An Objective Criteria for Ranking Extreme Cold Waves and their Relationship with the Polar Vortex Mike Cellitti University of Illinois 17 November 2003
Acknowledgements • Bob Rauber • John Walsh • Diane Portis
Outline • Motivation and Goals • Cold Wave Characteristics • Cold Wave Ranking Process • Cold Wave Climatological Trends • Polar Vortex Characteristics • Case Studies • Summary
Why Do We Care about Extreme Cold Waves? • On average, 35 deaths per year directly related to the extreme cold. • Economic losses can be in the billions • Heating costs increase considerably • Transportation related issues
Goals • Examine the role of the Polar Vortex for extreme cold air events • How does the location and intensity of the vortex affect the characteristics of a cold wave? • Do multiple polar vortices affect a cold wave? • Use this information to more accurately predict the onset of an extreme cold air event. • But first, need to identify the top extreme cold wave events!
Cold wave Characteristics • Highly amplified upper level ridge-trough system over North America. • Positive surface pressure anomalies due to an anticyclone that propagates southward from western Canada to the Gulf Coast. • Extensive snow cover over northern and central North America to keep the arctic airmass insulated. • Intensity of CAO most related to strength of anticyclone or its associated cold air advection.
Typical Upper-Level Pattern (Severe and Hazardous Weather. Rauber, Walsh, Charlevoix, 2002)
Objective Cold Wave Ranking Criteria • Rank cold waves quantitatively based on: • Duration • Intensity • Coverage area
Data Acquisition • Surface Airways Hourly from NCDC • Data range: 1948-2001 • November 1 – March 31 • Temperatures every six hours • Separation criteria: A cold wave event will be defined as the time period between temperatures decreasing below average and re-emerging above average
How does the Polar Vortex relate to these extreme CAO Events? • Lets first ask, what is the Polar Vortex and how does it form?
Vortex Characteristics • Forced planetary standing wave caused by westerlies impinging on the Rocky Mountains. • Nearly stationary over Hudson Bay during the winter. • Contains high values of isentropic potential vorticity (air parcels do not cross IPV gradients). • Contains relatively colder and drier air than its surroundings. • Acts as a barrier to transient weather systems – weather systems are steered around the vortex.
1994 and 1996 Vortex Comparison • Deep 500 mb vortex 15 days prior to CAO onset. • 1996 vortex was consistently deep prior to CAO unlike the 1994 vortex • Both CAO’s began when southern extension of the vortex infiltrated lower latitudes. • Most extreme cold air directly underneath and to the west side of both vortices.
Summary • No apparent trend in frequency of CAO’s. • Trend toward slightly greater intensity of CAO’s in Midwest and weaker intensity on the East Coast. • Deep polar vortex prior to onset of 1994 and 1996 CAO’s. • CAO onset when southern extension of the polar vortex penetrated southward.
References • R. Rauber, Charlevoix, D., Walsh, J. Severe and Hazardous Weather. Kendall/ Hunt Publishing Company, 2002. • J. Overland, J.M. Adams, N.A. Bond, 1997: Regional Variation of Winter Temperatures in the Arctic. J Climate, 10, 821-837. • J.E. Walsh, A.S. Phillips, D.H. Portis, W.L. Chapman, 2001: Extreme Cold Outbreaks in the United States and Europe, 1948-1999. J. Climate, 14, 2642-2657. • C.E. Konrad II, S.J. Colucci: An Examination of Extreme Cold Air Outbreaks over Eastern North America. Bull. Amer. Meteor. Soc., 117, 2687-2700.