Climatological Aspects of Freezing Rain in the Eastern U.S.

Christopher M. Castellano, Lance F. Bosart, and Daniel KeyserDepartment of Atmospheric and Environmental Sciences

University at Albany, State University of New York, Albany, NY

John Quinlan and Kevin LiptonNOAA/NWS/WFO Albany, NY

Northeast Regional Operational Workshop XIII3 November 2011, Albany, NY

NOAA/CSTAR Grant: NA01NWS4680002

Motivation and Objectives

Data and Methodology

Freezing Rain Climatology

Significant Freezing Rain Events

Summary

Outline

Freezing rain undermines public infrastructure, disrupts local and regional economies, and endangers human life and safety

Freezing rain presents a major forecast challenge due to the combined influence of synoptic, mesoscale and microphysical processes

Freezing rain is historically most prevalent and destructive in the eastern U.S.

Motivation

Fig 2. Changnon (2003). The amount of loss (millions of dollars expressed in 2000 values) from ice-storm catastrophes in each climate region during 1949–2000. Values in parentheses are the average losses per catastrophe.

Fig 3. Changnon (2003). The number of ice-storm catastrophes in each climate region during 1949–2000. Values in parentheses are those catastrophes that only occurred within the region.

Motivation

Construct a long-term (1975–2010) climatology of freezing rain for the eastern U.S.

Identify significant freezing rain events based on duration, ice accretion/damage, spatial characteristics, and thermal structure

Determine synoptic-scale environments and mesoscale features conducive to significant freezing rain events

Provide forecasters with greater situational awareness of synoptic and mesoscale processes that modulate the evolution of freezing rain events

Objectives

Construct a long-term (1975–2010) climatology of freezing rain for the eastern U.S.

Identify significant freezing rain events based on duration, ice accretion/damage, spatial characteristics, and thermal structure

Determine synoptic-scale environments and mesoscale features conducive to significant freezing rain events

Provide forecasters with greater situational awareness of synoptic and mesoscale processes that modulate the evolution of freezing rain events

Objectives

Freezing Rain Climatology:– Acquired hourly surface observations for 91 first-order stations

from NCDC’s Integrated Surface Database– Analyzed the temporal and spatial variability of freezing rain and

freezing drizzle

Significant Freezing Rain Events:– Identified significant freezing rain events at two stations:

Greensboro, NC [GSO] and Worcester, MA [ORH]– Generated synoptic composite maps for significant events at

each station using 2.5° NCEP/NCAR reanalysis data

Data and Methodology

Geographical domain: 32°N, 90°W – 48°N, 66°W

Freezing rain event: 1+ h of measureable freezing rain (FZRA) and/or freezing drizzle (FZDZ)

Significant freezing rain event: 6+ h of measureable freezing rain (FZRA) observed within a 24-h period

Data and Methodology: Definitions

* Sequential FZRA/FZDZ observations must be within 6 h of each other to be considered part of the same event

ORH

GSO

Domain and First-Order Stations

Average number of hours of freezing rain/freezing drizzle per season.

Climatology: Freezing Rain Hours

Average number of freezing rain events per season.

Climatology: Freezing Rain Events

Average number of significant freezing rain events per season.

Climatology: Significant FZRA Events

Percentage of freezing rain events qualifying as “significant.”

Climatology: Significant Event Ratio

Identified 38 significant events at GSO and 44 significant events at ORH

Examined 500-hPa geopotential heights and anomalies

Examined 300-hPa wind speed, 1000–500-hPa thickness, and mean sea-level pressure

Examined 850–700-hPa layer winds, precipitable water, and standardized precipitable water anomalies

Performed analyses at t = 0 and t − 24 h preceding events

Analysis of Significant Freezing Rain Events

500-hPa geopotential height (black contours, every 60 m) and anomalies (shaded, every 30 m)

N = 38

500-hPa geopotential height (black contours, every 60 m) and anomalies (shaded, every 30 m)

N = 38

N = 38

300-hPa wind speed (shaded, every 5 m s-1), 1000–500-hPa thickness (dashed contours, every 6 dam), and mean sea-level pressure (solid contours, every 4 hPa)

N = 38

300-hPa wind speed (shaded, every 5 m s-1), 1000–500-hPa thickness (dashed contours, every 6 dam), and mean sea-level pressure (solid contours, every 4 hPa)

850–700-hPa layer wind (arrows, m s-1), 850–700-hPa layer 0°C isotherm (dashed contour), precipitable water (green contours, every 4 mm), and standardized precipitable water anomalies (shaded, every 0.5 σ)

N = 38

850–700-hPa layer wind (arrows, m s-1), 850–700-hPa layer 0°C isotherm (dashed contour), precipitable water (green contours, every 4 mm), and standardized precipitable water anomalies (shaded, every 0.5 σ)

N = 38

500-hPa geopotential height (black contours, every 60 m) and anomalies (shaded, every 30 m)

N = 44

500-hPa geopotential height (black contours, every 60 m) and anomalies (shaded, every 30 m)

N = 44

300-hPa wind speed (shaded, every 5 m s-1), 1000–500-hPa thickness (dashed contours, every 6 dam), and mean sea-level pressure (solid contours, every 4 hPa)

N = 44

300-hPa wind speed (shaded, every 5 m s-1), 1000–500-hPa thickness (dashed contours, every 6 dam), and mean sea-level pressure (solid contours, every 4 hPa)

N = 44

850–700-hPa layer wind (arrows, m s-1), 850–700-hPa layer 0°C isotherm (dashed contour), precipitable water (green contours, every 4 mm), and standardized precipitable water anomalies (shaded, every 0.5 σ)

N = 44

850–700-hPa layer wind (arrows, m s-1), 850–700-hPa layer 0°C isotherm (dashed contour), precipitable water (green contours, every 4 mm), and standardized precipitable water anomalies (shaded, every 0.5 σ)

N = 44

Greatest frequency of freezing rain hours observed east of the Appalachian Mountains, in upstate New York, and in central New England

Sharp gradients in frequency exist across the Appalachians and along the Mid-Atlantic and New England coasts

Greatest frequency of significant events also occurs east of the southern Appalachians and in the interior Northeast

Higher percentage of freezing rain events qualify as significant in the Southeast than anywhere else

Summary: Freezing Rain Climatology

Freezing rain events are coincident with an amplifying 500-hPa ridge along the East Coast, an upstream trough between the Rockies and the Midwest, and negative 500-hPa geopotential height anomalies to the northeast

Freezing rain events occur near an equatorward entrance region of a confluent 300-hPa jet, within a 1000–500-hPa thickness ridge

Freezing rain events feature low-to-midlevel moisture transport and warm-air advection via deep southwesterly flow

Summary: Significant Freezing Rain EventsSimilarities

ORH events feature more amplified upper-level height anomalies, thermal trough/ridge patterns, and moisture anomalies than GSO events

GSO events primarily rely on Gulf of Mexico as sole moisture source, whereas ORH events likely rely on Atlantic Ocean as an additional moisture source

GSO events are typically associated with cold-air damming east of the Appalachians

ORH events are typically associated with ageostrophic northerlies that reinforce cold air on the poleward side of a mature warm front

Summary: Significant Freezing Rain EventsDifferences