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W. Drosdowsky and G. J. Holland

Abstract

A satellite classification and climatology of propagating mesoscale cloud fines in northern Australia is presented. These cloud fines range from long, narrow lines of shallow convection to extensive deep convective squall lines with mesoscale stratiform rain areas. These lines are the predominant weather feature during periods of easterly flow over the Australian tropics.

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N. E. Davidson and G. J. Holland

Abstract

Analyses of mean sea level pressure, wind, temperature and dewpoint am used to study the life cycles of two intense, heavy-rain-producing monsoon depressions over northern Australia. Two aspects are considered: (a) the large forcing, using both synoptic flow field changes and angular momentum budgets, and (b) the role of convective and stratiform clouds using kinematic and thermodynamic budgets.

For each situation, the Northern Hemisphere circulation becomes favorable well prior to genesis. The short-term trigger for development is the strengthening of the Southern Hemisphere subtropical ridge at the surface and an amplifying upper-level trough and subtropical jetstreak to the southwest of the formation point.

The outer region structure of these monsoon depressions is remarkably similar to that of a tropical cyclone, even though the systems develop over land. During development, maximum convective heating occurs at middle levels and within a region of already high cyclonic vorticity. Evidence suggests that the cloud population is mostly comprised of deep cumulonimbus clouds, middle-level stratiform cloud and shallow cumulus. The physical significance of these findings is discussed.

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Peter G. Black and Greg J. Holland

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The boundary layer structure of Tropical Cyclone Kerry (1979) is investigated using composite analysis of research aircraft, surface ship, and automatic weather station observations. The boundary layer was moist, convective, and strongly confluent to the east of the tropical cyclone center but was dry, subsident, and diffluent to the west. The vertical momentum transport in the eastern convective sector of Kerry was around two to three times the surface frictional dissipation. In contrast, the stable boundary layer in the western sector consisted of a shallow mixed layer capped by an equivalent potential temperature minimum and a low-level jet, which underwent a marked diurnal oscillation. Three mechanisms appear to have contributed to the observed asymmetry: 1) a general, zonal distortion arose from cyclonic rotation across a gradient of earth vorticity; 2) a westerly environmental vertical shear produced forced ascent on the east side of the storm and subsidence on the west side throughout the lower and midtroposphere; and 3) the western sector boundary layer was modified by an upstream cold tongue generated by the tropical cyclone passage. The authors present evidence that substantial drying also resulted from shear-induced mixing of the subsident environmental air in the region of the low-level jet.

Thermal boundary layer budgets are derived using both a general mixing theory approach and direct flux calculations from aircraft reconnaissance data. Use of actual sea surface temperature fields are essential. The surface flux estimates of latent heat are near the average of previous studies, but the sensible heat fluxes are downward into the ocean. Since horizontal advection also cooled the boundary layer, the thermal structure was maintained by downward fluxes of sensible heat from the top of the boundary layer of around 100 W m−2. We conclude that the pattern of oceanic cooling directly determines the pattern of vertical air-sea and advective sensible heat fluxes and indirectly determines the pattern of latent heat fluxes through forcing of PBL drying at the downwind end of the SST cold pool. It further enhances the inward penetration and negative feedback resulting from an easterly trade wind surge associated with a mobile trough in the westerlies.

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J. A. Curry, P. J. Webster, and G. J. Holland

The 2005 Atlantic hurricane season was the most active and costly season on record. Recent publications linking an increase in hurricane intensity to increasing tropical sea surface temperatures have fueled the debate on whether or not global warming is causing an increase in hurricane intensity. Because of the substantial implications of the hurricane–global warming issue for society and the immediate policy relevance associated with decision making related to Hurricane Katrina, attacks and rebuttals related to this research are being made in the media and on the World Wide Web without the rigor or accountability expected of scientific discourse. In this paper, we aim to promote a balanced and thoughtful examination of this subject by

  • clarifying the debate surrounding the subject as to whether or not global warming is causing an increase in global hurricane intensity,
  • illustrating a methodology of hypothesis testing to address multiple criticisms of a complex hypothesis that involves a causal chain, and
  • providing a case study of the impact of politics, the media, and the World Wide Web on the scientific process.

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J. A. Curry, J. Maslanik, G. Holland, and J. Pinto
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W. G. Large, W. R. Holland, and J. C. Evans

Abstract

A one-degree, flat bottom, eight-layer quasi-geostrophic model of the North Pacific Ocean is forced by six different wind stress curl datasets, all derived from seven years of twice daily analyses at the European Centre for Medium Range Weather Forecasts, 1980–86. The six datasets, with nominal averaging times of 1, 2, 3, 7, 14, and 30 days, are obtained by carefully filtering in the frequency domain. This filtering greatly reduces the variance, typically by 90% for 30-day averaging, because the wind stress curl spectrum is nearly white in frequency. It also smoothes spatially, reducing high wavenumber variance to a greater degree than variance near wavenumber zero. The climatology of the ECMWF wind stress curl does not show any unexpected differences from climatologies based on historical marine wind observations. The wind stress curl is neither temporally nor spatially stationary with the high frequency variance being much larger during the winter season and over the northern half of the North Pacific. Its spectrum does not appear to be isotropic in wavenumber.

From a common initial state, the baroclinic fields in the model ocean runs evolve nearly identically regardless of the forcing bemuse their frequencies are all lower than the Nyquist frequency of even 30-day sampling. The higher frequency forcing generates Rossby waves that dominate the instantaneous barotropic stream function throughout the basin. These barotropic waves are not found at frequencies above the Nyquist frequency of the forcing. There is negligible rectification into basin scale, six year mean flows. There are only small scale differences in mean monthly barotropic streamfunction fields. Thus, the barotropic ocean response diminishes as the nominal averaging time increases. Furthermore, these Rossby waves appear to be natural modes of the model basin, and they could be artificially forced unless the wind data processing carefully avoids aliasing unresolved frequencies. Overall the spectrum of ocean response is found to be more red in frequency than the nearly white wind curl spectrum and even more red in wavenumber.

High frequency forcing produces higher kinetic energies at all depths with an annual cycle that is related to the annual cycle of the high frequency variance in the wind stress curl. The deep kinetic energy and the intra-annual streamfunction variance are used to quantify the relative importance of the high frequency barotropic Rossby waves. There is considerable advantage in using 3-day average (over 7- and 14-day average) wind forcing, however, there is little more to be gained with 2-day averaging and nothing further added by 1-day averaging. When forced with 30-day averaged wind curls, the intra-annual stream function variance is typically only 30% of its value when forced with 3-day or shorter averages.

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J. Simpson, E. Ritchie, G. J. Holland, J. Halverson, and S. Stewart

Abstract

With the multitude of cloud clusters over tropical oceans, it has been perplexing that so few develop into tropical cyclones. The authors postulate that a major obstacle has been the complexity of scale interactions, particularly those on the mesoscale, which have only recently been observable. While there are well-known climatological requirements, these are by no means sufficient.

A major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development. Observations exist for only a few forming cases. In these, the moist convection in the preformation environment is organized into mesoscale convective systems, each of which have associated mesoscale potential vortices in the midlevels. Interactions between these systems may lead to merger, growth to the surface, and development of both the nascent eye and inner rainbands of a tropical cyclone. The process is essentially stochastic, but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental influences. For example a monsoon trough provides a region of reduced deformation radius, which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices. Further, a strong monsoon trough provides a vertical wind shear that enables long-lived midlevel mesoscale vortices that are able to maintain, or even redevelop, the associated convective system.

The authors develop this hypothesis by use of a detailed case study of the formation of Tropical Cyclone Oliver observed during . In this case, two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband. The eye developed on the edge of the major convective system, and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere, and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere. The importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western North Pacific.

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R. Rotunno, Y. Chen, W. Wang, C. Davis, J. Dudhia, and G. J. Holland
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N. E. Davidson, G. J. Holland, J. L. McBride, and T. D. Keenan

Abstract

The AMEX observational dataset, with its high temporal and spatial resolution, has been used to study the formation and structure of Tropical Cyclones Irma and Jason. These systems developed and evolved entirely within the experiment's special observing network. The study is mostly based upon six hourly numerical analyses of the mass and wind fields, on 11 vertical levels over a 1.25 lat/long grid. The systems are traced from prior to the formation of a resolvable closed surface circulation to when they were operationally classified as tropical cyclones.

The discussion focuses on the synoptic to cyclone scale changes during formation. Time sections of various kinematic variables, together with an index of deep convection obtained from digital satellite cloud imagery, are used to trace the development.

Both systems developed during active phases of the monsoon and initially were of maximum intensity in the middle troposphere. Low level spinup occurred in three stages. The first stage was associated with the establishment of a favorable large-scale environment and the development of a closed, low-level circulation. The second stage was marked by a strengthening in the low-level outer circulation and the development of a deep vortex. The final stage was the transformation of the tropical depressions into tropical cyclones, and was indicated by a large increase in low-level convergence, a burst in inner core convection, and intensification of the low-level inner circulation.

The evolution of the flow during development agrees well with the results of earlier tropical cyclogenesis studies. Large scale spinup appears to be at least partly associated with downstream Rossby-wave dispersion leading to increases in low-level horizontal wind shear and eventually the formation and strengthening of the low-level outer circulation. For the final transformation to cyclone status we suggest that the establishment of favorable patterns of vertical wind shear and inward propagation of eddy angular momentum flux convergence in the upper troposphere were important for intensification. Thermodynamic structure changes suggest that maintenance, rather than triggering of core convection, was dependent on surface evaporation.

The role of the observed structure changes, together with the processes operating during each phase of development are documented and discussed.

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G. J. Holland, P. J. Webster, J. A. Curry, G. Tyrell, D. Gauntlett, G. Brett, J. Becker, R. Hoag, and W. Vaglienti

The Aerosonde is a small robotic aircraft designed for highly flexible and inexpensive operations. Missions are conducted in a completely robotic mode, with the aircraft under the command of a ground controller who monitors the mission. Here we provide an update on the Aerosonde development and operations and expand on the vision for the future, including instrument pay loads, observational strategies, and platform capabilities. The aircraft was conceived in 1992 and developed to operational status in 1995–98, after a period of early prototyping. Continuing field operations and development since 1998 have led to the Aerosonde Mark 3, with ~2000 flight hours completed. A defined development path through to 2002 will enable the aircraft to become increasingly more robust with increased flexibility in the range and type of operations that can be achieved. An Aerosonde global reconnaissance facility is being developed that consists of launch and recovery sites dispersed around the globe. The use of satellite communications and internet technology enables an operation in which all aircraft around the globe are under the command of a single center. During operation, users will receive data at their home institution in near-real time via the virtual field environment, allowing the user to update the mission through interaction with the global command center. Sophisticated applications of the Aerosonde will be enabled by the development of a variety of interchangeable instrument payloads and the operation of Smart Aerosonde Clusters that allow a cluster of Aerosondes to interact intelligently in response to the data being collected.

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