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.

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).

scholarly journals Genesis of Tropical Storm Eugene (2005) from Merging Vortices Associated with ITCZ Breakdowns. Part II: Roles of Vortex Merger and Ambient Potential Vorticity

2009 ◽  
Vol 66 (7) ◽  
pp. 1980-1996 ◽  
Author(s):  
Chanh Q. Kieu ◽  
Da-Lin Zhang
Keyword(s):  
Potential Vorticity ◽  
Tropical Storm ◽  
Heat Fluxes ◽  
Vertical Shear ◽  
Tropical Cyclogenesis ◽  
Small Scale ◽  
Absolute Vorticity ◽  
Surface Heat Fluxes ◽  
Vortex Merger ◽  
Bottom Upward

Abstract In this study, the roles of merging midlevel mesoscale convective vortices (MCVs) and convectively generated potential vorticity (PV) patches embedded in the intertropical convergence zone (ITCZ) in determining tropical cyclogenesis are examined by calculating PV and absolute vorticity budgets with a cloud-resolving simulation of Tropical Storm Eugene (2005). Results show that the vortex merger occurs as the gradual capture of small-scale PV patches within a slow-drifting MCV by another fast-moving MCV, thus concentrating high PV near the merger’s circulation center, with its peak amplitude located slightly above the melting level. The merging phase is characterized by sharp increases in surface heat fluxes, low-level convergence, latent heat release (and upward motion), lower tropospheric PV, surface pressure falls, and growth of cyclonic vorticity from the bottom upward. Melting and freezing appear to affect markedly the vertical structures of diabatic heating, convergence, absolute vorticity, and PV, as well the production of PV during the life cycle of Eugene. Results also show significant contributions of the horizontal vorticity to the magnitude of PV and its production within the storm. The storm-scale PV budgets show that the above-mentioned amplification of PV results partly from the net internal dynamical forcing between the PV condensing and diabatic production and partly from the continuous lateral PV fluxes from the ITCZ. Without the latter, Eugene would likely be shorter lived after the merger under the influence of intense vertical shear and colder sea surface temperatures. The vorticity budget reveals that the storm-scale rotational growth occurs in the deep troposphere as a result of the increased flux convergence of absolute vorticity during the merging phase. Unlike the previously hypothesized downward growth associated with merging MCVs, the most rapid growth rate is found in the bottom layers of the merger because of the frictional convergence. It is concluded that tropical cyclogenesis from merging MCVs occurs from the bottom upward.

Life of a Six-Hour Hurricane

2009 ◽  
Vol 137 (1) ◽  
pp. 51-67 ◽  
Author(s):  
Kay L. Shelton ◽  
John Molinari
Keyword(s):  
Deep Convection ◽  
Potential Temperature ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Vertical Shear ◽  
Vortex Center ◽  
Dry Air ◽  
Sheared Flow ◽  
The Core ◽  
Equivalent Potential

Abstract Hurricane Claudette developed from a weak vortex in 6 h as deep convection shifted from downshear into the vortex center, despite ambient vertical wind shear exceeding 10 m s−1. Six hours later it weakened to a tropical storm, and 12 h after the hurricane stage a circulation center could not be found at 850 hPa by aircraft reconnaissance. At hurricane strength the vortex contained classic structure seen in intensifying hurricanes, with the exception of 7°–12°C dewpoint depressions in the lower troposphere upshear of the center. These extended from the 100-km radius to immediately adjacent to the eyewall, where equivalent potential temperature gradients reached 6 K km−1. The dry air was not present prior to intensification, suggesting that it was associated with vertical shear–induced subsidence upshear of the developing storm. It is argued that weakening of the vortex was driven by cooling associated with the mixing of dry air into the core, and subsequent evaporation and cold downdrafts. Evidence suggests that this mixing might have been enhanced by eyewall instabilities after the period of rapid deepening. The existence of a fragile, small, but genuinely hurricane-strength vortex at the surface for 6 h presents difficult problems for forecasters. Such a “temporary hurricane” in strongly sheared flow might require a different warning protocol than longer-lasting hurricane vortices in weaker shear.

A Vortex-Based Perspective of Eastern Pacific Tropical Cyclone Formation

2008 ◽  
Vol 136 (7) ◽  
pp. 2461-2477 ◽  
Author(s):  
Christopher Davis ◽  
Chris Snyder ◽  
Anthony C. Didlake
Keyword(s):  
Tropical Cyclone ◽  
Lower Troposphere ◽  
Hadley Circulation ◽  
Eastern Pacific ◽  
Synoptic Scale ◽  
Cyclonic Vorticity ◽  
Absolute Vorticity ◽  
Tropical Cyclone Formation ◽  
Environmental Prediction ◽  
Background State

Abstract Tropical cyclone formation over the eastern Pacific during 2005 and 2006 was examined using primarily global operational analyses from the National Centers for Environmental Prediction. This paper represents a “vortex view” of genesis, adding to previous work on tropical cyclone formation associated with tropical waves. Between 1 July and 30 September during 2005 and 2006, vortices at 900 hPa were tracked and vortex-following diagnostic quantities were computed. Vortices were more abundant during periods of an enhanced “Hadley” circulation with monsoon westerlies around 10°N in the lower troposphere. This zonally confined Hadley circulation was significantly stronger during the genesis of developing vortices. Developing vortices were stronger at the outset, with a deeper potential vorticity maximum, compared to nondeveloping vortices. This implies that developing disturbances were selected early on by favorable synoptic-scale features. The characteristic time-mean reversal of the meridional gradient of absolute vorticity in the lower troposphere was found to nearly vanish when the aggregate contribution of strong vortices was removed from the time-mean vorticity. This finding implies that it is difficult to unambiguously attribute development to a preexisting enhancement of vorticity on the synoptic scale. The time-mean enhancement of cyclonic vorticity primarily results from the accumulated effect of vortices. It is suggested that horizontal deformation in the background state helps distinguish developing vortices from nondevelopers, and also biases the latitude of development poleward of the climatological ITCZ axis.

scholarly journals Objective Identification of the Intertropical Convergence Zone: Climatology and Trends from the ERA-Interim

2014 ◽  
Vol 27 (5) ◽  
pp. 1894-1909 ◽  
Author(s):  
Gareth Berry ◽  
Michael J. Reeder
Keyword(s):  
Climate Model ◽  
Southern Oscillation ◽  
Weather Prediction ◽  
Lower Troposphere ◽  
Intertropical Convergence Zone ◽  
Eastern Pacific ◽  
Tropical Cyclogenesis ◽  
Convergence Zone ◽  
Operational Forecasts ◽  
The Mean

Abstract An objective method for the identification of the intertropical convergence zone (ITCZ) in gridded numerical weather prediction datasets is presented. This technique uses layer- and time-averaged winds in the lower troposphere to automatically detect the location of the ITCZ and is designed for use with datasets including operational forecasts and climate model output. The method is used to create a climatology of ITCZ properties from the Interim ECMWF Re-Analysis (ERA-Interim) dataset for the period 1979–2009 to serve as an indicator of the technique's ability and a benchmark for future comparisons. The automatically generated objective climatology closely matches the results from subjective studies, showing a seasonal cycle in which the oceanic ITCZ migrates meridionally and the land-based ITCZ features are predominantly summertime phenomena. Composites based on the phase of the El Niño–Southern Oscillation index show a major shift in the mean position and changes in intensity of the ITCZ in all ocean basins as the index varies. Under La Niña conditions, the ITCZ intensifies over the Maritime Continent and eastern Pacific, where the ITCZ weakens over the central and equatorial eastern Pacific. An analysis of changes in the ITCZ and its divergence during the period 1979–2009 indicates that the mean position of the ITCZ shifts southward in the western Pacific and a broad global intensification of the convergence into ITCZ regions. The relationship between tropical cyclogenesis and the ITCZ is also examined, finding that more than 50% of all tropical cyclones form within 600 km of the ITCZ.

scholarly journals The Effects of Topography on the Evolution of Typhoon Saomai (2006) under the Influence of Tropical Storm Bopha (2006)

2013 ◽  
Vol 141 (2) ◽  
pp. 468-489 ◽  
Author(s):  
Wook Jang ◽  
Hye-Yeong Chun
Keyword(s):  
Wrf Model ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Tropical Storm ◽  
Mountain Range ◽  
The West ◽  
Northerly Wind ◽  
Upper Troposphere ◽  
Significant Difference ◽  
Typhoon Saomai

Abstract The effects of topography on the evolution of Typhoon Saomai (2006) are investigated by conducting a series of numerical simulations with the Weather Research and Forecasting (WRF) Model using 100%, 75%, 50%, and 25% of terrain heights of the Central Mountain Range (CMR) in Taiwan. Differences in the track and intensity of Typhoon Saomai between the experiments are strongly related to those of Tropical Storm Bopha, which passed Taiwan earlier than the typhoon. In the sensitivity experiments, the higher CMR drifts Bopha more southward, which results in the weakening of Bopha by prohibiting the interaction between the CMR and Bopha, and the flows induced by Bopha force Saomai to propagate along a more southerly track. The higher CMR weakens the easterly flow in the lower troposphere and suppresses the northerly flow in the upper troposphere to the west of Saomai. The resultant weak vertical wind shear keeps warm air near the typhoon center in the upper troposphere, which promotes the intensification of the typhoon. To examine the direct effects of topography on the track and intensity of Saomai, additional simulations involving the removal of Bopha from the initial condition with 100% and 50% of CMR are conducted. The results without Bopha showed that Saomai moves more southward at a slower speed and with greater intensity, due to the stronger northerly wind to the west of Saomai, which was not canceled out by the southerly wind to the east of Bopha, and there is no significant difference in the tracks or intensity with respect to the mountain heights.

scholarly journals Surface Cold Pools in the Outer Rainbands of Tropical Storm Hanna (2008) Near Landfall

2012 ◽  
Vol 140 (2) ◽  
pp. 471-491 ◽  
Author(s):  
Matthew D. Eastin ◽  
Tiffany L. Gardner ◽  
M. Christopher Link ◽  
Kelly C. Smith
Keyword(s):  
Potential Temperature ◽  
Leading Edge ◽  
Tropical Storm ◽  
Vertical Shear ◽  
Cold Pool ◽  
Cold Pools ◽  
Pressure Anomaly ◽  
Cyclonic Flow ◽  
Convective Cells ◽  
Pool Formation

Surface mesonet observations were used to document the structure of prominent cold pools associated with two convective outer rainbands of Tropical Storm Hanna on 5 September 2008. The developing rainbands were located ~400 km north of the storm center as they crossed the Carolina coastline and passed over the mesonet. The combination of moderate CAPE, moderate low-level cross-band vertical shear, and dry midlevel air provided an environment supportive of surface cold pool formation and long-lived quasi-linear convection. Both rainbands exhibited multiple outward-tilting convective cells and discrete line segments centered within a narrow zone of nearly continuous stratiform precipitation. The mesonet observations provided a unique along- and cross-band perspective of discrete cold pools situated beneath and behind the convection portions of each rainband. Prominent cold pools extended 40–80 km behind the rainband’s leading edge and exhibited maximum potential temperature, mixing ratio, and equivalent potential temperature deficits of 2–4 K, 1–2 g kg−1, and 4–10 K, respectively. In the cross-band direction, convergence of storm-relative inflow along the cold pool leading edge was coincident with a modest high pressure anomaly, while inflow divergence prevailed through the cold pool and rainfall maxima. Several cold pools expanded along-band while being advected downband by the prevailing cyclonic flow. Cold pool wakes were observed more than 50 km behind the rainband leading edge and up to 20 km downband from intense convection. Variations in cold pool intensity were not well correlated with convective intensity, rainfall rate, or the degree of midlevel dryness. Implications of prominent cold pools on tropical cyclone convection, size, and intensity are discussed.

scholarly journals The West Coast Thermal Trough: Mesoscale Evolution and Sensitivity to Terrain and Surface Fluxes

2013 ◽  
Vol 141 (8) ◽  
pp. 2869-2896 ◽  
Author(s):  
Matthew C. Brewer ◽  
Clifford F. Mass ◽  
Brian E. Potter
Keyword(s):  
High Pressure ◽  
West Coast ◽  
Energy Equation ◽  
Potential Temperature ◽  
Wrf Model ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
The West ◽  
Surface Heat Fluxes ◽  
Thermodynamic Energy

Abstract Despite the significant impacts of the West Coast thermal trough (WCTT) on West Coast weather and climate, questions remain regarding its mesoscale structure, origin, and dynamics. Of particular interest is the relative importance of terrain forcing, advection, and surface heating on WCTT formation and evolution. To explore such questions, the 13–16 May 2007 WCTT event was examined using observations and simulations from the Weather Research and Forecasting (WRF) Model. An analysis of the thermodynamic energy equation for these simulations was completed, as well as sensitivity experiments in which terrain or surface fluxes were removed or modified. For the May 2007 event, vertical advection of potential temperature is the primary driver of local warming and WCTT formation west of the Cascades. The downslope flow that drives this warming is forced by easterly flow associated with high pressure over British Columbia, Canada. When the terrain is removed from the model, the WCTT does not form and high pressure builds over the northwest United States. When the WCTT forms on the east side of the Cascades, diabatic heating dominates over the other terms in the thermodynamic energy equation, with warm advection playing a small role. If surface heat fluxes are neglected, an area of low pressure remains east of the Cascades, though it is substantially attenuated.

scholarly journals The Use of Moist Potential Vorticity Vector as Diagnostic Variable of Rainfall Events in Tanzania

2017 ◽  
Vol 9 (5) ◽  
pp. 1
Author(s):  
Philbert Modest Luhunga ◽  
Agnes Kijazi ◽  
Ladislaus Chang a ◽  
Chuki A Sangalugembe ◽  
Doreen Mwara Anande ◽  
...  
Keyword(s):  
Potential Vorticity ◽  
Wrf Model ◽  
Lower Troposphere ◽  
Topographic Effects ◽  
New Paradigm ◽  
Rainfall Events ◽  
North Eastern ◽  
Vorticity Vector ◽  
Moist Potential Vorticity ◽  
Thermodynamic Variables

The work of this paper is a first step of the new paradigm, to use the Moist Potential Vorticity Vector (MPVV) as a diagnostic variable of rainfall events in Tanzania. The paper aims at computing and assessing the usefulness of MPVV in the diagnosis of rainfall events that occurred on 08th and 09th May 2017 over different regions in Tanzania. The relative contributions of horizontal, vertical components and the magnitude of MPVV on diagnosis of rainfall events are assessed. Hourly dynamic and thermodynamic variables of wind speed, temperature, atmospheric pressure and relative humidity from the numerical output generated by the Weather Research and Forecasting (WRF) Model, running at Tanzania Meteorological Agency (TMA) are used in computation of MPVV. The computed MPVV is then compared with WRF model forecasts and observed rainfall. It is found that in most parts of the country, particularly over coastal areas and North-Eastern Highlands, MPVV exhibited positive values in the lower troposphere (925hPa) and (850hPa) indicating local instability possibly associated with topographic effects, and continent/ocean contrast. MPVV is mostly positive with slightly negative values indicating instabilities (due to possible convective instability). Moreover, MPVV provides remarkably accurate tracking of the locations received rainfall, suggesting its potential use as a dynamic diagnostic variable of rainfall events in Tanzania.

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