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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 E. Cramer
and
Frank A. Record

Abstract

Direct measurements have been made of the vertical flux of heat and momentum in the layer from 2 to 12 meters. Eddy velocities were obtained from hot-wire anemometers and light bivanes, mounted at four levels; temperature fluctuations were measured with fast-response thermocouples, mounted at three levels. Data were recorded by taking photographs of indicating dials, at the rate of one exposure per second. Six sets of data have been analyzed, each set corresponding to one period of observation approximately 10 minutes in length. In four sets of data, the flow was over a rough land surface; in one set, the flow came directly from a water (ocean) surface; in the remaining set, the flow was principally over water except for a short land trajectory immediately upwind from the point of observation.

The flux data show a maximum variation from two- to four-fold within the layer. Over land, the shearing stress tends to decrease with height during the day and to increase with height at night; over water, both the heat flux and the momentum flux tend to decrease with height and also are significantly smaller at all levels than over a land surface. Values are presented for the coefficients of eddy viscosity K m, eddy conductivity K h, surface drag C d, and for von Kámán's constant K. For a land surface, the eddy coefficients are approximately equal near the ground; K m increases with height at a slower rate than K h during the day, while at night the reverse is true. Over a water surface, K m is considerably larger than K h at all levels. The values for c d are in good agreement with previous estimates, based on less direct measurements. The rather complicated variation of k with stability and height is discussed; the average of all determinations is approximately 0.4.

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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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D. E. Harrison
and
Paul S. Schopf

Abstract

The initial surface warming of the 1982 El Niño event was of quite different timing and pattern from that associated with most El Niño events; strong anomalous warming occurred first in July along the equator and subsequently along the South American coast. We show here that a simple advective model for tropical ocean surface warming can produce anomalous sea-surface temperature (SST) fields like those found in the first few months of the 1982 El Niño. The model physics assumes that the existing SST field is advected by anomalous currents to produce the anomalous warming, and that the anomalous currents are those induced subsequent to the passage of downwelling Kelvin wave front(s). With the initial SST field taken to be that of July 1982, the anomalous eastward currents of the model lead to a satisfactory prediction of the evolution of anomalous SST for several months. Numerical experiments with a fully nonlinear and thermally active ocean model support the physical relevance of the more idealized study.

The anomalous horizontal advection model can also account for the initial SST evolution during the more common type of El Niño event. The reason that a similar anomalous current field can produce two such different warming patterns is that the gradients of SST along the equator have strong seasonal variation. If anomalous eastward currents are generated along the equator between February and April, when the climatological zonal SST gradient is small, little equatorial warming will occur and so coastal warming is observed first; this is the case in most El Niño events. But if the same anomalous currents occur later in the year, when there is typically a strong zonal temperature gradient, strong equatorial surface warming will occur prior to coastal warming, as happened in 1982. The pattern of SST changes resulting from remote westerly wind changes in the tropical Pacific thus is very strongly linked to the annual cycle of SST.

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D. E. Harrison
and
D. S. Gutzler

Abstract

We examine the variability of monthly mean winds at 850 mb and the surface from five island stations in the tropical western Pacific Ocean. Climatological winds and (850 mb-surface) wind shear are evaluated and used to construct time series of monthly mean wind and shear anomalies. Wind variance at 850 mb tends to be substantially greater than at the surface, and large temporal variations in shear are found. Prominent anomalies are associated with El Niño–Southern Oscillation periods. Composite El Niño event anomalies are examined; it is found that the westerly wind anomalies associated with warm central Pacific sea surface temperatures are stronger at 850 mb than at the surface, and that the anomalous (850 mb-surface) shears are as large as the surface wind anomalies themselves.

Several simple techniques are described to investigate the feasibility of estimating surface wind anomalies from 850 mb wind anomalies. Because strong correlations exist between the zonal winds at these levels, zonal estimate errors can be reduced to ≈0.5 m s−1 if known shear statistics are included in the estimate algorithm. Estimates which extrapolate cloud level wind anomalies to the surface using only climatological shear are shown to produce much greater surface wind errors. If these results are representative and if accurate monthly mean winds at 850 mb can be obtained from cloud motion vectors, then very useful low-frequency surface wind fields can be derived from cloud motion data.

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Gabriel A. Vecchi
and
D. E. Harrison

Abstract

The 1997–98 El Niño was both unusually strong and terminated unusually. Warm eastern equatorial Pacific (EEqP) sea surface temperature anomalies (SSTAs) exceeded 4°C at the event peak and lasted well into boreal spring of 1998, even though subsurface temperatures began cooling in December 1997. The oceanic processes that controlled this unusual termination are explored here and can be characterized by three features: (i) eastward propagating equatorial Pacific thermocline (Z tc) shoaling beginning in the central Pacific in November 1997; (ii) persistent warm EEqP SSTA between December 1997 and May 1998, despite strong EEqP Z tc shoaling (and subsurface cooling); and (iii) an abrupt cooling of EEqP SSTA in early May 1998 that exceeded 4°C within two weeks.

It is shown here that these changes can be understood in terms of the oceanic response to changes to the meridional structure of the near-equatorial zonal wind field. Equatorial near-date-line westerly wind anomalies greatly decreased in late 1997, associated with a southward shift of convective and wind anomalies. In the EEqP, equatorial easterlies disappeared (reappeared) in late January (early May) 1998, driving the springtime extension (abrupt termination) of this El Niño event. The authors suggest that the wind changes arise from fundamentally meridional processes and are tied to the annual cycle of insolation.

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D. E. Harrison
and
Narasimhan K. Larkin

Abstract

Using COADS data for the period 1946–1993, the near-global sea level pressure (SLP) patterns associated with interannual variability and the climatological seasonal march are discussed. A particular focus concerns the patterns associated with the two years before and after the South American sea surface temperatures rise (El Niño). The ten El Niño events in this record are composited, and the robustness of the features of this composite is tested.

Many features of the composite are quite robust; they occur during most El Niño events and are infrequent during non-El Niño periods. The most robust feature is an area of negative SLP anomaly (SLPA) in the eastern equatorial Pacific during Year(0) of the composite. This feature exceeds significance thresholds during every El Niño year and never during non-El Niño years; it correlates better with central Pacific SST variability than does the SOI. A west-central North Pacific positive SLPA, occurring late in Year(0) and lasting into the spring of year (+1) is the second most robust feature. Strong SLPA signals occur in the eastern South Pacific and around Australia in many events, but the behavior varies greatly from event to event. Some events show interesting signals in the Indian and Atlantic Oceans, but the behavior is not sufficiently general to be a statistically meaningful element of the composite.

The largest signals in the composite occur in the eastern equatorial and west-central North Pacific and not in the Southern Hemisphere. Thus, the large-scale SLP variations associated with El Niño periods are not dominated by the classical Southern Oscillation. Little evidence is found for phase propagation of the signal in El Niflo years. Although several features of the composite occur during the same season in each El Niño period, so that the main signals are “phase locked” to the seasonal cycle, the patterns of variability have little in common with the patterns of the seasonal march of SLP.

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