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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 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

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

As a continuation of the previous experiments in this series, the 18-vertical-level general circulation model was used to investigate the large-scale diffusion of a further two tracers in the stratosphere. In the present experiment both tracers were initiated as photochemical ozone distributions, one distribution being based on photochemistry in an oxygen-only atmosphere, the other on photochemistry in an oxygen-hydrogen atmosphere. Photochemistry was included in the subsequent diffusion so that a joint photochemical-dynamical study resulted. The diffusion process was followed for 185 model days when the experiment was terminated, even though the tracer distributions had not attained a steady state. Despite this, substantial qualitative agreement was obtained with observation, particularly as regards the accumulation of ozone in the lower stratosphere at extratropical latitudes. The results also suggest that photochemistry for an oxygen-hydrogen atmosphere may be the more applicable to the actual atmosphere.

Both the meridional circulation and the eddies were important in the diffusion of the tracers, although they did not constitute as interactive a system as in the previous experiment. The direct meridional circulation at low latitudes was found to be the prime mover in the polewards transport of the ozone, the eddies continuing this transport polewards from the subtropics. Owing to photochemica1 dampening of the ozone concentration gradient, the eddies were of minor importance at low latitudes.

A schematic diagram summarizing the principal features of the large-scale diffusion mechanism is given.

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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

The transient behavior of the stratosphere and its relationship to fluctuations in the dynamical coupling between troposphere and stratosphere has been investigated. The analysis is based on a 124-day sequence of atmospheric statistics generated in an 18-level hemispheric general circulation model set up for annual mean conditions. The model results are assumed to be representative of the winter half-year because of a bias in the model toward winter conditions.

Examination of the various forcing terms demonstrates that the dynamical coupling is dominated by the w′ϕ′ flux. This flux maximizes at subpolar latitudes and experiences a quasi-periodic fluctuation which is controlled by the tropospheric vacillation cycle, thus indicating a plausible connection between this vacillation cycle and stratospheric warnings. Both long- and short-term stratosphere variations are largely controlled by the fluctuations of the w′ϕ′ flux. The convergence of this flux is shown to force corresponding variations in the eddy kinetic energy and the eddy and zonal available potential energies in the model stratosphere. Results are discussed for a number of different levels and latitudes. Fluctuations in the poleward flux of sensible heat and possibly relative angular momentum owing to large-scale eddies can also be related to variations in the w′ϕ′ flux. The induced responses in the mean zonal wind and zonal mean temperature distributions relate well to observations. These findings suggest that not only are intra-annual stratospheric variations governed by fluctuations in the tropospheric vacillation cycle, but that inter-annual variations are also attributable to the same cause. The synoptic response of the model stratospheric fields is illustrated for wavenumbers 2 and 5, and shown to vary considerably during the course of a vacillation cycle.

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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 Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mark 2 global coupled climatic model has been used to generate a 10 000-yr simulation of “present” climate. The resultant dataset has been used to investigate a number of aspects of extremes associated with annual mean rainfall. Multimillennial time series of normalized rainfall amounts for selected points are used to highlight secular variability, spatial variations, and the differences between pluvial and drought conditions. Global distributions are also presented for selected rainfall characteristics, including the frequency of occurrence of specified rainfall anomalies with annual durations, the frequency of occurrence of 5-yr sequences of specified rainfall anomalies, and the maximum and minimum normalized rainfall amounts attained in the simulation. Such features cannot be obtained from observations because of their limited duration. A case study is also made of a megadrought over the southwestern United States, together with an analysis of the associated causal mechanisms. Given the exclusion of all external forcing from the model, it is concluded that the extreme annual mean rainfall extremes presented in the paper are attributable to stochastic events.

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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 10 000-yr unforced simulation with the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mark 2 coupled global climatic model has been used to investigate the occurrence of heat waves over the globe. Results are presented for both seasonal (summer mean) and daily heat waves. Geographical distributions of the occurrence rates of these heat waves are shown for various magnitudes of surface temperature anomalies. The heat waves have specific geographical preferences with regions where relatively frequent, intense, and long-lasting heat waves occur. Time series over all 10 000 yr of the heat waves for the selected model grid boxes illustrate the differing temporal variabilities at these locations, as well as identifying the occurrences of extreme heat waves. To this end, the observed European heat wave of 2003 was simulated remarkably well in its overall characteristics; it occurs once in this simulation. Heat waves for various continental locations are shown to occur as isolated spatial and temporal events, and not as part of larger-scale systems over continental-size domains, suggesting stochastic forcing as a contributor to the initiation of the heat waves. Regional plots of selected heat waves at monthly intervals illustrated the considerable spatial variability, progression, and variation in the intensity of the heat waves. Comparison of year-long daily surface temperature anomalies for heat-wave years for simulated and observed conditions at individual model grid boxes indicated substantial agreement, while spatial plots permitted the progress of a short-term heat wave over the United States to be followed. Multidecadal time series plots of intense heat waves also showed basic similarities between the simulation and observations, despite the brevity of the latter. The simulated time series suggest that more extreme heat waves than currently are observed, owing to the brevity of the observations, may be a possibility as a consequence solely of natural variability. An examination of the physical processes associated with a heat wave showed mutually consistent climatic relationships, such that a heat wave was associated with reduced rainfall and consequently reduced soil moisture content, evaporation, and cloud cover, and increased insolation at the surface. These combined changes created the surface temperature increase intrinsic to the heat wave. All heat waves examined for different regions experienced negative rainfall anomalies prior to a heat wave. The cause of these rainfall anomalies was not readily apparent. While an ENSO influence on heat waves is shown to exist in the simulation, not all ENSO events produce heat waves, suggesting that stochastic influences may determine when a major heat wave occurs in conjunction with these events. The limitations of the adequacy of the model ENSO may, however, have had an influence in this regard.

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