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C-P. Chang
and
R. T. Williams

Abstract

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R. Saravanan
and
James C. Mc Williams

Abstract

An idealized coupled ocean–atmosphere is constructed to study climatic equilibria and variability. The model focuses on the role of large-scale fluid motions in the climate system. The atmospheric component is an eddy-resolving two-level global primitive equation model with simplified physical parameterizations. The oceanic component is a zonally averaged sector model of the thermohaline circulation. The two components exchange heat and freshwater fluxes synchronously. Coupled integrations are carried out over periods of several centuries to identify the equilibrium states of the ocean–atmosphere system. It is shown that there exist at least three types of equilibria, which are distinguished by whether they have upwelling or downwelling in the polar regions. Each of the coupled equilibria has a close analog in the ocean-only model with mixed boundary conditions. The oceanic circulation in the coupled model exhibits natural variability on interdecadal and longer timescales. The dominant interdecadal mode of variability is associated with the advection of oceanic temperature anomalies in the sinking regions. The sensitivity of the coupled model to climatic perturbations is studied. A rapid increase in the greenhouse gas concentrations leads to a collapse of the meridional overturning in the ocean. Introduction of a large positive surface freshwater anomaly in the high latitudes leads to a temporary suppression of the sinking motion, followed by a rapid recovery, due primarily to the high latitude cooling associated with the reduction of oceanic heat transport. In this evolution, the secondary roles played by the atmospheric heat transport and moisture transport in destabilizing the thermohaline circulation are compared, and the former is found to be dominant.

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C. J. R. Williams
,
D. R. Kniveton
, and
R. Layberry

Abstract

It is generally agreed that changing climate variability, and the associated change in climate extremes, may have a greater impact on environmentally vulnerable regions than a changing mean. This research investigates rainfall variability, rainfall extremes, and their associations with atmospheric and oceanic circulations over southern Africa, a region that is considered particularly vulnerable to extreme events because of numerous environmental, social, and economic pressures. Because rainfall variability is a function of scale, high-resolution data are needed to identify extreme events. Thus, this research uses remotely sensed rainfall data and climate model experiments at high spatial and temporal resolution, with the overall aim being to investigate the ways in which sea surface temperature (SST) anomalies influence rainfall extremes over southern Africa.

Extreme rainfall identification is achieved by the high-resolution microwave/infrared rainfall algorithm dataset. This comprises satellite-derived daily rainfall from 1993 to 2002 and covers southern Africa at a spatial resolution of 0.1° latitude–longitude. Extremes are extracted and used with reanalysis data to study possible circulation anomalies associated with extreme rainfall. Anomalously cold SSTs in the central South Atlantic and warm SSTs off the coast of southwestern Africa seem to be statistically related to rainfall extremes. Further, through a number of idealized climate model experiments, it would appear that both decreasing SSTs in the central South Atlantic and increasing SSTs off the coast of southwestern Africa lead to a demonstrable increase in daily rainfall and rainfall extremes over southern Africa, via local effects such as increased convection and remote effects such as an adjustment of the Walker-type circulation.

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R. T. Williams
and
Johnny C-L. Chan

Abstract

The motion of tropical vortices in east–west mean flows is studied with the barotropic vorticity equation on the beta plane. The vorticity equation is integrated numerically from an initially symmetric vortex embedded in (i) a linear shear flow or (ii) a parabolic jet. The first experiment with flow (i) has β = 0 and it is linearized about the mean flow. The vortex is distorted by the mean flow so that the even Fourier components around the vortex grow, but the vortex does not move. When nonlinear effects are included the distortion is damped in the inner part of the vortex, but wavenumber two grows in the outer region. The addition of the beta effect causes the vortex to move in the same direction as the no mean flow solution provided the mean flow advection is removed from the trajectories. The trajectory for the anticyclonic mean flow is significantly longer than the cyclonic and no mean flow trajectories, which are about equal. For mean flow (ii), with the same absolute vorticity gradient as β but on an f plane, the vortex has a much shorter trajectory and a more westerly direction of movement than the no mean flow solution with beta. This effect comes from the advective distortion of the vortex, which projects onto wavenumber one in the disturbance vorticity equation. It is shown with other experiments that beta has a stronger effect on vortex motion than the relative vorticity gradient.

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Johnny C. L. Chan
and
R. T. Williams

Abstract

The β-effect on tropical cyclone motion is studied using an analytical as well as a numerical model in a nondivergent barotropic framework. The analytical model and the linear version of the numerical model give essentially the same result: the linear β-effect causes a westward stretching of the model vortex but no significant movement of the vortex center. An east-west asymmetry in the meridional wind field is also created. It is the inclusion of the nonlinear term that produces the northwestward movement of the vortex previously found by other investigators (e.g., Kitade, 1981). This northwestward movement increases with both the maximum wind speed and the radius of maximum wind in a constant-shape vortex. A wind maximum is also found to the northeast of the vortex, which appears to be consistent with the observational findings of Shea and Gray. This asymmetry plays an important role in the vortex motion.

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L. C. Chou
,
C-P. Chang
, and
R. T. Williams

Abstract

A two-dimensional frontal model was used to study the structure and behavior of the Mei-Yu front over East Asia. The Mei-Yu front is characterized by mixed midlatitude-baroclinic and tropical-convective properties, with frequent occurrence of a low-level jet (LLJ) that is highly correlated with heavy convective rainfall.

The quasi-steady state responses to a large-scale stretching deformation forcing were obtained by integrating the perturbation equations from an initial state of seasonal-mean zonal flow. Two major sets of experiments were conducted to simulate different midlatitude and subtropical conditions. The midlatitude front extends deeply into the upper troposphere with a strong poleward tilt, whereas the subtropical front is confined to the lower troposphere with less tilt, in good agreement with observations. Along the sloping front, slantwise updrafts develop with a multiband structure. This updraft is more evident in the subtropical cases and in the more moist midlatitude cases.

For the subtropical cases, concurrent development of upper-level easterlies and low-level westerlies equatorward of the front is observed. The low-level westerly maximum at z=3–4 km resembles a LLJ, whose intensity increases when more moisture is included. The concurrent development suggests that the LIJ may be the result of a thermally direct secondary circulation that resembles a “reversed Hadley” cell. This circulation is revealed by a meridional–vertical streamfunction, with a strong lower branch return flow coinciding with the development of a LLJ in the more moist, subtropical cases. The Coriolis torque of the meridional circulation can develop and maintain the upper easterlies and the LLJ. Importance of cumulus convection and especially a slant-wise structure in developing the reversed Hadley cell and the LLJ is suggested.

These conclusions are consistent with the observed intense convection and heavy rainfall in the Mei-Yu front, and a sinking region south of the Baiu front as revealed by Matsumoto's moisture analysis.

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R. T. Williams
,
Lang C. Chou
, and
C. J. Cornelius

Abstract

A basic numerical model is described which contains diffusions of momentum and heat which balance frontogenetic advections, and also condensation heating and a moist convective adjustment mechanism. Numerical solutions show that frontal zones are strengthened above the planetary boundary layer by condensation heating, but there is little effect at the surface. Experiments with surface frontal motion show only a slight difference between warm and cold fronts, contrary to observations.

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R. T. Williams
,
H. Lim
, and
C-P. Chang

Abstract

In this study, nonlinear effects of barotropic instability in a downstream varying easterly jet are studied and compared with previous linear model results of Tupaz and others. The barotropic vorticity equation with Rayleigh friction and forcing is solved with finite differences. The initial mean flow is an easterly Bickley jet whose maximum speed and half-width vary downstream; the half-width ranges from 500 to 1200 km and the maximum speed is 30 m s−1. The time-independent forcing makes the initial mean flow, which is unstable in the central jet region, a steady-state solution to the vorticity equation. A disturbance with wavenumber 10, which is predicted to be locally unstable and most dominant based on linear model results, is added to the initial mean flow. The equation is then integrated numerically for 450 days.

The solutions may be separated into two phases: 1) an initial adjustment phase which consists of several ∼50-day cycles wherein an initial wavenumber 10 disturbance grows rapidly in the jet region, and then the disturbance energy shifts to a slightly longer wavelength and decays before the next cycle; and 2) a quasi-equilibrium phase which is achieved after 350 days. Fourier analysis of the disturbance streamfunction at each point during a typical interval in the adjustment phase shows two dominant modes with periods near 3.35 days and 3.58 days, respectively. After entering the quasi-equilibrium phase, a 4-day oscillation develops in the kinetic energy and the main periods of the streamfunction become 4 and 2 days, respectively. The former is the dominant mode and the latter is the result of the nonlinear self-interaction by the former. The frequency of the dominant mode is equal to the frequency of the most unstable mode from a parallel flow calculation based on the outflow region mean flow. However, in most of the unstable region, it is much less than the most unstable local frequency inferred from the parallel flow solution.

The dominant mode in the quasi-equilibrium phase propagates through the modified mean flow essentially as a linear wave, and its behavior can be compared with the linear model results. However, its maximum growth rate is 25% larger than the highest local growth rate for the parallel flow solution. This “enhancement effect” is also larger than was found by Tupaz and others. In addition, there is a hysteresis effect wherein the growth rate curve and the phase structure from the full model are shifted downstream relative to the parallel flow solution, similar to the linear model results. On the other hand, the wavelength is generally short in the jet region and much longer in the outer regions, opposite to the wavelength variation in Tupaz and others. With the help of a generalized Rossby wave formula, it is shown that two effects determine the downstream variation of the disturbance wavelength: 1) the variation of the latitudinal integral of the mean zonal wind and 2) the variation of the latitudinal integral of the mean absolute vorticity gradient. Due to the difference in disturbance scale, the second effect dominates in the quasi-equilibrium phase of this study while the first effect dominates the linear model used by Tupaz and others.

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J. B. Tupaz
,
R. T. Williams
, and
C-P. Chang

Abstract

The structure and behavior of barotropically unstable and stable waves in the vicinity of a zonally varying easterly jet are studied numerically with a linearized barotropic vorticity equation on a β plane. The easterly jet is approximated by a Bickley jet with a slow zonal variation. The numerical results are also compared with a simple mechanistic analytical model using the local phase speed and growth rate concepts. In several aspects the results are grossly similar to that expected from the parallel flow theory of barotropic instability. However, in the unstable region the resultant structure of the waves causes a spatial growth rate greater than predicted by the local growth rates computed with a parallel flow model. In the stable region, the structure leads to a strong dynamic damping. When a uniform advective velocity is added to a variable mean flow, the difference between the magnitude of the growth rate of the computed waves and that implied by the parallel flow theory is somewhat reduced. However, in this case a stronger zonal asymmetry in the spatial growth rate curve with respect to the jet maximum occurs as a result of slower adjustment of the wave structure to the local stability conditions.

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K. S. Gage
,
C. R. Williams
, and
W. L. Ecklund

Uncertainty in the magnitude and distribution of diabatic heating associated with precipitating cloud systems is one of the major factors giving rise to uncertainty in the simulation of large-scale atmospheric circulations in numerical models of the atmosphere. A major international effort is under way to develop an improved parameterization of the hydrological cycle within numerical models. Progress will require better observations of the distribution of the diabatic heating associated with cloud systems in the Tropics. In this paper new observations are presented demonstrating the potential of UHF profilers for diagnosing the vertical structure of convective systems in the Tropics. These preliminary results indicate that while mesoscale convective systems are prevalent in the Tropics there are important contributions to rainfall from smaller-scale warm rain systems that do not extend above the freezing level. They also show that extensive regions of upper-tropospheric precipitating clouds often exist at times when no rain is detected at the surface. These observations provide important information that should prove useful in developing improved methods for estimating precipitation from satellite observations.

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