UltraViolet Index

Craig S. LongNational Oceanic and Atmospheric Administration

National Weather ServiceNational Centers for Environmental Prediction

Climate Prediction Center

Outline

• Background information on UV radiation– Factors that affect UV radiation

– Health effects

• History of UV Index• WMO/WHO Standards• NWS UV Index Algorithm

– Current

– Future

– Products

Background

• Definition:– UV Index is a scaled erythemal (skin reddening) dose rate

integrated over the UV-B and UV-A spectral bands.

– WMO guidelines require forecasts to be generated at least for next day at solar noontime.

Background

• UltraViolet radiation has shorter/more powerful wavelengths than visible light.

• The most dangerous wavelengths of UV spectrum (UV-C) are completely absorbed in the stratosphere.

• Partial absorption of UV-B• Minor absorption of UV-A

UV Health Effects

• Short Term overexposure to UV-B:– UV-B causes sunburn (erythema)– Sunburn can prevent your body from eliminating heat efficiently and

can contribute to heat stress

• Long Term overexposure to UV-B:– Several severe sun burns linked to melanoma– One person dies of skin cancer per hour in the U.S.– Sustained exposure linked to squamous and basal skin cancers – non

lethal– Cataracts of eyes

• Long Term overexposure to UV-A:– Aging of skin– Loss of elasticity (leathery look)

• Small amounts of UV radiation needed for Vitamin-D

UV Health Effects

Background

• Factors affecting UV radiation from TOA to surface:– Flux at TOA

• Earth – Sun distance

– Ozone (total column)– Solar Zenith Angle

• Latitude, Day of Year

– Clouds– Aerosols– Elevation– Surface Albedo

• No Snow• Snow

• 1980’s: Australia started reporting yesterday’s UV radiation levels in conjunction with huge sun protection campaign.

• 1992: Environment Canada began issuing a next day forecast of clear sky UV levels called ‘UV Index’.

• 1994: NWS and EPA begin issuing a next day forecast w/cloud effects. Use slightly different scale than Canadians. Allowed to use name ‘UV Index’.

• 1995: WMO holds “meeting of experts” to establish UV Index definition.

• 1997: WMO holds second “meeting of experts” to standardize UV Index name and forecasts

• 2000: WHO holds meeting to standardize health messages and exposure categories.

History

UV Index Definition

• UV Index is a scaled erythemal dose rate integrated over the UV-B and UV-A spectral bands.

UV Spectrum

Weighted Irradiances

• EDR = Integral over the 290 to 400 nm range– EDR = ~0.250 Watts/m2 or 250 mW/m2.

• Erythemal Dose Rate is the instantaneous flux of skin reddening UV radiation.

• UV Index is the EDR(W/m2) x 40.– Or EDR(mW/m2)/25

– UV Index = 10.0

Erythemal Dose Rate

• RTMs are needed to determine “clear sky” UV flux at surface under prescribed conditions.– SZA – Ozone – Aerosols – Elevation– Albedo

• Several types of RTMs:– multiple scattering spectral models– fast spectral models– empirical models

• RTMs are computationally expensive– Operational forecasts of large gridded fields require the

creation of Look Up Tables

Radiative Transfer Models

Results from Spectral Radiative Transfer Model

Eythemal Dose Rate (no aerosols)25 mW/m 2 = 1 UV Index unit

0

50

100

150

200

250

300

350

400

450

500

80 120 160 200 240 280 320 360 400 440 480 520 560 600

Total Ozone (DU)

Ery

them

al D

ose

Rat

e (m

W/m

2 )

0

10

20

30

40

50

60

70

80

Ozone Hole

Tropics Extra Tropics

Zonal Mean Ozone

Zonal Mean UV Index (clear sky, sea level, no aerosol)

• Current UV Fcst– Ozone Forecast from GFS– Albedo = 5% (no snow)– Elevation (6% per Km)– Aerosol – clean atmosphere (AOD=0.2, SSA=1.0)– Clouds amounts from MOS– Products:

• 58 city bulletin• Clear sky UV Index on

global 1x1 grid• Cloudy UV Index for

CONUS and AK grids

UV Index Forecast

• Future UV Fcst– Ozone Forecasts from GFS– Albedo

• no snow = 3%• Snow = >30% (GFS Albedo)

– Elevation• 9% 1st km• 7% 2nd km• 6% 3rd km

– Aerosol• Variable from climatology

– Cloud attenuation from GFS shortwave code

– Products:• 58 city bulletin• Global Cloudy and Clear Sky

grids

UV Index Forecast

• 3 Forecasts in one– Ozone Forecast

– Cloud Forecast

– Aerosol Forecast (not there yet)

NCEP GFS Ozone Assimilation

Global Ozone Field

Ozone Hole

Global UV Index

Elevation Effect

Percent UV(ery) Increase with ElevationSZA=20, June 30, Toz=300

0%

4%

9%

12%

16%

18%

22%

2.8%

5.9%

9.0%

11.9%

14.7%

17.3%

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0% 5% 10% 15% 20% 25%

Percentage Increase

Ele

vati

on (

km)

New

Current

Binghamton, NY

Lubbock, TX

Boulder, CO

Jackson, WY

Yellowstone, WY

Breckenridge, CO

• Surface has low albedo is UV ~ 3%• Sand is more reflective ~30%• Snow is most reflective ~30-90%

Relation of UV Albedo with GFS Visible Albedo

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

UV Albedo

GF

S A

lbed

o

Albedo

UV Index Response to Albedo Changes

0%

10%

20%

30%

40%

50%

60%

70%

80%

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Albedo

% I

ncr

ease

of

UV

In

dex

Albedo (snow and sand)

• Aerosols scatter and absorb UV radiation• Reduce direct and Increase diffuse

– Global is usually attenuated

• Aerosol parameters include:Aerosol Optical DepthSingle Scattering Albedo

• Ratio of scattered /(scattered + absorbed)

o Asymmetry factor • Forward scattering vs backward scattering

Aerosol Effects

• Aerosol Optical Depth

• Single Scattering Albedo

Aerosol Climatology

Ratio UVaer/UV0 = f (SZA), SSA=0.7

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0.0 0.1 0.2 0.3 0.4 0.5 0.6

AOD350

Rat

io

0

10

20

30

40

50

60

70

80

0.8, 0

0.9, 0

1.0, 0

Aerosol Effect

Cloud Forecasts

• Current Method: MOS clouds – MOS fcsts Pct Clear, Scattered, Broken, Overcast

– Regression determined by comparing MOS fcsts against observed cloud transmission

– Clear (0-1 tenths): 100% transmission

– Scattered(2-5 tenths):89%

– Broken(6-8 tenths): 72%

– Overcast(9-10 tenths): 31%

• Proposed Method: From GFS Shortwave scheme– UV Transmission = UVCloud/UVNo Cloud

UV Attenuation from GFS Clouds

Clouds and UV Attenuation

Clouds and UV Attenuation

Clouds and UV Attenuation

Clouds and UV Attenuation

Clouds and UV Attenuation

Products: MOS Locations for CONUS and AK

Products: 58 City Bulletin

Products: Cloudy UV Index Output Grids

Eta Grid 207 Eta Grid 211

Products: UV Index Contoured Map

Products: Global UV Index

UV Index “to do” list

• Generate and validate global noon time UV Index fcsts

• EMC must operationally output UVcloud and UVno-cloud surface fluxes

• Generate UV Index forecasts out to day 5

• Generate UV Index forecasts at both 00Z and 12Z cycles

• Generate daily UV dosage– Diurnal variation

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