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K. S. Liu and Johnny C. L. Chan

Abstract

This paper presents the important climatological features of the tropical cyclones making landfall along the South China coast and proposes a statistical scheme for the prediction of the annual number of such tropical cyclones. This number is found to have a large variation, which is mainly due to the occurrence or nonoccurrence of the El Niño–Southern Oscillation (ENSO) phenomenon. A strong El Niño event is found to reduce the number of landfalling tropical cyclones whereas more tropical cyclones tend to make landfall in years associated with La Niña events. Such variations are more prominent in some seasons. The late season (October–November) activity is generally suppressed (enhanced) in El Niño (La Niña) years whereas the chance of a tropical cyclone striking the South China coast increases (decreases) significantly in the early season (May and June) after the mature phase of a La Niña (El Niño) event. These anomalous activities are apparently linked to the ENSO-induced anomalies in the low- and midlevel large-scale circulation.

Based on the ENSO-related indices such as the Niño-3.4 sea surface temperature anomaly and the equatorial Southern Oscillation index, a statistical prediction scheme for the annual number of such landfalling tropical cyclones by 1 April is developed using the projection–pursuit regression technique. This scheme provides a 40% skill improvement in root-mean-square error with respect to climatology. A real-time prediction made in 2001 gave reasonable results.

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Paul C. Liu and Gerald S. Miller

Abstract

The recently advanced approach of wavelet transforms is applied to the analysis of ocean currents. The conventional analyses of time series in the frequency domain can be readily generalized to the frequency.and time domain using wavelet transforms. An application of wavelet analysis to a set of observed current data acquired during the spring of 1991 in Lake Michigan shows some significant time-localized characteristics that would not be detected using the traditional Fourier transform approach.

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S. C. Liu and T. M. Donahue

Abstract

The thermal escape rate of hydrogen inferred from exospheric density measurements is too low by a factor of at least 2 to accord with the mixing ratio of H2O, H2 and CH4 observed in the stratosphere and mesosphere. The effect on the mixing ratio of adding hydrogen fluxes to support the polar wind by lateral flow and to supply a loss to charge transfer with fast protons is investigated. It is shown that these additional mechanisms can make up the deficit. The exobase hydrogen density must adjust locally to supply hydrogen to the three separate escape mechanisms at the rate demanded by the mixing ratio in the lower atmosphere and the exospheric temperature.

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K. S. Liu and Johnny C. L. Chan

Abstract

This study investigates the synoptic flow patterns associated with small and large tropical cyclones (TCs) that occurred over the western North Pacific between 1991 and 1996. The size of a TC is defined as the azimuthally averaged radius from the TC center at which the relative vorticity decreases to 1 × 10−5 s−1. Calculation of the relative vorticity is based on the satellite-derived surface winds of the European Remote Sensing Satellites 1 and 2 (ERS-1 and ERS-2). Operational analyses of the U.K. Met Office are employed to identify the synoptic patterns around the TCs.

Characteristic synoptic patterns at 850 hPa can be identified with TCs of different sizes. The southwesterly surge and late-season patterns are related to large TCs while the dominant ridge and monsoon-gyre patterns are associated with the occurrence of a small TC. A case study of Typhoon Bart demonstrates the time evolution of the synoptic pattern and its relationship with the TC size change. Bart exhibited a distinct transition from the dominant ridge synoptic pattern to the southwesterly surge synoptic pattern and, correspondingly, the size of Bart increased significantly.

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K. S. Liu and Johnny C. L. Chan

Abstract

The sizes of the tropical cyclones (TCs) occurring over the western North Pacific (WNP) and the North Atlantic between 1991 and 1996 are estimated to establish a database for the study of the climatology of TC size and the physical processes responsible for the size changes of TCs. Wind data from the scatterometer onboard the European Remote-Sensing Satellites 1 and 2 (ERS-1 and ERS-2) form the data source for defining the TC size. The size of a TC is defined as the mean radius at which the relative vorticity decreases to 1 × 10−5 s−1. The mean TC size is found to be 3.7° lat for WNP TCs and 3.0° lat for those in the North Atlantic. Such a difference in size between the two basins is statistically significant at the 95% confidence level. The mean TC size in the WNP is also found to vary seasonally, with a value larger in the late season (October and November) than in midsummer (July and August). These results generally agree with those from previous studies using other measures of size. The size changes (increasing or decreasing) of some TCs are also identified. The high-resolution surface wind data from the ERS satellites are shown to be a valuable tool in the study of TC sizes.

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S. C. Liu and T. M. Donahue

Abstract

A model for H2O, CH4, H2 and odd hydrogen is developed that properly relates the measured mixing ratios in the stratosphere to escape of H in the form of Jeans flux, charge exchange and polar wind. The resulting model predicts a temperature-dependent jeans flux in agreement with recent measurements.

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S. C. Liu and T. M. Donahue

Abstract

The distribution of hydrogen compounds between 50 km and the exobase is calculated subject to the condition that the upward flux of hydrogen atoms be sufficient to supply the thermal escape flux. The effect of varying many parameters, such as exospheric temperature, chemical rate constants, solar UV flux, and atmospheric transport coefficients is explored. It is found that H2 plays an important role in the chemistry and transport even above 100 km. It is found that the escape flux is determined mainly by the total mixing ratio and relatively insensitive to other factors at exospheric temperatures above 1000K, but is limited by the exosbase flow at lower temperatures. A thermal escape flux of 7×107 cm−2 sec−1 above 1000K is difficult to reconcile with a combine mixing ratio of H2O, H2 and CH4 greater than about 2 ppm at 50 km.

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Chester S. Gardner, Marcus S. Miller, and C. H. Liu

Abstract

During 13 nights of Rayleigh lidar measurements at Urbana Illinois in 1984–86, thirty-six quasi-monochromatic gravity waves were observed in the 35–50 km altitude region of the stratosphere. The characteristics of the waves are compared with other lidar and radar measurements of gravity waves and with theoretical models of wave saturation and dissipation phenomena. The measured vertical wavelengths (λ2) ranged from 2 to 11.5 km and the measured vertical phase velocities (c z) ranged from 10 to 85 cm s−1. The vertical wavelengths and vertical phase velocities were used to infer observed wave periods (T ob) which ranged from 100 to 1000 min and horizontal wavelengths (λx) which ranged firm 70 to 2000 km. There may be errors, in the inferred values of the horizontal wavelengths because they were calculated by assuming that the observed period inferred the intrinsic period. Dominant wave activity was found at vertical wavelengths between 2–4 km and 7–10 km. No significant seasonal variations were evident in the observed parameters. Vertical and horizontal wavelengths showed a clear tendency to increase with T ob, which is consistent with recent sodium lidar studies of quasi-monochromatic waves near the mesopause. An average amplitude growth length of 20.9 km for the rms wind perturbations was estimated from the data. Kinetic energy density associated with the waves decreased with height, suggesting that waves in this altitude region were subject to dissipation or saturation effects.

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K. N. Liou, S. C. Ou, Y. Takano, and Q. Liu

Abstract

The delta-four-stream polarized (vector) thermal radiative transfer has been formulated and numerically tested specifically for application to satellite data assimilation in cloudy atmospheres. It is shown that for thermal emission in the earth’s atmosphere, the [I, Q] component of the Stokes vector can be decoupled from the [U, V] component and that the solution of the vector equation set involving the four-stream approximation can be expressed in an analytic form similar to the scalar case. Thus, the computer time requirement can be optimized for the simulation of forward radiances and their derivatives. Computations have been carried out to illustrate the accuracy and efficiency of this method by comparing radiance and polarization results to those computed from the exact doubling method for radiative transfer for a number of thermal infrared and microwave frequencies. Excellent agreement within 1% is shown for the radiance results for all satellite viewing angles and cloud optical depths. For polarization, differences between the two are less than 5% if brightness temperature is used in the analysis. On balance of the computational speed and accuracy, the four-stream approximation for radiative transfer appears to be an attractive means for the simulation of cloudy radiances and polarization for research and data assimilation purposes.

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Zhengyu Liu, S. G. H. Philander, and R. C. Pacanowski

Abstract

Experiments with an oceanic general circulation model indicate that the tropical and subtropical oceanic circulations are linked in three ways. Far from coast in the oceanic interior, equatorial surface waters flow poleward to the southern part of the subtropical gyre, and then are subducted and returned in the thermocline to the upper part of the core of the Equatorial Undercurrent. There is, in addition, a surface western boundary current that carries waters from the equatorial region to the northern part of the subtropical gyre. After subduction, that water reaches the equator by means of a subsurface western boundary current and provides a substantial part (2/3 approximately) of the initial transport of the Equatorial Undercurrent. The eastward flow in the Equatorial Undercurrent is part of an intense equatorial cell in which water rises to the surface at the equator, drifts westward and poleward, then sinks near 3° latitude to flow equatorward where it rejoins the undercurrent.

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