unexpectedly heavy near- coastal precipitation due to mesoscale features induced by a landfalling...
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Unexpectedly Heavy Near-Coastal Precipitation Due to Mesoscale Features Induced
by a Landfalling Tropical Storm
Alan F. Srock, Lance F. Bosart, John MolinariUniversity at Albany/SUNY
NROW VIII – November 2006
Research supported by NSF Grant #ATM-0304254
Research Objectives
• Tropical cyclones can produce heavy precipitation both near and far from the storm center
• Mesoscale features which induce lift help account for this additional rainfall– Cold Air Damming (CAD)– Coastal Front (CF)
• These features can be caused or enhanced by the presence of a nearby TC
http://fermi.jhuapl.edu/states/us/big_us_color.gif
Data Sources
• NCEP’s NARR and ECMWF’s ERA-40
• Rainfall maps provided by W. Junker (NCEP/HPC) and D. Roth (NCEP/HPC)
• NHC Best Track (HURDAT)
• GIBBS Satellite Archive
• Surface Data from NCAR– U.S.A.F. DATSAV3, NCEP ADP, I-COADS
Overview of TS Marco (1990)
• In NHC Best Track from 1200 UTC 9 October to 1800 UTC 13 October
• Short lifetime as a tropical storm (~36 h)
• Other tropical cyclones in proximity (Klaus, Lili)
• Induced / enhanced two distinct episodes of mesoscale features– We’ll examine both instances in this study
Image courtesy D. Roth
24 h total rainfall ending 1200 UTC 11 October 1990
24 h total rainfall ending 1200 UTC 12 October 1990
Images courtesy W. Junker
Upper-Level Summary
• 200-hPa jet positioned so region of upper-level divergence located over coastal SE U.S.
• 500-hPa mid-latitude vorticity maxima never reached close proximity with Marco
• First synoptic trough likely enhanced precipitation well north of Marco
901011/0000 Surface θe gradient (K/100km) and wind (kt)
901012/1200 Surface θe gradient (K/100km) and wind (kt)
GIBBS Satellite Image (Visible) from 21 UTC 10 October 1990
925-hPa Back Trajectories from 1200 UTC 10 October 1990
925-hPa Back Trajectories from 0000 UTC 11 October 1990
Mesoscale Plan View Summary
• Synoptic front associated with midlatitude trough stays west of the Appalachians
• Signatures of CAD and a CF in near-surface winds, though Marco was still far south
• LL coastal frontogenesis indicates region of enhanced lift – collocated w/ heavy rain
• Back trajectories suggest shift to tropical source air coincident with first CF/CAD episode
901011/0000 Surface θe gradient (K/100km) and wind (kt)
Cross-Section Summary
• Highest θ air at the surface flows inland from offshore in both cases
• Cold air pocket inland advects in from the northeast while cooling, creating tight θ gradient near coast
• Strongest ascent throughout period located at and just inland of the CF
• Second CF moves inland more rapidly than first
Conclusions
• Cold-air damming and a coastal front induced/enhanced twice – once while Marco was still quite distant
• These mesoscale features led to greater precipitation totals inland of the coast, days before Marco arrived
• The CAD/CF appears to be dependent on a sufficient land/sea thermal contrast; thus, time of year can be important
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