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J. Gavin Esler

Abstract

The zonal modulation of baroclinic disturbances is studied in a quasigeostrophic two-layer periodic channel. The system is relaxed toward an unstable state with a uniform flow in each layer. For small criticality, two weakly nonlinear systems are then developed, which differ in the choice of boundary condition used for the correction to the basic flow. Each system is described by an amplitude equation that determines the evolution of the wave envelope over “long” time- and space scales. For the first system the amplitude equation allows wave packet formation. Depending upon the ratio of the length scale of the packets to the channel length, either a steady wave train, stable solitonlike wave packets, or chaotically evolving wave packets are observed. The mechanism that leads to wave packet formation is then discussed with reference to the instability criterion of the amplitude equation. For the second system the amplitude equation is found to allow convergence to a steady, uniform wave train only.

A numerical model is then used to investigate the finite criticality extension of the second weakly nonlinear system. At low criticality, the assumptions that underpin the weakly nonlinear theory are tested by analyzing the convergence to a uniform wave train. As the criticality is increased, the effects of full nonlinearity cause the weakly nonlinear theory to become invalid. Initially, resonant triads of waves that have fixed amplitudes become excited owing to the dissipative nature of the system. As the criticality is increased further, other waves are excited and the system approaches full baroclinic chaos. Wave packet–like structures are then observed that evolve rapidly, growing, decaying, merging, and dividing.

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G. Kukla
and
J. Gavin

Abstract

Autumns in the Northern Hemisphere during the 1974–78 pentad were substantially cooler than in the pentad ending in 1938. Zonally averaged surface air temperature in October along latitude 80°N was 4.8°C lower, while summers were 0.6°C warmer. The recent pentad is cooler between 20 and 80°N in all seasons except spring when virtually no change was detected. The largest temperature difference was observed in autumn and winter in the high latitudes, which is a region of negative surface heat balance. In latitudinal bands along 80, 65, 55 and 40°N, the average October temperatures were lower in the recent pentad than during 87% or more of the 1891–1978 period. However, July and August along 80°N were warmer than 91% of the 88-year long interval. Zonal mean temperatures used in the study were obtained from a recent publication of Gruza and Ran'kova (1979). Monthly surface air temperature anomalies during 1974–78 in the latitudinal belt close to the snowline were inversely related to anomalies of the total hemispheric snow and ice extent.

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Lewis J. Allison
,
Thomas J. Schmugge
, and
Gavin Byrne

The chronological development and diminution of six floods in eastern Australia during January, February, and March 1974 were mapped for the first time by the Nimbus Electrically Scanning Microwave Radiometer (ESMR). Day and nighttime ESMR (19.35 GHz) coverage was analyzed for the low gradient, flooded Darling River system in New South Wales. Apparent movement of surface water as indicated by low brightness temperatures (<250 K, day and <240 K, night) was easily followed around the curved runoff basin along the northern shoreline of the flooded Darling River during this 3-month period. This pattern was in good agreement with flood crest data at selected river height gage stations, even under cloudy conditions.

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J. Gavin Esler
,
Lorenzo M. Polvani
, and
R. Alan Plumb

Abstract

The effect of a simple representation of the Hadley circulation on the propagation and nonlinear reflection of planetary-scale Rossby waves in the winter hemisphere is investigated numerically in a single-layer shallow-water model.

In the first instance, waves are forced by a zonal wavenumber three topography centered in the extratropics. In the linear limit the location of the low-latitude critical line at which the waves are absorbed is displaced poleward by the Hadley circulation. At finite forcing amplitude the critical layer regions where the waves break are found to be displaced poleward by a similar distance. The Hadley circulation is also found to inhibit the onset of nonlinear reflection by increasing the dissipation of wave activity in the critical layer.

Second, for waves generated by an isolated mountain, the presence of the Hadley circulation further inhibits nonlinear reflection by generating a strong westerly flux of wave activity within the critical layer. This westerly flux is shown to be largely advective and is explained by the poleward displacement of the critical line into the region of westerly flow. A simple expression is derived for the minimum zonal wind strength allowing propagation in the case of a quasigeostrophic β-plane flow when the mean meridional wind υ > 0.

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Lorenzo M. Polvani
,
J. Gavin Esler
, and
R. Alan Plumb

Abstract

Using a global, one-layer shallow water model, the response of a westerly flow to a localized mountain is investigated. A steady, linear response at small mountain heights successively gives way first to a steady flow in which nonlinearities are important and then to unsteady, but periodic, flow at larger mountain heights. At first the unsteady behavior consists of a low-frequency oscillation of the entire Northern Hemisphere zonal flow. As the mountain height is increased further, however, the oscillatory behavior becomes localized in the diffluent jet exit region downstream of the mountain. The oscillation then takes the form of a relatively rapid vortex shedding event, followed by a gradual readjustment of the split jet structure in the diffluent region. Although relatively simple, the model exhibits a surprisingly high sensitivity to slight parameter changes. A linear stability analysis of the time-averaged flow is able to capture the transition from steady to time-dependent behavior, but fails to capture the transition between the two distinct regimes of time-dependent response. Moreover, the most unstable modes of the time-averaged flow are found to be stationary and fail to capture the salient features of the EOFs of the full time-dependent flow. These results therefore suggest that, even in the simplest cases, such as the one studied here, a linear analysis of the time-averaged flow can be highly inadequate in describing the full nonlinear behavior.

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G. Kukla
,
J. Gavin
, and
T. R. Karl

Abstract

Meteorological stations located in an urban environment in North America warmed between 1941 and 1980, compared to the countryside, at an average rate of about 0.12°C per decade. Secular trends of surface air temperature computed predominately from such station data are likely to have a serious warm bias.

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T. R. Karl
,
G. Kukla
, and
J. Gavin

Abstract

A network of 14 nonurban stations was used as a first step toward understanding the character of change of daily temperature between 1948 and 1983 across the United States. Overall, in the eastern two-thirds of the country more stations tend to reflect statistically significant cooler weather than would be expected due to chance alone. In the western one-third of the country the opposite is found. Despite the cooling in the east, air masses with comparable characteristics (dewpoints, cloud cover) affecting these stations have warmed. This finding is in qualitative agreement with the expected impact of rising concentrations of greenhouse gases.

Considering all stations regardless of location, a marked increase of daily minimum temperature was found in comparable air masses under overcast skies, which, when combined with the tendency for decreasing maxima, results in a highly significant (at 0.1% level) decrease of an implied diurnal temperature range. Both maxima and minima increased in clear weather after stratification by air mass characteristics. It is highly unlikely that the reported changes are related to boundary layer humidity or direction of air flow since the data were stratified for air mass identification by both surface dew-point temperature and wind direction.

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T. R. Karl
,
G. Kukla
, and
J. Gavin

Abstract

An appreciable number of nonurban stations in the United States and Canada have been identified with statistically significant (at the 90% level) decreasing trends in the monthly mean diurnal temperature range between 1941–80. The percentage of stations in the network showing the decrease is higher than expected due to chance throughout the year, with a maximum reached during late summer and early autumn and a minimum in December. Monte Carlo tests indicate that during five months the field significance of the decreasing range is above the 99% level, and in 12 months above the 95% level. There is a negligible probability that such a result is due to chance. In contrast, trends of increasing or decreasing monthly mean maximum or minimum temperatures have at most only two months with field significance at or above the 90% level. This is related to the tendency toward increasing temperature in the western portions of North America and decreasing temperature in the east.

The physical mechanism responsible for the observed decrease in the diurnal range is not known. Possible explanations include greenhouse effects such as changes in cloudiness, aerosol loading, atmospheric water vapor content, or carbon dioxide. Change in circulation is also a possibility, but it will be difficult to isolate since the patterns of the decreased diurnal temperature range have high field significance throughout much of the year, relatively low spatial coherence, and occur at many stations where individual trends in the maximum and minimum temperature are not statistically significant. Our data show that the trends in the maximum and minimum temperatures may differ considerably from trends in the mean.

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R. Giles Harrison
,
Keri A. Nicoll
,
Douglas J. Tilley
,
Graeme J. Marlton
,
Stefan Chindea
,
Gavin P. Dingley
,
Pejman Iravani
,
David J. Cleaver
,
Jonathan L. du Bois
, and
David Brus

Abstract

Electric charge is always present in the lower atmosphere. If droplets or aerosols become charged, their behavior changes, influencing collision, evaporation, and deposition. Artificial charge release is an unexplored potential geoengineering technique for modifying fogs, clouds, and rainfall. Central to evaluating these processes experimentally in the atmosphere is establishing an effective method for charge delivery. A small charge-delivering remotely piloted aircraft has been specially developed for this, which is electrically propelled. It carries controllable bipolar charge emitters (nominal emission current ±5 μA) beneath each wing, with optical cloud and meteorological sensors integrated into the airframe. Meteorological and droplet measurements are demonstrated to 2 km altitude by comparison with a radiosonde, including within cloud, and successful charge emission aloft verified by using programmed flight paths above an upward-facing surface electric field mill. This technological approach is readily scalable to provide nonpolluting fleets of charge-releasing aircraft, identifying and targeting droplet regions with their own sensors. Beyond geoengineering, agricultural, and biological aerosol applications, safe ionic propulsion of future electric aircraft also requires detailed investigation of charge effects on natural atmospheric droplet systems.

Open access
Clara Orbe
,
Luke Van Roekel
,
Ángel F. Adames
,
Amin Dezfuli
,
John Fasullo
,
Peter J. Gleckler
,
Jiwoo Lee
,
Wei Li
,
Larissa Nazarenko
,
Gavin A. Schmidt
,
Kenneth R. Sperber
, and
Ming Zhao

Abstract

We compare the performance of several modes of variability across six U.S. climate modeling groups, with a focus on identifying robust improvements in recent models [including those participating in phase 6 of the Coupled Model Intercomparison Project (CMIP)] compared to previous versions. In particular, we examine the representation of the Madden–Julian oscillation (MJO), El Niño–Southern Oscillation (ENSO), the Pacific decadal oscillation (PDO), the quasi-biennial oscillation (QBO) in the tropical stratosphere, and the dominant modes of extratropical variability, including the southern annular mode (SAM), the northern annular mode (NAM) [and the closely related North Atlantic Oscillation (NAO)], and the Pacific–North American pattern (PNA). Where feasible, we explore the processes driving these improvements through the use of “intermediary” experiments that utilize model versions between CMIP3/5 and CMIP6 as well as targeted sensitivity experiments in which individual modeling parameters are altered. We find clear and systematic improvements in the MJO and QBO and in the teleconnection patterns associated with the PDO and ENSO. Some gains arise from better process representation, while others (e.g., the QBO) from higher resolution that allows for a greater range of interactions. Our results demonstrate that the incremental development processes in multiple climate model groups lead to more realistic simulations over time.

Free access