ultraviolet index craig s. long national oceanic and atmospheric administration national weather...
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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
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.
• 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
• 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)
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
• 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
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 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