Ultra Violet Index
description
Transcript of Ultra Violet Index
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UltraViolet Index
Craig S. LongNational Oceanic and Atmospheric Administration
National Weather ServiceNational Centers for Environmental Prediction
Climate Prediction Center
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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
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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.
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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
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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
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UV Health Effects
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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
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• 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
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UV Index Definition
• UV Index is a scaled erythemal dose rate integrated over the UV-B and UV-A spectral bands.
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UV Spectrum
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Weighted Irradiances
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• 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
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• 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
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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
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Zonal Mean Ozone
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Zonal Mean UV Index (clear sky, sea level, no aerosol)
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• 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
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UV Index Forecast
• 3 Forecasts in one– Ozone Forecast
– Cloud Forecast
– Aerosol Forecast (not there yet)
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NCEP GFS Ozone Assimilation
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Global Ozone Field
Ozone Hole
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Global UV Index
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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
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• 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
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Albedo (snow and sand)
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• 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
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• Aerosol Optical Depth
• Single Scattering Albedo
Aerosol Climatology
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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
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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
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UV Attenuation from GFS Clouds
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Clouds and UV Attenuation
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Clouds and UV Attenuation
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Clouds and UV Attenuation
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Clouds and UV Attenuation
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Clouds and UV Attenuation
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Products: MOS Locations for CONUS and AK
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Products: 58 City Bulletin
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Products: Cloudy UV Index Output Grids
Eta Grid 207 Eta Grid 211
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Products: UV Index Contoured Map
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Products: Global UV Index
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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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fini