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B. G. Hunt

Abstract

In view of the possible climatic importance of volcanic eruptions an initial attempt has been made to simulate the particulate impact of a large eruption on the general circulation via its interaction with the incoming solar radiation. A principal aim of the experiment was to separate out the direct radiative effects of the debris on the general circulation from the indirect effects associated with the large-scale transport mechanisms, in the framework of an evolving volcanic debris distribution. An 18-level, hemispheric, general circulation model constrained to annual mean conditions was used. This model in a prior run had reproduced many features of the observed general circulation.

An amount of volcanic debris similar to that released by Krakatoa in IM was inserted as an initial zonal mean distribution in the model tropical stratosphere. The debris was subsequently permitted to be freely advected by the large-scale motions generated by the model for a total of 150 days. The debris's influence on the general circulation was restricted to backscattering the incoming solar radiation only, thus omitting the potentially important effect of the debris on the outgoing infrared radiation. Results presented show that the debris was diffused in a meaningful manner, and that the early debris distribution obtained was affected by whether or not coupling with the solar radiation was permitted.

The debris produced perturbations in the wind fields of such a nature that, in general, the mean zonal wind was slightly reduced, while some changes in the synoptic wind distributions were also apparent.

More marked differences were observed in the temperature distributions, although these were presumably overestimated owing to the omission of the thermal inertia of the oceans from the model. In particular, the hemispheric mean surface temperature was reduced by about 0.3 K, with cooling of approximately 0.7 K being noted in the tropics. At higher latitudes the perturbations caused by the volcanic debris tended to be obscured by “local weather variations.” This limited latitudinal response was a consequence of the short time-scale of the experiment and of having a “non-global” perturbation in the model, in contrast to most other model experiments that have involved global perturbations which produced maximum response at high latitudes. Outside of the tropics the temperature changes produced were a result of variations in the latitudinal transport of energy, especially latent heat, rather than an effect of the direct influence of the local debris distribution. The experiment suggests that individual large volcanic eruptions should have a transient, but not necessarily insignificant, impact on the climate.

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B. G. Hunt

Abstract

The hydrologic cycle has been included in a stereographic model of the Northern Hemisphere previously used as a dry model. The model had 18 vertical levels distributed between the surface and 37.5 km and was run out for annual mean conditions. The model had no topographical features.

A remarkably good simulation of many of the zonal mean features of the atmosphere was obtained. The tropospheric and stratospheric jet streams were well reproduced as regards intensity although they were displaced slightly equatorward. The meridional streamfunctions in the model agreed well with observation both in intensity and structure, subject to some distortion of the Hadley cell by the equatorial wall in the model. Considerable improvements were obtained in the representation of the large scale eddy flux of relative angular momentum resulting in good agreement with observation.

A detailed discussion is given concerning how the synoptic zonal and meridional wind distributions combine to produce a subtropical jet and a 3-cell mean meridional circulation pattern. The connection between the need to restrict the growth of westerly winds associated with conservation of absolute angular momentum in poleward trajectories, the production of the subtropical surface pressure highs, and the downward branch of the Hadley cell is explained, as is the location of the tropospheric jet stream. The dynamical factors which control the latitudinal extent of the Hadley and Ferrel cells are described. The production of a 3-cell mean meridional structure, and thus many of the basic characteristics of the atmosphere, are attributed to angular momentum requirements, rather than those associated with sensible or latent heat fluxes.

The zonal mean temperature distribution of the moist model was in rather good agreement with observation, and not too dissimilar to that of the previous dry model.

While the model produced a satisfactory tropospheric water vapor distribution its stratospheric distribution was as much as two orders of magnitude too low. This was attributed to the lack of subgrid scale vertical mixing in the model stratosphere. The stratospheric water vapor distribution was primarily maintained by a vertical flux generated by the large scale eddies, particularly at very low latitudes. The mean motions produced a net downward flux of water vapor in the vicinity of the tropical tropopause.

The energy balance of the model revealed that the model lacked eddy energy, although the ratio of eddy to zonal kinetic energy was much improved compared with previous versions of this model. The energy cycle of the lower and middle stratosphere is given, and shown to be fairly similar to that of the dry model, particularly as far as the forcing terms from the troposphere were concerned. Continuous coupling was found to exist between the troposphere and the stratospheric jet in the model, emphasizing the basic homogeneity of the troposphere and the lower and middle stratosphere.

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B. G. Hunt

Abstract

A new type of global general circulation model has been developed consisting of a finite difference representation in a single meridional-height plane, with the zonal variance of the atmospheric fields being treated by truncated Fourier series. The manipulation of the nonlinear terms of the equations was effected by using the so-called spectral-grid transform technique. Maintenance of stability in the model required the application of a filter designed by Shapiro which removed 2-grid-interval waves in the meridional grid from each of the variables. Stable integration of the model was achieved over a 100-day test.

The model was initiated from somewhat unrealistic, but particularly convenient, conditions generated by a zonally symmetric model corresponding to the mean term of the Fourier series. For the first 90 days of the integration the series was truncated at wavenumber 6, but this was increased to 10 for the last 10 days. Although the final results were not entirely realistic because some aspects of the initial conditions still persisted, primarily in the stratosphere, when the experiment was prematurely terminated, many features of the real atmosphere were reproduced. In particular the initial single Hadley cell was transformed into the conventional 3-cell structure of the troposphere, a subtropical jet stream evolved, and fairly realistic synoptic disturbances were obtained. However further integration with higher wavenumbers is needed to fully assess the potential of this approach.

Compared to conventional grid-point models, the principal advantage of the semi-spectral model is the improved accuracy of the differentiations in the zonal direction. In addition no problems were encountered in the polar regions of the model. This model is easy to understand and to use and is also particularly flexible, being readily convertible into hemispheric or channel models.

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B. G. Hunt

Abstract

Hindcasts for the period 1972–92 have been made using the two-tiered method. This involved forcing a global atmospheric model with sea surface temperature anomalies for the low-latitude Pacific Ocean generated with the Zebiak and Cane intermediate coupled model. Outside of this Pacific Ocean domain sea surface temperatures were specified from climatology. All hindcasts were for a duration of 12 months, with each hindcast commencing on 1 January of the individual years. An ensemble of 10 hindcasts was made for each year using different initial atmospheric conditions obtained from a long control run. In addition, a four-member ensemble simulation was made with the same global atmospheric model forced with the annually varying Global Sea Ice and Sea Surface Temperature (GISST) distribution for the period 1971–91. This provided a criterion against which the limited sea surface temperature forcing for the hindcasts could be compared. In the analysis of the results no attempt was made to remove systematic errors or to minimize other possible model deficiencies.

The hindcasts reproduced the observed temporal variability of the Southern Oscillation index, with transitions between ENSO events particularly well defined. The interannual variability of the anomalous zonal wind stress over the equatorial Pacific Ocean was hindcast satisfactorily, but an index of the Pacific–North America oscillation was poorly represented in the hindcasts. This deficiency may have implications for predictability in regions influenced by this oscillation.

Rainfall hindcasts are presented in some detail, particularly time series for individual model grid boxes or averages over regions. The results are presented as monthly (rather than seasonal) totals for individual years in order to provide some indication of their potential temporal limits. The highest accuracy for rainfall was achieved over the low-latitude Pacific Ocean where anomaly correlation coefficients with observations greater than 0.6 were obtained over an extended region. The quality of hindcasts over ENSO-influenced land regions varied noticeably with location, but the marked interannual variation in rainfall associated with ENSO events was quite well captured for northeast Australia. The influence of chaos on the hindcasts is illustrated by providing outputs for individual members of the ensemble generated. In general, limited systematic improvements could be identified for the GISST simulation compared to the hindcasts. This outcome is attributable to deficiencies in the model used.

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B. G. Hunt

Abstract

A hemispheric general circulation model, with fixed zonally averaged cloud cover, was used to investigate the climatic impact of increased albedo of low-level clouds caused by atmospheric pollution. The albedo of these clouds was increased from 0.69 to 0.87, corresponding to rather high levels of pollution. The albedo was modified in the midlatitudes and subtropics of the model in two separate experiments. In the midlatitude experiment, a cooling of 2.5 K at the surface occurred. The cooling was confined to the zone of the albedo increase. The subtropical experiment had a maximum surface cooling of only 1 K, but it extended beyond the immediate zone of the albedo increase. These coolings were much less than those produced by radiative-convective models. Time series plots of surface temperature at an individual point indicated that it would be difficult to detect this cooling against the day-to-day variability of the model atmosphere. A number of other perturbations to the model's climate occurred, which could be related to the cooling. These included variations in convective activity, precipitation and evaporation. Dynamical changes were also identified, including stronger mean zonal winds in midlatitudes, variations in eddy kinetic energy, and modifications to the mean meridional streamfunction.

Despite the perturbations induced by the cloud albedo increases, no systematic changes in the synoptic properties of the model atmosphere were detected.

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B. G. Hunt

Abstract

A primitive equation general circulation model has been developed to simulate the atmosphere from the surface to 100 km. The model was based on the semispectral technique and has 15 zonal wavenumbers and 40 grid points between poles. The simulation was for fixed January conditions, was nondiurnal, had land-sea contrast with specified sea surface temperatures but omitted both orography and the hydrologic cycle. For convenience and brevity only the zonal mean state of the model is presented in this paper.

The overall simulation revealed several interesting and novel features of the atmosphere. The mean meridional wind exhibited a distinctly layered structure which was confined to the tropics throughout the stratosphere and mesosphere. This was attributed to inertial instability in the winter hemisphere resulting from the intense mesospheric jet: the actual mesosphere also may experience such an instability to a much lesser extent. Some observational evidence exists to support layering in the synoptic meridional velocities. A single mean meridional circulation in the mesosphere extending from pole to pole was not obtained, rather the principal component was only between the midlatitudes. This circulation was an extension of the winter tropospheric Hadley cell and it was the most dominant feature of the stratosphere and mesosphere. The mesospheric easterlies generated by this circulation were confined to the summer hemisphere of the model by an intense southward flux of relative angular momentum owing to the inertial instability in the tropics centered on 60 km altitude. The extension of this mean circulation into the winter hemisphere was the primary cause of the westerly jet. A detailed explanation of the angular momentum balance of the mesosphere and stratosphere, with particular emphasis on the role of the troposphere is presented.

A leakage of wave energy from the troposphere to 100 km occurred in both hemispheres of the, model, with the winter hemispheric energy flux being about three times the larger at 100 km compared with 10 times at 25 km. The tropospheric energy flux into the upper atmosphere by large-scale eddies alone was almost three times the local generation of available potential energy. This energy flux was sufficiently strong to reverse the baroclinic cycle over most of the upper atmosphere. Regions with large eddy kinetic energy were found to exist in the tropical mesosphere and throughout most of the lower thermosphere of the model.

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B. G. Hunt

Abstract

The role of nonlinear interactions in the climatic system in contributing to variability at both spatial and temporal levels is poorly understood. To gain some insight into the possible role of nonlinearities an elementary model has been devised. The model is capable of simulating interannual fluctuations substantially in agreement with observation. Very long term integrations (100 000 years) indicate that nonlinearities alone can produce noticeable climatic extremes, which could account for much of the observed anomalous behavior of the atmosphere. The nonlinearities in this particular model, however, are incapable of maintaining an extended climatic change. Short-term external forces in the model, less than one year, were not able to produce climatic perturbations extending beyond about two years. Finally it is shown that even for quite strong solar modulation of the model climate, nonlinearities are capable of intermittently disrupting this modulation. This suggests that a reassessment is needed of the rejection of claims for solar-induced climatic fluctuations on the grounds that such fluctuations are observed to breakdown.

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B. G. Hunt

Abstract

It is shown that when rate constants confirmed by recent laboratory studies are used in photochemical O3 calculations unacceptably high O3 concentrations and total O3 amounts are obtained. In order to account for this disagreement, an investigation has been made to see whether reactions between O3 and excited forms of molecular and atomic oxygen are of importance in the atmosphere, following recent laboratory work in this field. It was found that excited molecular oxygen may be neglected in the O3 reaction scheme but that reactions between O3 and O(1 D could be of importance in the stratosphere. The importance of this reaction depends very markedly on the rate of deactivation of O(1 D in the atmosphere, and a conflict of requirements exists between the O3 and 6300 Å airglow values for this rate. Hence, in view of this conflict, the photochemical O3 problem has been left unresolved.

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B. G. Hunt

Abstract

A general circulation model has been used to investigate the climatic and dynamical consequences of increasing and decreasing the rotation rate of the earth by a factor of 5. The model was hemispheric, devoid of topographical features, non-diurnal, and set up for annual mean conditions based on fixed, specified cloud, ozone, water vapor and surface albedo.

The high rotation rate model tended toward a multicellular mean meridional streamfunction with a weak tropospheric jet at very low latitudes, while the slow rotation rate model had a two-cell structure with an intense tropospheric jet at middle latitudes. These differences were interpreted in terms of the requirements for conservation of absolute angular momentum, and they indicate that such conservation was the single most important constraint in determining the dynamical structure of the model atmospheres. The latitudinal extent of the Hadley cell and the associated region of high surface pressure, the location and intensity of the tropospheric jet, and the conservation requirements were found to be mutually and dynamically related for both the fast and slow rotation rates as well as the control experiment.

The slow rotation rate model had quasi-axisymmetric synoptic distributions, a small tropospheric latitudinal temperature gradient, a sufficiently warm polar region to question the existence of permanent ice cover, and an enlarged and zone in the subtropics.

The fast rotation rate model exhibited irregular small-scale synoptic features, a marked tropospheric latitudinal temperature gradient, a very narrow arid zone in the tropics, and a very dry and cold high-latitude region.

Energy comparisons showed that the high rotation rate model had almost twice as much total energy as the control and slow rotation rate models combined, with most of this energy in the form of zonal available potential energy. In all three models the baroclinic cycle prevailed in the atmosphere, but the overall “efficiency” of the atmosphere declined with increasing rotation rate.

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B. G. Hunt

Abstract

A 124-day time series of simulated atmospheric data has been generated for annual mean conditions with a hemispheric general circulation model. Analysis of these data based on discrete 4-day time means revealed a vacillatory behavior, particularly in the eddy kinetic and available potential energies, with a period of about 20 days in substantial agreement with a number of observational studies. This quasi-periodic behavior was also found to occur in the coupling terms between the various energy components and to a lesser extent in the forcing functions. A fairly complete analysis has been produced which accounts for the various relationships occurring in the vacillation cycle, and essentially explains the basic behavior of the cycle. This indicates that a vacillation is a natural feature of the atmosphere, which occurs because of fluctuations in the intensity of baroclinic activity resulting from a tendency of the atmosphere to overrespond to an initial imbalance in its latitudinal temperature gradient. The analysis in wavenumber space of the synoptic behavior of the 500 mb geopotential surface reveals that drastic changes occur between maximum and minimum phases of the vacillation for wavenumbers associated with baroclinic activity.

Finally some implications regarding the possible impact of the vacillation cycle on numerical weather forecasting are noted.

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