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Tsuyoshi Nitta

Abstract

Large-scale mass, heat and moisture budgets have been computed over the GATE A/B-scale area during two priority periods in Phase 3. The computed budget results are well-correlated with the activities of the cloud clusters which develop and decay during the analyzed period. Strong upward motion exists with two maxima at 700 and 350 mb during the active period of the cloud cluster, but sinking motion appears in the middle troposphere during the dissipating period. Large amounts of apparent heat sources and moisture sinks occur in the whole troposphere and large amounts of heat energy are transported upward by cumulus clouds during the active period; but an apparent heat sink, moisture source and slight downward heat flux are observed in the middle troposphere during the dissipating period.

A diagnostic method for determination of cloud properties has been improved by including downdraft effects and tested on the mean Marshall Islands data. In this method both spectral properties of updrafts and bulk properties of downdrafts are determined without prescribing downdraft parameters. The downdraft acts as a heat sink and a moisture source and neglect of the downdraft overestimates the mass flux of shallow clouds. The downdraft originates from air of the environment somewhere in the middle troposphere and extends below the cloud base keeping temperatures cooler than the surrounding air.

This diagnostic method is applied to three different budget results in GATE which are classified according to the activities of the cloud clusters. Bimodal cloud mass flux distributions for updrafts with dominance of very shallow and very deep clouds are obtained during the period when the GATE area is not affected by cloud clusters. However, unimodal distributions are found with dominance of very deep clouds during the active stage of cloud clusters and with dominance of shallow clouds during the dissipating stage. Greater cloud downdraft mass flux is observed in GATE than in the Marshall Islands area and, specifically, net downward cloud mass flux results from the dominance of downdrafts during the dissipating period. The relation between vertical distributions of the downdraft mass flux and vertical shear of ambient winds is also discussed.

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Tsuyoshi Nitta

Abstract

Using Arakawa's spectral representation of cumulus convection, the cloud mass flux distributions and other cloud properties are diagnostically determined from large-scale observations. Observational data over the tropical Atlantic Ocean during the BOMEX Phase 3 (22–30 June 1969) are used. The analyzed period is subdivided into three parts according to different synoptic situations. The cloud properties and the manner of interaction between large-scale fields and cumulus convection under the different situations are examined.

During the period in which a strong trade inversion lies near 800 mb, most clouds detrain below the trade inversion and cloud mass flux is confined below this level. The large apparent heat sink and apparent moisture source found near the inversion base are mainly due to the cooling and moistening effects of the detrainment from clouds. During the period affected by an upper-level extended trough, the trade inversion is weakened and some clouds penetrate through the 800-mb level but the cloud mass flux is still confined below 500 mb. During the period disturbed by an organized cloud cluster, not only small, shallow clouds but large, deep clouds make large contributions to the vertical mass flux. The total cloud mass flux extends into the upper troposphere.

Approximate distributions of cloud area ratio are also estimated from the budget of cloud liquid water using a simple pararmeterization of the rainfall rate. Computed results of cloud mass flux distributions are qualitatively verified by aircraft cloud photographs and radar echo analyses.

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Tsuyoshi Nitta and Steven Esbensen

Abstract

Large-scale heat and moisture budgets over the tropical Atlantic Ocean are examined during Phase 3 (22–30 June 1969) of the Barbados Oceanographic and Meteorological Experiment (BOMEX). From the satellite cloud photographs of ATS-3, the analyzed period is subdivided into an undisturbed part and a disturbed part. During the undisturbed period, downward motion predominates from the surface to about 500 mb and a large apparent heat sink and apparent moisture source are found near the top of the trade inversion layer. The upward heat flux due to cumulus clouds is confined below the 700-mb level. On the other hand during the relatively disturbed period, upward motion takes place at low levels and the heat flux due to cumulus convection extends to at least 500 mb. Values of the total heat flux estimated by large-scale budgets agree well with those obtained independently by bulk aerodynamic computations.

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Masahiro Watanabe and Tsuyoshi Nitta

Abstract

In association with extreme anomalies in the extratropical atmosphere, numerical experiments using an atmospheric general circulation model are performed to investigate the relative impact of the anomalous snow with SST anomalies on the atmospheric circulation. Large negative anomalies in the Eurasian snow cover and global SST anomalies observed in 1988/89 are employed as the respective boundary forcings because winter atmospheric states largely shifted in 1989.

The model is integrated for half a year from 1 September. Five-member ensemble states are obtained by conducting the light snow (LSNW) run, in which the snowfall was suppressed over eastern Eurasia during the first 3 months with prescribed SSTs, and another experiment, which employed observed SST anomalies instead of snow anomalies (the SST run). The LSNW run simulated dipole (positive in midlatitudes and negative in polar regions) anomalies in 500-hPa height similar to those observed in 1989, although the amplitude was smaller over the North Pacific. Surface warming over Eurasia found in winter 1989 is also reproduced through albedo feedback. On the other hand, the SST run reveals large height anomalies over the North Pacific in addition to the significant dipole similar to that in the LSNW run, but failed to reproduce observed surface warming as well as negative snow anomalies over Eurasia. SST anomalies in the equatorial Pacific corresponding to La Niña in 1988/89 are responsible for simulated height anomalies over the North Pacific in the SST run, whereas an influence of extratropical SST anomalies appears to be tenuous relative to either tropical SST anomalies or Eurasian snow anomalies. The amplitude of response in the LSNW run is roughly 60% of that in the SST run.

An analysis of the dynamics emphasizes that, in the upper troposphere, interactions of anomalies themselves with climatological zonal asymmetries as well as changes in transient eddy activities contribute to the height response found in the model. This suggests that the nonlinearities in the atmosphere are also important in addition to the snow and SST anomalies for the extreme anomalies in winter 1989 atmospheric circulation.

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Masahiro Watanabe and Tsuyoshi Nitta

Abstract

This study attempts to investigate decadal-scale climate changes in the mid- and high latitudes of the Northern Hemisphere in winter 1989 by using various observational data for atmospheric parameters, sea surface temperature (SST), and snow cover.

Decadal-scale changes in the winter atmosphere after 1989 are characterized as follows: a dipole pattern of height anomalies between midlatitudes and polar regions with an equivalent barotropic structure, temperature changes with cooling in the polar region and warming in midlatitudes in the middle troposphere, and associated reduction of the subtropical jet stream. Statistical tests applied to the 500-hPa height field reveal that the changes in 1989 are a distinct discontinuity or shift on the decadal scale with hemispheric extent. The spatial structure is interpreted as a linear combination of three teleconnections: the North Atlantic oscillation, Pacific–North American, and Eurasian patterns, in addition to the zonally symmetric dipole. On the other hand, the sharpness of decadal changes in 1989 arises from synchronous phase shifts of interdecadal variations over the Pacific Ocean and quasi-decadal variations over the North Atlantic. Similar concurrence is also found in winter 1977. In agreement with previous studies, the interdecadal atmospheric variations over the North Pacific reveal a strong coupling with tropical/extratropical SST anomalies in the Pacific Ocean. A relationship between the quasi-decadal variability of the North Atlantic atmosphere and underlying SST anomalies is relatively tenuous.

Surface air temperatures show a warming over all of Eurasia after winter 1989. It is shown by the analysis of snow cover in the Northern Hemisphere derived from NOAA satellites that a large decrease (increase) of the snow extent over the eastern part of the Eurasian continent occurred in autumn 1988 (1976) prior to winter 1989 (1977). It is suggested that the anomalies in the Eurasian snow cover during autumn play a role as an amplifier in the atmospheric shifts. These snow anomalies may be as important for the atmospheric changes as changes in SSTs.

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M. Yanai, J-H. Chu, T. E. Stark, and Tsuyoshi Nitta

Abstract

A spectral diagnostic method of cumulus ensemble (Nitta, 1975) is compared with a bulk diagnostic method (Yanai et al., 1973), using the same data set taken in the Marshal Islands area for a 100-day period in 1956. For the total vertical cloud mass flux and mass detrainment, both methods give nearly identical results. Using the spectral diagnostic method, daily spectra of cloud base mass flux as functions of the detrainment height are obtained. The effects of radiative cooling upon the background cloud mass spectrum is examined. Typical radiative cooling rates in the tropics tend to produce a bi-modal distribution of mass spectrum showing deep and shallow clouds. Then the dependence of daily cloud mass spectra on the large-scale vertical motion and the evaporation from the sea is examined by data stratification, by correlation analysis, and by time-spectrum analysis. Under disturbed conditions, the bi-modal distribution is further enhanced. Under suppressed conditions, a uni-modal distribution consisting of only shallow clouds prevails. Deeper clouds are more highly correlated with the large-scale vertical motion at higher levels than that at lower levels, showing the controlling influence of the upper tropospheric vertical motion upon the deep clouds. Shallow clouds persist nearly all the time due to radiative cooling and surface evaporation, but they are enhanced by the large-scale ascent. Time-spectrum analysis applied to the computed mass flux and the large-scale parameters shows that both deep and shallow clouds are modulated by large-scale disturbances.

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Masahiro Watanabe, Masahide Kimoto, Tsuyoshi Nitta, and Misako Kachi

Abstract

Decadal climate variations in the Atlantic Ocean found in observational fields and a coupled general circulation model (CGCM) are investigated. In particular, physical processes responsible for the phase reversal are compared.

Observed and modeled decadal variations have dominant periodicities around 12.3 and 9.9 yr, respectively. Both variations show similar spatial features: sea surface temperature (SST) anomalies in the western subtropical Atlantic sandwiched by those with opposite sign to the north and south, and a dipole of sea level pressure anomalies, which resemble the North Atlantic Oscillation. Their temporal evolutions are, however, different from each other, suggestive of different dynamics of the oscillation. In the observations, SST and surface-layer (0–100 m) temperature anomalies move eastward from the subtropical western Atlantic to the European coast along the Gulf Stream. Northward propagation of SST anomalies are also found along the western boundaries including the Gulf of Mexico. A budget analysis for the temperature equation shows that these features are the manifestation of the advection of SST anomalies by the mean current, which acts to switch one phase of the oscillation to another. Anomalous gyre intensity appears to have little contribution to the phase switching process of the near-surface variability, although the influence of the anomalous gyre is found in the lower subsurface up to 500 m. In contrast, SST anomalies in the CGCM are more strongly tied with subsurface temperature anomalies that propagate westward, consistent with a slow gyre adjustment by the baroclinic Rossby wave propagation. The wave-induced advection acts to change the phase of SST as well as the subsurface temperature anomalies in the model. Subduction of temperature anomalies is found to occur on decadal timescales both in the observation and in the model over the eastern basin where the winter mixed layer is deepened, although the consequence of such a process is not very clear.

In agreement with previous studies, it is suggested that the atmosphere–ocean interaction is important for the decadal variability. The anomalous heat flux originated from the wind–evaporation feedback appears to play a dominant role in the formation of the tripolar structure of oceanic thermal anomalies both in observations and in the CGCM. On the other hand, the dominant timescales of observed and simulated decadal modes are largely dominated by the mean subtropical gyre velocity, and by the propagation speed of long Rossby waves, respectively, both of which happen to have similar timescales.

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