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33
2004: The importance of the precipitation mass sink in tropical cyclones and other heavily precipitating systems
- J. Atmos. Sci
"... When water vapor is converted to cloud and precipitation and subsequently removed to the surface via precipitation, there is a corresponding hydrostatic pressure decrease due to the reduction of mass in the overlying column. Pressure changes resulting from the addition or removal of water vapor are ..."
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When water vapor is converted to cloud and precipitation and subsequently removed to the surface via precipitation, there is a corresponding hydrostatic pressure decrease due to the reduction of mass in the overlying column. Pressure changes resulting from the addition or removal of water vapor are currently neglected in most meteorological applications. However, in heavily precipitating systems such as tropical cyclones, where precip-itation rates may exceed 250 mm day21, the pressure equivalent of the precipitation mass sink is not negligible (;25 hPa day21). Pressure decreases due to this mechanism are most pronounced in the lower troposphere, particularly below the melting level. The resulting unbalanced pressure-gradient force can enhance convergence, which precludes full realization of the pressure decrease but may contribute to vorticity generation and moisture convergence. The importance of the precipitation mass sink is investigated for the case of Hurricane Lili (2002) through the computation of mass and potential vorticity (PV) budgets and numerical sensitivity experiments. The pre-cipitation mass reaching the surface within 100 km of the storm center is of the same order as the mass loss needed to explain the area-averaged pressure decrease during the intensification stage of Lili. The PV is altered by precipitation mass flux divergence across isentropic layers. A volume-integrated PV budget reveals that the
Evaluation of an analytical model for the maximum intensity of tropical cyclones
- J. Atmos. Sci
"... Several studies have shown that the intensity of numerically simulated tropical cyclones can exceed (by 50%) a theoretical upper limit. To investigate the cause, this study evaluates the underlying components of Emanuel’s commonly cited analytic theory for potential intensity (herein referred to as ..."
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Cited by 11 (2 self)
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Several studies have shown that the intensity of numerically simulated tropical cyclones can exceed (by 50%) a theoretical upper limit. To investigate the cause, this study evaluates the underlying components of Emanuel’s commonly cited analytic theory for potential intensity (herein referred to as E-PI). A review of the derivation of E-PI highlights three primary components: a dynamical component (gradient-wind and hydrostatic balance); a thermodynamical component (reversible or pseudoadiabatic thermodynamics, al-though the pseudoadiabatic assumption yields greater intensity); and a planetary boundary layer (PBL) closure (which relates the horizontal gradients of entropy and angular momentum at the top of the PBL to fluxes and stresses at the ocean surface). These three components are evaluated using output from an axi-symmetric numerical model. The present analysis finds the thermodynamical component and the PBL closure to be sufficiently accurate for several different simulations. In contrast, the dynamical component is clearly violated. Although the balanced portion of the flow (yg, to which E-PI applies) appears to also exceed E-PI, it is shown that this difference is attributable to the method used to calculate yg from the model output. Evi-dence is shown that yg for a truly balanced cyclone does not exceed E-PI. To clearly quantify the impact of unbalanced flow, a more complete analytic model is presented. The model is not expressed in terms of ex-
Equilibrium vs. activation control of large-scale variations of tropical deep convection
- The Physics and Parameterization of Moist Atmospheric Convection
, 1997
"... What processes control large-scale variations of deep convection (LSVDC) in the trop-ics? Here ‘large-scale ’ is taken to mean any coherent variations, in either space or time, comprised of statistical populations of separate convective cloud systems. This essay highlights the distinction between pr ..."
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What processes control large-scale variations of deep convection (LSVDC) in the trop-ics? Here ‘large-scale ’ is taken to mean any coherent variations, in either space or time, comprised of statistical populations of separate convective cloud systems. This essay highlights the distinction between processes which supply moisture or available energy over the depth of the convecting layer (equilibrium control), versus inhibition and initia-tion processes at low levels (activation control), as hypotheses for explaining LSVDC. Conceptual separations of the LSVDC problem are reviewed. Scale separation, though rigorous, is artificial, since net heating makes deep convective clouds multiscale, or spectrally red. Moist-dry, or diabatic-adiabatic, separation is more useful. An ill-posed hybrid separation- ‘the interaction of moist convection with large scales ’- has spawned confusion. Correlations between deep convection and its own large-scale components (suggestively labeled ‘forcing’) have been misinterpreted as evidence for equilibrium control. This externalization of large-scale vertical velocity also encourages overinter-pretation of a fictitious ‘compensating subsidence ’ term.
The extratropical transition of tropical cyclones : Forecast challenges, current understanding, and future directions, Wea
- Forecasting
, 2003
"... A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that ..."
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Cited by 9 (0 self)
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A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition are described and potential vorticity thinking is presented as an aid to understanding ET. Further sections discuss
2011: Mesoscale simulation of tropical cyclones in the South Pacific: Climatology and interannual variability
- J. Climate
"... The Weather Research and Forecast model at 1/38 resolution is used to simulate the statistics of tropical cyclone (TC) activity in the present climate of the South Pacific. In addition to the large-scale conditions, the model is shown to reproduce a wide range of mesoscale convective systems. Tropic ..."
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Cited by 5 (4 self)
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The Weather Research and Forecast model at 1/38 resolution is used to simulate the statistics of tropical cyclone (TC) activity in the present climate of the South Pacific. In addition to the large-scale conditions, the model is shown to reproduce a wide range of mesoscale convective systems. Tropical cyclones grow from the most intense of these systems formed along the South Pacific convergence zone (SPCZ) and sometimes develop into hurricanes. The three-dimensional structure of simulated tropical cyclones is in excellent agreement with dropsondes and satellite observations. The mean seasonal and spatial distributions of TC genesis and occurrence are also in good agreement with the Joint Typhoon Warning Center (JTWC) data. It is noted, however, that the spatial pattern of TC activity is shifted to the northeast because of a similar bias in the environmental forcing. Over the whole genesis area, 8.26 3.5 cyclones are produced seasonally in the model, compared with 6.6 6 3.0 in the JTWC data. Part of the interannual variability is associated with El Niño– Southern Oscillation (ENSO). ENSO-driven displacement of the SPCZ position produces a dipole pattern of correlation and results in a weaker correlation when the opposing correlations of the dipole are amalgamated over the entire South Pacific region. As a result, environmentally forced variability at the regional scale is relatively weak, that is, of comparable order to stochastic variability (61.7 cyclones yr21), which is estimated from a 10-yr climatological simulation. Stochastic variability appears essentially related to mesoscale in-teractions, which also affect TC tracks and the resulting occurrence. 1.
On the role of upper-tropospheric potential vorticity advection in tropical cyclone formation: Case studies from 1991. M.S. thesis, Dept. of Earth, Atmospheric and Planetary Sciences
- Available from Massachusetts Institute of Technology, Room 14-0551, 77 Massachusetts Avenue
, 1992
"... Several cases of western North Pacific tropical cyclogenesis from the 1991 season are studied, with special emphasis on the antecedent conditions in the upper troposphere. Specifically, I test the hypothesis that tropical cyclogenesis takes place through an interaction between a pre-existing lower-t ..."
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Several cases of western North Pacific tropical cyclogenesis from the 1991 season are studied, with special emphasis on the antecedent conditions in the upper troposphere. Specifically, I test the hypothesis that tropical cyclogenesis takes place through an interaction between a pre-existing lower-tropospheric disturbance (ITCZ or easterly wave disturbance) and an independent upper-level trough. The upper-level trough provides a source of upper-tropospheric lifting via differential potential vorticity advection which may aid in the genesis process. In this study, locations of pre-cyclone disturbances are obtained from JTWC best-track information. A measure of potential vorticity advection by the upper-tropospheric shear or the "forcing " of ascent is then calculated from NMC gridded analyses. It is found that the majority of tropical cyclones studied are subjected to periods of positive forcing during their pre-genesis stages. In addition, positive forcing is implicated in the generation of the pre-cyclone disturbances for the majority of cases. The findings support the notion of tropical cyclogenesis as an externally triggered phenomenon.
Effects of Convective Heating on Movement and Vertical Coupling of Tropical Cyclones: A Numerical Study*
, 2000
"... The influence of convective heating on movement and vertical coupling of tropical cyclones (TCs) is inves-tigated using a hurricane model with different environmental flows. The authors identify two processes by which convective heating may affect TC motion. One is the advection of symmetric potenti ..."
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Cited by 2 (1 self)
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The influence of convective heating on movement and vertical coupling of tropical cyclones (TCs) is inves-tigated using a hurricane model with different environmental flows. The authors identify two processes by which convective heating may affect TC motion. One is the advection of symmetric potential vorticity (PV) by heating-induced asymmetric flow. The other is the direct generation of a positive PV tendency by asymmetric heating, which acts to shift a TC to the region of maximum downward gradient of asymmetric heating. A steering level exists that is located at the level where the direct influence of asymmetric heating vanishes, normally in the lower troposphere. At that level, a TC moves with the asymmetric flow averaged within a radius of 200 km, because the influence of asymmetric flows on TC motion is weighted by the horizontal PV gradient that is primarily confined within the TC core. Although the vertical shear in the asymmetric flow (including environ-mental and heating-induced flows) could tilt the vortex, the influence of asymmetric heating tends to offset the vertical tilt caused by the vertical shear through a fast adjustment between the asymmetric wind and diabatic heating. Therefore, diabatic heating enhances the vertical coupling. 1.
2012: Impact of physics representations in the HWRFX on simulated hurricane structure and pressure–wind relationships.Mon.Wea
- Rev
"... Aseries of idealized experiments with theNOAAExperimentalHurricaneWeatherResearchandForecasting Model (HWRFX) are performed to examine the sensitivity of idealized tropical cyclone (TC) intensification to various parameterization schemes of the boundary layer (BL), subgrid convection, cloud microphy ..."
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Aseries of idealized experiments with theNOAAExperimentalHurricaneWeatherResearchandForecasting Model (HWRFX) are performed to examine the sensitivity of idealized tropical cyclone (TC) intensification to various parameterization schemes of the boundary layer (BL), subgrid convection, cloud microphysics, and ra-diation. Results from all the experiments are compared in terms of the maximum surface 10-m wind (VMAX) and minimum sea level pressure (PMIN)—operational metrics of TC intensity—as well as the azimuthally averaged temporal and spatial structure of the tangential wind and its material acceleration. The conventional metrics of TC intensity (VMAX and PMIN) are found to be insufficient to reveal the sensitivity of the simulated TC to variations inmodel physics. Comparisons of the sensitivity runs indicate that (i) different boundary layer physics parameterization schemes for vertical subgrid turbulence mixing lead to differences not only in the intensity evolution in terms of VMAX and PMIN, but also in the structural characteristics of the simulated tropical cyclone; (ii) the surface drag coefficient is a key parameter that controls the VMAX–PMIN relationship near the surface; and (iii) different microphysics and subgrid convection parameterization schemes, because of their different realizations of diabatic heating distribution, lead to significant variations in the vortex structure.
A.: Evaluation of a reduced model for investigating hurricane formation from turbulence
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a: Viscous Destabilization of Stratified Shear Flow b: Organization of Rainfall by an Unstable Jet Aloft
, 1990
"... Viscous Destabilization of Stratified Shear Flow: Viscosity may allow insta-bility in a stratified shear flow even though the Richardson number (Ri) is everywhere greater than one-quarter. This is in contrast to an inviscid fluid, where any flow with Ri> 1 is stable. 4 While a rigorous upper boun ..."
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Viscous Destabilization of Stratified Shear Flow: Viscosity may allow insta-bility in a stratified shear flow even though the Richardson number (Ri) is everywhere greater than one-quarter. This is in contrast to an inviscid fluid, where any flow with Ri> 1 is stable. 4 While a rigorous upper bound has not been obtained, normal mode instabilities were found for Ri as large as 0.349. A description of the instability as an over-reflecting wave suggests why viscosity, which typically damps perturbations, can in this instance lead to their amplification. The calculations suggest that the stability of a stratified shear flow is determined by the index of refraction for waves propagating across the shear-a function of the basic state velocity and stratification-rather than by these latter two quantities considered individually. Organization of Rainfall by an Unstable Jet Aloft: During GATE, ' it
