a statistical analysis and synoptic climatology of u.s. heat waves scott c. runyon and lance f....

A Statistical Analysis and Synoptic A Statistical Analysis and Synoptic Climatology of U.S. Heat WavesClimatology of U.S. Heat Waves

Scott C. RunyonScott C. Runyonandand

Lance F. BosartLance F. Bosart

Department of Earth and Atmospheric Sciences, Department of Earth and Atmospheric Sciences,

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

IntroductionIntroduction::

Why study heat waves?Why study heat waves?• Heat waves are a major contributor to Heat waves are a major contributor to

weatherweather––related fatalitiesrelated fatalities

• Understanding the characteristics of heat Understanding the characteristics of heat waves would lead to improved forecastswaves would lead to improved forecasts

• These forecasts may become more critical These forecasts may become more critical given the possibility of an increase in the given the possibility of an increase in the frequency and intensity of heat wavesfrequency and intensity of heat waves

Introduction (cont.)Introduction (cont.)::

• Previous work has largely focused on specific Previous work has largely focused on specific heat wave events (e.g. July 1995) or extended heat wave events (e.g. July 1995) or extended “heat wave-droughts” (e.g. 1980 & 1988)“heat wave-droughts” (e.g. 1980 & 1988)

• Published synoptic climatologies have been Published synoptic climatologies have been limited in scope to the Midwest or Great Plains limited in scope to the Midwest or Great Plains (e.g. Chang & Wallace, 1984)(e.g. Chang & Wallace, 1984)

PurposePurpose::

• Document differences in heat waves as a Document differences in heat waves as a function of both season and region function of both season and region

• Understand both the dynamical and thermo-Understand both the dynamical and thermo-dynamical contributions to regional heat wavesdynamical contributions to regional heat waves

• Resolve annual and decadal trends in heat Resolve annual and decadal trends in heat wave frequencywave frequency

OverviewOverview::

• Methodology/DefinitionsMethodology/Definitions

• ResultsResults– Skewed datasetsSkewed datasets– Northeast heat wave statisticsNortheast heat wave statistics– Case Study: 7-11 June 1984 Case Study: 7-11 June 1984

• SummarySummary– ConclusionsConclusions– Future WorkFuture Work

MethodologyMethodology::

• Temperature data was extracted from the Temperature data was extracted from the National Climatic Data Center’s (NCDC) high National Climatic Data Center’s (NCDC) high resolution surface datasetresolution surface dataset

• Database contains daily high temperatures Database contains daily high temperatures for 54 stations over a 54-year period (1948-for 54 stations over a 54-year period (1948-2001)2001)

• Surface stations were selected on the basis Surface stations were selected on the basis of dataset continuity and geographical of dataset continuity and geographical coveragecoverage

Methodology (cont.)Methodology (cont.)::

Stations in Database

Methodology (cont.)Methodology (cont.)::

• Definitions:Definitions:– AnAn anomalously hot day anomalously hot day was initially defined as a was initially defined as a

day having a high temperature at least 2 standard day having a high temperature at least 2 standard deviations (deviations (σσ) above the normal high temperature ) above the normal high temperature for the datefor the date

– This definition was later changed to a day having a This definition was later changed to a day having a high temperature above the climatological 97.5 high temperature above the climatological 97.5 percentile threshold for the datepercentile threshold for the date

– A A heat waveheat wave was defined as three or more was defined as three or more consecutive anomalously hot days – no matter the consecutive anomalously hot days – no matter the seasonseason

Methodology (cont.)Methodology (cont.)

• Regions mirror the NCDC Standard Regions Regions mirror the NCDC Standard Regions for Temperature and Precipitationfor Temperature and Precipitation

• Heat waves were considered regional when Heat waves were considered regional when two or more stations within that region had two or more stations within that region had overlapping warm days overlapping warm days

• All anomalously hot days, heat waves, and All anomalously hot days, heat waves, and regional heat waves were identified for each regional heat waves were identified for each station and each region across the countrystation and each region across the country

ResultsResults

““Skewness” of Daily High Temperature DataSkewness” of Daily High Temperature Data

• Initially using a 2Initially using a 2σσ high temperature anomaly high temperature anomaly to define a hot day to define a hot day a surprising amount of a surprising amount of variability in number of heat waves identified variability in number of heat waves identified from one city to the nextfrom one city to the next

• A serendipitous discovery: a discrepancy A serendipitous discovery: a discrepancy between number of anomalously cold daysbetween number of anomalously cold days and the number of anomalously hot days for and the number of anomalously hot days for most stations in the datasetmost stations in the dataset

Results:Results: Top Ten “Skewed” Cities Top Ten “Skewed” Cities

Positively SkewedPositively SkewedNegatively SkewedNegatively Skewed

Most Negatively Skewed City: DenverMost Negatively Skewed City: Denver

• In 54-year dataset, only 49 days met old In 54-year dataset, only 49 days met old criteria as anomalously hot days (Tcriteria as anomalously hot days (T≥≥22σσ))

• Only 2 heat waves were identified in the entire Only 2 heat waves were identified in the entire datasetdataset

March 1 – May 31

Denver, CO Daily High Temperatures –– Spring

MeanMean

Mean + 2Mean + 2σσ

Mean - 2Mean - 2σσ

Denver, CO Daily High Temperatures –– Summer

June 1 – August 31

Denver, CO Daily High Temperatures –– Fall

September 1 – November 30

Denver, CO Daily High Temperatures –– Winter

December 1 – February 29

Most Positively Skewed City: Los AngelesMost Positively Skewed City: Los Angeles

• In 54-year dataset, 989 days met old criteria In 54-year dataset, 989 days met old criteria as anomalously hot days (Tas anomalously hot days (T≥≥22σσ))

• Hence, over 130 heat waves were identified in Hence, over 130 heat waves were identified in the datasetthe dataset

Los Angeles, CA Daily High Temperatures –– Spring

March 1 – May 31

Los Angeles Daily High Temperatures –– Summer

June 1 – August 31

Los Angeles, CA Daily High Temperatures –– Fall

September 1 – November 30

Los Angeles, CA Daily High Temperatures –– Winter

December 1 – February 29

New MethodNew Method::

• AnAn anomalously hot day anomalously hot day is now defined as a is now defined as a day having a high temperature above the daily day having a high temperature above the daily climatological 97.5 percentile threshold climatological 97.5 percentile threshold

• Heat wave definitions remained unchangedHeat wave definitions remained unchanged

• Updated Figures:Updated Figures:– DNR: 736 Days, 41 Heat WavesDNR: 736 Days, 41 Heat Waves– LAX: 669 Days, 66 Heat WavesLAX: 669 Days, 66 Heat Waves

Stat. Analysis of Northeast Heat WavesStat. Analysis of Northeast Heat Waves

Stat. Analysis of Northeast Heat WavesStat. Analysis of Northeast Heat Waves

Stat. Analysis of Northeast Heat WavesStat. Analysis of Northeast Heat Waves

Stat. Analysis of Northeast Heat WavesStat. Analysis of Northeast Heat Waves

Stat. Analysis of Northeast Heat WavesStat. Analysis of Northeast Heat Waves

Stat. Analysis of Northeast Heat WavesStat. Analysis of Northeast Heat Waves

Stat. Analysis of Northeast Heat WavesStat. Analysis of Northeast Heat Waves

Stat. Analysis of Northeast Heat WavesStat. Analysis of Northeast Heat Waves

Stat. Analysis of Northeast Heat WavesStat. Analysis of Northeast Heat Waves

Stat. Analysis of Northeast Heat WavesStat. Analysis of Northeast Heat Waves

Results:Results:

• A trend toward more Winter and Spring heat A trend toward more Winter and Spring heat waves with timewaves with time

• A trend toward fewer Summer and Autumn A trend toward fewer Summer and Autumn heat waves with timeheat waves with time

• Decades of the 1950’s, 1980’s and 1990’s had Decades of the 1950’s, 1980’s and 1990’s had the highest frequency of heat wavesthe highest frequency of heat waves

A Case Study: 7-11 June 1984A Case Study: 7-11 June 1984

• A long-lasting, region-wide Northeast heat A long-lasting, region-wide Northeast heat wavewave

• Both thermodynamic and dynamic signatures Both thermodynamic and dynamic signatures can be seencan be seen

June 4, 1984June 4, 1984

ALB: 80ALB: 80° F° F

LGA: 85LGA: 85° F° F

BOS: 85BOS: 85° F° F

June 5, 1984June 5, 1984

ALB: 84° F

LGA: 88° F

BOS: 89° F

June 6, 1984June 6, 1984

ALB: 81° F

LGA: 88° F

BOS: 70° F

June 7, 1984June 7, 1984

ALB: 89° F

LGA: 93° F

BOS: 89° F

June 8, 1984June 8, 1984

ALB: 93° F

LGA: 96° F

BOS: 97° F

June 9, 1984June 9, 1984

ALB: 93° F

LGA: 95° F

BOS: 96° F

June 10, 1984June 10, 1984

ALB: 91° F

LGA: 95° F

BOS: 95° F

June 11, 1984June 11, 1984

ALB: 90° F

LGA: 96° F

BOS: 98° F

June 12, 1984June 12, 1984

ALB: 82° F

LGA: 84° F

BOS: 85° F

June 4, 1984June 4, 1984

ALB: 80° F

LGA: 85° F

BOS: 85° F

June 5, 1984June 5, 1984

ALB: 84° F

LGA: 88° F

BOS: 89° F

June 6, 1984June 6, 1984

ALB: 81° F

LGA: 88° F

BOS: 70° F

June 7, 1984June 7, 1984

ALB: 89° F

LGA: 93° F

BOS: 89° F

June 8, 1984June 8, 1984

ALB: 93° F

LGA: 96° F

BOS: 97° F

June 9, 1984June 9, 1984

ALB: 93° F

LGA: 95° F

BOS: 96° F

June 10, 1984June 10, 1984

ALB: 91° F

LGA: 95° F

BOS: 95° F

June 11, 1984June 11, 1984

ALB: 90° F

LGA: 96° F

BOS: 98° F

June 12, 1984June 12, 1984

ALB: 82° F

LGA: 84° F

BOS: 85° F

Case Study ResultsCase Study Results

• Origin of hot air: Rockies (elevated heat Origin of hot air: Rockies (elevated heat source)source)

• Downsloping plays a role:Downsloping plays a role:– Westward extension of the Bermuda High into Westward extension of the Bermuda High into

Southeast allowed for west-northwesterly flow Southeast allowed for west-northwesterly flow throughout periodthroughout period

– Warm air coming off RockiesWarm air coming off Rockies

• Anticylonic shear side of jet: subsidenceAnticylonic shear side of jet: subsidence• June 9June 9thth: Northeast located in the equator-: Northeast located in the equator-

ward jet exit region: enhanced subsidenceward jet exit region: enhanced subsidence

ConclusionsConclusions::

• From initial heat-wave identifying From initial heat-wave identifying methodology: daily max temperatures are not methodology: daily max temperatures are not normally distributednormally distributed

• Stations located adjacent to cool (warm) water Stations located adjacent to cool (warm) water seem to have positively (negatively) skewed seem to have positively (negatively) skewed high temperature datahigh temperature data

• Most stations have a cool biasMost stations have a cool bias

Conclusions (cont.) Conclusions (cont.) ::

• From Northeast statisticsFrom Northeast statistics: :

– Positive heat wave trend in Winter and Positive heat wave trend in Winter and SpringSpring

– Negative heat wave trend in Summer and Negative heat wave trend in Summer and AutumnAutumn

• From case studyFrom case study: :

– Heat waves can have both dynamic and Heat waves can have both dynamic and thermodynamic aspectsthermodynamic aspects

– Local wind direction is importantLocal wind direction is important

Conclusions (cont.)Conclusions (cont.)::

• Future Work:Future Work:– Apply statistical methods to other regionsApply statistical methods to other regions

Create composite analyses to:Create composite analyses to:– Illustrate typical synoptic signatures of heat Illustrate typical synoptic signatures of heat

waves in each seasonwaves in each season– Determine regional “flavor” of heat wavesDetermine regional “flavor” of heat waves