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  • Author or Editor: Roger A. Pielke Sr. x
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Melville E. Nicholls
and
Roger A. Pielke Sr.

Abstract

A three-dimensional, fully compressible cloud model is used to simulate a convective storm in order to investigate the properties of compression waves and gravity waves induced by latent heat release. Time series of the low-level pressure perturbations caused by the propagating waves are examined at various distances from the storm. A compression wave that is close to hydrostatic balance and can be considered to be a Lamb wave, which propagates in the horizontal plane, emerges from the storm. This latter property gives the wave a distinctly two-dimensional character that is clarified by comparison with a linear model of a two-dimensional thermally induced compression wave. This has implications for its shape and results in a decay rate with distance propagated from the source of 1/(distance)1/2. The period of the Lamb wave is determined primarily by the time it takes for the storm to develop and decay. The fast-moving Lamb wave is trailed by slower-moving thermally induced gravity waves. It is found that the amplitude of the gravity waves decay with 1/distance. Distinct gravity wave modes can be identified. The first mode propagates the fastest and results in deep subsidence warming. The second mode propagates at half the speed of the first and causes weak low-level uplift, which in some convective situations might aid the development of new convection.

An analysis of the transfer of internal and gravitational potential energies showed that the net transfer by the Lamb wave was approximately equal to the net increase of total energy in the atmosphere brought about by the convective storm. This result suggests that physical interpretations of total energy transfer in the atmosphere need to take into account that it can be transferred in a wavelike manner at the speed of sound.

An interesting buoyancy oscillation occurred when the downdraft air overshot its buoyant equilibrium level, which resulted in a resurgence of convection. The convection was able to obtain moderate strength by feeding on moist environmental air that had been advected over the top of the cold pool. This mechanism may be a factor contributing to the early meso-β convective cycle that has been observed in many convective systems.

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Warren P. Smith
,
Melville E. Nicholls
, and
Roger A. Pielke Sr.

Abstract

Recent numerical modeling studies indicate the importance of radiation in the transformation from a tropical disturbance to a tropical depression, a process known as tropical cyclogenesis. This paper employs a numerical modeling framework to examine the sensitivity to radiation in idealized simulations for different initial vortex strengths, and in doing so highlights when during tropical cyclogenesis radiation is most important. It is shown that all else being equal, a stronger initial vortex reduces the impact that radiation has on accelerating tropical cyclogenesis. We find that radiation’s primary role is to moisten the core of a disturbance through nocturnal differential radiative forcing between the disturbance and its cloud-free surroundings, and after sufficient moistening occurs over a deep layer and the winds are sufficiently strong at the surface, radiation no longer plays as significant a role in tropical cyclogenesis.

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Alexandre A. Costa
,
William R. Cotton
,
Robert L. Walko
, and
Roger A. Pielke Sr.

Abstract

A cloud-resolving model coupled to an ocean model with high vertical resolution is used to investigate air–sea interactions in 10-day long simulations. Modeled fields showed good agreement with two different convective regimes during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Research Experiment (TOGA COARE) Intensive Observing Period. The model simulates the formation of precipitation-produced, stable freshwater lenses at the top of the ocean mixed layer, with a variety of horizontal dimensions and lifetimes. The simulated fresh anomalies show realistic features, such as a positive correlation between salinity and temperature, the development of a surface jet in the direction of the wind, and, as a consequence, downwelling (upwelling) on its downwind (upwind) edge. The dataset generated by the coupled model is used to evaluate the contribution from several factors (ocean currents, gustiness, and correlations between wind speed and air temperature, wind speed and water vapor mixing ratio, and wind speed and SST) to the surface heat fluxes. Gustiness was shown to be a major contribution to the simulated surface heat fluxes, especially when convection is active. In a multiday average, the contributions from the other effects (currents and wind speed–air temperature, wind speed–water vapor mixing ratio, and wind speed–SST correlations) are small; however, they cannot be neglected under certain circumstances.

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Alexandre A. Costa
,
William R. Cotton
,
Robert L. Walko
,
Roger A. Pielke Sr.
, and
Hongli Jiang

Abstract

A two-dimensional cloud-resolving model (CRM) was used to simulate the evolution of convection over the western Pacific between 19 and 26 December 1992, during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. A control simulation (CONTROL) was performed in which observed, time-evolving, spatially homogeneous SSTs were used as a lower boundary condition. It showed that the CRM was able to properly represent the evolution of the cloud systems.

Sensitivity experiments were carried out, in which the sea surface temperature was increased (SST+) or decreased (SST−) by 1°C and the same evolving large-scale forcing used in CONTROL. The similarities among all simulations suggested that the large-scale forcing is the dominant mechanism controlling the statistics of the cloud systems, including the total precipitation. However, the convective–stratiform partition of the cloud systems was altered, the convective part being favored in SST+ and the stratiform part favored in SST−. In terms of the radiative budget, the reduced low-level cloud coverage in SST+ acted to compensate the enhancement of high-cloud coverage produced by more vigorous convection (the opposite occurred in SST−). As a consequence, the surface downward radiation was approximately the same in CONTROL, SST+, and SST−.

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David A. Schecter
,
Melville E. Nicholls
,
John Persing
,
Alfred J. Bedard Jr.
, and
Roger A. Pielke Sr.

Abstract

This paper addresses the physics and numerical simulation of the adiabatic generation of infrasound by tornadoes. Classical analytical results regarding the production of infrasound by vortex Rossby waves and by corotating “suction vortices” are reviewed. Conditions are derived for which critical layers damp vortex Rossby waves that would otherwise grow and continually produce acoustic radiation. These conditions are similar to those that theoretically suppress gravity wave radiation from larger mesoscale cyclones, such as hurricanes. To gain perspective, the Regional Atmospheric Modeling System (RAMS) is used to simulate the infrasound that radiates from a single-cell thunderstorm in a shear-free environment. In this simulation, the dominant infrasound in the 0.1–10-Hz frequency band appears to radiate from the vicinity of the melting level, where diabatic processes involving hail are active. It is shown that the 3D Rossby waves of a tornado-like vortex (simulated with RAMS) can generate stronger infrasound if the maximum wind speed of the vortex exceeds a modest threshold. Technical issues regarding the numerical simulation of tornado infrasound are also addressed. Most importantly, it is shown that simulating tornado infrasound likely requires a spatial resolution that is an order of magnitude finer than the current practical limit (10-m grid spacing) for modeling thunderstorms.

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Gregory S. Poulos
,
James E. Bossert
,
Thomas B. McKee
, and
Roger A. Pielke Sr.

Abstract

Via numerical analysis of detailed simulations of an early September 1993 case night, the authors develop a conceptual model of the interaction of katabatic flow in the nocturnal boundary layer with mountain waves (MKI). A companion paper (Part I) describes the synoptic and mesoscale observations of the case night from the Atmospheric Studies in Complex Terrain (ASCOT) experiment and idealized numerical simulations that manifest components of the conceptual model of MKI presented herein. The reader is also referred to Part I for detailed scientific background and motivation.

The interaction of these phenomena is complicated and nonlinear since the amplitude, wavelength, and vertical structure of the mountain-wave system developed by flow over the barrier owes some portion of its morphology to the evolving atmospheric stability in which the drainage flows develop. Simultaneously, katabatic flows are impacted by the topographically induced gravity wave evolution, which may include significantly changing wavelength, amplitude, flow magnitude, and wave breaking behavior. In addition to effects caused by turbulence (including scouring), perturbations to the leeside gravity wave structure at altitudes physically distant from the surface-based katabatic flow layer can be reflected in the katabatic flow by transmission through the atmospheric column. The simulations show that the evolution of atmospheric structure aloft can create local variability in the surface pressure gradient force governing katabatic flow. Variability is found to occur on two scales, on the meso-β due to evolution of the mountain-wave system on the order of one hour, and on the microscale due to rapid wave evolution (short wavelength) and wave breaking–induced fluctuations. It is proposed that the MKI mechanism explains a portion of the variability in observational records of katabatic flow.

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