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Benjamin J. Moore Cyclone Research Group Meeting 26 Aug 2009

The Development of Predecessor Rainfall Events in Advance of Landfalling Tropical Cyclones in the United States of America. Benjamin J. Moore Cyclone Research Group Meeting 26 Aug 2009. Outline. Overview of the Predecessor Rain Event (PRE) Overview of Composites

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Benjamin J. Moore Cyclone Research Group Meeting 26 Aug 2009

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  1. The Development of Predecessor Rainfall Events in Advance of LandfallingTropical Cyclones in the United States of America Benjamin J. Moore Cyclone Research Group Meeting 26 Aug 2009

  2. Outline • Overview of the Predecessor Rain Event (PRE) • Overview of Composites • Central U.S. Case Study: Grace (2003) • Eastern U.S. Case study: Ernesto (2006) • Concluding remarks

  3. The Predecessor Rain Event • Coherent mesoscale region of heavy precipitation developing as moisture from a tropical cyclone (TC) interacts with a poleward region of baroclinity. • Occur most commonly as bands of convective precipitation along a low-level thermal boundary aligned with mid/upper level flow • This mode favors heavy, prolonged rainfall over a given area • corresponds the training line/adjoingingstratiform extreme rainfall event from Schumacher and Johnson (2005) • Persistent, deep tropical moisture favors heavy rainfall rates

  4. PRE-relative composites kts g kg −1 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 S −1, blue), 850 hPa equivalent potential temperature (K, red) 925 hPaZ (black, dam), potential temperature (K, red), mixing ratio (g kg −1, shaded), Q-vectors (10 −11 K m −2 s −1), Q-vector convergence (10 −16 K m −2 s −1, pink)

  5. TC-relative composites for Central U.S. PREs kts g kg −1 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 S −1, blue), 850 hPa equivalent potential temperature (K, red) 925 hPaZ (black, dam), potential temperature (K, red), mixing ratio (g kg −1, shaded), Q-vectors (10 −11 K m −2 s −1), Q-vector convergence (10 −16 K m −2 s −1, pink)

  6. TC-relative composites for Eastern U.S. PREs kts g kg −1 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 S −1, blue), 850 hPa equivalent potential temperature (K, red) 925 hPaZ (black, dam), potential temperature (K, red), mixing ratio (g kg −1, shaded), Q-vectors (10 −11 K m −2 s −1), Q-vector convergence (10 −16 K m −2 s −1, pink)

  7. Common synoptic characteristics for PREs • Develop in the equatorward entrance region of an upper-tropospheric jet • Form along a low-level boundary in a region of low-level frontogenesis • Flanked by a trough upstream and a ridge downstream

  8. Central vs. Eastern U.S. PREs Central U.S. PREs Eastern U.S. PREs • Typically occur after recurvature; TC has been taken up by strong westerlies associated with a trough • Less likely to be long-lived; more transient • Middle-/upper-level flow is characterized by a shortwave trough just upstream of the TC and a ridge overlying the TC • Trough and jet are much closer to the TC • Moisture typically is not able to advance as far poleward of the TC as in the central U.S. cases due to weaker low-/mid-level anticyclone • Strong deformation flow associated with TC and ridge about a southwest to northeast oriented barocliniczone • TC overlaps baroclinic zone • Forcing associated with upper-level jet and low-level baroclinic zone much closer to the TC • Typically occur before or during recurvatureprocess • Are more often longer-lived and produce more rain • Middle-/upper-level flow is dominated by a positively tilted upstream trough and a ridge poleward of the TC maintain a strong southwesterly jet over the north central U.S. • Low-level flow is characterized by a strong anticyclone over the eastern U.S. – facilitates strong poleward moisture transport well ahead (i.e. poleward) of the TC and enhances frontogenesis • Low-level baroclinic zone is zonally oriented, parallel to the mid-level shear JUST KIDDING

  9. Central vs. Eastern U.S. PREs Central U.S. PREs Eastern U.S. PREs • Typically occur after recurvature; TC has been taken up by strong westerlies associated with a trough • Less likely to be long-lived; more transient • Middle-/upper-level flow is characterized by a shortwave trough just upstream of the TC and a ridge overlying the TC • Trough and jet are much closer to the TC • Moisture typically is not able to advance as far poleward of the TC as in the central U.S. cases due to weaker low-/mid-level anticyclone • Strong deformation flow associated with TC and ridge about a southwest to northeast oriented barocliniczone • TC overlaps baroclinic zone • Forcing associated with upper-level jet and low-level baroclinic zone much closer to the TC • Typically occur before or during recurvatureprocess • Are more often longer-lived and produce more rain • Middle-/upper-level flow is dominated by a positively tilted upstream trough and a ridge poleward of the TC maintain a strong southwesterly jet over the north central U.S. • Low-level flow is characterized by a strong anticyclone over the eastern U.S. – facilitates strong poleward moisture transport well ahead (i.e. poleward) of the TC and enhances frontogenesis • Low-level baroclinic zone is zonally oriented, parallel to the mid-level shear

  10. Case Study: Grace (2003) Average 200 hPageopotential height, wind, standardized height anomalies, total precipitation for 30 Aug – 2 Sep 2003 200 mm

  11. Radar Evolution 0000 UTC 31 Aug – 2300 UTC 1 Sep 2003

  12. Synoptic Environment 1200 UTC 30 Aug 2003 200 hPa 850 hPa kts mm Precipitable water (mm, shaded) , 850 hPaZ (black, dam), potential temperature (K, red), Q-vectors (10 −11 K m −2 s −1), 700-950 hPa layer average relative vorticity (10−5 s−1, white) 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 s−1, blue)

  13. Synoptic Environment 0000 UTC 31 Aug 2003 200 hPa 850 hPa kts mm Precipitable water (mm, shaded) , 850 hPaZ (black, dam), potential temperature (K, red), Q-vectors (10 −11 K m −2 s −1), 700-950 hPa layer average relative vorticity (10−5 s−1, white) 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 s−1, blue)

  14. Synoptic Environment 1200 UTC 31 Aug 2003 200 hPa 850 hPa kts mm Precipitable water (mm, shaded) , 850 hPaZ (black, dam), potential temperature (K, red), Q-vectors (10 −11 K m −2 s −1), 700-950 hPa layer average relative vorticity (10−5 s−1, white) 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 s−1, blue)

  15. Synoptic Environment 0600 UTC 1 Sep 2003 200 hPa 850 hPa kts mm Precipitable water (mm, shaded) , 850 hPaZ (black, dam), potential temperature (K, red), Q-vectors (10 −11 K m −2 s −1), 700-950 hPa layer average relative vorticity (10−5 s−1, white) 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 s−1, blue)

  16. Synoptic Environment 1800 UTC 1 Sep 2003 200 hPa 850 hPa kts mm Precipitable water (mm, shaded) , 850 hPaZ (black, dam), potential temperature (K, red), Q-vectors (10 −11 K m −2 s −1), 700-950 hPa layer average relative vorticity (10−5 s−1, white) 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 s−1, blue)

  17. Vertically Integrated Moisture Flux 1800 UTC 30 Aug 2003 mm * m s−1 Precipitable water flux (mm * m s−1, shaded), pwat flux vectors, 850 hPa height (dam, black), 850 hPa relative vorticity (10−5 s−1, white)

  18. Vertically Integrated Moisture Flux 1200 UTC 31 Aug 2003 mm * m s−1 Precipitable water flux (mm * m s−1, shaded), pwat flux vectors, 850 hPa height (dam, black), 850 hPa relative vorticity (10−5 s−1, white)

  19. Vertically Integrated Moisture Flux 0600 UTC 1 Sep 2003 mm * m s−1 Precipitable water flux (mm * m s−1, shaded), pwat flux vectors, 850 hPa height (dam, black), 850 hPa relative vorticity (10−5 s−1, white)

  20. Summary of the Grace PRE • Deep, strong southeasterly moisture flux from the TC/Gulf of Mexico, associated with brawny anticyclone, towards a quasi-stationary baroclinic zone over Central Plains initiated a large region of convection (PRE-1) • Grace moved in the direction of the low-level flow, allowing persistent moisture transport towards the region of frontogenesis, maintaining a region of heavy stationary precipitation • A upper-level shortwave well poleward of the TC crossed the Continental Divide into the Plains and acted to “backbuild” and strengthen the jet overlying the PRE • The shortwave advanced eastward, carrying the low-level baroclinic zone with it and maintaing the jet • Resulted in the dissipation of the initial PRE • Allowed the development of a second PRE south of the Great Lakes as moisture transport from Grace continued • PRE-2 gradually dissipated as the anticyclone weakened and Grace dissipated

  21. Case Study: Ernesto (2006) Average 200 hPageopotential height, wind, standardized height anomalies, total precipitation for 30 Aug – 31 Aug 2006 mm

  22. Radar Evolution 1800 UTC 30 Aug – 1200 UTC 31 Aug 2006

  23. Synoptic Environment 0600 UTC 30 Aug 2006 200 hPa 850 hPa kts mm Precipitable water (mm, shaded) , 850 hPaZ (black, dam), potential temperature (K, red), Q-vectors (10 −11 K m −2 s −1), 700-950 hPa layer average relative vorticity (10−5 s−1, white) 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 s−1, blue)

  24. Synoptic Environment 1800 UTC 30 Aug 2006 200 hPa 850 hPa kts mm Precipitable water (mm, shaded) , 850 hPaZ (black, dam), potential temperature (K, red), Q-vectors (10 −11 K m −2 s −1), 700-950 hPa layer average relative vorticity (10−5 s−1, white) 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 s−1, blue)

  25. Synoptic Environment 0000 UTC 31 Aug 2006 200 hPa 850 hPa kts mm Precipitable water (mm, shaded) , 850 hPaZ (black, dam), potential temperature (K, red), Q-vectors (10 −11 K m −2 s −1), 700-950 hPa layer average relative vorticity (10−5 s−1, white) 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 s−1, blue)

  26. Synoptic Environment 0600 UTC 31 Aug 2006 200 hPa 850 hPa kts mm Precipitable water (mm, shaded) , 850 hPaZ (black, dam), potential temperature (K, red), Q-vectors (10 −11 K m −2 s −1), 700-950 hPa layer average relative vorticity (10−5 s−1, white) 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 s−1, blue)

  27. Synoptic Environment 1200 UTC 31 Aug 2006 200 hPa 850 hPa kts mm Precipitable water (mm, shaded) , 850 hPaZ (black, dam), potential temperature (K, red), Q-vectors (10 −11 K m −2 s −1), 700-950 hPa layer average relative vorticity (10−5 s−1, white) 200 hPaZ (dam, black), winds (kts, barbs), wind speed (kts, shaded), divergence (10−5 s−1, blue)

  28. Vertically Integrated Moisture Flux 1800 UTC 30 Aug 2006 mm * m s−1 Precipitable water flux (mm * m s−1, shaded), pwat flux vectors, 850 hPa height (dam, black), 850 hPa relative vorticity (10−5 s−1, white)

  29. Vertically Integrated Moisture Flux 0000 UTC 31 Aug 2006 mm * m s−1 Precipitable water flux (mm * m s−1, shaded), pwat flux vectors, 850 hPa height (dam, black), 850 hPa relative vorticity (10−5 s−1, white)

  30. Vertically Integrated Moisture Flux 0600 UTC 31 Aug 2006 mm * m s−1 Precipitable water flux (mm * m s−1, shaded), pwat flux vectors, 850 hPa height (dam, black), 850 hPa relative vorticity (10−5 s−1, white)

  31. Summary of the Ernesto PRE • Stalled cold front and associated cold air damming in NC and VA provided focus for convection • Southerly flow afforded by anticyclone east of the TC enhanced low-level frontogenesis along NC/VA border and transported moisture from TC poleward • Advancing upper-level shortwave aided in the strengthening of the jet streak along the east coast leading to enhanced forcing • PRE dissipated as: • flow became less favorable for frontogenesis • the shortwave weakened and moved off to the east • ‘Nesto accelerated northeastward overtaking the baroclinic zone and the PRE

  32. A proposition concerning PRE duration • PRE longevity is a function of TC velocity: both direction and magnitude. • PREs tend to last longer when TC motion vector is ~perpendicular to the vertical shear in the region of the PRE • As TC move perpendicular to the shear and presumably a low-level boundary it provides persistent moisture to be exploited in the region of forced ascent • As precipitation is advected away new precipitation develops upstream along the boundary (i.e. backbuilds) • More transient PREs are produced when the TC is moving at more acute angles to the vertical shear overlying the PRE • Moisture transport is more fleeting in the region of the PRE • TC overtakes or moves past the PRE • A slower moving TC has greater potential to produce a more prolific PRE provided that forcing is present • Provides persistent moisture to the PRE

  33. A PRE-liminary theory for PRE development • Opposing forces of poleward moving TC (and its associated outflow ridge) and an eastward advancing shortwave trough act to enhance the upper-level jet streak during the PRE. • Low-/mid-level northwesterly cold air advection associated with the advancing trough and southerly warm, moist advection associated with the TC and downstream anticyclone act to enhance frontogenesis in the vicinity of the PRE. • The end result is a troposphere-deep baroclinic zone with the upper-level jet overlying the low-level front in the presence of deep moisture from the TC.

  34. Future work • Continue to look at the intricacies of PREs by analyzing individual cases • Start to identify and analyze null cases to verify key ingredients • Sharpen up composites • Identify dynamical mechanisms (if any) for LOT vs. ROT vs. AT PREs • Beer

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