taken, heating of sensors due to solar radiation and ship effects may be as large as 2C. Such an error in dry bulb and wet bulb temperatures with a mean wind speed of 6 m sec−1 yields a 100% error in the sensible heat flux and a 50% error in the latent heat flux calculations. Nomograms of sensible and latent heat flux are constructed to demonstrate the manner in which such errors influence these quantities.

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K. L. Echternacht and M. Garstang

Abstract

Results of the analysis of measurements of temperature, mixing ratio, and horizontal wind speed are presented. The data were collected using a tethered ballon system during the 1969 BOMEX during periods of undisturbed and disturbed weather conditions. The results show that substantial changes occur in the character and stratification of the lower subcloud layer from the undisturbed to disturbed states. The lapse rate, height of the mixed layer, and estimated surface fluxes under disturbed conditions are to be obtained from the observations. These values are applied to currently suggested parametric relationships of the mixed layer under undisturbed conditions. Totally unrealistic values of vertical motion are obtained. It is concluded that convective cloud scale downdrafts driven by evaporation are extremely important in determining the budget of the mixed layer under disturbed conditions.

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R. D. Diab and M. Garstang

Abstract

A two-dimensional numerical model is used to predict near surface wind velocities, and consequently wind power, for five distinct synoptic regimes for contrasting east and west coasts of South Africa. The model results suggest that no one solution for optimizing a wind generator location would satisfy both coastlines owing to their differing characteristics. On the east coast, the most favorable location is approximately 55 km inland on the high-lying terrain, whereas on the west coast, the best location is generally about 10 km offshore. The application of the two-dimensional form of a primitive equation model in areas where the topography and the coastline are in accordance with model constraints is shown to provide a useful framework for measurements and analysis of the potential wind power of such a region.

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H. J. Cooper, M. Garstang, and J. Simpson

Abstract

Surface wind measurements taken during the summers of 1973 and 1975 in the Florida Area Cumulus Experiments (FACE) fine-mesh networks are used to calculate surface divergence on the convective scale and mesoscale.

Examination of the daily time series of divergence averaged over a 30 km × 25 km grid showed that on days with deep convective activity over and around the network, a definable sequence in the network-averaged surface wind divergence is observed. The sequence consists of five stages: persistent convergence, peak convergence, non-divergence, peak divergence and the return to a non-divergent state.

Use is made of observations of individual cases and of radar composites built around the above sequence in the network-averaged divergence fields to demonstrate that a convective-scale feedback mechanism consisting of outflows from previous storms triggering new growth is frequently observed during both experimental periods.

It is shown that the observations taken over the FACE 1975 network are consistent with the model results of Pielke (1974) and estimates of peninsular-scale convergence. Comparison of the convective-scale vertical transports over the network at the near surface (4 m) level with the peninsular-scale forcing reveals a relationship between the two. After the initiation of convective processes by the peninsular-scale forcing, downdraft-induced convergence maintains and intensifies the convective-scale activity long after the peninsular-scale forcing has passed its peak. The daily cycle of initiation, intensification, and the eventual decline of convective activity is related to the rate of change of the peninsular-scale divergence.

The links established between the various scales are of fundamental importance to the understanding of the initiation, maintenance, and decay of deep precipitating convection and to its theoretical parameterization.

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J. Levine, M. Garstang, and N. E. LaSeur

Abstract

A 3-hr flight of a constant-volume balloon which is entrained into a cumulus congestus cloud and lifted to an altitude of nearly 3 km provides an opportunity to determine the trajectory and vertical velocity of the balloon and, in turn, relate these to the circulation of the cloud. Simultaneous supplementary observations by radar, cameras, satellite, and other ground based instruments provide the meteorological context for the event. Computations show that the vertical velocities of the cloud may approach 10 m sec−1, and sustained upward velocities of 3–5 m sec−1 are maintained for considerable periods of time and height. Comparison with a “jet” model shows remarkable agreement, while the “bubble” model would require unreasonably large values of the radius to generate the observed vertical velocities.

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P. D. Tyson, M. Garstang, and R. Swap

Abstract

Kinematic air parcel trajectory analysis is used to determine patterns of horizontal air transport in 2000 km × 2000 km areas over southern Africa. From these, composite zonal and meridional transport fields are derived for the subcontinent to estimate the extent to which recirculation of air and aerosols may take place in the lower troposphere between the surface and 500 hPa. The nature and degree of recirculation beneath the persistent 500-hPa absolutely stable layer is demonstrated, and transport by recirculation in discrete streams is shown to constitute 44% of the total transport over the region.

From a determination of air volume fluxes and estimates of aerosol concentrations, the total mass flux of aerosols by direct transport and by recirculation in conditions during which semipermanent, subtropical, continental anticyclones prevail is estimated to be about 51 Mton yr−1 in the surface-to-hPa layer. Recirculation comprises approximately 22 Mton yr−1 of this amount. Of the recirculated transport, about 5 Mton yr−1 is recirculated to the west in easterly transport and 17 Mton yr−1 to the east in westerly transport.

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K. L. Warsh, K. L. Echternacht, and M. Garstang

Abstract

Currents at 25 m at 13.14N, 59.20W were measured from 2 July 1968 through 30 August 1968. The current meter was attached to a stable spar buoy and the position of the buoy monitored with radar based on Barbados. Mean currents were 20 cm set−1. Current direction varied from westerly, to southerly, and then to northerly over the period. Strong semidiurnal current variations were observed. Oscillations corresponding to the inertial period for this latitude are clearly observed; the intensity of these oscillations is modulated at a period near four days, corresponding to a similar near-periodicity in atmospheric disturbances. Measurements and analysis suggest an interaction between variations in the basic ocean surface currents and impulses due to identifiable atmospheric systems.

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