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Robert T. Merrill

Abstract

In this paper the climatology and structure of, and possible reasons for, tropical cyclones of different sizes are examined. The climatology of tropical cyclone sizes confirms that tropical cyclones of the western North Pacific are characteristically twice as large as their Atlantic counterparts, and also reveals that the typical size of tropical cyclones varies seasonally and regionally and is only weakly correlated with cyclone intensity (maximum surface wind or minimum surface pressure). Rawinsonde composities of large and small tropical cyclones show that large cyclones have much more relative angular momentum (RAM) than small cyclones, while the differences in RAM between intense and weaker cyclones of equivalent size are less. Some of the implications of this observance are discussed, and a hypothesis that cyclones grow as a result of an increased convergence of angular momentum forced by their environment is presented.

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Robert T. Merrill

Abstract

A method for estimating the horizontal structure of the upper-tropospheric warm anomaly of tropical cyclones from 55-GHz microwave observations is presented. Because the peak warming occurs over an area smaller than that viewed by current and planned satellite antenna systems, it is necessary to model explicitly the interaction of the warm anomaly structure and antenna gain pattern. The anomaly is approximated by an analytic function whose parameters are estimated using a maximum-likelihood algorithm with constraints analogous to that used for thermodynamic sounding retrieval or optimal interpolation. Simulation studies demonstrate the overall soundness of the technique and its possible performance and limitations when applied to two different polar-orbiting microwave sensors, MSU (Microwave Sounding Unit) and SSM/T (Special Sensor Microwave/Temperature).

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Robert T. Merrill

Abstract

Return flow is the moist southerly wind that develops over the Gulf of Mexico after an outbreak of polar air. Surface, aircraft, and special rawinsonde data collected during the Gulf of Mexico Experiment (GUFMEX) are used to describe the return-flow event of 10–12 March 1988. The return flow at the surface contained both modified polar air and prefrontal air. The surface moist layer was capped by a stable subsident layer except over its western extremity, where an elevated mixed layer was observed. At later stages, a moist layer aloft was present as well.

These complex airmass structures arose because both advective and diabatic processes are significant in a return flow. The roles of each are inferred qualitatively by comparing the observed mixing-ratio distribution to the equilibrium conditions expected for the observed sea surface temperatures. The surface moisture distribution can be explained by rapid modification of offshore flow to near equilibrium, followed by onshore (return) flow of the modified air with little additional change. The structure above the surface moist layer is explained by differential advection that juxtaposed three different airstreams. Though no significant severe weather followed this particular case, the processes that typically lead to a favorable severe-weather environment are evident.

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Robert T. Merrill

Abstract

The upper-tropospheric flow out to a radius of 2000 km around Atlantic hurricanes is described using rotated coordinate composite analysis of the NOAA National Hurricane Center operational wind set. The rotated coordinate methodology, designed to preserve some of the asymmetry of hurricane outflow during compositing, is described in detail.

A rotated coordinate composite of all hurricanes from a five-year period is used to study the general properties of the hurricane outflow layer. Coordinate rotation improves the representation of the outflow jet and the associated extrema of radial and tangential wind, but tends to obscure the geographically persistent features of the upper-tropospheric environment such as the midlatitude westerlies. The amplitude of the asymmetric radial wind is twice that of the symmetric, while the amplitudes of tangential winds are equivalent. A comparison of geographic and rotated coordinate composites indicates that both the outflow jet and the midlatitude westerlies are important structures for the import of angular momentum into the hurricane by horizontal eddy fluxes.

Separate composites of eight characteristic outflow patterns are also presented. Pattern variability arises from the juxtaposition of the hurricane circulation with surrounding synoptic features.

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Robert T. Merrill

Abstract

Although driven by internal processes, hurricanes are also regulated by conditions in their oceanic and atmospheric surroundings. Sea surface temperature determines an upper bound on the intensity of hurricanes, but most never reach this potential, apparently because of adverse atmospheric conditions.

Winds measured by satellite cloud tracking, commercial aircraft, and rawinsondes are composited using a rotated coordinate system designed to preserve the asymmetries in the upper-tropospheric environment. Composites of upper-tropospheric environmental flows for intensifying and nonintensifying hurricanes for a five-year period are compared. Nonintensifying composites indicate stronger mean environmental flow relative to the hurricane motion, unidirectional flow over and near the hurricane center, and slightly weaker radial outflow and/or more pronounced anticyclonic flow surrounding the center in the upper troposphere.

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Robert T. Merrill
and
Christopher S. Velden

Abstract

Isentropic coordinate analyses of rawinsondes and cloud motion wind vectors derived from geostationary satellite imagery are employed to describe the three-dimensional upper-tropospheric and lower-stratospheric circulation attending western North Pacific Supertyphoon Flo during September 1990. Outflow from the storm is concentrated in several evolving channels in the horizontal. In terms of vertical structure, net outflow evaluated at 6° latitude (666 km) radius is found to occur at higher levels and over an increasing range of potential temperature θ as the tropical cyclone intensifies. Outflow on the equatorward side of the tropical cyclone tends to occur at greater θ values (higher altitudes) than poleward outflow. Potential vorticity also decreases within the corresponding isentropic layers associated with the outflow. The implications of the vertical variability of outflow structure in terms of the interactions between storm and environment, and effects on storm structural changes, are considered briefly.

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Robert W. Burpee
,
Donald G. Marks
, and
Robert T. Merrill

Omega dropwindsondes (ODWs) were released from two NOAA WP-3D aircraft to measure the environmental wind field in the middle and lower troposphere within 1000 km of the center of Hurricane Debby on 15 and 16 September 1982. The observations were coded in standard formats and transmitted from the aircraft to the National Hurricane Center (NHC) and the National Meteorological Center (NMC) before operational forecast deadlines. The ODW winds clearly indicated the location and strength of a midtropospheric trough in the westerlies that was the major synoptic-scale feature affecting Debby's motion. On 16 September, the dropwindsondes also identified a smaller scale cutoff low in the northern part of the trough. The cutoff low that was centered about 500 km to the north northwest of Debby affected the hurricane's motion from midday on the 16th to midday on the 17th.

The ODWs provided NHC with timely information that was used subjectively in determining the official forecasts of Debby's track. The potential of the ODWs to improve the track models that serve as guidance for the forecasters at NHC depends upon both the quality of the ODW data and the ability of the operational objective analyses to respond to the ODW data. In 1982, the objective analysis that initialized several of the track models was a spectral analysis with a global domain. At 500 mb, the scale of the wind circulations of Debby and the cutoff low was approximately 500 km. The global operational objective analysis did not resolve these important features. The ODW data can help to improve the objective guidance for the hurricane forecasters only if the operational objective analyses and the track models are designed to make use of the ODW information. To obtain the data needed to revise current models and to develop new models, ODW experiments are planned in the next few years when hurricanes threaten the Atlantic or Gulf coasts of the United States.

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Christopher S. Velden
,
Brian M. Goodman
, and
Robert T. Merrill

Abstract

A method is examined for estimating the intensity of western North Pacific tropical cyclones from satellite passive microwave observations. Vertical profiles of atmospheric temperature derived from radiances remotely sensed by the Microwave Sounding Unit (MSU) onboard the current NOAA series of polar orbiting satellites are used to depict upper-tiopospheric warm anomalies associated with these storms. Data from a large sample of western North Pacific tropical cyclones are used to develop a nonlinear statistical relationship between the satellite-depicted warm core anomalies and the surface intensifies as measured by reconnaissance aircraft. Results based on an 82-case dependent sample indicate standard errors of 13 mb and 15 kt for estimates of the surface pressure anomalies and maximum wind speeds. These errors are reduced considerably when a bias in the sample intensity distribution is taken into account. Comparisons of results and method accuracy are made with a previous study of North Atlantic tropical cyclones.

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Greg J. Holland
,
I. A. Berzins
, and
Robert T. Merrill

A brief description is given of the Cyclone Game, a direct simulation of many of the operational aspects of the Perth Tropical Cyclone Warning Centre. The game's main purpose is to provide tropical-cyclone forecasters with additional operational experience and skills. It may also be used to familiarize others with some of the complexities of tropical-cyclone forecasting. The game is controlled by a microcomputer and may be played by one or a team of “forecasters.” It has a modular structure and is designed for easy modification to suit local requirements.

We suggest that microcomputer-based simulations of severe weather events, such as described in this paper, have considerable potential as an educational and training tool.

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Robert T. Merrill
,
W. Paul Menzel
,
Wayman Baker
,
James Lynch
, and
Eugene Legg
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