CONTRAILS & CLIMATE STUDIES

Langley Research Center

CONTRAILS & CLIMATE STUDIES

Patrick MinnisNASA Langley Research Center

Hampton VA, USA

30 October 2003


MOTIVATION

• Air traffic increasing 2 - 5%/year over the globe

• Ice supersaturation exists 10-20% of the time at flight altitude

• Aircraft produce persistent contrails => cirrus aviaticus

• Cirrus clouds affect radiation budget, possibly water budget

• Aircraft exhaust might affect microphysics of extant cirrus

• Contrail/cirrus impact least certain effect of aircraft on climate

Can contrails have an effect large enough for concern?

- Mitigation efforts by aircraft industry (new technology)

- Mitigation efforts by air traffic control (new routing)


Aircraft exhaust short circuits natural cirrus formation

- high humidities normally needed to make cirrus (C)

- cirrus can exist at lower humidities (B) but need formation boost

- no cirrus for RHI < 100% (A)

T < -39°C


GOES-8 IR LOOP FOR NOVEMBER 18, 2001, 1015 - 2115 UTC

QuickTime™ and aGIF decompressor

are needed to see this picture.


11-12 µm temperature difference from 1-km satellite data 24 October 2003; Okla, Ark, Kan, Missouri

NOAA-15 1250 UTC

NOAA-17 1738 UTC


<= Terra MODIS 2025 UTC

NOAA-12 IR 2251 UTC

Contrails have become cirrus clouds

11-12 µm temperature difference from 1-km satellite data 24 October 2003, Okla, Ark, Kan, Missouri


11-12 µm temperature difference from 1-km satellite data 24 October 2003, Midwest

<= NOAA-15, 1250 UTC

NOAA-12, 2111 UTC =>

More contrail cirrus


11-12 µm temperature difference from 1-km satellite data 24 October 2003, Other areas

TEXAS

CA coast

Idaho

Pacific NW


CONTRAILS

• Ubiquitous feature of our skies

- increase cirrus coverage over areas with air traffic

• Can affect climate by altering

- Radiation budget (warming, cooling)

- Changing moisture budget of upper troposphere?


Fig. 11. LW, SW, and net radiative forcing at TOA and surface (SFC) for cirrus cloud with D = 24

and 60 µm for ice water path of 5.3 gm-2 at 250 mb over land with surface albedo of 20% andmean surface temperature of 283 K with diurnal range of 17 K.

THEORETICAL RADIATIVE EFFECTS OF CONTRAILS


Fig. 12. Change in daily mean atmospheric heating rates for contrail over clear and cloudyscenes. Contrail has = 0.3 and D = 60 µm. Calculations for midlatitude spring atmosphereduring April at 45°N with surface albedo of 0.2 and cloud optical depth of 20.

MEAN DAILY HEATING RATES DUE TO CONTRAILS

= 0.3


CONTRAIL UNKNOWNS

• contrail-cirrus coverage

- geographical & temporal

- now & future (requires modeling)

• microphysics: optical depth, particle size

- mean between 0.1 and 0.4, varies between 0.01 & 2

- De changes over life cycle (5 -100 µm)

• radiative forcing

- depends on when and where it occurs

• vertical spreading

- dries the UT?


APPROACH

Can it be significant?

• Estimate lower & upper bounds of current contrail-cirrus impact

- use empirical-theoretical estimates in RTM

- relate cirrus change to air traffic

Can we accurately predict it?

• Develop climatology of contrail coverage, frequency, microphysics, radiative forcing

- surface & satellite observations

• Relate contrail observations to meteorological conditions

- develop empirical-theoretical models to predict contrail coverage & properties


• European studies

- regional coverage from linear features in AVHRR imagery

- tuned global coverage from ECHAMP/ECMWF output (Sausen et al. 98)

- recently estimated = 0.11 from AVHRR, similar from GCM

- GCM simulations of CRF (Contrail Radiative Forcing)

• US studies

- LaRC estimated = 0.30 from AVHRR & GOES data over US

- NASA GISS GCM simulation of CRF (Rind et al. 2000)

- LaRC simulation of CRF from European tuned output/ISCCP/ERBE

- lower bound (Minnis et al. 99)

BACKGROUND


LaRC Contrail Minimum Radiative Forcing Estimates

=> Global contrail forcing:

FSW = -0.003 to -0.012 Wm-2, FLW = 0.011 to 0.033 Wm-2

Fnet = 0.008 - 0.020 Wm-2 ; 0.017 Wm-2 for = 0.3

European estimate 0.003 Wm-2 for = 0.15

Greater over areas with heavy air traffic!


Current Estimates of contrail radiative forcing

-90

-60

-30

0

30

60

90

0 0.2 0.4

Latitude (°)

F (Wm

-2

)

0 0.2 0.4

F / c (Wm

-2

%

-1

)

a) b)

Minnis et al. 1999, GRL


LaRC Contrail Maximum Radiative Forcing Estimates

• Estimate change in cirrus cloudiness due to air traffic

- primary : sfc obs 1971-1995

• Repeat CRF calculations with cirrus change estimate

- assume linear scaling with coverage, = 0.15 - 0.25

• Use GCM conversion factors to estimate temperature changes

Rind et al. (2000):

= > 0.025 Wm-2 for = 0.25

New range of global radiative forcing = >0.006 - 0.025 Wm-2


To estimate upper bound contrail radiative forcing:

• Measure trends in cirrus where contrails form & do not form (air traffic patterns)

• Estimate impact of relative humidity

• Estimate cirrus change for no humidity change

- no trend over USA

Study in "Contrails, Cirrus, and Climate," Minnis et al., 2003, accepted J. Climate


EVIDENCE FOR CHANGE IN CIRRUS CLOUDINESS DUE TO CONTRAILS IS

PILING UP!


Trends in cirrus cover ( SFC OBS, 15+ yrs) & RH(300 hPA), 1971-1995

1992 contrail cover RH

Cirrus trend Cirrus trend, conf level 90%

from Minnis et al. 2004



Cirrus & contrail seasonal trends

Satellite contrail coverage: 1990s (Mannstein et al. 1998, Palikonda et al. 2002)

Surface cirrus: 71-95

USA Europe

Satellite contrail frequencies: 1993-94 & 98-99 (Minnis et al. 2002)

-Contrails consistent with cirrus trend over USA, not Europe


Table 2. Contrails, mean cirrus cover, and cloudiness trends (%/decade) over air traffic regions from surface (CC) and ISCCP (CCI) data. The numbers in parentheses indicate the interannual variability in CC. The 1971-95 trends in CC are all significant at the 99% confidence level, except over WEUR where no trend is apparent.

Region

1992 ECO

N

(%)

Mean CC

(%)

1971-95

CC Trend

1971-95

Total Cloud Trend

1971-95

CC Trend,

1983-95

CCI Trend,

1983-95

WASIA 0.08 36.2 (1.0) -0.9 -0.7 -2.0 -2.1

EUR 0.60 18.5 (1.3) -1.2 -0.4 -0.4 0.0

WEUR 1.52 19.8 (1.4) 0.0 -0.7 1.8 0.9

USA 1.75 29.2 (1.1) 1.0 0.5 0.3 2.3

LOR 0.09 24.5 (0.5) -1.6 -1.4 -1.5 -0.6

NA 0.32 15.3 (0.7) 0.7 0.0 0.3 0.2

NP 0.16 15.7 (0.8) 0.9 0.8 1.6 -0.4

OOR 0.13 14.4 (0.6) 0.7 1.2 0.8 0.1

Minnis et al. 2004,

J. Climate


Zerefos et al. 2003, JGR

Cirrus coverage trends more positive over areas of heavy air traffic in a given region

Based on ISCCP data


Mannstein et al., AAC, 2003

Over Europe, cirrus coverage, especially thin cirrus, coverage highly dependent on air traffic

Based on Meteosat imagery

linear contrail coverage over Europe only 0.3%

cirrus delta = 3%

Contrail spreading a factor of 10!


INTERIM SUMMARY

• Cirrus coverage is increasing over USA (consistent w/ seasonal contrail frequency

steady over Europe (inconsistent)

decreasing over western Asia, but not in areas of heavy air traffic

decreasing most over other land areas

• Cirrus is increasing over ocean (not many obs in pristine areas)

Is increase due to air traffic or weather changes?

- Zerefos and Mannstein results suggest the former

- Minnis et al. (2004) agree



ESTIMATION OF TEMPERATURE CHANGE OVER USA DUE TO CONTRAIL CIRRUS BASED ON GCM STUDY & CIRRUS OBS

Minnis et al. 2004, J. Climate


IMPACT OF CIRRUS TREND

• Contrail cirrus can account for all of observed warming over USA between 1975 & 1994

- Ozone impact not included!


con < 0.2%

QuickTime™ and aGraphics decompressorare needed to see this picture.

-0.15

-0.05

0.05

0.15

0.25

0.35

-90 -60 -30 0 30 60 90LATITUDE (°)

MSU Observed

min contrail

max contrail

CHANGES IN ATMOSPHERIC TEMPERATURE, 1979-1997 200-850 mb FROM SATELLITE DATA AND ESTIMATED

CONTRAIL RADIATIVE FORCING

data from J. R. Christy

Minnis et al. GRL (1999)

present study


ESTIMATES OF ALL AVIATION FORCING WITHOUT CONTRAIL SPREADING

- IPCC (1999)


BETTER UNDERSTANDING & PREDICTION

• IMPROVED OBSERVATIONS OF LINEAR CONTRAILS & SPREADING

- provides data for developing & validating models

• RELATIONSHIP BETWEEN AIR TRAFFIC, CONDITIONS, & CONTRAILS

- gives basis for parameterizing cirrus aviaticus

• PREDICT WHEN & WHERE CONTRAILS WILL FORM

Contrails are a problem, what can we do?


Linear Contrail Climatology

Automated Contrail Detection

NOAA-12 AVHRR, April 1997

10.8-µm image detected contrailsmethodology from Mannstein et al. 1999

VA

NC


DECEMBER

LINEAR CONTRAIL COVERAGE DURING 2001 FROM AVHRR

730 AM APRIL 230 PM

DECEMBER

Palikonda et al. 2003)


CONTRAIL DETECTION FROM SATELLITES

• Very sensitive to particular imaging instrument response

- need careful tuning of technique

• Sometimes mistakes cirrus streaks as contrails

- need error budget

• Sometimes misses larger contrails

- need more error budget

This work is underway!


Optical depths nearly identical for both NOAA-15 & 16


Comparison of contrail coverage (%)

USA

Time Sausen et al. 98 Palikonda et al. 98 present (NOAA-16, 15)

1993-94 2001

Dec 1.6 2.1 (Dec) 0.8 (0.9)

Apr 2.0 2.0 0.7 (1.3)

Jul 0.5 1.3 0.3 (1.1)

Oct 1.9 1.9 0.8 (1.0)

-------------------

Sausen et al. 98, Global Annual 0.087


Comparison of contrail properties

Source Time OD NCLRF (Wm-2)

Minnis et al. 98, USA Apr 0.30

Minnis et al. 99, Global

(theoretical) Annual 0.30 27

Palikonda et al. 98 Apr (0.27) 12.4 (14.2)

N14, 93-94, USA Jul (0.30) 16.0 (22.3)

( N15&16, 01, USA) Oct (0.27) (10.4))

Dec (0.27) 11 (12.0)

Meyer et al. 02, Europe Annual 0.11 14

(NOAA14, 95-97)


CONTRAIL PREDICTION

• To relate contrails to the conditions, we have acquired a database of flight tracks for commercial air traffic over USA for 3 years

- Garber et al. 2003 (NASA RP in review)

• Use NCEP Rapid Update Cycle (RUC-2) experimental product to predict contrail occurrence

- realtime USA contrail predictor online

new RUC data not very good for contrails

• Compare with satellite data


Contrail boundaries and relative humidity with respect to ice (RHI)

1600 UTC, November 18, 2001

MODIS T4-T5 ImageRHI from RUC-2 analysis, 225 mb




CONTRAIL OUTBREAK OVER GREAT LAKES, 9 OCT 2000

"Fly" aircraft through RUC fields, simulate formation & spreading

from Duda et al. 2003, JAS, accepted

Langley Research Center Garber et al. 2003, NASA


OCTOBERSEPTEMBER

Comparison of contrail amount from satellite data and frequency of potential contrail conditions from RUC-2 data


• Humidity fields critical for contrail formation, but correlations not always apparent because

- extant cirrus may prevent detection of contrails from satellite

contrail/cirrus conditions often equivalent

- afternoon contrails may be less detectable because of overlap

• 2001 coverage much less than 1993-94

- 1993-94 period one of moistest upper troposphere in 30 years (45.5%)

- 2001 one of driest at high altitudes in 30 years (39.4%)

- NOAA-16 may be less sensitive to contrails than NOAA-11

- NOAA-11 tendency for overestimation (~0.5%)

SUMMARY


UPPER TROPOSPHERIC HUMIDITY

OLD CONTRAILS

ROADBLOCKS TO ACCURATE CONTRAIL PREDICTION

THE AIR TRAFFIC SHUTDOWN CASE

2001 air traffic shutdown removed some impediments for contrail study

- System cleared of commercial air traffic contrails by 0000 UTC, 12 September 2001


GOES-8 IR LOOP FOR SEPTEMBER 12, 2001, 1045 - 2345 UTC




By studying the few contrails that occurred during the shutdown, we can tune a model that simulates contrails


12 SEPTEMBER CONTRAIL ANALYSES

• Use GOES images to track & compute spreading

• Estimate heights using stereographic analysis

- GOES-8 & AVHRR, MODIS, or GOES-10

- Flight levels between 10.5 and 12.5 km

• Compute optical depth using RUC temperature at 11.5 km (225 hPa) and adjacent clear-sky temperature


Terra MODIS 1 -km Infrared Image

1545 UTC, 12 September 2001

NOAA-15 AVHRR 1-km Infrared Image

1245 UTC, 12 September 2001

3-hour change in observed contrails

H

E


DURING THIS EVENT

MEAN CONTRAIL LIFETIME IS 6.5 hr

MEAN AREAL COVERAGE FOR EACH CONTRAIL IS 2270 km2

MEAN OPTICAL DEPTH IS 0.23

In the mean, over a 6.5 hour period, 8 contrails covered 18,000 km2

=> We need to relate the contrails to the humidity fields at altitude!


Model humidity


Comparison of RHI profiles from radiosondes & RUC-2 analyses 12 UTC, September 12, 2001

AE,F,G B,C,D,H


NEED TO ARTIFICIALLY ENHANCE MOISTURE PROFILES

OBSERVED & MODELED HUMIDITIES TOO LOW!

RHI for adiabatic cirrus ~ 150%

RHI for persistent contrails > 100%

Nevertheless, there appears to be a relationship between the vertical structure of RHI and the lifetime, spreading, and optical depths of the observed contrails


Comparison of persistent contrail occurrence and sonde RH, SLC, Utah

RHI corrections based on frost-point hygrometer data

Miloshevich et al. 2001, JAS

Sassen, 1999, BAMS

RHI correction for sonde profiles used here


Radiosonde profiles of RHI in contrail areas after correcting for dry bias, 12Z, September 12, 2001


Contrail simulation for September 12 over northeastern USA• Assume air traffic is equal to September 5

- use database from Garber et al. 2003

• Apply Appleman criteria, use RHI > 70% to define persistence

• Assume RHI for a flight track = that of nearest RUC level

• Compute spreading, assuming fall speed of 3 cm/s

- max width = 12 km, shear determines spread rate (mean = 6 km/h, same as observed for military contrails)

- no new nucleation, optical mass = OD*width

- OD = f(t), peaks at 2 hours

• Advect old & add new contrails

• Delete trails if RHI < 70% or older than 6 hr


Hourly simulation of contrails over northeastern USA September 12 2001, assuming air traffic for September 5






ANOTHER VIEW


Estimate of linear contrail coverage over northeastern US for September 12, 2001 assuming air traffic for Sept. 5

Coverage could have been at least 200,000 km2 if traffic were normal

Daily global mean is 200,000 - 400,000 km2!


ALTERNATIVES FOR MITIGATION

• Predict locations & altitudes where contrails most likely

- provide alternate routing (height change)

- our proposal

• Fly lower all the time

- German study 2003

• Use liquid hydrogen fuels

- German study 2003






CONCLUDING REMARKS

• Understanding has increased rapidly but remaining issues

- accuracy of contrail coverage

- where does contrail impact occur

how much is local? spread around?

- contrail optical depths

0.1 - 0.3?

geographic dependence?

- indirect effects on cirrus clouds

does AC exhaust increase opt depth, decrease De?

do AC exhaust aerosols reduce formation threshold

leading to more cirrus?

- is there an effect on the moisture budget?

- how accurately can we model cirrus aviaticus?

need good natural cirrus model (satellite comparisons)

- what are best mitigation options?


FUTURE RESEARCH AT LANGLEY

• Continue climatology development

- examine relationship of natural cirrus to environment

• Compute radiative forcing for simulations

• Apply more sophisticated contrail model, account for natural Ci

- match flights to vertical details of RHI

- improve precip, spreading, dissipation

• Improve threshold for contrail persistence from models

- find a new data source (RUC changed April 19)

• Determine source of differences in lifetimes and spreading

• Push for improved UT RH

• Provide realistic real time predictions of contrails in clear air


REFERENCES & MISCELLANEOUS

• All references can be found on our main web page

http://www-pm.larc.nasa.gov/

click on "SASS", then on "Related References"

• Other imagery and examples are available on the same main web page, click on "PATHFINDER", "SUCCESS" or "SASS"

• A near-real-time contrail predictor is available on the main web page, click on "Contrail Forecast" (not so good since RUC change)

Documents

CONTRAILS & CLIMATE STUDIES