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