School of Earth Sciences - Research Publications
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ItemNo Preview AvailableThe generation of near-cloud turbulence in idealized simulations(American Meteorological Society, 2014-01-01)This study explores the generation of turbulence in the upper outflow regions of simulated idealized mesoscale convective systems. The simulated storms are shown to generate parameterized turbulence that occurs significant distances (>100 km) from the main convective regions, in both the clear air surrounding the convection and low simulated reflectivity regions with cloud ice but negligible amounts of graupel and snow. The source of the turbulence is related to Kelvin-Helmholtz instabilities that occur in the shear zones above and below the storm-induced upper-level outflow jet that is centered near the tropopause; the model produces resolved-scale billows within regions of low gradient Richardson number. Short-scale gravity waves are also coincident with the regions of turbulence, become trapped within the jet core, and appear to be generated by the shear instability. Additional experiments with different initial upper-level wind shear show similar mechanisms to those simulations with no initial upper-level shear. These results help elucidate the dynamics of turbulence generation near convection, which has important implications for the aviation industry and the fundamental understanding of how convective clouds interact with their environment.
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ItemIntensity of thunderstorm-generated turbulence revealed by large-eddy simulation(American Geophysical Union, 2014-03-28)Thunderstorms are characterized by turbulent processes that constitute an important aviation hazard and cause vertical transport of atmospheric constituents. Turbulence occurs within cloud and in the surrounding clear air, but, despite its importance, the characteristics of thunderstorm-generated turbulence and its spatial distribution are poorly understood, especially outside of cloud. Here we use large-eddy simulation to characterize turbulence generated by a canonical thunderstorm. The simulation identifies regions of notable three-dimensional anisotropic turbulence more than 5 km above the storm, in a shallow layer above the storm's anvil, and a horizontally asymmetric pattern of weaker turbulence that extends more than 50 km horizontally away from the cloud. Our results provide the first continuous estimate of turbulence intensity in and around thunderstorms and represent a major step toward improved turbulence avoidance methods. The results have broader implications for understanding the fundamental aspects of how thunderstorms affect their environment through vertical exchange processes.
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ItemGround-based observations of overshooting convection during the tropical warm pool-international cloud experiment(American Meteorological Society, 2014-01-27)This study uses gridded radar data to investigate the properties of deep convective storms that penetrate the tropical tropopause layer (TTL) and overshoot the cold-point tropopause during the Tropical Warm Pool-International Cloud Experiment (TWP-ICE). Overshooting convection during the observed break period is relatively more intense and exhibits lesser diurnal variability than severe monsoonal storms in terms of mean overshooting area in the TTL (as covered by >20 dBZ echoes). However, ground-based radar has geometrical constraints and sampling gaps at high altitude that lead to biases in the final radar product. Using synthetic observations derived from model-based data, ground-based radar is shown to underestimate the mean overshooting area in the TTL across both TWP-ICE regimes. Differences range from ∼180 km2(∼100 km2) to ∼14 km2(∼8 km2) between 14 and 18 km for the active (break) period. This implies that the radar is underestimating the transport of water and ice mass into the TTL by convective overshoots during TWP-ICE. The synthetic data is also used to correct profiles of the mean observed overshooting area. These are shown to differ only marginally between the two sampled regimes once the influence of a large mesoscale convective system, considered as a departure from normal monsoon behavior, was removed from the statistics. The results of our study provide a useful cross-validation comparison for satellite-based detections of overshooting top areas over Darwin, Australia.
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ItemGravity waves generated by convection during TWP-ICE: 2. High-frequency gravity waves(American Geophysical Union, 2014-05-16)High-frequency gravity waves are analyzed using radiosonde soundings taken during the Tropical Warm Pool-International Cloud Experiment (TWP-ICE). The intrinsic periods of these waves are estimated to be between 10 and 50 min. The high-frequency wave activity in the stratosphere, defined by mass-weighted variance of the vertical motion of the sonde, has a maximum following the afternoon local convection indicating that these waves are generated by local convection. The wave activity is the strongest in the lower stratosphere below 22 km and, during the suppressed monsoon period, is modulated with a 3-4 day period. The concentration of the wave activity in the lower stratosphere is consistent with the properties of the environment in which these waves propagate, whereas its 3-4 day modulation is explained by the variation of the convection activity in the TWP-ICE domain. For shallow convection, the wave activity has a weak tendency to increase as the rainfall intensity increases. The wave activity associated with deep convection, which typically occurs at high rainfall intensities, is larger and has more spread than that associated with shallow convection.
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ItemGravity waves generated by convection during TWP-ICE: I. inertia-gravity waves(American Geophysical Union, 2014-05-16)Gravity waves are analyzed in radiosonde soundings taken during the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) campaign. The properties of the inertia-gravity waves are analyzed in Part I, whereas Part II focuses on high-frequency gravity waves. Two groups of inertia-gravity waves are detected: group L (Long vertical wavelength) in the middle stratosphere during the suppressed monsoon period and group S (Short vertical wavelength) in the lower stratosphere during the monsoon break period. Waves belonging to group L propagate to the southeast with a mean intrinsic period of 35 h and have vertical and horizontal wavelengths of about 5-6 km and 3000-6000 km, respectively. Ray-tracing calculations indicate that these waves originate from a deep convective region near Indonesia. Waves belonging to group S propagate to the south-southeast with an intrinsic period, vertical wavelength, and horizontal wavelength of about 45 h, 2 km, and 2000-4000 km, respectively. These waves appear to originate from convection in the vicinity of New Guinea.
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ItemNo Preview AvailableFurther sensitivities of orographic precipitation to terrain geometry in idealized simulations(American Meteorological Society, 2014-01-01)This study examines how variations to the nondimensional mountain height Ĥ and the horizontal aspect ratio β of a straight ridge and a concave ridge influence orographic precipitation. An idealized threedimensional model is used to simulate a moist flow impinging upon these two ridges with Ĥ 50.66-2.0 and β 5 1.0-8.0. The concave ridge generates substantially more precipitation than the straight ridge via an established precipitation-enhancing funneling mechanism near the ridge vertex when the flow is unblocked. Based on previous work, it was hypothesized that when the approaching flow becomes blocked, the strength of the precipitation enhancement by the concave ridge relative to the straight ridge becomes negligible. This study reveals that, if Ĥ is sufficiently large to induce flow reversal on the windward slope, then a secondary circulation develops that is strengthened by the concave ridge with a subsequent enhancement of precipitation. It is also shown that the competing effects of the ridge length and width render the strength of the precipitation enhancement largely insensitive to β. A flow regime diagram for the straight ridge and the concave ridge is also constructed to illustrate the sensitivity of the critical Ĥ value for flow regime transition to changes in the terrain geometry; variations to the low-level relative humidity are also explored.
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ItemConvectively generated gravity waves simulated by NAM-SCA(AMER GEOPHYSICAL UNION, 2014-08-16)Abstract Convectively generated gravity waves are an unresolved process in global models. However, parameterization of their effects is difficult because the parameterized wave spectrum must be linked to the convective source, which is itself parameterized. Here we use the NAM‐SCA model (nonhydrostatic anelastic model with segmentally constant approximation), which is midway between a cloud‐resolving model and convection parameterization. Unlike conventional convection parameterizations, NAM‐SCA is built directly upon a nonhydrostatic anelastic model, and thus it describes dynamical processes consistently but in a “compressed” manner (i.e., with truncation via a spatiotemporal adaptive mesh) that facilitates its potential use as a parameterization. As a result, under the dynamical framework of NAM‐SCA, convection generates a spectrum of gravity waves in a physically consistent manner without any additional assumptions or simplifications. The present study examines the capacity of NAM‐SCA for generating gravity waves from convection under varying degree of “compression” and assesses its viability as a physically consistent convective gravity wave source parameterization. By taking the TWP‐ICE (Tropical Warm Pool‐International Cloud Experiment) period for the case study, NAM‐SCA successfully represents the convectively generated gravity waves even down to the compression rate of 0.1. Analysis in the wave number frequency space shows that the compression behaves as a weak low‐pass filter in wave number, but higher‐frequency components generated by dynamic adaptation partially compensate the filtering effect making the net Reynolds stress relatively insensitive to compression.