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Benjamin J. Moore, Lance F. Bosart , and Daniel Keyser

Mechanisms for the Development of Predecessor Rain Events in Advance of Landfalling Tropical Cyclones. Benjamin J. Moore, Lance F. Bosart , and Daniel Keyser Department of Atmospheric and Environmental Sciences, University at Albany/SUNY, Albany, NY 12222 Michael L. Jurewicz , Sr.

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Benjamin J. Moore, Lance F. Bosart , and Daniel Keyser

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  1. Mechanisms for the Development of Predecessor Rain Events in Advance of Landfalling Tropical Cyclones Benjamin J. Moore, Lance F. Bosart, and Daniel Keyser Department of Atmospheric and Environmental Sciences, University at Albany/SUNY, Albany, NY 12222 Michael L. Jurewicz, Sr. NOAA/NWS, Binghamton, NY 35th Annual Northeastern Storm Conference, Saratoga Springs, NY 5−7 March 2010 NOAA/CSTAR Grant NA07NWS4680001

  2. Outline • Objectives • Definition of a predecessor rain event (PRE) • Data and methodology • PRE-relative composite analysis • PRE associated with TC Rita (2005) • Concluding remarks

  3. Objectives • Identify preferential dynamic and thermodynamic configurations for PREs to improve forecasts • Establish physical mechanisms accounting for different spatial and temporal characteristics of PREs

  4. Definition of a Predecessor Rain Event • Defined by Cote (2007) as distinct mesoscale regions of heavy rainfall [~100 mm (24 h)−1] ~1000 km downstream of landfalling tropical cyclones (TCs) • Develop as a poleward stream of deep moisture from the TC interacts with an environment favoring ascent PRE TC Rita 06Z 25 Sep 2005

  5. Data • 1988–2008 PRE climatology: • WSR-88D base reflectivity mosaics (cases during 1995–2008) • 1º GFS analyses and 2.5º NCEP–NCAR reanalysis data • NCDC HPD • NPVU QPE • UPD • Rita case analysis: • NCEP 1º GFS analyses • WSI NOWRAD radar data • NOAA profiler data • NCDC HPD

  6. Methodology • PRE stratification: • Three distinct categories based upon upper-level jet configuration/position relative to the TC: • Southwestery jet • Jet in ridge • Downstream confluence • PRE-relative compositing: • 6 hourly 2.5º NCEP–NCAR grids for PRE cases were shifted and centered on PRE centroid • PRE cases were binned into synoptic categories and composited at time of initiation

  7. PREs 1988−2008 Number of TCs/PREs PREs relative to TC track Num. of PREs Jet in ridge Southwesterly jet Downstream confluence Misc. Jet in ridge Southwesterly jet Downstream confluence Misc. Category

  8. PREs 1988−2008 Separation Distance N=17 N=11 • Longest-lived (24 h or greater) PREs are generally “jet in ridge” category • “Jet in ridge” category tends to produce most extreme maximum rainfall • “Southwesterly jet” category has shortest separation distance 29 N=27 Ridge Southwesterly Confluence Maximum Rainfall Longevity N=11 N=11 N=17 N=27 N=27 N=17 Confluence Southwesterly Ridge Southwesterly Confluence Ridge

  9. PRE-Relative Composites “Jet in ridge” Category 925 hPa 200 hPa H L mm kt 925 hPa heights (dam, black), potential temperature (K, red), total precipitable water (>25 mm, shaded) 200 hPa heights (dam, black), winds (≥50 kt, barbs), wind speed (>50 kt, shaded); 925 hPa relative vorticity (10−5 s−1, blue)

  10. PRE-Relative Composites “Southwesterly jet” Category 925 hPa 200 hPa L H mm kt 925 hPa heights (dam, black), potential temperature (K, red), total precipitable water (>25 mm, shaded) 200 hPa heights (dam, black), winds (≥50 kt, barbs), wind speed (>50 kt, shaded); 925 hPa relative vorticity (10−5 s−1, blue)

  11. PRE-Relative Composites “Downstream confluence” Category 925 hPa 200 hPa H H mm kt 925 hPa heights (dam, black), potential temperature (K, red), total precipitable water (>25 mm, shaded) 200 hPa heights (dam, black), winds (≥50 kt, barbs), wind speed (>50 kt, shaded); 925 hPa relative vorticity (10−5 s−1, blue)

  12. “Jet in ridge” PRE associated with TC Rita 24–25 Sep 2005 26 25 24 00 UTC 12 UTC 23 20 21 22 mm 1200 UTC 24 Sep –0000 UTC 26 Sep 2005: 200 hPa time mean geopotential height (dam, black), time mean wind speed (≥70 kt, red), and total precipitation (mm, shaded)

  13. NPVU QPE 12Z 24 Sep – 00Z 26 Sep Overview of the Rita PRE 3 2 4 1 • Quasi-stationary band of convective and stratiform rainfall developed well poleward of Rita in the Upper Midwest • Southern Minnesota received≥200 mm of rain during 24–25 Sep 2005 mm Hourly rainfall 24−25 Sep 2005: select stations in MN Worthington (1) Amboy (2) Rochester (3) La Crescent (4) mm

  14. Radar Evolution 00Z 25 Sep 03Z 25 Sep 06Z 25 Sep 12Z 25 Sep 06Z 19 Aug WSI NOWRAD reflectivity mosaic

  15. Synoptic Environment 0600 UTC 25 Sep 2005 925 hPa 200 hPa mm kt 200 hPa heights (dam, black), winds (≥50 kt, barbs), wind speed (>70 kt, shaded), 850 hPa relative vorticity (10−5 s−1, blue) 925 hPa heights (dam, black), potential temperature (K, red), total precipitablewater (>30 mm, shaded)

  16. Moisture Supply from Rita 0600 UTC 25 Sep 2005 Low-level moisture flux Back trajectories 1 06Z/25 12Z/24 06Z/22 hPa kg m−1 s−1 1000−850 hPa vertically integrated WV flux (>12 kg m−1 s−1,shaded), WV flux vectors, WV flux convergence (10−3 kg m−2 s−1, blue) 72 h back trajectories beginning at 700 hPa

  17. Influence of the Low-level Jet Amboy Worthington NOAA Profiler winds (kt, barb) and wind speed (kt, red) from Slater, IA 18Z/24−12Z/25 Sep 2005 Slater Height above MSL (km) Pressure (hPa) 500 700 850 925 25 Sep 24 Sep 1800 UTC 1200 UTC 0600 UTC 0900 UTC 2100 UTC 0000 UTC 0300 UTC

  18. Mechanisms for Heavy Rainfall 0600 UTC 25 Sep 2005 kt K (100 km)−1 (3 h)−1 200 hPa heights (dam, black), wind speed (>70 kt, shaded), irrotational wind vectors, 925 hPa WV flux convergence (10−7 s−1, blue) 925 hPa heights (dam, black), potential temperature (K, red), winds (kt, barbs), frontogenesis [K (100 km)−1 (3 h)−1, shaded]

  19. Mechanisms for Heavy Rainfall 0600 UTC 25 Sep 2005 A J Pressure (hPa) B Potential Temperature (K) PRE RITA 10 m s−1 A B kts g kg−1 θ(K, purple), ω < 0(μb s−1, black), mixing ratio (>6 g kg−1, blue shading), wind speed (>70 kt, gray shading), horizontal wind vectors > 5 m s−1 below 700 hPa in plane of cross section, frontogenesis [10−1 K (100 km)−1 (3 h)−1, red]

  20. Schematic Depiction of the Rita PRE Pressure (hPa) J F Latent Heat Release F Moist low-level flow S N 1200 km

  21. Concluding Remarks Key features of composites • PREs preferentially develop in equatorward entrance region of an upper-level jet streak • Strong low-level flow downstream of TC directed towards baroclinic zone  warm air advection, frontogenetical forcing, moisture transport from the TC • Categories differ with regard to: • Position of TC relative to key features (i.e., trough, jet, ridge, baroclinic zone) • Amplitude and configuration of upper-/middle-tropospheric flow • Nature of the interaction between the TC and the midlatitude flow

  22. Concluding Remarks Rita case summary • PRE developed as continuous, strong poleward moisture transport from Rita impinged upon a quasi-stationary baroclinic zone • Low-level convergence and deformation at the terminus of the southerly low-level jet likely enhanced frontogenesis along baroclinic zone • Anticyclonic outflow associated with Rita and the PRE promoted confluent flow over Ontario, and supported southwesterly upper-level jet

  23. Concluding Remarks Rita case summary • Upper-/middle-level diabatic heating in the mature PRE likely promoted frontogenesis and contributed to the strengthening of upper-level jet • Long-lived PRE was likely due to a combination of: • Continuous moisture transport towards and moisture convergence within PRE region • Quasi-stationary low-level frontogenesis • Diabatically enhanced secondary circulation within upper-level jet entrance region

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