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William M. Frank

Abstract

An unusually large quantity of aircraft data was obtained within the core of Hurricane Frederic (1979). These data are combined with Powell's surface observations of the same storm and composited. The data are sufficiently abundant to allow three-dimensional kinematic analyses of poorly documented parameters such as divergence and vertical velocity. Patterns of related but independently determined parameters such as radar reflectivities, cloud water concentrations and kinematically-derived vertical velocities agree well. Diagnostic budget analyses of sensible heat and angular momentum are computed for the storm inflow layer.The analysis provides a unique quantitative picture of the inflow layer of a mature, asymmetric hurricane.

The depth of the inflow layer decreases with decreasing radius. As a result, surface drag coefficients derived from angular momentum budgets do not appear to increase with increasing wind speed. The sensible heat budget shows downward subgrid-scale heat fluxes near cloud base and indicates that the net flux of sensible heat from the sea to the air in the core region is probably on the order of 50 W m2, which is much smaller than most previous estimates.

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William M. Frank

Abstract

Rawinsonde composites from the west Pacific and West Indies are used to analyze some of the properties of tropical cyclones and their influences upon larger scale circulations. Composites of storms with different intensities are compared to determine the scales of different circulation features. The radial wind and vertical motion anomalies which occur during the transition from pre-storm convective system to mature tropical cyclone are found to be confined to the inner 6° and 2°, respectively, while the tangential circulation increases at least to the edge of the 15° grid.

Tropical cyclones are found to be net sources of kinetic energy and sinks of relative angular momentum. Of the storms studied, only typhoons made significant contributions to large-scale meridional fluxes of westerly momentum and kinetic energy through a plane 10° north of the storm centers. Fluxes of other quantities were negligible. All of the tropical weather systems were found to be largely thermodynamically self-contained when viewed on a scale of 12° radius.

The wake regions of westerly-moving tropical cyclones are favorable locations for subsequent tropical cyclogenesis while the path areas ahead of the cyclone are generally suppressed for storms exhibiting significant upper level outflow to the southwest.

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William M. Frank

Abstract

Diagnostic moisture and dry static energy budgets for the GATE A/B-scale area are performed for individual time periods using rawinsonde and satellite data. The data are sufficiently accurate to permit quantitative analysis of that area with 3–6 h time resolution. Each budget is used to estimate the net condensation rate for every time period.

The budget condensation rates slightly exceed radar rainfall estimates for the GATE master array and the radar values lag the budget condensation by 4–6 h during major rainfall episodes. Storage of liquid water and unsampled vapor in clouds can account for a large portion of the observed lag. The present results indicate that observed 3–6 h lags between, low-level mass convergence and echo growth during GATE result primarily from the lag between condensation and precipitation during convective system evolution. The release of latent heat in a tropical convective system appears to be closely related to the instantaneous mass convergence below 700 mb.

Budget-derived condensation is maximum at about 0900 GMT (∼0730 LT). This agrees well with long-term gage measurements in many other tropical oceanic regimes, but contradicts the GATE radar rainfall. Part of the discrepancy is due to the storage of liquid water and cloud vapor, but solar heating of the rawinsonde also contributes.

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William M. Frank

Abstract

A composite study of 10 years of northwest Pacific rawinsonde data is used to analyze the large-scale structure of tropical cyclones. The temperature, height, moisture, wind and vertical motion fields are analyzed for various storm regions. Mean soundings for all regions from the eye through 12° radius are presented. Rainfall characteristics of the area inside 4° radius are discussed. Hurricane flight data are used to augment the analyses in the inner regions.

Many important features are noted. Strong persistent asymmetries in storm structure exist, particularly at large radii. The storm circulation has very broad horizontal extent and appears to conform to a constant scale regardless of inner core intensity. Inflow in the middle troposphere is substantial from 4° outward. A mean subsidence region is observed from about 4–6° radius. Humidities are extremely high in the inner regions, and conditional instability exists everywhere outside the eye. Significant diurnal variations in rainfall and temperature are observed.

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William M. Frank

Abstract

This study uses the data composited in Paper I to analyze the dynamics and energetics of a mature tropical cyclone in term of budget studies of moist static energy, angular momentum and kinetic energy. Sea surface to air fluxes of moisture and sensible heat are found to be much smaller than the estimates of most previous researchers. Horizontal eddy fluxes of momentum and kinetic energy are substantial at outer radii in the upper troposphere. The Coriolis torque term in the angular momentum equation does not integrate to zero over the large-scale storm circulation but rather becomes a dominant term. Tropical cyclones are net sources of kinetic energy over their large-scale circulations. This probably results from strong transient eddy generation of kinetic energy within the storm.

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William M. Frank

Abstract

In the past two decades there has been extensive research into the nature of atmospheric convection and scale interactions in cumulus regimes. A major goal of these efforts has been to advance the state of the art in cumulus parameterization. This paper reviews the cumulus parameterization problem in terms of fundamental principles, goals and dynamics constraints as they apply to parameterization in mesoscale and large scale numerical models. Several popular current schemes are discussed in terms of their relationships to these overall aspects of the problem.

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Lisa M. Briegel
and
William M. Frank

Abstract

Objectively analyzed data from the European Centre for Medium-Range Weather Forecasts are used to examine the large-scale aspects of the formation of tropical cyclones. It is hypothesized that tropical cyclogenesis occurs when external atmospheric forcing on the synoptic or larger scale provides uplift through a deep layer, enhancing convection, in a region with environmental conditions favorable for genesis. Emphasis is placed on the roles of upper-level troughs, low-level wind surges, preexisting tropical cyclones, and propagating wave disturbances in triggering tropical cyclogenesis. Composites of the 200-hPa and 850-hPa flows reveal the presence of both upper-level troughs and low-level wind surges, respectively, prior to genesis. In the composites, the wind surges also appear to be related to the presence of a prior circulation located approximately 2000 km to the west of the genesis location. An examination of the individual cases demonstrates that approximately 85% of all storms had either an upper-level trough or an identifiable low-level feature, while 49% had both an upper- and a lower-level feature. Given the limitations of the objective analyses over the tropical oceans, this provides strong evidence for the role of large-scale external forcing in triggering tropical cyclogenesis.

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Jeffrey S. Gall
and
William M. Frank

Abstract

This is the second of two papers examining the role of equatorial Rossby (ER) waves in tropical cyclone (TC) genesis. Based on results from , it is hypothesized that genesis resulting from the circulation of an ER wave alone is uncommon and that the majority of ER wave–related genesis events occur when a sufficiently intense ER wave interacts with a favorable background flow environment. This paper examines this contention by performing a series of simulations in which ER waves are imposed upon idealized background flows. The background flows are designed to resemble a region of a monsoon trough (MT), a flow feature observed at certain times of the year in all of the TC basins, and most dramatically, in the western North Pacific basin. It is believed that ER wave interactions with the MT may speed up the internal breakdown genesis mechanism of the MT, or even result in genesis when the MT is too weak to breakdown from in situ processes alone. The latter scenario is examined here. When just the MT is simulated without the ER wave anomaly fields, the MT remains quasi-steady and TC genesis does not occur. It is only when the ER wave is imposed on the MT that TC genesis is initiated. The results imply that the ER wave–MT interactions produce more TCs than would otherwise occur if no such interactions took place. Results demonstrate that wave breaking of the ER wave is a mechanism by which vorticity is organized on the scale of a TC. This process features a decrease in the initial horizontal scale of the cyclonic gyre of the ER wave to a scale comparable with a TC. This genesis mechanism is sensitive to the magnitude of the background cyclonic vorticity of the MT, as TC genesis is only initiated when the value of the 850-mb relative vorticity of the MT is larger than 2 × 10−5 s−1. This genesis pathway provides a unique interpretation of TC genesis and is compared with previous theories on TC genesis.

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William M. Frank
and
George S. Young

Abstract

This paper examines the interannual variability of tropical cyclones in each of the earth’s cyclone basins using data from 1985 to 2003. The data are first analyzed using a Monte Carlo technique to investigate the long-standing myth that the global number of tropical cyclones is less variable than would be expected from examination of the variability in each basin. This belief is found to be false. Variations in the global number of all tropical cyclones are indistinguishable from those that would be expected if each basin was examined independently of the others. Furthermore, the global number of the most intense storms (Saffir–Simpson categories 4–5) is actually more variable than would be expected because of an observed tendency for storm activity to be correlated between basins, and this raises important questions as to how and why these correlations arise. Interbasin correlations and factor analysis of patterns of tropical cyclone activity reveal that there are several significant modes of variability. The largest three factors together explain about 70% of the variance, and each of these factors shows significant correlation with ENSO, the North Atlantic Oscillation (NAO), or both, with ENSO producing the largest effects. The results suggest that patterns of tropical cyclone variability are strongly affected by large-scale modes of interannual variability. The temporal and spatial variations in storm activity are quite different for weaker tropical cyclones (tropical storm through category 2 strength) than for stronger storms (categories 3–5). The stronger storms tend to show stronger interbasin correlations and stronger relationships to ENSO and the NAO than do the weaker storms. This suggests that the factors that control tropical cyclone formation differ in important ways from those that ultimately determine storm intensity.

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Elizabeth A. Ritchie
and
William M. Frank

Abstract

Numerical simulations of tropical cyclones are performed to examine the effects of a variable Coriolis parameter on the structure and intensity of hurricanes. The simulations are performed using the nonhydrostatic fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model using a 5-km fine mesh and fully explicit representation of moist processes. When a variable Conolis parameter ( f ) environment is applied to a mature tropical cyclone, a persistent north-northwesterly shear develops over the storm center as a result of an interaction between the primary circulation of the storm and the gradient in absolute vorticity. As a result, the variable-f storm quickly develops a persistent wavenumber-1 asymmetry in its inner-core structure with upward motion and rainfall concentrated on the left side of the shear looking downshear, in agreement with earlier studies. In comparison, the constant-f storm develops weak transient asymmetries in structure that are only partially related to a weak vertical wind shear. As a result, it is found that the tropical cyclone with variable f intensifies slightly more slowly than that with constant f, and reaches a final intensity that is about 5 mb weaker. It is argued that this “beta shear” is not adequately represented in large-scale analyses and so does not figure into calculations of environmental shear. Although the effect of the beta shear on the tropical cyclone intensity seems small by itself, when combined with the environmental shear it can produce a large net shear or it can reduce an environmental shear below the apparent threshold to impact storm intensity. If this result proves to be generally true, then the presence of an additional overlooked beta shear may well explain differences in the response of tropical cyclone intensification to westerly versus easterly shear regimes.

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