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William E. Johns

Abstract

During November 1986, a 6-day record was collected from a 150 kHz Acoustic Doppler Current Profiler (ADCP) mounted in the upward-looking mode on a subsurface mooring in the Gulf Stream near Cape Hatteras. The flotation unit used for the ADCP was a newly developed streamlined float, designed to minimize the effects of drag-induced tilt and high-frequency buoy motion on the range and precision of the Doppler measurements. The overall performance of the float was found to be excellent, with a mean tilt of less than 2° in up to 2 kt of current and a high apparent stability to vortex-induced oscillations. As a result, good velocity data were obtained to within 30 m of the surface from a mean depth of 375 m. A comparison of the near-field ADCP velocity data with a conventional Aanderra current meter moored 20 m below the ADCP yielded mean and root-mean-square speed and direction differences of 1.0 ± 3.7 cm s−1 and 0.5 ± 2.9°, respectively. Also, a comparison with Pegasus velocity profiles taken within 1 n mi of the mooring site showed qualitatively good agreement, with the ADCP reproducing well the small-scale vertical structure. Significant fluctuations in the vertical component were also observed, related to diurnal migration of biological scatterers, with vertical “speeds” often in excess of 3–4 cm s−1.

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William H. Klein
and
John E. Walsh

Abstract

A comparison is made between two types of specification of monthly wintertime surface temperatures over the United States. The specifications are obtained by multiple regression of station temperature anomaly at each of 37 stations onto 700 mb height anomalies represented by 1) grid-point values selected by a forward stepwise screening procedure, and 2) coefficients of the dominant empirical orthogonal functions (EOF's). Various measures of skill show that specifications derived from the pointwise screening are superior in both developmental (dependent) and independent samples. The differences in the skill levels are interpreted as a disadvantage of the spatial generality inherent in the EOF representations.

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William L. Chapman
and
John E. Walsh

Abstract

Gridded fields of sea ice concentration are used to evaluate weekly and monthly anomalies of sea ice coverage in 22 Arctic subregions. The primary period of study is 1972–1988, although statistical comparisons are made with data of lesser quality from 1953–1971. The various time series of regional ice coverage permit the evaluation of ice anomaly persistence as a function of region, season, and lag (forecast range). The fractions of variance explained by anomaly persistence in most regions are considerably larger than corresponding atmospheric values. The fractions typically decrease from 50% to 10% as the forecast range increases from several weeks to several months. Anomaly persistence from the winter months is generally largest, although the regions of greatest persistence-derived forecast skill tend to migrate seasonally with the marginal ice zone. Biases in the regional analyses of the 1950s and 1960s inflate the apparent persistences in the North Atlantic during 1953–1971, but the persistences in most other regions are generally similar in the pre-1972 and post-1972 data. The inclusion of lagged regional cross-correlations provides little increment of forecast skill over persistence at the 1-month range, but this strategy appears to have the potential to enhance the usefulness of ice forecasts at ranges of several months. Analog-based forecasts show statistically significant skill but are generally unable to outperform persistence at the 1-month range.

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John E. George
and
William M. Gray

Abstract

Ten years of rawinsonde data for 30 stations in the western North Pacific have been composited relative to tropical cyclone center positions. This information is used to study tropical cyclone motion and surrounding parameter relationships. Tropical cyclone motion and lower troposphere surrounding actual and geostrophic flow fields from 1°–7° radius are very well correlated. This general correlation of surrounding flow features applies equally well for cyclones with different directions of motion, speeds of propagation, intensities and intensity changes. The 700 mb level best specifies cyclone speed. The 500 mb level best specifies cyclone direction. The steering flow concept of cyclone motion appears to be quite valid in the statistical sense. These results may be useful to tropical cyclone forecasters.

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John E. George
and
William M. Gray

Abstract

Ten years of rawinsonde data for 30 stations in the western North Pacific have been composited relative to tropical cyclone center positions. This is the same data set described in a previous paper by the same authors. These data were used to study prior (12–72 h) differences in surrounding environmental fields between tropical cyclones which recurved and those which did not. Differences in the wind, height and temperature fields out to 21° radius to the north between these two classes of storms are presented. A strong recurvature correlation is found in the 200 mb wind and height fields at large radii to the north. It is suggested that an operational long-range (2–3 days) tropical cyclone recurvature forecast scheme can be developed using upper tropospheric wind-height data.

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John R. Gerhardt
and
William E. Gordon

Abstract

Selected portions of microtemperature data obtained continuously and with near simultaneity at several levels up to six feet over a desert surface are plotted on expanded height-time coordinates. The resulting isotherm patterns are shown to be strikingly consistent at all levels and are qualitatively analyzed in relation to the turbulence field present. Correlation coefficients between temperature fluctuations simultaneously at two levels and at a point for various time intervals are evaluated and their variation with separation, time, wind speed, and thermal stability is discussed. Tentative intensity and scale measures of turbulence derived from iemperature data are presented.

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William Bluman
and
John E. Hart

Abstract

Airbone Doppler lidar wind measurements were obtained in the lee of Mount Shasta in northern California on 28 August 1984. These data consist of line of sight wind vectors at flight level (3000 m) and along planes tilted at 1, 2 and 3 degree below the 3000 m level. The observed field is confined to a rectangular box, encompassing the mountain, that extends about 40 km downwind and about 20 km crosswind. The spatial resolution of the measured wind field is approximately 330 m.

The upstream southwesterly flow tended to circumvent the mountain although some air did rise over the peak (at 4317 m) to initiate three-dimensional internal gravity waves in the lee. These waves are delineated in the two-dimensional divergence field D, determined from the downwind velocity components on each of the tilted planes with line of sight wind vector measurements. The observed field of D exhibits a peak in its power spectrum, determined along the downstream direction, at a wavelength of about 8 km with a secondary peaks at about 17 km. Data from upper air soundings at Medford, Oregon and from onboard sensors establish that the 8 km wavelength represents the free wave response, which is determined by the airstream characteristics. Comparison with the power spectrum of the mountain slope indicates that the longer wavelength is a forced response.

Qualitative aspect of the lee-wave pattern are reproduce in a linear model with uniform airstream characteristics. However, the amplitude of the free wave response is underestimated by a factor of two, and the forced wave amplitude is about three time that of the free wave. In addition, the wave disturbance produced by the linear model decays more rapidly in the downstream direction than the observed wave. These discrepancies are interpreted in relation to physical features that are contained in the linear model.

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William L. Chapman
and
John E. Walsh

Abstract

Simulations of Arctic surface air temperature and sea level pressure by 14 global climate models used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change are synthesized in an analysis of biases and trends. Simulated composite GCM surface air temperatures for 1981–2000 are generally 1°–2°C colder than corresponding observations with the exception of a cold bias maximum of 6°–8°C in the Barents Sea. The Barents Sea bias, most prominent in winter and spring, occurs in 12 of the 14 GCMs and corresponds to a region of oversimulated sea ice. All models project a twenty-first-century warming that is largest in the autumn and winter, although the rates of the projected warming vary considerably among the models. The across-model and across-scenario uncertainties in the projected temperatures are comparable through the first half of the twenty-first century, but increases in variability associated with the choice of scenario begin to outpace increases in across-model variability by about the year 2070. By the end of the twenty-first century, the cross-scenario variability is about 50% greater than the across-model variability. The biases of sea level pressure are smaller than in the previous generation of global climate models, although the models still show a positive bias of sea level pressure in the Eurasian sector of the Arctic Ocean, surrounded by an area of negative pressure biases. This bias is consistent with an inability of the North Atlantic storm track to penetrate the Eurasian portion of the Arctic Ocean. The changes of sea level pressure projected for the twenty-first century are negative over essentially the entire Arctic. The most significant decreases of pressure are projected for the Bering Strait region, primarily in autumn and winter.

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John E. Walsh
and
William L. Chapman

Abstract

Associations between cloudiness, radiative fluxes, and surface air temperature in the central Arctic are evaluated from 1) measurements made at Russian drifting ice stations, and 2) atmospheric reanalyses of the National Centers for Environmental Prediction (NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF). In the ice station data, cloudiness is associated with an increase of downward longwave radiation in all months and an increase of net (downward minus upward) total radiation from September through March. The surface air temperatures under overcast skies are 6°–9°C higher than under clear skies during September–March, and the differences are even larger when the observations are stratified by wind as well as cloudiness. The warming by the radiative flux enhancement after a transition from clear skies to overcast has a 1–2-day timescale, while the cooling after the transition to clear skies has a somewhat shorter timescale. The NCEP reanalysis exaggerates slightly the association between cloudiness and surface air temperature, while the ECMWF reanalysis shows a considerably weaker association.

The maximum cloud-radiative forcing (MCRF), defined as the difference between the ice station measurements of net surface radiation under cloudy and clear skies, ranges from −59 W m−2 in June to positive values of 20–30 W m−2 in September–March. The annual mean is small but positive, 3 W m−2, despite the approximately three-month summer period of substantially negative MCRF. These findings are consistent with the conventional cloud-radiative forcing obtained in earlier studies using satellite data and one-dimensional models of the Arctic atmosphere and sea ice. Neither reanalysis captures the seasonality of the observationally deduced effects of clouds on surface radiation. The NCEP reanalysis does not capture the seasonality of the actual cloudiness (as defined by the reported cloud fractions), while the ECMWF reanalysis does not show an impact of clouds on the surface solar flux.

Issues needing further attention in the model–data comparison are the effects of surface heterogeneities, the characterization of Arctic clouds, the formulational reasons for the discrepancies between the model-derived reanalyses and the observational data, and the implications for model-derived projections of climate change in the Arctic.

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John R. Gerhardt
and
William E. Gordon

The propagation of radio waves above about 30 megacycles is seriously affected by certain weather phenomena. The meteorological aspects of this effect for a particular case are considered and a forecasting technique proposed.

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