Tropical Cyclogenesis Associated with Rossby Wave Energy Dispersion of a Preexisting Typhoon. Part II: Numerical Simulations*

Abstract The cyclogenesis events associated with the tropical cyclone (TC) energy dispersion are simulated in a 3D model. A new TC with realistic dynamic and thermodynamic structures forms in the wake of a preexisting TC when a large-scale monsoon gyre or a monsoon shear line flow is present. Maximum vorticity generation appears in the planetary boundary layer (PBL) and the vorticity growth exhibits an oscillatory development. This oscillatory growth is also seen in the observed rainfall and cloud-top temperature fields. The diagnosis of the model output shows that the oscillatory development is attributed to the discharge and recharge of the PBL moisture and its interaction with convection and circulation. The moisture–convection feedback regulates the TC development through controlling the atmospheric stratification, raindrop-induced evaporative cooling and downdraft, PBL divergence, and vorticity generation. On one hand, ascending motion associated with deep convection transports moisture upward and leads to the discharge of PBL moisture and a convectively stable stratification. On the other hand, the convection-induced raindrops evaporate, leading to midlevel cooling and downdraft. The downdraft further leads to dryness and a reduction of equivalent potential temperature. This reduction along with the recharge of PBL moisture due to surface evaporation leads to reestablishment of a convectively unstable stratification and thus new convection. Sensitivity experiments with both a single mesh (with a 15-km resolution) and a nested mesh (with a 5-km resolution in the inner mesh) indicate that TC energy dispersion alone in a resting environment does not lead to cyclogenesis, suggesting the important role of the wave train–mean flow interaction. A proper initial condition for background wind and moisture fields is crucial for maintaining a continuous vorticity growth through the multioscillatory phases.

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Tropical Cyclogenesis Associated with Rossby Wave Energy Dispersion of a Preexisting Typhoon. Part I: Satellite Data Analyses*

Journal of the Atmospheric Sciences ◽

10.1175/jas3692.1 ◽

2006 ◽

Vol 63 (5) ◽

pp. 1377-1389 ◽

Cited By ~ 70

Author(s):

Tim Li ◽

Bing Fu

Keyword(s):

Rossby Wave ◽

Satellite Data ◽

Wave Energy ◽

Large Scale ◽

Wave Train ◽

Vertical Shear ◽

Energy Dispersion ◽

Data Analyses ◽

Wave Trains ◽

Rossby Wave Train

Abstract The structure and evolution characteristics of Rossby wave trains induced by tropical cyclone (TC) energy dispersion are revealed based on the Quick Scatterometer (QuikSCAT) and Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) data. Among 34 cyclogenesis cases analyzed in the western North Pacific during 2000–01 typhoon seasons, six cases are associated with the Rossby wave energy dispersion of a preexisting TC. The wave trains are oriented in a northwest–southeast direction, with alternating cyclonic and anticyclonic vorticity circulation. A typical wavelength of the wave train is about 2500 km. The TC genesis is observed in the cyclonic circulation region of the wave train, possibly through a scale contraction process. The satellite data analyses reveal that not all TCs have a Rossby wave train in their wakes. The occurrence of the Rossby wave train depends to a certain extent on the TC intensity and the background flow. Whether or not a Rossby wave train can finally lead to cyclogenesis depends on large-scale dynamic and thermodynamic conditions related to both the change of the seasonal mean state and the phase of the tropical intraseasonal oscillation. Stronger low-level convergence and cyclonic vorticity, weaker vertical shear, and greater midtropospheric moisture are among the favorable large-scale conditions. The rebuilding process of a conditional unstable stratification is important in regulating the frequency of TC genesis.

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Four Years of Tropical ERA-40 Vorticity Maxima Tracks. Part II: Differences between Developing and Nondeveloping Disturbances

Monthly Weather Review ◽

10.1175/2008mwr2545.1 ◽

2009 ◽

Vol 137 (8) ◽

pp. 2576-2591 ◽

Cited By ~ 23

Author(s):

Brandon Kerns ◽

Edward Zipser

Keyword(s):

Large Scale ◽

Deep Convection ◽

Vertical Wind Shear ◽

Skill Score ◽

Relative Vorticity ◽

Prediction Skill ◽

Tropical Cyclogenesis ◽

Linear Discriminant ◽

Predictive Skill ◽

Heidke Skill Score

Abstract Using a subset of the relative vorticity maxima (VM) tracks described in Part I, large-scale environmental fields, cold cloud area, and rainfall area are used to discriminate between developing and nondeveloping tropical disturbances in the eastern North Pacific (EPAC) and Atlantic Oceans. By using a minimum cold cloud coverage requirement, the nondeveloping VM are limited to disturbances with enhanced low-level relative vorticity and widespread deep convection. Linear discriminant analysis is used to determine the overall discrimination and the relative importance of each predictor for each basin separately. It is important to distinguish the two basins because, for many predictors, the differences between the basins are greater than the differences between developing and nondeveloping VM in each basin. Using the parametric forecast method, there is greater discrimination and prediction skill in the EPAC than in the Atlantic. There are also significant differences between the two basins in terms of the degree of discrimination provided by each of the predictors. Surprisingly, the mean vertical wind shear magnitude is greater for EPAC developing VM than for EPAC nondeveloping VM. Incorporating the satellite-derived predictors marginally improves the potential forecast skill in the EPAC but not in the Atlantic. The prediction skill (Heidke skill score) of tropical cyclogenesis in the Atlantic is similar to what has been obtained in previous studies using cloud cluster tracks. There is greater predictive skill in the EPAC.

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Modulation of Internal Gravity Waves in a Multiscale Model for Deep Convection on Mesoscales

Journal of the Atmospheric Sciences ◽

10.1175/2010jas3269.1 ◽

2010 ◽

Vol 67 (8) ◽

pp. 2504-2519 ◽

Cited By ~ 7

Author(s):

Daniel Ruprecht ◽

Rupert Klein ◽

Andrew J. Majda

Keyword(s):

Gravity Waves ◽

Large Scale ◽

Energy Transport ◽

Deep Convection ◽

Potential Temperature ◽

Internal Gravity Waves ◽

Small Scale ◽

Closed Model ◽

Large Scale Flow ◽

Effective Stability

Abstract Starting from the conservation laws for mass, momentum, and energy together with a three-species bulk microphysics model, a model for the interaction of internal gravity waves and deep convective hot towers is derived using multiscale asymptotic techniques. From the leading-order equations, a closed model for the large-scale flow is obtained analytically by applying horizontal averages conditioned on the small-scale hot towers. No closure approximations are required besides adopting the asymptotic limit regime on which the analysis is based. The resulting model is an extension of the anelastic equations linearized about a constant background flow. Moist processes enter through the area fraction of saturated regions and through two additional dynamic equations describing the coupled evolution of the conditionally averaged small-scale vertical velocity and buoyancy. A two-way coupling between the large-scale dynamics and these small-scale quantities is obtained: moisture reduces the effective stability for the large-scale flow, and microscale up- and downdrafts define a large-scale averaged potential temperature source term. In turn, large-scale vertical velocities induce small-scale potential temperature fluctuations due to the discrepancy in effective stability between saturated and nonsaturated regions. The dispersion relation and group velocity of the system are analyzed and moisture is found to have several effects: (i) it reduces vertical energy transport by waves, (ii) it increases vertical wavenumbers but decreases the slope at which wave packets travel, (iii) it introduces a new lower horizontal cutoff wavenumber in addition to the well-known high wavenumber cutoff, and (iv) moisture can cause critical layers. Numerical examples reveal the effects of moisture on steady-state and time-dependent mountain waves in the present hot-tower regime.

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Transition from Suppressed to Active Convection Modulated by a Weak Temperature Gradient–Derived Large-Scale Circulation

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-14-0041.1 ◽

2015 ◽

Vol 72 (2) ◽

pp. 834-853 ◽

Cited By ~ 5

Author(s):

C. L. Daleu ◽

S. J. Woolnough ◽

R. S. Plant

Keyword(s):

Temperature Gradient ◽

Large Scale ◽

Deep Convection ◽

Potential Temperature ◽

Mean Value ◽

Transition Time ◽

Large Scale Circulation ◽

Transition Times ◽

Active Convection ◽

Fully Coupled

Abstract Numerical simulations are performed to assess the influence of the large-scale circulation on the transition from suppressed to active convection. As a model tool, the authors used a coupled-column model. It consists of two cloud-resolving models that are fully coupled via a large-scale circulation that is derived from the requirement that the instantaneous domain-mean potential temperature profiles of the two columns remain close to each other. This is known as the weak temperature gradient approach. The simulations of the transition are initialized from coupled-column simulations over nonuniform surface forcing, and the transition is forced in the dry column by changing the local and/or remote surface forcings to uniform surface forcing across the columns. As the strength of the circulation is reduced to zero, moisture is recharged into the dry column and a transition to active convection occurs once the column is sufficiently moistened to sustain deep convection. Direct effects of changing surface forcing occur over the first few days only. Afterward, it is the evolution of the large-scale circulation that systematically modulates the transition. Its contributions are approximately equally divided between the heating and moistening effects. A transition time is defined to summarize the evolution from suppressed to active convection. It is the time when the rain rate in the dry column is halfway to the mean value obtained at equilibrium over uniform surface forcing. The transition time is around twice as long for a transition that is forced remotely compared to a transition that is forced locally. Simulations in which both local and remote surface forcings are changed produce intermediate transition times.

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Coarse, intermediate and high resolution numerical simulations of the transition of a tropical wave critical layer to a tropical storm

Atmospheric Chemistry and Physics ◽

10.5194/acp-10-10803-2010 ◽

2010 ◽

Vol 10 (22) ◽

pp. 10803-10827 ◽

Cited By ~ 42

Author(s):

M. T. Montgomery ◽

Z. Wang ◽

T. J. Dunkerton

Keyword(s):

High Resolution ◽

Deep Convection ◽

Potential Temperature ◽

Wrf Model ◽

Tropical Storm ◽

Critical Layer ◽

Tropical Cyclogenesis ◽

Cyclonic Vorticity ◽

Easterly Wave ◽

Tropical Depressions

Abstract. Recent work has hypothesized that tropical cyclones in the deep Atlantic and eastern Pacific basins develop from within the cyclonic Kelvin cat's eye of a tropical easterly wave critical layer located equatorward of the easterly jet axis. The cyclonic critical layer is thought to be important to tropical cyclogenesis because its cat's eye provides (i) a region of cyclonic vorticity and weak deformation by the resolved flow, (ii) containment of moisture entrained by the developing flow and/or lofted by deep convection therein, (iii) confinement of mesoscale vortex aggregation, (iv) a predominantly convective type of heating profile, and (v) maintenance or enhancement of the parent wave until the developing proto-vortex becomes a self-sustaining entity and emerges from the wave as a tropical depression. This genesis sequence and the overarching framework for describing how such hybrid wave-vortex structures become tropical depressions/storms is likened to the development of a marsupial infant in its mother's pouch, and for this reason has been dubbed the "marsupial paradigm". Here we conduct the first multi-scale test of the marsupial paradigm in an idealized setting by revisiting the Kurihara and Tuleya problem examining the transformation of an easterly wave-like disturbance into a tropical storm vortex using the WRF model. An analysis of the evolving winds, equivalent potential temperature, and relative vertical vorticity is presented from coarse (28 km), intermediate (9 km) and high resolution (3.1 km) simulations. The results are found to support key elements of the marsupial paradigm by demonstrating the existence of a rotationally dominant region with minimal strain/shear deformation near the center of the critical layer pouch that contains strong cyclonic vorticity and high saturation fraction. This localized region within the pouch serves as the "attractor" for an upscale "bottom up" development process while the wave pouch and proto-vortex move together. Implications of these findings are discussed in relation to an upcoming field experiment for the most active period of the Atlantic hurricane season in 2010 that is to be conducted collaboratively between the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation (NSF), and the National Aeronautics and Space Adminstration (NASA).

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Cyclogenesis Simulation of Typhoon Prapiroon (2000) Associated with Rossby Wave Energy Dispersion*

Monthly Weather Review ◽

10.1175/2009mwr3005.1 ◽

2010 ◽

Vol 138 (1) ◽

pp. 42-54 ◽

Cited By ~ 7

Author(s):

Xuyang Ge ◽

Tim Li ◽

Melinda S. Peng

Keyword(s):

Rossby Wave ◽

Wave Energy ◽

Large Scale ◽

Wave Train ◽

Energy Dispersion ◽

Sensitivity Experiment ◽

Low Level ◽

Filtering Technique ◽

Upper Level

Abstract The genesis of Typhoon Prapiroon (2000), in the western North Pacific, is simulated to understand the role of Rossby wave energy dispersion of a preexisting tropical cyclone (TC) in the subsequent genesis event. Two experiments are conducted. In the control experiment (CTL), the authors retain both the previous typhoon, Typhoon Bilis, and its wave train in the initial condition. In the sensitivity experiment (EXP), the circulation of Typhoon Bilis was removed based on a spatial filtering technique of Kurihara et al., while the wave train in the wake is kept. The comparison between these two numerical simulations demonstrates that the preexisting TC impacts the subsequent TC genesis through both a direct and an indirect process. The direct process is through the conventional barotropic Rossby wave energy dispersion, which enhances the low-level wave train, the boundary layer convergence, and the convection–circulation feedback. The indirect process is through the upper-level outflow jet. The asymmetric outflow jet induces a secondary circulation with a strong divergence tendency to the left-exit side of the outflow jet. The upper-level divergence boosts large-scale ascending motion and promotes favorable environmental conditions for a TC-scale vortex development. In addition, the outflow jet induces a well-organized cyclonic eddy angular momentum flux, which acts as a momentum forcing that enhances the upper-level outflow and low-level inflow and favors the growth of the new TC.

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Thermodynamic Environments of Deep Convection in Atlantic Tropical Disturbances

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-12-0278.1 ◽

2013 ◽

Vol 70 (7) ◽

pp. 1912-1928 ◽

Cited By ~ 24

Author(s):

Christopher A. Davis ◽

David A. Ahijevych

Keyword(s):

Large Scale ◽

Inner Core ◽

System Development ◽

Statistical Tests ◽

Deep Convection ◽

Lower Troposphere ◽

Thermodynamic Characteristics ◽

Static Stability ◽

Tropical Cyclogenesis ◽

Sea Temperature

Abstract Conditional composites of dropsondes deployed into eight tropical Atlantic weather systems during 2010 are analyzed. The samples are conditioned based on cloud-top temperature within 10 km of the dropsonde, the radius from the cyclonic circulation center of the disturbance, and the stage of system development toward tropical cyclogenesis. Statistical tests are performed to identify significant differences between composite profiles. Cold-cloud-region-composite profiles of virtual temperature deviations from a large-scale instantaneous average indicate enhanced static stability prior to genesis within 200 km of the center of circulation, with negative anomalies below 700 hPa and larger warm anomalies above 600 hPa. Moist static energy is enhanced in the middle troposphere in this composite mainly because of an increase in water vapor content. Prior to genesis the buoyancy of lifted parcels within 200 km of the circulation center is sharply reduced compared to the buoyancy of parcels farther from the center. These thermodynamic characteristics support the conceptual model of an altered mass flux profile prior to genesis that strongly favors convergence in the lower troposphere and rapid increase of circulation near the surface. It is also noted that the air–sea temperature difference is greatest in the inner core of the pregenesis composite, which suggests a means to preferentially initiate new convection in the inner core where the rotation is greatest.

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A New Perspective on the Excitation of Low-Tropospheric Mixed Rossby–Gravity Waves in Association with Energy Dispersion

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-11-0331.1 ◽

2012 ◽

Vol 69 (4) ◽

pp. 1397-1403 ◽

Cited By ~ 7

Author(s):

Guanghua Chen ◽

Chi-Yung Tam

Keyword(s):

El Niño ◽

El Nino ◽

Wave Train ◽

La Niña ◽

Boreal Summer ◽

Mean Flow ◽

Energy Dispersion ◽

Low Level ◽

La Nina ◽

Equatorial Wave

Abstract This study investigates the synoptic-scale equatorial response to Rossby wave energy dispersion associated with off-equatorial wave activity sources and proposes a new mechanism for triggering low-level mixed Rossby–gravity (MRG) waves. A case study based on observations in boreal summer 2002 reveals that a vortex related to tropical cyclogenesis generated a coherent wave train through southeastward energy dispersion. The southeastward-propagating energy packet gave rise to the equatorial atmospheric response with a temporal scale similar to the wave train and with a structure consistent with the equatorially trapped MRG wave. A baroclinic multilevel anomaly model is employed to verify the excitation of MRG associated with the energy dispersion originating outside of the equatorial region and to explore the discrepancy in the equatorial responses under the different background flows corresponding to El Niño and La Niña. The results show that the prevalence of the low-level westerly flow, the associated zonal wind convergence, and the easterly vertical wind shear can be more favorable for the enhancement of southeastward-propagating energy dispersion and equatorial MRG response in the low troposphere during El Niño than those during La Niña. In addition, the strength of the mean flow can strongly affect the extent of equatorial wave response and modulate its phase and group velocity due to the Doppler shift effect.

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Range of validity of an extended WKB theory for atmospheric gravity waves: one-dimensional and two-dimensional case

Journal of Fluid Mechanics ◽

10.1017/jfm.2013.307 ◽

2013 ◽

Vol 729 ◽

pp. 330-363 ◽

Cited By ~ 12

Author(s):

Felix Rieper ◽

U. Achatz ◽

R. Klein

Keyword(s):

Wave Packet ◽

Gravity Wave ◽

Large Scale ◽

Equations Of Motion ◽

Potential Temperature ◽

Multiple Scale ◽

Wave Number ◽

Momentum Balance ◽

Mean Flow ◽

Governing Parameter

AbstractA computational model of the pseudo-incompressible equations is used to probe the range of validity of an extended Wentzel–Kramers–Brillouin theory (XWKB), previously derived through a distinguished limit of a multiple-scale asymptotic analysis of the Euler or pseudo-incompressible equations of motion, for gravity-wave packets at large amplitudes. The governing parameter of this analysis had been the scale-separation ratio $\varepsilon $ between the gravity wave and both the large-scale potential-temperature stratification and the large-scale wave-induced mean flow. A novel feature of the theory had been the non-resonant forcing of higher harmonics of an initial wave packet, predominantly by the large-scale gradients in the gravity-wave fluxes. In the test cases considered a gravity-wave packet is propagating upwards in a uniformly stratified atmosphere. Large-scale winds are induced by the wave packet, and possibly exert a feedback on the latter. In the limit $\varepsilon \ll 1$ all predictions of the theory can be validated. The larger $\varepsilon $ is the more the transfer of wave energy to the mean flow is underestimated by the theory. The numerical results quantify this behaviour but also show that, qualitatively, XWKB remains valid even when the gravity-wave wavelength approaches the spatial scale of the wave-packet amplitude. This includes the prevalence of first and second harmonics and the smallness of harmonics with wave number higher than two. Furthermore, XWKB predicts for the vertical momentum balance an additional leading-order buoyancy term in Euler and pseudo-incompressible theory, compared with the anelastic theory. Numerical tests show that this term is relatively large with up to $30\hspace{0.167em} \% $ of the total balance. The practical relevance of this deviation remains to be assessed in future work.

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A modified lightning potential index for numerical models with parameterized deep convection

10.5194/ems2021-253 ◽

2021 ◽

Author(s):

Guido Schröder

Keyword(s):

Central Europe ◽

Vertical Velocity ◽

Large Scale ◽

Numerical Models ◽

Deep Convection ◽

Potential Temperature ◽

Wrf Model ◽

Lightning Flash ◽

Potential Index ◽

Forecasting System

A modified lightning potential index (MLPI) for numerical models with parameterized deep convection is presented. It is based on the LPI formula of Lynn and Yair (2010). Following the idea of Lopez (2016), the quantities (e.g. vertical velocity) needed in the LPI formula are derived from the updraft of the Bechtold-Tiedtke parameterization scheme (Bechtold et al., 2014). The formula is further improved by taking into account the vertical equivalent potential temperature gradient.The LPI and MLPI are tested in ICON with 20km resolution (ICON-20) over central Europe. A key component in the LPI is the vertical velocity. To assess its quality, the vertical velocity of the updraft in the convection scheme in ICON-20 is compared to updrafts in the convection-resolving COSMO model with 2.2 km resolution (COSMO-D2). It is shown that in ICON-20 the extension of the vertical velocity is generally broader with the maximum located in higher altitudes. In the charge separation area where the vertical velocity is relevant, the ICON-20 vertical velocity is less than in COSMO-D2. Consequently, the LPI values in ICON-20 are lower by a factor of 2 compared to COSMO-D2.The MLPI is verified against LINET lightning data (Betz et al. 2009) over central Europe for summer 2020 and compared to LPI in COSMO-D2. The MLPI is also compared to the LPI and the lightning flash density (LFD, &#160;Lopez, 2016), all computed in ICON-20. For the test period the MLPI outperforms the LPI and LFD. However, the quality of the LPI in COSMO-D2 cannot quite be reached.&#160;Bechtold et al. 2014: Representing Equilibrium and Nonequilibrium Convection in Large-Scale Models. J. Atmos. Sci. 71, 734-753.Betz et al., 2009: &#160;LINET - An international lightning detection network in Europe. Atmos. &#160;Res. 91 564&#8211;573.Lopez, 2016: A Lightning Parameterization for the ECMWF Integrated Forecasting System. Mon. Wea. Rev., 144, 3057-2075.Lynn and Yair, 2010: Prediction of lightning flash density with the WRF model &#160;Adv. Geosci., 23, 11&#8211;16.

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