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

2010 ◽  
Vol 10 (22) ◽  
pp. 10803-10827 ◽  
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).

scholarly journals Intermediate and high resolution numerical simulations of the transition of a tropical wave critical layer to a tropical storm

2009 ◽  
Vol 9 (6) ◽  
pp. 26143-26197 ◽  
Author(s):  
M. T. Montgomery ◽  
Z. Wang ◽  
T. J. Dunkerton
Keyword(s):  
High Resolution ◽  
Deep Convection ◽  
Potential Temperature ◽  
Wrf Model ◽  
Tropical Storm ◽  
Critical Layer ◽  
Trade Wind ◽  
Tropical Cyclogenesis ◽  
Cyclonic Vorticity ◽  
Easterly Wave

Abstract. Recent work has hypothesized that tropical cyclones in the deep Atlantic and eastern Pacific basins develop from the cyclonic Kelvin cat's eye of a tropical easterly wave critical layer located equatorward of the easterly jet axis that typifies the trade wind belt. 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 problem of 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) 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 vorticity dominant region with minimal strain/shear deformation within 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).

Genesis of Pre–Hurricane Felix (2007). Part I: The Role of the Easterly Wave Critical Layer

2010 ◽  
Vol 67 (6) ◽  
pp. 1711-1729 ◽  
Author(s):  
Zhuo Wang ◽  
M. T. Montgomery ◽  
T. J. Dunkerton
Keyword(s):  
Large Scale ◽  
Focal Point ◽  
Model Simulation ◽  
Deep Convection ◽  
Wrf Model ◽  
Lower Troposphere ◽  
Critical Layer ◽  
African Easterly Wave ◽  
Easterly Wave ◽  
Vorticity Balance

Abstract The formation of pre–Hurricane Felix (2007) in a tropical easterly wave is examined in a two-part study using the Weather Research and Forecasting (WRF) model with a high-resolution nested grid configuration that permits the representation of cloud system processes. The simulation commences during the wave stage of the precursor African easterly-wave disturbance. Here the simulated and observed developments are compared, while in Part II of the study various large-scale analyses, physical parameterizations, and initialization times are explored to document model sensitivities. In this first part the authors focus on the wave/vortex morphology, its interaction with the adjacent intertropical convergence zone complex, and the vorticity balance in the neighborhood of the developing storm. Analysis of the model simulation points to a bottom-up development process within the wave critical layer and supports the three new hypotheses of tropical cyclone formation proposed recently by Dunkerton, Montgomery, and Wang. It is shown also that low-level convergence associated with the ITCZ helps to enhance the wave signal and extend the “wave pouch” from the jet level to the top of the atmospheric boundary layer. The region of a quasi-closed Lagrangian circulation within the wave pouch provides a focal point for diabatic merger of convective vortices and their vortical remnants. The wave pouch serves also to protect the moist air inside from dry air intrusion, providing a favorable environment for sustained deep convection. Consistent with the authors’ earlier findings, the tropical storm forms near the center of the wave pouch via system-scale convergence in the lower troposphere and vorticity aggregation. Components of the vorticity balance are shown to be scale dependent, with the immediate effects of cloud processes confined more closely to the storm center than the overturning Eliassen circulation induced by diabatic heating, the influence of which extends to larger radii.

scholarly journals Genesis of Tropical Storm Eugene (2005) from Merging Vortices Associated with ITCZ Breakdowns. Part I: Observational and Modeling Analyses

2008 ◽  
Vol 65 (11) ◽  
pp. 3419-3439 ◽  
Author(s):  
Chanh Q. Kieu ◽  
Da-Lin Zhang
Keyword(s):  
Exchange Process ◽  
Potential Temperature ◽  
Wrf Model ◽  
Lower Troposphere ◽  
Tropical Storm ◽  
Eastern Pacific ◽  
Vertical Shear ◽  
Dynamical Instability ◽  
Tropical Cyclogenesis ◽  
Environmental Prediction

Abstract Although tropical cyclogenesis occurs over all tropical warm ocean basins, the eastern Pacific appears to have the highest frequency of tropical cyclogenesis events per unit area. In this study, tropical cyclogenesis from merging mesoscale convective vortices (MCVs) associated with breakdowns of the intertropical convergence zone (ITCZ) is examined. This is achieved through a case study of the processes leading to the genesis of Tropical Storm Eugene (2005) over the eastern Pacific using the National Centers for Environmental Prediction reanalysis, satellite data, and 4-day multinested cloud-resolving simulations with the Weather Research and Forecast (WRF) model at the finest grid size of 1.33 km. Observational analyses reveal the initiations of two MCVs on the eastern ends of the ITCZ breakdowns that occurred more than 2 days and 1000 km apart. The WRF model reproduces their different movements, intensity and size changes, and vortex–vortex interaction at nearly the right timing and location at 39 h into the integration as well as the subsequent track and intensity of the merger in association with the poleward rollup of the ITCZ. Model results show that the two MCVs are merged in a coalescence and capture mode due to their different larger-scale steering flows and sizes. As the two MCVs are being merged, the low- to midlevel potential vorticity and tangential flows increase substantially; the latter occurs more rapidly in the lower troposphere, helping initiate the wind-induced surface heat exchange process leading to the genesis of Eugene with a diameter of 400 km. Subsequently, the merger moves poleward with characters of both MCVs. The simulated tropical storm exhibits many features that are similar to a hurricane, including the warm-cored “eye” and the rotating “eyewall.” It is also shown that vertical shear associated with a midlevel easterly jet leads to the downshear tilt and the wavenumber-1 rainfall structures during the genesis stage, and the upshear generation of moist downdrafts in the vicinity of the eyewall in the minimum equivalent potential temperature layer. Based on the above results, it is concluded that the ITCZ provides a favorable environment with dynamical instability, high humidity, and background vorticity, but it is the merger of the two MCVs that is critical for the genesis of Eugene. The storm decays as it moves northwestward into an environment with increasing vertical shear, dry intrusion, and colder sea surface temperatures. The results appear to have important implications for the high frequency of development of tropical cyclones in the eastern Pacific.

Observations and Predictability of a Nondeveloping Tropical Disturbance over the Eastern Atlantic

2018 ◽  
Vol 146 (9) ◽  
pp. 3079-3096 ◽  
Author(s):  
Alan Brammer ◽  
Chris D. Thorncroft ◽  
Jason P. Dunion
Keyword(s):  
Deep Convection ◽  
Recirculation Region ◽  
Tropical Cyclogenesis ◽  
Short Term ◽  
Ensemble Forecasts ◽  
Dry Air ◽  
Easterly Wave ◽  
Closed Circulation ◽  
West African Coast ◽  
Favorable Conditions

Abstract A strong African easterly wave (AEW) left the West African coast in early September 2014 and operational global numerical forecasts suggested a potential for rapid tropical cyclogenesis of this disturbance in the eastern Atlantic, despite the presence of a large region of dry air northwest of the disturbance. Analysis and in situ observations show that after leaving the coast, the closed circulation associated with the AEW trough was not well aligned vertically, and therefore, low-level or midlevel dry air was advected below or above, respectively, areas of closed circulation. GPS dropwindsonde observations highlight the dry air undercutting the midlevel recirculation region in the southwestern quadrant. This advection of dry air constrains the spatial extent of deep convection within the AEW trough, leading to the vortex decaying. As the column continues to be displaced horizontally, losing vertical alignment, this enables increased horizontal advection of dry air into the system further limiting convective activity. Ensemble forecasts indicate that short-term errors in precipitation rate and vorticity generation can lead to an over intensified and well-aligned vortex, which then interacts less with the unfavorable environment, allowing for further convection and intensification. The stronger vortex provides more favorable conditions for precipitation through a more vertically coherent closed circulation and thus a positive feedback loop is initiated. The short-term forecasts of precipitation were shown to be sensitive to lower-tropospheric moisture anomalies around the AEW trough through ensemble sensitivity analysis from Global Ensemble Forecast System real-time forecasts.

scholarly journals Hurricane Fred (2015): Cape Verde’s First Hurricane in Modern Times: Observations, Impacts, and Lessons Learned

2017 ◽  
Vol 98 (12) ◽  
pp. 2603-2618 ◽  
Author(s):  
Gregory S. Jenkins ◽  
Ester Brito ◽  
Emanuel Soares ◽  
Sen Chiao ◽  
Jose Pimenta Lima ◽  
...  
Keyword(s):  
Wrf Model ◽  
Heavy Precipitation ◽  
Adaptation Strategy ◽  
Tropical Storm ◽  
Cape Verde ◽  
Lessons Learned ◽  
Coastal Zones ◽  
Sea Level Pressure ◽  
Tropical Oceans ◽  
Easterly Wave

Abstract During 31 August 2015, Hurricane Fred traversed through the Cape Verde islands, passing near Boa Vista and possibly making landfall over São Nicolau during the evening hours. Hurricane Fred was a category 1 storm with maximum winds of 75 kt and a sea level pressure of 986 hPa during 31 August. The African easterly wave associated with Tropical Cyclone (TC) Fred emerged from Guinea on 29 August and was named Tropical Storm Fred on 30 August. Prior to impacting Cape Verde, the storm caused the sinking of a ship and the loss of seven lives off the coast of Guinea-Bissau and damage along coastal zones of Senegal. In Cape Verde, there was no loss of life but there was damage on 7 of the 10 islands. Station data show tropical-storm-force winds in Boa Vista and Sal and hurricane-force winds in São Nicolau. Heavy precipitation with amounts in excess of 200 mm occurred on the islands of Santiago, São Nicolau, and Santo Antão. The Weather Research and Forecasting (WRF) Model and Global Forecast System (GFS) were used as primary guidance several days prior to the formation of TC Fred. Challenges for protecting populations from tropical cyclones can be addressed through annual hurricane awareness and preparedness programs in several languages. This is also an adaptation strategy related to anthropogenic climate change as warming tropical oceans may increase the risk of damage and threaten populations. Radar and buoy observations are needed now, and the movement of vulnerable populations and other adaptation strategies will need to be implemented in the coming decades.

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

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

<p>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.</p><p>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.</p><p>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,  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.</p><p> </p><p>Bechtold et al. 2014: Representing Equilibrium and Nonequilibrium Convection in Large-Scale Models. J. Atmos. Sci. 71, 734-753.</p><p>Betz et al., 2009:  LINET - An international lightning detection network in Europe. Atmos.  Res. 91 564–573.</p><p>Lopez, 2016: A Lightning Parameterization for the ECMWF Integrated Forecasting System. Mon. Wea. Rev., 144, 3057-2075.</p><p>Lynn and Yair, 2010: Prediction of lightning flash density with the WRF model  Adv. Geosci., 23, 11–16.</p>

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

2006 ◽  
Vol 63 (5) ◽  
pp. 1390-1409 ◽  
Author(s):  
Tim Li ◽  
Xuyang Ge ◽  
Bin Wang ◽  
Yongti Zhu
Keyword(s):  
Large Scale ◽  
Wave Train ◽  
Deep Convection ◽  
Potential Temperature ◽  
Stable Stratification ◽  
Temperature Fields ◽  
Tropical Cyclogenesis ◽  
Mean Flow ◽  
Energy Dispersion ◽  
Vorticity Generation

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.

Examining the Roles of the Easterly Wave Critical Layer and Vorticity Accretion During the Tropical Cyclogenesis of Hurricane Sandy

2014 ◽  
Author(s):  
III Lussier ◽  
Rutherford Louis L. ◽  
Montgomery Blake ◽  
Boothe Michael T. ◽  
Dunkerton Mark A. ◽  
...  
Keyword(s):  
Hurricane Sandy ◽  
Critical Layer ◽  
Tropical Cyclogenesis ◽  
Easterly Wave

Simulation of the Downshear Reformation of a Tropical Cyclone

2015 ◽  
Vol 72 (12) ◽  
pp. 4529-4551 ◽  
Author(s):  
Leon T. Nguyen ◽  
John Molinari
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Diabatic Heating ◽  
Wrf Model ◽  
Relative Vorticity ◽  
Horizontal Resolution ◽  
Cyclonic Circulation ◽  
Tropical Storm ◽  
Weather Research And Forecasting ◽  
Cyclonic Vorticity

Abstract The downshear reformation of Tropical Storm Gabrielle (2001) was simulated at 1-km horizontal resolution using the Weather Research and Forecasting (WRF) Model. The environmental shear tilted the initial parent vortex downshear left and forced azimuthal wavenumber-1 kinematic, thermodynamic, and convective asymmetries. The combination of surface enthalpy fluxes and a lack of penetrative downdrafts right of shear allowed boundary layer moist entropy to increase to a maximum downshear right. This contributed to convective instability that fueled the downshear convection. Within this convection, an intense mesovortex rapidly developed, with maximum boundary layer relative vorticity reaching 2.2 × 10−2 s−1. Extreme vortex stretching played a key role in the boundary layer spinup of the mesovortex. Cyclonic vorticity remained maximized in the boundary layer and intensified upward with the growth of the convective plume. The circulation associated with the mesovortex and adjacent localized cyclonic vorticity anomalies comprised a developing “inner vortex” on the downshear-left (downtilt) periphery of the parent cyclonic circulation. The inner vortex was nearly upright within a parent vortex that was tilted significantly with height. This inner vortex became the dominant vortex of the system, advecting and absorbing the broad, tilted parent vortex. The reduction of tropical cyclone (TC) vortex tilt from 65 to 20 km in 3 h reflected the emerging dominance of this upright inner vortex. The authors hypothesize that downshear reformation, resulting from diabatic heating associated with asymmetric convection, can aid the TC’s resistance to shear by reducing vortex tilt and by enabling more diabatic heating to occur near the center, a region known to favor TC intensification.

Convective Bursts, Downdraft Cooling, and Boundary Layer Recovery in a Sheared Tropical Storm

2013 ◽  
Vol 141 (3) ◽  
pp. 1048-1060 ◽  
Author(s):  
John Molinari ◽  
Jaclyn Frank ◽  
David Vollaro
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Potential Temperature ◽  
Tangential Velocity ◽  
Vertical Wind Shear ◽  
Surface Heat ◽  
Tropical Storm ◽  
Convective Bursts ◽  
Moisture Fluxes ◽  
Heat And Moisture

Abstract Tropical Storm Edouard (2002) experienced episodic outbreaks of convection downshear within the storm core in the presence of 11–15 m s−1 of ambient vertical wind shear. These outbreaks lasted 2–6 h and were followed by long periods with no deep convection. Flights from U.S. Air Force reconnaissance aircraft within the boundary layer were used to investigate the cause of one such oscillation. Low equivalent potential temperature θe air filled the boundary layer as convection ceased, creating a 4–6-K deficit in θe within the convective region. Soundings within 110 km of the center were supportive of convective downdrafts, with midlevel relative humidity below 15% and large downdraft CAPE. Deep convection ceased within 75 km of the center for more than 8 h. Tangential velocity reached hurricane force locally during the convective outbreak, then became nearly symmetric after convection stopped, arguably as a result of axisymmetrization, and the storm weakened. Nevertheless, the corresponding lack of convective downdrafts during this period allowed surface heat and moisture fluxes to produce substantial increases in boundary layer entropy. A new burst of convection followed. Consistent with recent papers it is argued that tropical cyclone intensification and decay can be understood as a competition between surface heat and moisture fluxes (“fuel”) and low-entropy downdrafts into the boundary layer (“antifuel”).

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