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John T. Ball, Stephen J. Thoren, and Marshall A. Atwater

Abstract

The spatial and temporal characteristics of cloud coverage during the 21-day GATE Phase 111 period were studied using SMS-1 infrared hourly digitized data and standard hourly meteorological surface observations taken on 18 ships positioned within the A/B and C scale regions. Results were obtained for the entire sample and for portions stratified according to enhanced (E) or depressed (D) convective activity. Separate areal analyses based on ship data and on satellite data were obtained for total coverage, low-cloud amount including cumulonimbus, middle-cloud amount and high-cloud amount. Both the satellite and the ship data indicate that the average coverage for the GATE A/B array area during Phase 111 is close to 80%. The zone of maximum cloudiness exhibits a basic east-west orientation and is centered near 8°N.

Analysis of hourly variations yields a nighttime maximum of total coverage near 0300 GMT on D days while on E days the maximum occurs in late afternoon or early evening. Analysis of low clouds showed a double maxima at 0400 and 1500 GMT. The early afternoon maximum predominates on E days while only the nighttime maximum is present on D days. On days of significant activity, the high cloud maximum occurs in the late afternoon (1900 GMT), ∼4 h later than the low-cloud maximum. The results of this study emphasize that generalizations about the diurnal variations of clouds, convective activity and precipitation over tropical oceans must be carefully evaluated in terms of regional location and prevalent degree of convective activity.

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Stephen D. Pierce, John A. Barth, and Robert L. Smith

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Accurate ship velocity is important for determining absolute currents from acoustic Doppler current profiler (ADCP) measurements. In this paper, the authors describe the application of two methods to improve the quality of ship velocity estimates. The first uses wide-area differential global positioning system (WADGPS) navigation to improve ship positioning. During the cruise, raw global positioning system (GPS) pseudorange data are collected. The pseudorange measurement is the difference between satellite transmission time and receiver reception time of a GPS signal. A few days after the cruise, satellite clock corrections from the Canadian Active Control System and orbital parameters from the U.S. Coast Guard Navigation Center are used to derive WADGPS positions that remove the position degradation effects of selective availability. Two-dimensional root-mean-square (rms) position accuracies reduce from ±34 to ±9 m. The authors’ second method of improving the ship velocity applies an adaptive local third-order polynomial smoother to the raw ship velocities. This smoothing method is particularly effective at handling the nonstationary nature of the signal when the ship is starting, stopping, or turning, which is typical of oceanographic cruises. Application of the smoother in this case reduces overall rms noise in the ship velocity by 16%. The combination of both methods reduces the uncertainty due to navigation of a 20-min ADCP absolute velocity from ±0.063 to ±0.038 m s−1—a 40% reduction. These methods also improve the calibration for sensitivity error and ADCP–gyrocompass misalignment angle.

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John T. Ball, Marshall A. Atwater, and Stephen J. Thoren

Abstract

A radiation model was used to simulate daily incoming solar radiation at four ships during Phase III of GATE. The accuracy of the simulations from several different cloud analyses based on ship or satellite data was estimated by comparison with measured values. The cloud-data source, objective analysis characteristics and analysis grid interval affected the accuracy of the radiation computations.

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Stephen F. Corfidi, Robert H. Johns, and Mark A. Darrow

Abstract

A significant, convectively induced windstorm known as a derecho occurred over parts of Utah, Wyoming, Idaho, and Colorado on 31 May 1994. The event was unusual in that it occurred not only in an environment of relatively limited moisture, but also one with a thermodynamic profile favorable for dry microbursts in the presence of moderate midtropospheric flow. The development and evolution of the severe wind-producing convective system is described, with emphasis on the synoptic and mesoscale features that may have contributed to its strength and maintenance. A very similar derecho that affected much the same region on 1 June 2002 is more briefly introduced. Questions are raised regarding the unique nature of these events and their potential utility in achieving an increased understanding of the mechanics of derecho-producing convective systems in more moisture-rich environments.

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John R. Lanzante, Stephen A. Klein, and Dian J. Seidel

Abstract

Trends in radiosonde-based temperatures and lower-tropospheric lapse rates are presented for the time periods 1959–97 and 1979–97, including their vertical, horizontal, and seasonal variations. A novel aspect is that estimates are made globally of the effects of artificial (instrumental or procedural) changes on the derived trends using data homogenization procedures introduced in a companion paper (Part I). Credibility of the data homogenization scheme is established by comparison with independent satellite temperature measurements derived from the microwave sounding unit (MSU) instruments for 1979–97. The various analyses are performed using monthly mean temperatures from a near–globally distributed network of 87 radiosonde stations.

The severity of instrument-related problems, which varies markedly by geographic region, was found, in general, to increase from the lower troposphere to the lower stratosphere, although surface data were found to be as problematic as data from the stratosphere. Except for the surface, there is a tendency for changes in instruments to artificially lower temperature readings with time, so that adjusting the data to account for this results in increased tropospheric warming and decreased stratospheric cooling. Furthermore, the adjustments tend to enhance warming in the upper troposphere more than in the lower troposphere; such sensitivity may have implications for “fingerprint” assessments of climate change. However, the most sensitive part of the vertical profile with regard to its shape was near the surface, particularly at regional scales. In particular, the lower-tropospheric lapse rate was found to be especially sensitive to adjustment as well as spatial sampling. In the lower stratosphere, instrument-related biases were found to artificially inflate latitudinal differences, leading to statistically significantly more cooling in the Tropics than elsewhere. After adjustment there were no significant differences between the latitude zones.

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John C. H. Chiang, Stephen E. Zebiak, and Mark A. Cane

Abstract

Elevated heating by cumulus convection and sea surface temperature gradients are both thought to contribute to surface winds over tropical oceans. The relative strength and role of each mechanism is examined by imposing forcing derived from data on a linear primitive equation model with idealized parameterizations for the two forcings, and comparing the response with observed surface winds. Two test cases are studied: one related to the El Niño–Southern Oscillation, and the other related to the “dipole” mode in the tropical Atlantic. It is found that in both cases, elevated heating dominates the surface zonal wind response, and contributes significantly to the meridional wind response, especially in the subtropics and the South Pacific and South Atlantic convergence zone regions. Surface temperature gradients dominate the meridional wind forcing in regions near the equator with strong meridional temperature gradients.

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Jason A. Sippel, John W. Nielsen-Gammon, and Stephen E. Allen

Abstract

This study explores the extent to which potential vorticity (PV) generation and superposition were relevant on a variety of scales during the genesis of Tropical Storm Allison. Allison formed close to shore, and the combination of continuous Doppler radar, satellite, aircraft, and surface observations allows for the examination of tropical cyclogenesis in great detail.

Preceding Allison’s genesis, PV superposition on the large scale created an environment where decreased vertical shear and increased instability, surface fluxes, and low-level cyclonic vorticity coexisted. This presented a favorable environment for meso-α-scale PV production by widespread convection and led to the formation of surface-based, meso-β-scale vortices [termed convective burst vortices (CBVs)]. The CBVs seemed to form in association with intense bursts of convection and rotated around each other within the meso-α circulation field. One CBV eventually superposed with a mesoscale convective vortex (MCV), resulting in a more concentrated surface vortex with stronger pressure gradients.

The unstable, vorticity-rich environment was also favorable for the development of even smaller, meso-γ-scale vortices that formed within the cores of deep convective cells. Several meso-γ-scale convective vortices were present in the immediate vicinity when a CBV developed, and the smaller vortices may have contributed to the formation of the CBV. The convection associated with the meso-γ vortices also fed PV into existing CBVs.

Much of the vortex behavior observed in Allison has been documented or simulated in studies of other tropical cyclones. Multiscale vortex formation and interaction may be a common aspect of many tropical cyclogenesis events.

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John R. Lanzante, Stephen A. Klein, and Dian J. Seidel

Abstract

Historical changes in instrumentation and recording practices have severely compromised the temporal homogeneity of radiosonde data, a crucial issue for the determination of long-term trends. Methods developed to deal with these homogeneity problems have been applied to a near–globally distributed network of 87 stations using monthly temperature data at mandatory pressure levels, covering the period 1948–97. The homogenization process begins with the identification of artificial discontinuities through visual examination of graphical and textual materials, including temperature time series, transformations of the temperature data, and independent indicators of climate variability, as well as ancillary information such as station history metadata. To ameliorate each problem encountered, a modification was applied in the form of data adjustment or data deletion. A companion paper (Part II) reports on various analyses, particularly trend related, based on the modified data resulting from the method presented here.

Application of the procedures to the 87-station network revealed a number of systematic problems. The effects of the 1957 global 3-h shift of standard observation times (from 0300/1500 to 0000/1200 UTC) are seen at many stations, especially near the surface and in the stratosphere. Temperatures from Australian and former Soviet stations have been plagued by numerous serious problems throughout their history. Some stations, especially Soviet ones up until ∼1970, show a tendency for episodic drops in temperature that produce spurious downward trends. Stations from Africa and neighboring regions are found to be the most problematic; in some cases even the character of the interannual variability is unreliable. It is also found that temporal variations in observation time can lead to inhomogeneities as serious as the worst instrument-related problems.

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Stephen E. Schwartz, Robert J. Charlson, Ralph A. Kahn, John A. Ogren, and Henning Rodhe
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Stephen E. Schwartz, Robert J. Charlson, Ralph A. Kahn, John A. Ogren, and Henning Rodhe

Abstract

The observed increase in global mean surface temperature (GMST) over the industrial era is less than 40% of that expected from observed increases in long-lived greenhouse gases together with the best-estimate equilibrium climate sensitivity given by the 2007 Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Possible reasons for this warming discrepancy are systematically examined here. The warming discrepancy is found to be due mainly to some combination of two factors: the IPCC best estimate of climate sensitivity being too high and/or the greenhouse gas forcing being partially offset by forcing by increased concentrations of atmospheric aerosols; the increase in global heat content due to thermal disequilibrium accounts for less than 25% of the discrepancy, and cooling by natural temperature variation can account for only about 15%. Current uncertainty in climate sensitivity is shown to preclude determining the amount of future fossil fuel CO2 emissions that would be compatible with any chosen maximum allowable increase in GMST; even the sign of such allowable future emissions is unconstrained. Resolving this situation, by empirical determination of the earth’s climate sensitivity from the historical record over the industrial period or through use of climate models whose accuracy is evaluated by their performance over this period, is shown to require substantial reduction in the uncertainty of aerosol forcing over this period.

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