baroclinic influences on tropical cyclogenesis

Baroclinic Influences on Tropical CyclogenesisRon McTaggart-Cowan, Tom Galarneau Jr., Lance Bosart, Rich Moore and Olivia MartiusNPS Seminar15 August 2013

Early WorkTC DianaPV ThinkingTTClimatologyClassificationsEfficiency

In the Beginning ...Tropical cyclone (TC) development has been described by numerous theories since the advent of regular tropical observations in the mid-1950'sRiehl (1948) emphasizes the importance of upper-level disturbances in enhancing the growth rate of instabilities in the easterly trades and proposes three patterns:Strong polar westerlies aloft: no TC formationModerate polar westerlies within 10o of the equatorial shear line: TCs can form during interactionsTropical-only upper and lower-level disturbancesIn Riehl's conceptual model, the isallobaric field created by upper-level divergence enhances growth ratesEarly Work


The TUTTs and TC-GenesisSadler (1975) further emphasized the importance of upper-level features by proposing outflow-enhancement by either midlatitude troughs (b) or the tropical upper-tropospheric trough (TUTT; c)The TUTT is a time-mean feature of the tropical upper-troposphere (200 mb). It is also referred to as the mid-ocean trough, and appears to form through a combination of radiative and dynamic (anticyclonic wave breaking) processes.

TUTT cells are isolated vorticity maxima that break from the base of the TUTT (Kelley and Mock 1982).Early Work


However, Gray does estimate 15% subtropical (Type-B) and 1-2% hybrid (Type-C) development, distinguished primarily by formation latitudeTextbook TC DevelopmentAlthough Gray (1968) describes elements of these theories, he focuses primarily on lower-level structures and the initial development of TCs through a convective instability of the second kind (CISK)-type processThe primary impact of the upper level flow described by Gray is through vertical shearEarly Work


Using the TS Diana (1984) case study, Bosart and Bartlo (1991) describe the leading role that baroclinic features play in the development of the storm:Stage 1

An upper-level trough fractures as it crosses the Eastern Seaboard, leaving an isolated cyclonic PV anomaly over Florida, upstream of the remnant low-level baroclinic zone.

Anticyclogenesis behind the progressive trough leads to cool, dry northerlies along the coast that enhance surface heat fluxes behind the offshore front.SLP isobars (solid, 8 mb) and 1000-500 mb thickness (dashed, 6dam) at 0000 UTC 7 September 1984 (Bosart and Bartlo 1991).Something Different: Dianna (1984)TC Diana



The upper-level isolated cyclonic PV (cold) anomaly drifts eastward: cyclonic vorticity advection by the thermal wind yields deep ascent along uplifted isentropic surfaces.

Surface sensible and latent heat fluxes are maximized as the northeasterly flow behind the front wraps into the developing lower-level center.Surface sectional map for 1200 UTC 8 September 1948 show SLP (solid, 2 mb) and temperature (dashed, 2oC). Stations are plotted conventionally (Bosart and Bartlo 1991).Something Different: Dianna (1984)TC Diana



The cold dome aloft collapses as diabatic heating from the convectively-active cyclone actively erodes the PV feature aloft.

Southwesterly geostrophic flow between an upstream trough and the retreating ridge leads to enhancement of the poleward outflow channel as TS Diana develops a warm core.Heights at 300 mb (solid, 4 dam), and PV (dashed) for 0000 UTC 9 September (top) and 0000 UTC 10 September 1984 (bottom). Figure 19 from Bosart and Bartlo (1991).Something Different: Dianna (1984)TC Diana


PV Analysis of TC-GenesisKey conceptual elements proposed by Bosart and Bartlo (1991), new to TC formation theories, include:The impact of sustained deep ascent on mid-level moisture, permitting convection upshear of the developing centre and reducing downdraft formationStrong surface heat fluxes behind the baroclinic zone enhance instability and enhances upright convection at the southwestern end of the frontMontgomery and Farrell (1993) employ the conceptual model developed for TS Diana to investigate the interaction between upper- and lower-level PV anomalies during the initial stage of development before a circulation of sufficient strength for CISK or WISHE-type feedbacks exists.TC Diana


PV Analysis of TC-GenesisMontgomery and Farrell (1993) propose that there is a range of admissable shears for a given environment, which will allow an upper-level PV disturbance to constructively interact (couple) with a developing lower-level feature to produce rapid growth ratesWithout an upper-level PV anomaly growth rates are smaller, but lower-level baro-clinicity can promote sufficient ascent for developmentCoupling is most effective in moist neutral environments, where balanced vertical motions are enhanced

Cross-section of PV through a developing vortex in a moist environment (Fig. 4b from Montgomery and Farrell 1993).PV Thinking


Composite StructuresBracken and Bosart (2000) identify two dominant upper-level patterns in the western and eastern North AtlanticShear and QG forcing for ascent in both configurations900 mb Stream & Div200 mb QG ForcingStorm-centered composite streamlines (lines) and divergence (shading) for the Bahamas (top) and Cape Verde (bottom) regions in the left columns. The Sutcliffe-derived QG forcing for ascent in each domain is shown on the right (Bracken and Bosart 2000).1) Cyclonic lower-level 2) Forcing for deep ascent (destabilization; cap removal; stretching; humidification; convective organization)3) Local shear reduction near ascent maximumPV Thinking


Shear ReductionDavis and Bosart (2003) show that reduction of the environmental shear is critical to high latitude late-season TC formation from precursors of different initial intensitiesBaroclinic growth rate is small, but the features focus latent heating by moistening and destabilizationShear is reduced in the heating regions by effective vertical redistribution of PVElimination of cyclonic PV anomaly aloft reduced zonal PV gradient

CtrlDryPV Thinking


Introducing the TT ParadigmDavis and Bosart (2004) formalize these processes and coin the phrase tropical transition (TT)The concept of weak and strong precursors is also developed by Davis and Bosart (2004):WEC: convective organization by the precursor progressively weakens shear and strengthens flow

SEC: extratropical cyclogenesis creates a mesoscale vortex that is self-sustaining through WISHE

Convection along a bent-back front is most effective for shear reduction (diabatic occlusion)

Combined radar/satellite image for Hurricane Michael ~ 1200 UTC 15 October 2000.TT


TC-Genesis as OcclusionHulme and Martin (2009) show that the upstream position of convection along the bent-back front is crucial to the development of a strong vortexCompare TT process directly to occlusion with an enhanced diabatic componentDiabaticAdvectivePV and PV TendenciesLayer-mean (330-340 K) PV (solid) and PV tendencies (dark negative, light positive) for in a WRF simulation of Hurricane Karen at 1800 UTC 11 October 2001 (Hulme and Martin 2009).Convection along bent-back front: creates low-level PV that is transported into the centre (possible LLJ) enhances frontal strength by downdrafts eliminates PV aloft to create a PV notch enhances advective reduction of the local gradient by backing aloftTT


Tropical Cyclogenesis - ClimatologyMcTaggart-Cowan et al. (2008) use the conceptual model of TT to create a pair of metrics that define the baroclinic nature of the formation environment:Lower-level thickness asymmetry (Th) represents the strength of remnant frontal structures that can lead to lower-level ascent and PV generation/advection

Upper-level Q-vector convergence (Q) represents the combined effects of destabilization, moistening and convective organization by an upper-level featurePercentage of TC developments following each pathway in the 1948-2004 North Atlantic dataset.CategoryFractionNon-baroclinic 40%Low-Level 13% BaroclinicTransient Trough 16% InteractionWeak TT 13%Strong TT 15%Climatology


Tropical Cyclogenesis - ClimatologyThe latent trajectory model (LTM) technique used by McTaggart-Cowan et al. (2008) classifies environments by their evolution over the 36 h prior to TC development; however, it is not extensible to other seasons or basinsA linear discriminant analysis (LDA) allows for ~90% correct reclass-ification of the original dataset without the restrictions of the LTMNorth Atlantic TC in metric space, where storms (dots) are colour-coded by the LTM classification and LDA classification regions are shaded: a dot on the same-coloured background is a correct reclassification.Climatology


Tropical Cyclogenesis - ClimatologyMean 400-200 mb PV (shaded) and 1000-700 mb thickness (black lines at 6 dam intervals) for the development of Typhoon Irving (1992) in (a). In (b), the associated pathway classifications are plotted from 50oS to 50oN as shown in the legend.Metric values from a set of reanalysis products (NCEP, ERA-40, ERA-Interim, JRA) are combined to compute metrics over the ocean regions at 6-hourly intervalsThis allows both for the construction of a climatological description of local environments, and for individual TC classification by sampling of the categorized stateClimatology


Environment ClimatologyFrequency of occurrence of pathways as identified on each panel. The early- and late-summer seasons are defined as MJJ and ASO for the Northern Hemisphere, NDJ and FMA in the Southern Hemisphere. The 2 PVU contour on the 350 K (equatorward) and 330 K (poleward) surfaces are shown with heavy dark lines.The non-baroclinic pathway environment dominates (a,b) near the equator except where local jets are associated with low level baroclinicity (c,d).

The trough induced environment (e,f) is most common downshear of the TUTT axes, where isolated PV features are extruded from breaking Rossby waves in the mid-ocean regions.Climatology


Environment ClimatologyFrequency of occurrence of pathways as identified on each panel. The early- and late-summer seasons are defined as MJJ and ASO for the Northern Hemisphere, NDJ and FMA in the Southern Hemisphere. The 2 PVU contour on the 350 K (equatorward) and 330 K (poleward) surfaces are shown with heavy dark lines.Weak TT environments (g and h) are most common along TUTT axes, where subtropical fronts, precursor vortices or SST gradients can provide lower-level baroclinicity.

Strong TT environments (i and j) occur at higher latitudes, although breaking Rossby waves enhance their frequency downshear of the TUTT axes, primarily in the Northern Hemisphere.Climatology


Baroclinic PathwaysThe geographical separation of the classes suggests that the classifications are physically relevant, since there was no spatial information used to define the categoriesTropical cyclone development locations for the full climatology, displayed for all categories (a), and colour-coded by pathway classification in all other panels.Non-baroclinicLow Level BaroclinicTrough InducedWeak TTStrong TTLow latitude and warm SSTCoastal TCsAEJTUTT cellsFractured PV TUTT axesContinental troughsClassifications


Baroclinic PathwaysWhile the non-baroclinic pathway dominates in all basins, the secondary development pathway varies regionally: overall, 30% of events display some baroclinic influenceFrequency of development by pathway (bar group) in each basin (bar colour).HighlightsThe North Atlantic has the most varied set of baroclinic develop-ment typesThe trough induced pathway is most prominent in the West PacificThe North Indian Ocean experiences numerous weak TT formationsClassifications


North Atlantic BasinTropical cyclones forming in the he North Atlantic basin follow a broad range of baroclinic development pathwaysDistribution of TC development events by month (indigo, early summer; violet, late summer).The non-baroclinic pathway is strongly peaked between July and SeptemberBaroclinic pathways have a flatter distribution during the hurricane seasonThe relative importance of the baroclinic pathways (particularly weak and strong TT) is thus maximal during the shoulder seasons

Classifications


West Pacific BasinThe West Pacific is the most prolific basin, representing >30% of all TC formation eventsDistribution of TC development events by month (indigo, early summer; violet, late summer).The West Pacific warm pool effectively eliminates lower-level baroclinicity via differential fluxesThe short weak TT season coincides with the subtropical western North Pacific monsoon, during which the TUTT shifts westwardTUTT cells appear to play an important role in the trough induced pathway

Classifications


Arabian Sea (North Indian Subbasin)The Arabian Sea is the only region in which the non-baroclinic pathway is not clearly dominantDistribution of TC development events by month (indigo, early summer; violet, late summer).Easterly shear and poleward displacement of the monsoon trough suppress development during the monsoonThe Somali jet induces cold upwelling in the western Arabian Sea and generates cyclonic shear at lower levelsDuring monsoon break periods, westerly troughs interact with lower level baroclinicity for strong TT

Classifications


TC Development Efficiency (Yield)Development efficiency (shading) by pathway and season as indicated on each panel. The efficiency levels correspond approximately to per-event periods of 0.2=1.5yr, 0.5=6mo, 1=3mo, 2=2mo, 4=1mo, and 6=2wks. The 26.5oC SST isotherm is shown with a dashed line.Late-summer increase in non-baroclinic yield (a and b) is likely a result of increased SSTs.

Late-summer yield of low level baroclinic TCs (c and d) is highest off the west African coast in response to warming SSTs beneath the AEJ.

Despite the rarity of trough induced environments, TUTT cells appear to be highly efficient TC generators (e and f).

Efficiency


TC Development Efficiency (Yield)Development efficiency (shading) by pathway and season as indicated on each panel. The efficiency levels correspond approximately to per-event periods of 0.2=1.5yr, 0.5=6mo, 1=3mo, 2=2mo, 4=1mo, and 6=2wks. The 26.5oC SST isotherm is shown with a dashed line.Elevated climatological shear along the TUTT axes restricts efficient weak TT formation regions to the edges of the basins (g and h).

Troughs dropping equatorward across North America lead to increased late-summer yields following both the weak TT (g and h) and strong TT (i and j) pathways in the western North Atlantic basin.

Efficiency


Metric-Space AnalysisSince each point in space and each TC development event has its own metrics, the analysis can be recast from geographical to metric spaceThe structure of the metric-space climatologies suggests both preferred environments (a) and pathways (b)Extensions of event frequencies into strong TT and low level baroclinic categories are indicative of extreme events in these environmentsEnvironmental climatology (a) and TC developments (b) recast in metric space. Classification regions are colour-coded, with the class centroids indicated by squares. The kernel density estimate for development events (contours in b) is computed using the cell area in (a).Efficiency


Metric-Space AnalysisDevelopment efficiency in metric space. Classification regions are colour-coded, with the class centroids indicated by squares. The yield is computed as the ratio of the development KDE (b) to the environmental frequency (a). A value of unity corresponds to the average yield, while higher and lower values represent more and less effective development positions in metric space, respectively.The non-baroclinic environment does not represent the optimal state for TC development: an upper-level disturbance is beneficial (Montgomery and Farrell 1993, Bracken and Bosart 2000)Minimum yield along the low level baroclinic / strong TT boundary represents the negative impact of intermediate shear valuesEfficiency


DiscussionOur appreciation for the influence of baroclinic features on TC development has been increasing since the early theories on TC formation, some of which have led quite directly to our current understanding of these eventsWhile developments involving large-amplitude baroclinic features are not the norm from a global perspective, they represent an important subset and can be regionally dominantConsistent with previous results involving case studies, idealized simulations, limited climatologies, it appears that the presence of an upper-level baroclinic disturbance of limited amplitude is required to create the optimal environment for TC development


ReferencesBracken, W. E., and L. F. Bosart, 2000: The role of synoptic-scale flow during tropical cyclogenesis over the North Atlantic Ocean. Mon. Wea. Rev., 353-376.

Bosart, L. F. and J. A. Bartlo, 1991: Tropical storm formation in a baroclinic environment. Mon. Wea. Rev., 119, 1979-2013.

Davis, C. A. and L. F. Bosart, 2003: Baroclinically influenced tropical cyclogenesis. Mon. Wea. Rev., 131, 2730-2747.

Davis, C. A. and L. F. Bosart, 2004: The TT problem: forecasting the tropical transition of cyclones. Bull. Amer. Meteor. Soc., 85, 1657-1662.

Gray, W. M., 1968: Global view of the origin of tropical disturbances and storms. Mon. Wea. Rev., 96, 669-700.

Hulme, A. L. and J. E. Martin, 2009: Synoptic- and frontal-scale influences on tropical transition events in the Atlantic basin. Part II: Tropical transition of Hurricane Karen. Mon. Wea. Rev., 137, 3636-3650.

Kelley, W. E. and Mock, D. R., 1982: A diagnostic study of upper tropospheric cold lows over the western North Pacific. Mon. Wea. Rev., 110, 471-480.


ReferencesMcTaggart-Cowan, R., G. D. Deanne, L. F. Bosart, C. A. Davis and T. J. Galarneau Jr., 2008: Climatology of tropical cyclogenesis in the North Atlantic (1948-2004). Mon. Wea. Rev., 136, 1284-1304.

McTaggart-Cowan, R., T. J. Galarneau Jr., L. F. Bosart, R. W. Moore and O. Martius, 2013: A global climatology of baroclinically influenced tropical cyclogenesis. Mon. Wea. Rev., (in press).

Montgomery, M. T., and B. F. Farrell, 1993: Tropical cyclone formation. J. Atmos. Sci., 50, 285-310.

Riehl, H., 1948: On the formation of typhoons. J. Meteor., 5, 247-264.

Sadler, J., 1975: A role of the tropical upper tropospheric trough in early season typhoon development. Mon. Wea. Rev., 104, 1266-1278.


Baroclinic PathwaysWhile the non-baroclinic pathway dominates in all basins, the secondary development pathway varies regionally: overall, 30% of events display some baroclinic influenceFrequency of development by pathway (bar group) in each basin (bar colour).North AtlanticSmaller fraction of traditional events than all other basinsFrequent baroclinic trough penetrations promote TT pathsStrong temperature gradients in the Cape Verde region promote low-level baroclinic pathway


Baroclinic PathwaysWhile the non-baroclinic pathway dominates in all basins, the secondary development pathway varies regionally: overall, 30% of events display some baroclinic influenceFrequency of development by pathway (bar group) in each basin (bar colour).North IndianUpwelling beneath the Somali jet creates SST gradients in the Arabian Sea for baroclinic pathwaysBreaks in the monsoon allow trough penetration and a temporarily enhanced relative frequency of baroclinic pathways


Baroclinic PathwaysThe geographical separation of the classes suggests that the classifications are physically relevant, since there was no spatial information used to define the categoriesTropical cyclone development locations for the full climatology, displayed for all categories (a), and colour-coded by pathway classification in all other panels.Non-baroclinicLow Level BaroclinicTrough InducedWeak TTStrong TT

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baroclinic influences on tropical cyclogenesis

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