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Harrison E. Cramer

Abstract

The principle of dynamic entrainment is applied to the turbulent-mixing problem. With use of the velocity fluctuations characteristic of turbulent flow and the equation of continuity, an expression is derived for the mass exchange between an accelerated fluid and its environment, under steady-state conditions. A two-stage diffusion mechanism is postulated, in which it is held that the dilution and spreading of aerosols occur in consequence of the inflow and outflow required by continuity in regions of accelerated motion. It is assumed that the inflow and outflow are orderly, that the atmosphere is incompressible, that changes in density are negligible, and that the entrained air is uniformly mixed with the aerosol. Several elementary models are described, and the results obtained by numerical integration in selected cases are discussed. It is indicated that the turbulent-mixing process depends upon the scale of the velocity fluctuations, and upon the ratio of the amplitude of these fluctuations to the mean wind speed.

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D. E. Harrison

Abstract

The monthly mean surface wind changes during recent ENSO events, as observed from 11 islands in the tropical Pacific, are described. Two different composite ENSO wind fields are evaluated and compared. The month-to-month wind changes during each event are also discussed.

The wind changes for each event between 1953 and 1980 except 1969 show several common features:

(i) Westerly anomalies appear first west of the date line and then at the date line sometime in summer (0) to fall (0), then intensify over the following several months. The anomalies are confined to within ±3° of the equator during this stage.

(ii) In either November (0), December (0), or January (+1) there is an abrupt southward shift of the narrow band of westerly anomalies, so that the maximum anomaly is then at ∼5°S latitude at the date line, and nearly normal conditions prevail north of the equator.

(iii) Westerly anomalies are gone or greatly reduced one to two months after the southward shift.

The event-to-event variations are considerable, particularly prior to July (0) and after February (+1), so that composites show much reduced anomaly amplitude and much smaller month-to-month anomaly changes than are typical of any given event. The large amplitude months of the composites show similarities with a composite by Rasmusson and Carpenter, but a number of significant differences are identified. These findings, and their relationship to existing simple ideas concerning tropical Pacific coupled ocean-atmosphere interactions, are discussed.

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D. E. Harrison

Abstract

Using a version of the global surface marine observation historical data set, a new 1° spatial resolution global ocean surface wind stress climatology has been evaluated using the Large and Pond surface drag coefficient formulation. These new results are compared, after spatial smoothing, with those of Hellerman and Rosenstein, who used a different drag coefficient form. It is found that the new stresses are almost everywhere smaller than those of Hellerman and Rosenstein, often by 20%–30%, which is greater than the formal error estimates from their calculations. The stress differences show large-scale spatial structure, as would he expected given the spatial variation of the surface stability parameter and the known different wind variability regions. Basin zonally averaged Ekman transports are computed to provide perspective on the significance of the stress differences; annual mean differences can exceed 10 Sv (Sv = 106 m3 s−1) equatorward of 20° lat, but are smaller poleward. Wind stress curl and Sverdrup transport calculations provide a different perspective on the differences; particularly noticeable differences are found in the regions of the Gulf Stream and Kuroshio separation. Large annual variations in midlatitude wind stress curl suggest that study of the forced response at annual periods should be of interest.

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D. E. Harrison

Abstract

The question of the importance of mesoscale motions in the long time averaged ocean circulation is examined from the viewpoint offered by Eulerian scale estimates of the magnitudes of the explicit eddy and largest inviscid mean flow terms in the mean heat, momentum and vorticity equations. Comparisons of these estimates reveal the quantities that must be known to obtain reliable estimates of the importance of eddy terms in the mean balances. Using historical information and long time series of data from the western North Atlantic, two distinct regimes (“near field” and “mid-ocean”) are identified for this ocean region and the appropriate term comparisons are made for each regime. From estimates of the reliability of the ocean values used in these comparisons the robustness of the comparisons is examined. The momentum and vorticity equation estimates suggest that terms based on the eddy Reynolds stress can generally be neglected compared to terms involving f 0 and β in both the near field of the Gulf Stream and the mid-ocean. In the near field, mean advective terms appear to be at least as important as the eddy terms, but the eddy terms dominate these advective ones in the mid-ocean. The heat equation comparisons suggest that the eddy term is comparable to the mean horizontal advection of heat in the mid-ocean but is of somewhat reduced importance in the near field. Some remarks on the generality of results from numerical ocean models that contain mesoscale motions to the question of eddy importance in the ocean are offered.

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D. E. Harrison

Abstract

The importance of mesoscale eddies in the basin energy budgets of closed-basin numerical model oceanic systems that attempt to resolve such motions varies greatly from calculation to calculation. In existing calculations, eddy importance has been found to depend strongly on the dissipation mechanism(s) selected. These energy budget results can be understood by examination of how eddy and mean flow kinetic energy are dissipated in the long-time mean in the different regions of the model flow. Scale analysis arguments are presented, assuming that the characteristics of the flows satisfy certain mild quasi-oceanic constraints, to investigate these dissipation terms. From these scale. estimates it appears that many of the model ocean results can he understood in terms of a nondimensional parameter that measures the relative importance of horizontal and bottom friction dissipation. When horizontal friction dissipation dominates, eddies can only be of modest importance in basin energy budgets, but when bottom friction dissipation dominates, eddies generally must be important. This follows simply from the assumed flow characteristics. The implications of these results on the interpretation of present modeling results are described.

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Harrison E. Cramer
and
Frank A. Record

Abstract

Power spectra of the eddy-velocity components have been determined at four levels within the layer from 2 to 12 meters under varying conditions of mean wind speed, trajectory and thermal stability. A filtering technique suggested by J. W. Tukey has been used to obtain rough estimates of contributions to the total variance for seven consecutive frequency intervals within the range from about 0.5 to 0.005 cycles per second. At the higher frequencies studied, variances for all three components are approximately equal and equipartition of turbulent energy is thus indicated. Spectra for the u- and v-components appear to be invariant with frequency at the lowest level, and tend to increase slowly with decreasing frequency at the higher levels. The w-spectra at all levels fall off sharply with decreasing frequency, contributions to the vertically-directed energy becoming almost negligible at the lowest frequencies investigated.

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Harrison Woodson Bowles
and
Sarah E. Strazzo

Abstract

Florida’s summertime precipitation patterns are in part influenced by convergence between the synoptic-scale wind and local sea-breeze fronts that form along the east and west coasts of the peninsula. While the National Weather Service previously defined nine sea-breeze regimes resulting from variations in the synoptic-scale vector wind field near Tampa, Florida, these regimes were developed using a shorter 18-yr period and examined primarily for the purposes of short-term weather prediction. This study employs reanalysis data to develop a full 30-yr climatology of the Florida sea-breeze regime distribution and analyze the composite mean atmospheric conditions associated with each regime. Further, given that 1) the synoptic-scale wind primarily varies as a result of movement in the western ridge of the North Atlantic subtropical high (NASH), and 2) previous studies suggest long-term shifts in the mean position of the NASH western ridge, this study also examines variability and trends in the sea-breeze regime distribution and its relationship to rainy-day frequency over a longer 60-yr period. Results indicate that synoptic-scale flow from the west through southwest, which enhances precipitation probabilities along the eastern half of the peninsula, has increased in frequency, while flow from the east through northeast has decreased in frequency. These changes in the sea-breeze regime distribution may be partially responsible for increases in rainy-day frequency during June–August over northeastern Florida, though results suggest that other factors likely contribute to interannual variability in precipitation across the southern peninsula.

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Henry G. Houghton
and
Harrison E. Cramer

Abstract

It is held that entrainment is a necessary dynamic consequence of the vertical stretching of an accelerated convective column. On this basis, equations are developed for the rate of entrainment, the vertical divergence and the lapse rate, for both unsaturated air and cloud air. It is assumed that a steady state exists, the cross section of the rising column is invariant with height, the entrained air is uniformly mixed with the rising air and the environment is at rest. The equations are integrated numerically over height in a number of selected cases. In unsaturated air, entrainment results in a lapse rate which is always greater than the dry adiabatic; if the environmental lapse-rate is superadiabatic, the lapse rate of the rising air is intermediate between the lapse rate of the environment and the dry adiabatic lapse-rate. In a cloud, entrainment results in a lapse rate intermediate between the environmental lapse-rate and the moist adiabatic lapse-rate. The lapse rate of the rising air increases as the relative humidity of the environment decreases. As a result of entrainment, the cloud liquid-water content increases with height at a significantly slower rate than would result from a simple lifting process. A decrease in the humidity of the environment reduces the rate of increase of liquid water with height, but it does not appear possible to “dry out” the cloud even in a dry environment. The horizontal velocity-convergence is found to be of the order of 10−3 sec−1, and the computed vertical velocities in the cloud are in general agreement with those observed by the Thunderstorm Project. It is pointed out that the entrained air may be added through an ordered inflow, by turbulent exchange or by a combination of the two. It is assumed here that an ordered inflow occurs.

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David S. Gutzler
and
D. E. Harrison

Abstract

The longitude–height–time evolution of seasonally averaged wind anomalies over the near-equatorial eastern Indian and western Pacific Oceans is examined, using multiyear time series derived from a network of eight rawinsonde stations. Data at six pressure levels, between 850 and 150 mb, are considered. The first two modes of an empirical orthogonal function analysis of zonal wind fluctuations are cross correlated at lag, with spatial structures suggesting that the dominant pattern of variability on seasonal time scales is best described as a propagating oscillation. This space–time structure is confirmed using a complex empirical orthogonal function analysis, which indicates that over half of the interseasonal zonal wind variance at these stations is associated with an eastward-propagating mode (denoted E1). Wind anomalies described by E1 are negatively correlated in the upper and lower troposphere at each station, and are out of phase between the southern tip of India and the central Pacific, so that E1 can be interpreted as an eastward-propagating pattern of convergence/divergence along the equator. Variations in the phase of this mode are “phase-locked” to the annual cycle, and are highly correlated with a conventional Southern Oscillation Index. The wind anomaly field described by E1 evolves through a characteristic life cycle during El Niño events, which begins before the onset of ocean surface warming in the eastern Pacific; the anomaly pattern then propagates eastward during the course of the event.

These results are further confirmed by compositing wind anomalies with respect to the phones of the six most recent El Niño events. During the Northern Hemisphere autumn season prior to the onset of El Niño, anomalous low level convergence and upper level divergence are observed in the vicinity of Indonesia. This pattern subsequently propagates eastward, until the opposite pattern of anomalies is observed during the fully developed phase of El Niño, one year after the initial appearance of the atmospheric anomaly pattern. The eastward phase speed is much slower than an atmospheric Kelvin wave, suggesting that the wind anomalies are part of an air–sea interactive system.

The interevent variability for each phase of the six El Niño events in the data record is substantial; the significance of the composite anomaly pattern varies considerably from phase to phase. The composite is most robust for the Northern Hemisphere autumn season during the year in which ocean surface warming first occurs. It is particularly noteworthy that the evolution of wind anomalies over the far western Pacific prior to the 1982 event was not significantly different from previous events.

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D. S. Luther
and
D. E. Harrison

Abstract

The utility of studying low-frequency surface weather phenomena with long time series of meteorological data from tropical Pacific islands is demonstrated. The wind stress changes associated with El Niño events in the period 1950–78 are examined at seven locations. Zonal wind stress anomalies at the equator near the date line often exhibit strengthening and subsequent weakening of the trade winds prior to each El Niño, as originally suggested by Wyrtki. An exception is the weak 1963 El Niño, which is preceded by meridional wind stress anomalies at the equator. The strongest zonal and meridional wind stress anomalies, however, occur well after the first occurrence of anomalously warm water off the coast of Peru for each El Niño, in agreement with prior analyses of merchant marine data. Away from the equator, variability of the wind stress anomalies from one El Niño to the next is strong, leading to numerous discrepancies with published profiles of the “mean” El Niño wind changes.

Power spectra of wind stress from three island stations are compared with concurrent wind stress spectra computed from merchant marine data. Many disparities are found and can be attributed to (sometimes severe) aliasing in the ship data. Possible aliasing errors in the ship data time series are estimated by randomly subsampling the island data in order to mimic the ship data sampling. Sampling criteria, which depend upon the scientific application, are suggested in order to limit the alias noise in the ship data to acceptable amounts.

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