Mesoscale Precipitation Structures Accompanying Landfalling and Transitioning Tropical Cyclones in the Northeast United States Jared Klein, Lance F. Bosart, and Daniel Keyser University at Albany/SUNY; Albany, NY CSTAR II Grant NA04NWS David Vallee NWS Weather Forecast Office; Taunton, MA The Eighth Northeast Regional Operational Workshop 1-2 November 2006

Objectives  Examine the distribution of rainfall in relation to tropical cyclone (TC) track and identify smaller scale areas of enhanced rainfall within the overall precipitation shield.  Better understand how the observed mesoscale distribution of heavy rainfall in landfalling and transitioning TCs is modified by the interactions of mesoscale and synoptic-scale features.  Create a conceptual model applicable to operational forecasters to use once the research is completed.

Motivation Difficulty in predicting the exact timing and location of mesoscale features accompanying heavy precipitation. Approximately 4 out of every 5 fatalities from landfalling TCs are directly caused by inland flash flooding (Rappaport 2000). Recent active TC seasons (2004 and 2005) have led to an increase in the frequency of TC related flooding events over the Northeast U.S. –1950 – 2003: Approximately 1 event every year –2004 – 2005: Approximately 5 events every year

Complex Topography of the Northeast United States

Previous CSTAR Work (DeLuca 2004)

Methodology Identify TCs which produced ≥100 mm (4 in.) of rainfall from (present). Construct detailed surface analysis. Identify surface mesoscale processes associated with heavy precipitation: –Coastal frontogenesis (CF) –Frontogenesis along a pre-existing synoptic boundary. –Orographic upsloping of low-level warm and moist air over the Northeast U.S. Construct analyses using NCEP NARR gridded datasets.

67 tropical cyclones producing ≥ 4 inches of rainfall in the Northeast U.S. during the period 1950 – 2006 Case list provided by David Vallee, NWS Taunton 1950 Able 1950 Dog 1952 Able 1953 Barbara 1953 Carol 1954 Carol 1954 Dolly 1954 Edna 1954 Hazel 1955 Connie 1955 Diane 1955 Ione 1958 Daisy 1958 Helene 1959 Cindy 1959 Grace 1960 Brenda 1960 Cleo 1960 Donna 1961 Unnamed 1961 Esther 1961 Frances 1961 Gerda 1962 Alma 1962 Daisy 1962 Ella 1963 Ginny 1968 Gladys 1969 Gerda 1971 Beth 1971 Doria 1971 Heidi 1972 Agnes 1972 Carrie 1974 Dolly 1975 Blanche 1976 Belle 1979 David 1985 Gloria 1985 Henri 1986 Charley 1988 Alberto 1988 Chris 1991 Bob 1996 Bertha 1996 Edouard 1996 Fran 1996 Hortense 1997 Danny 1998 Bonnie 1999 Floyd 2001 Allison 2002 Isidore 2003 Bill 2003 Isabel 2003 Juan 2004 Alex 2004 Bonnie 2004 Charley 2004 Frances 2004 Gaston/Hermine 2004 Ivan 2004 Jeanne 2005 Cindy 2005 Katrina 2005 Ophelia 2006 Ernesto

Track vs. Precipitation Distribution Analyses Determine track versus precipitation distribution. – Rainfall Analyses: NCEP 24 h daily ( UTC) Unified Precipitation Dataset (UPD) for cases prior to –Coarse 0.25 o x 0.25 o resolution. –Insufficient for identifying mesoscale rainfall structures. Archived 6 h and 24 h RFC analysis of QPE from the NWS National Precipitation Verification Unit (NPVU). –NCEP HPC analyzed 24 h precipitation and storm total precipitation/track maps. –NHC best-track data to determine tracks of TCs.

UPD vs. HPC Precipitation Maps Isabel (2003) UPDHPC

Preliminary Results Enhanced precipitation associated with frontogenesis along a mesoscale (e.g., a coastal front) or pre-existing synoptic-scale boundary occurred in 16 of 17 cases. –Frontogenesis area generally located left and poleward of storm track. –Enhanced precipitation region along or in cold sector of coastal front. –Slight θ gradient of approximately 2-4 o C/100 km commonly occurred. –θ gradient was stronger when interacting with a synoptic- scale frontal boundary. Orographically enhanced precipitation occurred in 9 of 17 cases where low-level easterly flow was upslope on the eastern sides of the Appalachian Mountains from northern Georgia to New England.

Summary of 17 landfalling and transitioning TCs in the northeast U.S. containing orographic enhancement and/or coastal fronts during Mesoscale FeatureOrographic EnhancementFrontogenesis

Track map of all 17 storms ( ) Mesoscale Features : Blue shades- Frontogenesis Green- Orographic Red/Orange Shades- Both

Case Study: Ivan September 2004 Mesoscale frontogenesis along a pre-existing synoptic boundary and orographic precipitation enhancement

Summary of Preliminary Results Interaction between a landfalling and transitioning TC, an advancing upper-level trough, and a mesoscale coastal or pre-existing synoptic-scale front occurred in almost every recent heavy rainfall case examined. Orographically enhanced precipitation was found in nearly half the cases studied where low-level easterly flow was present over the eastern slopes Appalachian Mountains. Ivan’s large circulation induced an influx of tropical air into the pre-existing boundary, which led to both mesoscale frontogenesis and topographic rainfall enhancement.

Future Work ‘Go back in time’ and examine earlier cases from Obtain more detailed precipitation data and analyses in order to better locate mesoscale areas of heavy rainfall. Examine mesoscale structures in further detail by constructing cross-sectional analyses using the NARR to better understand the key forcing ingredients.

REFERENCES DeLuca, D. P., 2004: The distribution of precipitation over the Northeast accompanying landfalling and transitioning tropical cyclones. M.S. Thesis, Department of Earth and Atmospheric Sciences, University at Albany-SUNY, 178 pp.