Extreme weather events and air quality by CESM and WRF/CMAQ Yang Gao1, Joshua S. Fu1, John B. Drake1, Jean-Francois Lamarque2 and Yang Liu3

1Department of Civil and Environmental Engineering, University of Tennessee, Knoxville, TN 2Atmospheric Chemistry and Climate and Global Dynamics Divisions, National Center for Atmospheric Research, Boulder, CO

3Rollins School of Public Health, Emory University, Atlanta, GA

Acknowledgements

This research was supported in part by the National Science Foundation through TeraGrid resources provided by National Institute for

Computational Sciences (NICS) under grant number [TG-ATM110009] and [UT-TENN0006].

This research also used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported

by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725.

This work was partially sponsored by the Climate and Health program led by George Luber at the Centers for Disease Control and Prevention

(CDC) under a research project cooperative agreement (5 U01 EH000405).

References

Gao Y, Fu J S, Drake J B, Liu Y and Lamarque J-F (2012). Projected changes of extreme weather events in the Eastern United States

based on a high-resolution climate modeling system. Environmental Research Letters, 7, 044025.

Gao, Y., Fu, J. S., Drake, J. B., Lamarque, J.-F., and Liu, Y.: The impact of emissions and climate change on ozone in the United States

under Representative Concentration Pathways (RCPs), Atmos. Chem. Phys. Discuss., 13, 11315-11355, doi:10.5194/acpd-13-

11315-2013, 2013

Huth R, Kysely J and Pokorna L (2000). A GCM simulation of heat waves, dry spells, and their relationships to circulation. Climatic Change,

46(1-2), 29-60.

Karl T R and Knight R W (1997). The 1995 Chicago heat wave: how likely is a recurrence? Bulletin of the American Meteorological Society,

78, 1107-19.

Meehl G A and Tebaldi C (2004). More intense, more frequent, and longer lasting heat waves in the 21st century. Science, 305, 994-7.

Salinger M J and Griffiths G M (2001). Trends in New Zealand daily temperature and rainfall extremes. International Journal of Climatology,

21, 1437-52.

Introduction

High resolution dynamical downscaling technique was used in this study to link

global climate model Community Earth System Model (CESM) and regional climate

model Weather Research and Forecasting (WRF) Model. The fossil fuel intensive

scenario Coupled Model Intercomparison Project Phase 5 (CMIP 5)

Representative Community Pathways (RCP) 8.5 was selected to investigate the

changes of extreme weather events in future climate (2057-2059) compared with

present climate (2001-2004). The 4km by 4km high resolution eastern U.S.

domain was the major focus in this study. More detailed information was

described by Gao et al. [2012].

Evaluation of extreme precipitation

Summay

This study is the first assessment on a 4km by 4km high resolution downscaling in the entire eastern

US using the CESM and WRF. Comparison with observations shows a significant improvement in

high resolution modeling, with improvement for heat wave frequency as high as 98%. The study of

fossil fuel intensive scenario RCP 8.5 indicates the heat waves become more severe in future (2057-

2059) across the entire Eastern US and the total annual extreme rainfall in the Northeast and

Southeast increase 35% compared to that of the present climate (2001-2004). The Northeastern US

shows large increase in both heat wave intensity (3.05 ºC) and annual extreme rainfall (107.3 mm

more per year). The reduced anthropogenic emissions play dominant roles in ozone reduction in RCP

4.5, while the increased methane emissions and stratosphere intrusion in RCP 8.5 could drive ozone

increase.

Figure 1. WRF simulation domains: D1 (36

km by 36 km resolution), D2 (12 km by 12

km) and D3 (4 km by 4 km). The points

represent NCDC US COOP network station

observation points in three regions:

Northeast (red color), Eastern Midwest (blue

color) and Southeast (green color).

A rainy day is defined as a day when the daily rainfall totals at least 1 mm [Salinger and

Griffiths, 2001]. In the current analysis, extreme precipitation is defined as the 95th

percentile of all the rainy days [Salinger and Griffiths, 2001].

• Total extreme rainfall (mm/year): Total amount of annual extreme rainfall

• Annual extreme events (days/year): Total annual extreme rainfall days

Evaluation of heat waves

• Heat wave intensity (ºC) is defined as the highest three continuous nighttime minima [Karl and

Knight, 1997].

• Heat wave duration (number of days during a heat wave) and frequency (number of heat wave

events per year) is based upon two thresholds (T1 and T2). T1 and T2 were defined as 97.5th and

81st percentile of a given period of time. Then the heat wave was defined as the longest continuous

period during which (1) the maximum daily temperature reach T1 for at least 3 days (2) the

mean daily maximum temperature can not be less than T1 and the daily maximum

temperature must reach T2 every day [Huth et al., 2000; Meehl and Tebaldi, 2004].

Annual

extreme

precipitation (mm/year)

Annual

extreme

precipitation

days (days/year)

Intensity

(ºC)

Duration

(days/event)

Frequency

(events/year)

Figure 2. Probability distributions of precipitation from NCDC, CESM and WRF outputs. The

probability distributions of daily rainfall 40 mm or more (extreme rainfall) is zoomed in and

plotted in the middle of each plot. Total annual extreme rainfall amounts and days were listed in

the upper portion of each plot. The numbers on the left represent total annual extreme rainfall,

with NCDC in black, bias in CESM (CESM-NCDC) in blue, bias in WRF (WRF-NCDC) in red

and the bias reduction in WRF over CESM ((|CESM-NCDC|-|WRF-NCDC|)/(|CESM-

NCDC|)*100%, in green); The numbers on the right are similar to the left but apply to the

annual extreme rainfall days.

Present RCP8.5-Present

The CESM tends to yield larger percentages of rainy days with daily

rainfall from 1- 5 mm, but lower percentages with daily rainfall of 10 mm or

more. The probability distributions of extreme rainfall in WRF agree more

closely with NCDC data, while CESM data substantially underestimate the

frequency of extreme rainfall.

After downscaling, there are 16 and 14 states showing statistically significant

improvement for heat wave intensity and duration, respectively. The greatest

improvements in WRF over CESM include: heat wave intensity in Florida (97%),

heat wave duration in Maryland (91%) and heat wave frequency in Kentucky (98%).

Wide spread increase of heat

waves and extreme precipitations

was projected by the end of 2050s

(2057-2059) in RCP 8.5 compared

to present (2001-2004)

City-level heat waves

After downscaling, the mean improvement in WRF for the 20 cities is 21%,

71% and 57% for heat wave intensity, duration and frequency, respectively.

In future (RCP 8.5, 2057-2059), widespread increase of heat waves occurs

in the 20 major cities in the eastern US, with a mean increase of 3.10 ºC for

intensity, 1.85 days per event, and 4.38 events per year.

Present RCP8.5-Present

Top 20 cities by population in

Eastern US

Note: Among the top 50 cities by

population in US, 20 cities are

located in the eastern US, shown

in the figure above

Impact of climate/emissions on

ozone

The cumulative distribution of RCP 4.5 shifts

to the left, indicating reduced ozone

concentrations by the end of 2050s due to

dramatic reduction in anthropogenic

emissions (both VOCs and NOx)

In RCP 8.5, the ozone reduction is smaller

than RCP 4.5, and in the western US, the

ozone concentration may even increase due

to increased methane and stratosphere

intrusion [Gao et al., 2013].