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Global daily reanalysis fields from the Navy Operational Global Atmospheric Prediction System (NOGAPS) are used to analyze Northern Hemisphere summertime (June–September) developing and nondeveloping disturbances for tropical cyclone (TC) formation from 2003 to 2008. This is Part II of the study focusing on the western North Pacific (WNP), following Part I for the North Atlantic (NATL) basin. Tropical cyclone genesis in the WNP shows different characteristics from that in the NATL in both large-scale environmental conditions and prestorm disturbances. A box difference index (BDI) is used to identify parameters in differentiating between the developing and nondeveloping disturbances. In order of importance, they are 1) 800-hPa maximum relative vorticity, 2) rain rate, 3) vertically averaged horizontal shear, 4) vertically averaged divergence, 5) 925–400-hPa water vapor content, 6) SST, and 7) translational speed. The study indicates that dynamic variables are more important in TC genesis in the WNP, while in Part I of the study the thermodynamic variables are identified as more important in the NATL. The characteristic differences between the WNP and the NATL are compared.

This study investigates the characteristic differences of tropical disturbances that eventually develop into tropical cyclones (TCs) versus those that did not, using global daily analysis fields of the Navy Operational Global Atmospheric Prediction System (NOGAPS) from the years 2003 to 2008. Time filtering is applied to the data to extract tropical waves with different frequencies. Waves with a 3–8-day period represent the synoptic-scale disturbances that are representatives as precursors of TCs, and waves with periods greater than 20 days represent the large-scale background environmental flow. Composites are made for the developing and nondeveloping synoptic-scale disturbances in a Lagrangian frame following the disturbances. Similarities and differences between them are analyzed to understand the dynamics and thermodynamics of TC genesis. Part I of this study focuses on events in the North Atlantic, while Part II focuses on the western North Pacific. A box difference index (BDI), accounting for both the mean and variability of the individual sample, is introduced to subjectively and quantitatively identify controlling parameters measuring the differences between developing and nondeveloping disturbances. Larger amplitude of the BDI implies a greater possibility to differentiate the difference between two groups. Based on their BDI values, the following parameters are identified as the best predictors for cyclogenesis in the North Atlantic, in the order of importance: 1) water vapor content within 925 and 400 hPa, 2) rain rate, 3) sea surface temperature (SST), 4) 700-hPa maximum relative vorticity, 5) 1000–600-hPa vertical shear, 6) translational speed, and 7) vertically averaged horizontal shear. This list identifies thermodynamic variables as more important controlling parameters than dynamic variables for TC genesis in the North Atlantic. When the east and west (separated by 40°W) Atlantic are examined separately, the 925–400-hPa water vapor content remains as the most important parameter for both regions. The SST and maximum vorticity at 700 hPa have higher importance in the east Atlantic, while SST becomes less important and the vertically averaged horizontal shear and horizontal divergence become more important in the west Atlantic.

Eleven (10 Pacific, 1 Atlantic) tropical cyclones (TCs), which include typhoons/hurricanes and tropical storms, are examined using the latest 40-yr ECMWF Re-Analysis (ERA-40) dataset and Joint Typhoon Warning Center (JTWC) best-track data to determine if and how tropical upper-tropospheric trough (TUTT) cells influence TC tracks. This type of interaction has led to rather large TC track forecast errors at 72 h (2000+ km) in the northwest Pacific and is often ignored or poorly forecast due to inadequate numerical model TUTT cell predictions. Ten selected cases out of the initial 25 potential Pacific cases exhibited a “nonstandard” TC track; a TUTT cell was the sole large-scale transient feature within 2000 km of the TC’s center, and the TC intensity was >17 m s−1. The circulations’ separation distance, orientation, intensity, and TUTT cell’s closed circulation size are critical characteristics in determining the likelihood of a TUTT cell influencing a TC track. Interactions occur at distances greater than 1700 km, continue for periods from 24 to 48 h, and occur 2–3 times per year in the NW Pacific. Examination of the TC’s tropospheric winds’ deep layer mean (100–1000 hPa), and upper (100–500 hPa), middle (300–850 hPa), and lower (500–1000 hPa) layers, along with various quadrants of the upper layer, demonstrate a link between the TUTT cell’s wind field and the nonstandard TC tracks. A conceptual model of how a TUTT cell can influence TC track is presented. The model provides decision-grade operational guidance for TC forecasters using pattern recognition scenarios. Application of the conceptual model at the JTWC is currently under way.

Using 1979–2003 radiosonde data at Hilo and Līhu‘e, Hawaii, the trade wind inversion (TWI) is found to occur approximately 82% of the time at each station, with average base heights of 2225 m (781.9 hPa) for Hilo and 2076 m (798.8 hPa) for Līhu‘e. A diurnal pattern in base height of nighttime high and afternoon low is consistently found during summer at Hilo. Inversion base height has a September maximum and a secondary maximum in April. Frequency of inversion occurrence was found to be higher during winters and lower during summers of El Niño years than non–El Niño years. Significant upward trends were found for inversion frequency at Hilo for March–May (MAM), June–August (JJA), and September–November (SON) seasons, and at Līhu‘e for all seasons and for annual values.

On 26 June 2003 (approximately 1030 local time) the Helios ultralight aircraft broke apart off the west coast of Kauai Island, Hawaii as it was climbing out of the Kauai wind shadow. Following the aircraft mishap, a study was carried out to understand the conditions on the day of the crash and to better characterize the wind in the lee of Kauai. As part of this effort, both aircraft measurements and numerical modelling studies were carried out. Measurements and models showed the trade wind flow was enhanced around the island creating a region of wind shear surrounding the leeside calm zone. This wind shear region was found to be vertically oriented along the south side but tilted northward with height along the northern side of the calm zone. Several other factors on the day of the crash were investigated including water vapour gradients, diurnal Island heating, and gravity waves but their possible influences on the crash could not be confirmed. While the numerical model captured the general features of the Kauai leeside winds, the orientation of the calm zone was north of the observed one.

Emissions from the Kilauea volcano pose significant environmental and health risks to the Hawaiian community. This paper describes progress toward simulating the concentration and dispersion of plumes of volcanic aerosol after they emanate from the Pu'u O'o vent of the Kilauea volcano. In order to produce an accurate regional forecast of the concentration and dispersion of volcanic aerosol, the Hybrid Single-Particle Lagrangian Integrated Trajectory (HY-SPLIT) model was used. Wind fields and thermodynamic data from the non-hydrostatic Mesoscale Spectral Model (MSM) were employed as input for the HY-SPLIT model. A combination of satellite remote sensing, aircraft, and ground-based observations collected during a field experiment was used to validate the model simulation of aerosol distribution. The HY-SPLIT model shows skill in reproducing the plume shape, orientation, and concentration gradients as deduced from satellite images of aerosol optical depth. Comparison of the modelled and observed values suggests that the model was able to produce reasonable plume concentrations and spatial gradients downwind of the source. Model concentrations were generally less than those observed on the leeward side of the Island of Hawaii. This deficiency may be explained by a lack of (i) background concentrations, (ii) local sources of pollution and/or (iii) sea-breeze circulation in the prognostic input wind field. These results represent early progress toward the goal of future operational application of the HY-SPLIT model to predict volcanic aerosol concentrations in Hawaii. This may help mitigate their negative impacts of plumes respiratory health, agriculture, and general aviation.

On the island of Maui, the dispersion of air pollutants associated with the field burning of biomass is complicated because the persistent Maui Vortex in the lee of Haleakala tends to trap smoke making pollution concentration worse in the central valley. In the present study we describe the spatially as well as temporally continuous short-term climatology of the airflow in the central valley of Maui during a summer month under trade wind conditions, simulated by a high-resolution mesoscale model initialized and forced by large-scale objective analyses. These simulations are compared with station observations. Under persistent trade wind conditions, an eddy would form in the central valley, and would remain steady and stationary regardless of the time of the day with little indications of vortex shedding even if the Froude number Fr = U/NH < 0.4, where U is the uniform flow speed, H the height of the obstacle, and N the BruntVaisala frequency. However, vortex shedding could occur if the West Maui mountains were removed. Two major factors contributing to formation, maintenance, and steadiness of the Maui Vortex, solar heating and the accelerated northerly flow over the valley resulting from deflection of the trades by West Maui, are discussed. Suggestions are made for preferred synoptic conditions for field burning.

The numerical study presented here focuses on baroclinic processes that contribute to tropical cyclone (TC) propagation. A three-dimensional, semispectral, primitive equation model of baroclinic vortex was developed to study TC motion. In a tilted vortex, interaction between upper- and lower-level vorticity anomalies leads to vortex propagation relative to the steering flow. On a, β plane, with no environmental flow, the vortex is tilted toward the south and the interaction between the layers reduces the westward movement of the vortex. The vortex tilting can also occur due to the vertical shear in the environmental wind. On an f plane, the interaction between the layers causes the northward movement of the vortex in westerly linear shear, and southward movement in easterly linear shear, with a meridional velocity of about 1 m s−1. Ibis velocity increases with increasing vortex intensity and vertical motion.

The paper examines the role of the development of outflow-layer instabilities on the motion of tropical cyclones. The influence of barotropic instability is examined by comparing the time changes in the storm tracks with the frequencies of free, unstable barotropic modes. For intense vortices barotropic instability is shown to contribute to the slow (periods of a few days) trochoidal motion of a cyclone. The development of instability depends on the horizontal distribution and frequency of environmental forcing. The strongest response occurs when the frequency of the forcing matches the frequency of an unstable mode.

The Hawaiian Islands consist of dramatic terrain changes over short distances, resulting in a variety of microclimates in close proximity. To handle these challenging conditions, weather models must be run at very fine vertical and horizontal resolutions to produce accurate forecasts. Computational demands require Weather Research and Forecasting (WRF) to be executed in parallel on the Maui High Performance Computing Center's Mana system, a Power Edge M610 Linux cluster. This machine has 1,152 compute nodes, each with two 2.8GHz quad-core Intel® Nehalem processors and 24GB RAM. Realizing maximum performance on Mana relied on the determination of an optimal number of cores to use per socket, the efficiency of an Message Passing Interface (MPI)-only implementation, an optimal set of parameters for adaptive time-stepping, a way to meet the strict stability requirements necessary for Hawaii, effective choices for processor and memory affinity, and parallel automation techniques for producing forecast imagery.

We propose a quasi-balance dynamical system of equations for slowly evolving, large-scale motions in the tropics. Unlike other balance schemes, equatorially trapped Kelvin waves are included. Following the lead of Gill, this system is based on the neglect of the meridional acceleration (the so-called long-wave approximation). Consistent budgets for kinetic energy and potential vorticity are derived, with those quantities appropriately redefined. Unlike the quasi-geostrophic system, vertical advection of potential vorticity as well as of potential temperature is relevant. A primary circulation is defined, involving the zonal wind, geopotential, and potential temperature, such that potential vorticity depends on latitudinal and vertical gradients of the primary circulation. A residual secondary circulation is governed by an elliptic equation for a streamfunction in the meridional plane. The invertibility principle is affected by the Kelvin waves with zero potential vorticity. The eigenfrequencies f...

The barotropic instability of basic states with downstream and asymmetric cross-stream variations is investigated using the linearized, non-divergent barotropic vorticity equation in a periodic beta channel. A single expression for streamfunction defines the basic state; the variations are introduced by parameter changes. Eigenvalue and time integration methods are employed to determine the instabilities. When downstream variation is present, the maximum amplitude of the unstable streamfunction is always located downstream from the location of maximum latitudinal shear. This occurs because the disturbance propagates through a range of longitudes where the flow is locally barotropically unstable. The instability is sensitive to the degree of downstream variation. When there is strong variation there is more concentration of streamfunction amplitude in the regions downstream of the jet maximum, smaller disturbance scales and smaller growth rates than when the flow is more parallel. The smaller grow...

A critical element to the accurate prediction of fire/weather behaviour is the knowledge of near-surface weather. Weather variables, such as wind, temperature, humidity and precipitation, make direct impacts on the practice of managing prescribed burns and fighting wild fires. State-of-the-art Numerical Weather Prediction (NWP), coupled with the use of high performance computing, now enable significantly improved short-term forecasting of near-surface weather at a 1-3 km grid resolution. This proof of concept project integrates two complementary model types to aid federal agencies in real-time management of fire. (1) A highly complex, full-physics mesoscale weather prediction model (MM5) which is applied in order to estimate the weather fields up to 72 hours in advance. (2) A diagnostic fire behavior model (FARSITE) takes the near-surface weather fields and computes the expected spread rate of a fire driven by wind, humidity, terrain, and fuels (i.e. vegetation).

In Hawaii, where weather and climate variations are strongly affected by the steep island topography, there is a clear and acknowledged need for improved weather and climate forecasts at increased spatial resolution. The Hawaii Weather, Climate,Modeling Ohana (HWCMO) was therefore formed at the Maui High Performance Computing Center in 1997 to establish an experimental weather/climate mesoscale modeling effort for near real-time support to the local National Weather service (NWS), decision makers in federal and state agencies, and local researchers. This operational mesoscale forecasting effort is currently providing on an almost regular schedule, daily forecast products {http://www.mhpcc.edu/ wswx/), using 5 nodesof the MHPCC multi-node IBM SP2 cluster.

The movement of a set of Lagrangian parcels in the two-dimensional pressure field associated with a tropical convective line is considered. The results are compared with observational characteristics of GATE quasi-two-dimensional convective lines. It is shown that for the composite, slow-moving GATE convective line it is necessary to account for horizontal pressure gradients in order to obtain realistic momentum flux. The role of vertical pressure gradients and initial conditions for an air parcel's movement is studied. Based on the results, some suggestions concerning proper formulation of parameterization of the convective momentum flux are made.

An approximation to the thermodynamic equation was introduced by Holton which was motivated by an energy principle involving a quadratic expression in temperature deviation, which he identified as available potential energy. The approximation consists of neglecting the component of adiabatic warming/cooling due to the temporally and horizontally variable part of the specific volume on a constant pressure surface, WKT *. If the approximation is made, the system lacks a total energy principle involving either total potential energy or available potential energy (except in one special case). Although this approximate form of the primitive equations does not cause substantial error in a numerical simulation of the annual cycle of a zonally-symmetric circulation, there appears to be no significant advantage to its application. Use of the standard set of primitive equations is suggested. DOI: 10.1111/j.1600-0870.1984.tb00260.x

A revised version Of Raymond's wave-CISK model is used to study the propagation of mesoscale modes. The sensitivity of unstable modes to variations in basic state stability and wind show (characteristics of the eastern Atlantic during GATE), as well as parameterized cumulus heating is examined. Unstable, modes exist for mesoscale wavelengths, which are sensitive to the details of the cumulus heating parameters as well as the structure of the large-scale wind shell. In many cases, the most unstable mode has a two-dimensional structure.

A linear primitive equation model has been used to test the hypothesis that the vertical structure of observed Caribbean easterly waves is determined by the interaction between convective heating and the environmental wind. The model determines the response to a propagating heat source in a specified basic state. The model allows for the inclusion of diffusion and cumulus momentum transports. The linear perturbations are assumed to have the form of a single Fourier component in the zonal direction. The frequency and zonal wavelength of the disturbance are taken from observations of the three-dimensional structure of a series of Caribbean easterly waves made by Shapiro. The structure of the basic state zonal wind, assumed to be a function of height, is based on observations near the latitude of largest observed wave amplitude. The maximum heating rate is 5 K day−1, centered at about 19°N. Very good agreement is found between the model-derived vertical structure of the waves and that observed by Sh...

A version of Holton's numerical model of the stratospheric sudden warming is found to be very sensitive to the assumed initial mean zonal wind distribution. In tests with three initial wind profiles, in only one case can we simulate a wavenumber 1 warming involving flow reversal over a deep layer of the polar atmosphere and substantial warming (i.e., a major warming). In the other two cases, flow reversal is noted in restricted areas and warming is less intense. Examination of Eliassen-Palm cross sections and of Matsuno's refractive index squared throughout each integration reveals how the different warnings develop, and why they differ. Examination of refractive index squared in the atmosphere at the time of enhanced wave propagation out of the troposphere may be valuable aid in predicting the likelihood of a warming. Some characteristics of an “ideal” mean zonal wind profile (with which a major warming can develop) are discussed.

The tropical response to a localized thermal forcing with approximately 45-day period is investigated for several models of increasing complexity consisting of two equivalent shallow water systems and two fully stratified systems. The fully stratified models appear to be able to reproduce a number of observed features of the tropical 40-50 day oscillation including the modulation of the subtropical jet and the eastward and poleward propagation of zonal wind anomalies.

The shallow water, linear model in cylindrical coordinates is used to investigate barotropic and inertial instabilities in the tropical cyclone outflow layer. When the mean state wind satisfies the condition |ζ|«|f 0 the instabilities found in our model are equivalent to the barotropic instabilities calculated with the quasi-geostrophic potential vorticity equation. The maximum instability is internal and appears at azimuthal wavenumber one. When an observed (composite) hurricane outflow layer is assumed as a basic state, the largest barotropic growth rate (e-folding time scale ≈7 days) appears at azimuthal wavenumber s = 1 and equivalent depth about 30 m. The maximum growth for s = 2 is about four times smaller than for s = 1 and occurs with the external mode. Inertial instability dominates at the very small equivalent depths and is likely to be stabilized by friction.

A two level, global, spectral model using pressure as a vertical coordinate was developed. The system of equations describing the model is nonlinear and quasi-geostrophic (linear balance). Static stability is variable in the model. A moisture budget is calculated in the lower layer only. Convective adjustment is used to avoid supercritical temperature lapse rates. The mechanical forcing of topography is introduced as a vertical velocity at the lower boundary. Solar forcing is specified assuming a daily mean zenith angle. The differential diabatic heating between land and sea is parameterized. On land and sea ice surfaces, a steady state thermal energy equation is solved to calculate the surface temperature. On the oceans, the sea surface temperature is specified as the climatological average for January. The model is used to simulate the January, February and March circulations. Previously announced in STAR as N83-14822

Through the use of a zonal balance model we investigate the response of the mean meridional circulation to a specified diabatic forcing for both resting and nonresting zonal flows. The use of a potential latitude coordinate and transformed meridional circulation components results in a simplified meridional circulation equation in which the variable coefficients are the normalized potential vorticity and inertial stability. Solutions of this equation illustrate how latent heat release away from the equator forces a winter hemisphere Hadley cell that is more intense than the summer hemisphere cell. This asymmetric response is due primarily to the anisotropy associated with the spatial variation of the inertial stability field. Despite the sensitivity of the meridional circulation to the location and breadth of the forcing, the low latitude thermodynamic response is for the most part insensitive as long as the total latent heat release remains the same. Numerical solutions of the zonal balance mode...

A comparison is made between the magnitudes of observed large-scale weather waves over the tropical Pacific and the magnitudes of the corresponding waves, predicted by wave-CISK theories, which are driven by the observed amount of latent heating (i.e., precipitation). The theoretical wave fields of meridional velocity, vorticity, and temperature rate are shown to exceed the observed quantities by an order of magnitude. An attempt is made to simulate the observed balance between the diabatic heating and adiabatic cooling within the context of the inviscid theories. For a broad class of heating profiles, geometries and basic states, it is found that this compensation without temperature change cannot be satisfactorily modelled, regardless of the vertical shape of the heating, when the vertical wavelength of the disturbance exceeds about 6 km. Scale analysis demonstrates that an important dynamical term has been neglected in the inviscid models, viz., the vertical transport of horizontal momentum by cumulus clouds. When this process is included in the wave model, velocities, relative vorticity, and the time rate of temperature change are all comparable to the observed values. The general behavior of a system where both forcing and cumulus “friction” are proportional to each other has not been previously examined. We find the behavior of such a system to have several novel features. For example, we find that for a wide range of precipitation amplitudes (from 1 4 to 4 times the precipitation amplitudes of waves in the western Pacific) we get essentially constant amplitudes for wind. The implications of this and other features for various aspects of tropical wave modelling are discussed.

The wave-CISK problem is investigated in a linearized context with two significant departures from previous models: 1) the cumulus heating parameterization is made thermodynamically consistent by requiring that cumulus heating exactly equal the latent heat of moisture converged in the wave; and 2) cumulus friction is included in the model. The parameterization of vertical transport of horizontal momentum by cumulus clouds follows the formulation of Schneider and Lindzen (1976); with a constant mean zonal wind, cumulus friction is proportional to the mean cloud mass flux. Results from the model of Stevens et at. (1977) are presented for various scales of motion. For magnitudes of cloud mass flux typical of the ITCZ, no wave-CISK modes appear on the synoptic scale of several thousand kilometers. As the horizontal scales of these long-period waves are decreased, the disturbances become frictionally controlled, neutral wave-CISK modes. When the mean cloud mass flux is reduced by a factor of 2, instab...

We present a detailed analysis of a mesoscale circulation phenomenon that appeared on satellite images as a group of propagating cloud wavelets. These wavelets developed on the anticyclonic side of a strong, upper-level subtropical jet in a region of negative potential vorticity (P). During their lifetime, the wavelets were located over the North American synoptic sounding network; the meteorological conditions surrounding their occurrence are examined. We investigate the various physical mechanisms that may have been responsible for generating the observed cloud wavelets. In consideration of the time scale of the wavelets and their proximity to the region where P < 0, inertial instability appears to be the most likely dynamical mechanism for explaining their existence. Using hourly satellite images, we determined that the wavelets propagated downstream at a phase speed 25 m s−1 slower than the maximum wind speed of the jet with life cycles of 4–5 h, horizontal wavelengths of ∼400 km, and periods...

We investigate the temporal and spatial characteristics of unstable normal modes in a horizontally sheared flow on a sphere using the shallow water equations. Both inertial and barotropic instabilities are identified in cases where the appropriate necessary conditions are satisfied. A primary focus is determining what conditions favor asymmetric modes of inertial instability rather than symmetric modes. With the Bickley jet profile, the region of instability [f(f + &xi) ≤ 0] is confined to the anticyclonic side of the jet in a limited region. We find that symmetric instability is preferred only for modes of very small vertical wide, for which the pressure gradient force is secondary. Relatively small dissipation is needed to stabilize these modes. With deeper vertical scales, asymmetric instabilities are preferred in which the zonal scale of the instability is comparable to the width of the unstable region. This study extends previous results for linear shear on an equatorial beta plane to the mi...

Obserations of longitudinally-averaged zonal flows in the atmosphere and ocean tend to display north–south symmetry about the equator, with a characteristic wind maximum or minimum, and therefore little horizontal wind shear locally near the equator. It is shown that this configuration is required for balanced flow on a sphere to be inertially stable. If dissipation can be neglected, any horizontal wind shear at the equator will cause inertial instability to develop. effectively eliminating the horizontal shear. It follows that the potential vorticity (q) must vanish at the equator for the symmetric circulation to be stable; and that it must increase to the north and decrease to the south. Balanced cross-equatorial flow can occur only if there is a north–south gradient in the torque or the diabatic heating at the equator. These conclusions are obtained under the assumption of a balanced zonal flow; i.e., acceleration and dissipation are explicitly neglected in the meridional momentum equation. Th...

In recent years there has been a great deal of interest in a quasi-periodic tropical oscillation of zonal winds, which was first reported by Madden and Julian. An attempt to determine the temporal variation of the oscillation parameters is presented here. Using a 4-year duration global time series and a 25-year station time series, it is found that although the nonseasonal variations are large, any seasonal cycle in the oscillation amplitude and frequency must be very small. The small seasonal signal in the oscillation frequency seems to argue against explanations for the time scale based on Doppler-shifted traveling waves.

The spherical geometry of the earth is replaced by polar cylindrical geometry, with a plane tangential to the earth at the pole. The resulting frequency and structure of free motions in an isothermal, adiabatic atmosphere with a resting basic state is studied in both geometries. The solutions for ν (meridional wind) may be written as a single Bessel function if certain approximations are made. For positive equivalent depths, the geometrical approximation is best when the Lamb parameter ϵ≳ 10, so that Rossby waves are well modeled, while fast moving gravity waves are not well approximated. The impact of setting f to a constant value when undifferentiated, as in the usual midlatitude beta-plane approximation, is examined. It is found that the value of f is as important in determining how well the model behaves as are the geometrical and other approximations.

The term Walker Circulation is used to refer to the zonal overturning across the equatorial Pacific driven by enhanced convection over the Indonesian region. In this work, an attempt is made to simulate the Walker Circulation using a linear model that includes a cumulus friction parameterization. The work of Geisler is extended by including a realistic mean zonal wind field obtained from the FGGE dataset and a prescribed mean Hadley cell that is computed from an analytical streamfunction. The model is forced by a stationary tropical heat source. The sensitivity of the model circulation to changes in the basic state is examined. Model results show that the inclusion of a nonzero mean zonal wind field tends to enhance the extratropical response in the winter hemisphere. Including a cumulus friction parameterization tends to damp the zonal wind response near the heating center and also lower the level of zero zonal wind in the model Walker Circulation. Including a mean Hadley cell in the basic state...

The linear, zonally symmetric modes of the basic state of a Hadley cell are examined. It is found that the inclusion of the divergent basic state leads to the formation of a new class of slowly oscillating modes, some of which have periods in the range of 40-50 days. The modes have many features in common with the observed tropical 40-50-day oscillation; however, an explanation for the observed fluctuations in convective cloudiness remains a topic for future work.

The budgets of vorticity, momentum and divergence are evaluated for the average synoptic-scale wave composited by Thompson et al. (1979) from Phase III data of the GARP Atlantic Tropical Experiment (GATE). The data are analyzed into a Phase III mean component and a time-varying wave component. The geopotential field which is required for the momentum and divergence budgets is obtained from vertical integration of the hydrostatic equation. It is found that nonlinear terms in all wave budgets tend to be small, so that the disturbances are governed by linear dynamics. In contrast with the composite wave analyzed by Shapiro (1978), the waves are not approximately advected by the horizontal wind. Accelerations have the same amplitude as the Coriolis force in the momentum balances of the disturbances, indicating that the disturbances are not quasi-geostrophic. However, thermal wind balance appears to be a good approximation for the mean zonal wind. None of the usual simplifications to the divergence eq...

This paper presents an analysis of the vertical structure of steady motion in a dissipative tropical atmosphere forced by steady isolated diabatic heating. Vertical modes appropriate to the problem are obtained, and the forcing is projected onto these modes. With the use of an analytic expression obtained by Gill (1980) for the horizontal structure, these modes are summed to obtain the amplitude and the vertical structure of the response in the region to the east of the heating.

Measuremenits of the momentum transport in tropical convective lines suggest that horizontal momentum can be generated by the pressure low located near the center of the convective part of the line. A simple convective parameterization is used to evaluate this effect. The parameterization is a version of the Fritsch and Chappell scheme, modified in order to evaluate the influence of the horizontal pressure gradients on momentum transport. The results suggest that in modeling of convective (particularly slow-moving) lines with 20 km resolution, subgrid horizontal pressure gradients should be taken into account. Sensitivity studies show that the magnitude of the calculated momentum flux strongly depends on the average vertical velocity and the vertical velocity in clouds.

Dynamic budgets of an average synoptic-scale wave have been made by Stevens (1979) from GARP Atlantic Tropical Experiment Phase III B- and A/B-scale data composited by Thompson et al. (1979). In the present study the apparent sources of momentum and vorticity, computed from the large-scale budgets, are compared with parameterized sources from independently derived cumulus mass fluxes and one-dimensional steady-state cloud models. The cloud models include spectral and bulk, as well as single-cloud models. The cumulus mass fluxes are determined from a thermodynamic budget analysis of Johnson (1 978). A simple single-cloud model is found to adequately account for the net effect of the cumulus transport and production of vorticity. The one-dimensional cloud models, however, do not account for the apparent momentum source in the upper troposphere. An evaluation is made of the sensitivity of the results to the assumed cloud-base vorticity and radiative heating rate. The limitations of the simple cloud ...

A linear model of wave-CISK consisting of the primitive equations formulated on an equatorial beta-plane with a resting basic state, is used to compare the relatively simple cumulus parameterization scheme developed by Stevens and Lindzen with the relatively complex one developed by Arakawa and Schubert. The comparison is based mainly on the instability characteristics and vertical structure of the wave. Discrete forms of the equations are developed and the inhomogeneous vertical structure equation is solved as a generalized matrix eigenvalue problem. The eigenvalues correspond to complex equivalent depths which determine the growth rates through the use of the dispersion relation for an equatorial beta-plane. The eigenvectors, which are the vertical velocities at discrete levels in the model, determine the vertical structure. Results indicate that the growth rates obtained with Arakawa and Schubert's parameterization are smaller than those obtained with Stevens and Lindzen's parameterization. Co...

Large-amplitude planetary waves in the southern winter stratosphere are observed to occur episodically, the result of episodic tropospheric forcing. This work is an observational study of the dynamics of the planetary waves, focusing on the evolution through a typical life cycle. Time lag correlations of wave amplitude with the Eliassen-Palm flux vector reveal the characteristic heat and momentum flux patterns associated with wave evolution. Energetic studies clearly show that the stratospheric waves can be understood in terms of a life cycle of vertical propagation from the troposphere, followed by decay from barotropic interactions with the zonal mean flow. Although usually of secondary importance baroclinic decay of stratospheric wave energy is also observed, resulting from equatorward heat flux in the lower stratosphere. Good agreement in the energy balances discounts in situ instability in the stratosphere as a source of wave activity. An average or composite over several clearly propagating...