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Gary K. Greenhut

Abstract

After reanalyzing the potential temperature data of Peixoto (1958, 1960), a criterion is derived for locating the mean latitude of the subtropical high over West Africa. Using this criterion, it is found that the subtropical high tends to move to higher latitudes, and therefore the amount of rainfall in the Sahel tends to increase, as the concentration of carbon dioxide and particulate matter in the atmosphere increases. We conclude that increasing amounts of atmospheric pollutants are probably not the cause of the severe drought experienced in the Sahel in the early 1970's. This conclusion contradicts that of Bryson (1973) who used the so called Z criterion to predict an opposite response of the subtropical high to changes in the concentration of atmospheric pollutants.

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Gary K. Greenhut

Abstract

No abstract available.

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Gary K. Greenhut

Abstract

Lead and lag correlation coefficients are obtained between sea surface temperature and rainfall in the B-scale region during GATE. Significant negative correlation is found for 1-day lag and positive correlation for 1-day lead of sea surface temperature, implying negative feedback with a response delay of about 1 day. Mechanisms that may contribute to the correlations are discussed.

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Gary K. Greenhut

Abstract

Using the temperature departure data of Angell and Korshover (1978), models relating the movement of the Intertropical Convergence Zone (ITCZ) to hemispheric temperature gradients are compared with rainfall data from the Sahel. The Northern Hemisphere models (Bryson, 1973, 1974; Greenhut, 1977), in general, do not correlate with rainfall. Rainfall is well correlated with meridional temperature gradients taken with constant vertical lapse rate—a result which parallels that of Korff and Flohn (1969) in a study of the motion of the subtropical high. The Southern Hemisphere model relating temperature gradients to ITCZ motion (Kraus, 1977) also is not correlated with rainfall in the Sahel. However, highly significant correlation between rainfall and summer-winter change in the Southern Hemisphere meridional temperature gradient with the latter lagged by 0.5–1 year is evidence that the cross-equatorial transport of energy may be driven to some extent by the movement of the ITCZ in the Northern Hemisphere.

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Gary K. Greenhut and Giangiuseppe Mastrantonio

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Doppler sodar wind measurements made in light wind conditions in September 1979 near a power plant in Turbigo, Italy, are used to derive terms in the turbulence kinetic energy (TKE) budget. Measurements on five days are grouped into two classes: fair-weather conditions and boundary layer interaction with subsiding air. Profiles of σ3 w/z are used to obtain the surface heat flux and the vertical velocity scaling parameter, as well as the buoyancy production term in the TKE budget. The vertical transport of TKE is derived from the profiles of (w′)3. The horizontal components are approximated using a parameterization based on the data of Lenschow et al. Dissipation in the TKE budget is obtained from a spectral analysis of the Doppler sodar data in the inertial subrange. The resulting TKE budget profiles for the two classes are compared with budgets obtained previously, using a number of different methods.

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Siri Jodha Singh Khalsa and Gary K. Greenhut

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Data from two aircraft flights in the marine atmospheric boundary layer are analyzed in an investigation of entrainment processes at the top of the well-mixed layer. Both days had strong wind shear across the inversion, which is reflected in the horizontal wind statistics. Conditional sampling is used to show that near the top of the mixed layer most updrafts are cool, moist and deficient in along-wind momentum (slow) with respect to their environment. About half of these updrafts are still positively buoyant. Downdrafts that are warm, dry and have an excess of along-wind momentum (fast) occupy the greatest area of any downdraft type. Most of them are positively buoyant. Also found near the top of the mixed layer are large numbers of warm/dry/fast updrafts and cool/moist/slow downdrafts, i.e., drafts that have overturned. Time series from this level reveal large masses of cool/moist/slow and warm/dry/fast air, usually containing both upward-moving and downward-moving elements. These observations are related to features seen in returns from clear convective boundary layers, using various remote sensing systems.

Conditional sampling is also used to determine the net buoyant production or consumption of turbulence kinetic energy by each convective element. The results support the process partitioning method of entrainment closure but deviate from the assumption of linear buoyancy flux profiles for each proem.

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Gary K. Greenhut and Siri Jodha Singh Khalsa

Abstract

The properties of updrafts and downdrafts through the entire depth of the marine atmospheric boundary layer are investigated using conditional sampling based on an indicator function derived from the vertical velocity time series. Statistics on event size, number density and area occupied are obtained, along with conditional averages of the meteorological variables and percent contributions to the fluxes by updrafts, downdrafts and the environment. A single profile is obtained for convective mass flux based on conditional averaging of updrafts and downdrafts applicable to the fluxes of latent and sensible heat and momentum.

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Gary K. Greenhut and Siri Jodha Singh Khalsa

Abstract

Conditional sampling has been applied to aircraft turbulence measurements in order to study updrafts and downdrafts over the central equatorial Pacific Ocean. Average event size, number density and proportion of time series occupied are obtained for the drafts, along with conditional averages of horizontal momentum, moisture and virtual temperature and the draft contributions to the total fluxes. It is found that updrafts are usually cool/moist and warm/moist while downdrafts are most often warm/dry. Convective mass flux parameterizations of the sensible and latent heat flux are tested. Results consistent with previous workers are obtained when the flux production is mainly by updrafts. However, when downdrafts dominate, they must be taken into account explicitly in order for the parameterization to be accurate.

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Siri Jodha Singh Khalsa and Gary K. Greenhut

Abstract

The properties of updrafts and downdrafts in the lower third of the marine atmospheric boundary layer (MABL) over the central Pacific Ocean are investigated using a conditional sampling technique. When the drafts are classified according to their heat and moisture content, the properties of the major classes (moist updrafts and dry downdrafts) are in agreement with a parcel displacement model of vertical mixing. The minor class events appear to be the result of the reversal of motion of the major class events. Drafts that consume turbulent kinetic energy (TKE), although small in number in the lower third of the MABL, have spatial scales comparable with drafts that produce TKE. At the lowest level, the area occupied by positively buoyant downdraft exceeds the area occupied by negatively buoyant updrafts by a factor of 2. Updrafts and downdrafts produce a large fraction of the total fluxes of heat moisture and momentum, increasing from 75% at 0.07zi to 85% at 0.32zi. Of the draft contribution to the fluxes, 95% is due to the mean properties of the events and only 5% is due to the correlated fluctuations within the events. A convective man flux parameterization, based on the mean conditions within updraft and downdrafts, is obtained for the lower third of the mixed layer.

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Tom Beer, Gary K. Greenhut, and S. E. Tandoh

Abstract

We use radiosonde data from Tamale in northern Ghana to evaluate the Z criterion latitude ϕz. We find it correlates well with other Hadley cell parameters since its variations are apparently controlled by the meridional movement of the subtropical westerly jet. Since the Hadley cell parameters correlate well with Tamale rainfall, we conjecture that it should be possible to predict northern Ghana rainfall on the basis of the southward extension of the subtropical jet once more data have been collected by the Tamale radiosonde. This conjecture is supported by data from other stations in West Africa.

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