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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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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

Rawinsonde and satellite data from GATE are used to construct composite analyses of the Intertropical Convergence Zone (ITCZ) of the summertime eastern Atlantic. The latitude of the ITCZ varies substantially with time, and the data are analyzed relative to the latitude of maximum convection. Sensible heat, moisture and kinetic energy budgets are performed.

Significant mean upward motion occurs over a band approximately 900 km wide. The ITCZ is centered in a broad surface low pressure trough with a weak upper-level warm core structure. It is found to be a direct circulation which generates an excess of kinetic energy, most of which is exported meridionally in the upper troposphere by transient eddy circulations.

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William M. Frank

Abstract

The net vertically integrated temperature of the GATE B army is analyzed at each rawinsonde observation time for all three phases. Temperatures are adjusted to remove persistent intership biases and errors induced by solar heating of the rawinsondes. Modulation of the net temperature by direct solar heating, secondary circulation processes and latent heat release are explored.

Latent heat release does not warm the troposphere on the observed time (3–6 h) and space scales indicating that condensation heating is dispersed very rapidly from cloud-cluster-scale systems to larger circulations. The net temperature undergoes a regular diurnal temperature oscillation modulated primarily by direct radiational heating and indirect circulation. These processes are of comparable magnitudes.

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William M. Frank

Abstract

The life cycles of GATE squall lines and loosely organized cloud clusters are analyzed and documented using radar and composited rawinsonde data. Time variations of the temperature, moisture, wind and vertical motion fields are presented for both types of systems. The convective systems are usually triggered by the approach of a middle-level trough in the easterlies. Low-level convergence increases prior to intensification of convection, possibly by several hours. Once the convection begins, the systems largely cause the changes in their upper and lower level divergence profiles through cumulus-induced and radiational heating. Squall line and cluster systems are found to be essentially similar except for their propagation speeds and their vertical wind shears.

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William M. Frank

Abstract

Vertical fluxes of mass, static energy and angular momentum in tropical cyclones and their environments are analyzed using composite rawinsonde data. Cumulus-scale eddy fluxes are derived using a spectral cloud model incorporating the effects of downdrafts, overshoot cooling, multiple-level cloud detrainment and ice-phase transitions.

Deep convection exists in all regions in approximately direct proportion to the low-level mass convergence, and large amounts of shallow cloudiness are found everywhere. A bimodal cloud distribution is found only in the outer convective area. Direct warming of the troposphere through cloud-induced subsidence is very small in the upper troposphere inside a radius of 4° latitude.

Angular momentum appears to be conserved in deep and middle-level convection, while shallow clouds and dry turbulence transport momentum downward in the lower levels. These principles could be incorporated into cumulus parameterization schemes.

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